Tibial component

Abstract

A reciprocating surgical cutting tool 3, for cutting a slot 21 into a resected proximal tibia to receive the keel of a joint replacement tibial component, comprises an elongate body defining the reciprocating axis of the tool. A cutting portion 36 extends outward from the elongate body and has an inclined distal cutting profile (37, Fig. 15) that overhangs the reciprocating axis to form a correspondingly shaped undercut 23 in the bone at the end of the keel slot. The cutting head 36 may also have a non-overhanging proximal cutting profile that shapes the other end 24 of the keel slot without forming an undercut. The cutting edges may have teeth. The cutting tool may be supplied with a slotted tibial cutting guide and a tibial implant, e.g. for partial knee arthroplasty, with the ends of the implant keel shaped to correspond to the tool’s cutting edges, including an overhanging end (121", Fig. 6). The implant keel may have a lattice or framework structure, e.g. a grid or a porous, cancellous, or trabecular structure.

Description

This invention relates generally, but not exclusively, to a tibial component for an orthopaedic implant and to a surgical saw for knee arthroplasty. Total or partial knee replacement surgery, also known as total or partial knee arthroplasty (TKA or PKA), is a surgical procedure to resurface a damaged knee joint. During knee arthroplasty the medial and / or lateral side of the tibia of the knee joint are removed and replaced with a tibial component. There are numerous known designs of tibial component for knee arthroplasty. For example some tibial components require bone cement to secure the tibial component to the tibia, and some are cementless, meaning that they are held in place by an interference fit and / or by one or more fasteners. In most cases, the tibial component has a bone insertion feature which extends from a bone-interfacing side of a plate portion of the tibial component, and which is configured to be received in a corresponding slot or cavity in the tibia. In cases where fixation relies upon an interference fit, it is important that the bone-insertion feature is strong enough to withstand insertion forces during surgery, which can lead to such features being overly bulky. Otherwise, this may lead to the opting of a looser fit between the keel and bone-insertion feature and the bone material, which may lead to loosening of the implant. In order to perform knee arthroplasty, a surgeon typically makes a cut down the anterior side of the knee and moves the patella to one side to access the knee joint. The tibia is then resected, and one or more cuts or cavities are formed in the resected tibia for receipt of the one or more bone insertion features. When the surgeon places the bone-insertion features into the tibia, they must align the tibial component correctly, and might have to rely upon their cuts and / or cavities to correctly position the tibial component. Due to limited access, misalignment can occur, especially as the surgeon has limited visibility of the posterior side of the knee joint. Furthermore, the surgeon may find it difficult to fully bed the bone-interfacing side of the plate portion onto the resected tibia, which is even more difficult on the posterior side which the surgeon might find it difficult to see, especially when an interference fit is used. Inaccurate placement may result in poor fixation of the tibial component. Poor fixation of the tibial component might cause damage to the surrounding bone surface through wear which can cause loosening of the implant. Loosening of the implant may reduce the effectiveness of the implant and may cause the patient problems with mobility and pain. Furthermore, the problems that may arise from inaccurate placement of the tibial component can result in further surgical intervention being required, or being required sooner than would be necessary with a correctly positioned implant. It would therefore be advantageous to overcome at least some of the aforementioned limitations. Accordingly, a first aspect of the invention provides a tibial component for, e.g., for use in, an orthopaedic implant for knee arthroplasty, the tibial component comprising: a plate portion which comprises a bone-interfacing side; and a keel extending from the bone-interfacing side of the plate portion, the keel having a length with a posterior side at one end of the length, an anterior side at the other end of the length and an inferior side extending between the posterior and anterior sides, the keel being configured to be inserted into a keel slot which extends in a posterior to anterior direction in a tibia of a patient. It will be understood that the keel slot is a cut in the tibia of the patient, the keel slot being configured to receive the keel of the tibial component. It will be understood that the tibial component may also be referred to as a tibial implant component. That is, the tibial component is configured to provide a part of the orthopaedic implant. The keel may comprise a central portion which is a porous structure. The keel may comprise a substantially non-porous keel peripheral rim extending around a perimeter of, and connected to, the central portion. A posterior part of the keel peripheral rim may provide the posterior side of the keel. An anterior part of the keel peripheral rim may provide the anterior side of the keel. An inferior part of the keel peripheral rim may provide the inferior side of the keel. A posterior end of the inferior part of the keel peripheral rim may be connected to the posterior part of the keel peripheral rim and an anterior end of the inferior part of the keel peripheral rim may be connected to the anterior part of the keel peripheral rim. The posterior side of the keel may comprise an overhanging portion to define an undercut between the overhanging portion and the plate portion. The undercut may be configured to engage with bone material when the keel is inserted into the keel slot. The overhanging portion of the posterior side of the keel may be substantially straight. The overhanging portion of the posterior side of the keel may extend away from the plate portion with a posterior direction component. It will be understood that the posterior direction is a posterior direction when the keel is located in the keel slot, in use. According to a further aspect of the invention, there is provided a tibial component for, e.g., for use in, an orthopaedic implant for knee arthroplasty, the tibial component comprising: a plate portion which comprises a bone-interfacing side; and a keel extending from the bone-interfacing side of the plate portion, the keel having a length with a posterior side at one end of the length, the keel being configured to be inserted into a keel slot which extends in a posterior to anterior direction in a tibia of a patient; wherein the posterior side of the keel comprises an overhanging portion to define an undercut between the overhanging portion and the plate portion, the undercut being configured to engage with bone material when the keel is inserted into the keel slot. Advantageously, when a surgeon is performing knee arthroplasty from an anterior side, the overhanging portion of the posterior side of the keel can be located at a posterior end of the keel slot such that bone material is present in the undercut between the overhanging portion and the plate portion. This may robustly secure the posterior side of the implant in place and prevent posterior lift-off of the implant during insertion into the knee. This may enable better and more secure positioning of the tibial tray during total or partial knee arthroplasty. It will be understood that the tibial component may also be referred to as a tibial implant component. That is, the tibial component is configured to provide a part of the orthopaedic implant. According to a further aspect of the invention, there is provided a tibial component for, e.g., for use in, an orthopaedic implant for knee arthroplasty, the tibial component comprising: a plate portion which comprises a bone-interfacing side; and a keel extending from the bone-interfacing side of the plate portion, the keel having a length with a posterior side at one end of the length, an anterior side at the other end of the length and an inferior side extending between the posterior and anterior sides, the keel being configured to be inserted into a keel slot which extends in a posterior to anterior direction in a tibia of a patient; wherein the keel comprises a central portion which is a porous structure, and a substantially non-porous keel peripheral rim extending around a perimeter of, and connected to, the central portion, a posterior part of the keel peripheral rim providing the posterior side of the keel, an anterior part of the keel peripheral rim providing the anterior side of the keel, and an inferior part of the keel peripheral rim providing the inferior side of the keel. A posterior end of the inferior part of the keel peripheral rim may be connected to the posterior part of the keel peripheral rim and an anterior end of the inferior part of the keel peripheral rim may be connected to the anterior part of the keel peripheral rim. It will be understood that the tibial component may also be referred to as a tibial implant component. That is, the tibial component is configured to provide a part of the orthopaedic implant. The inferior part of the keel peripheral rim may extend along an inferior side of the central portion. The anterior part of the keel peripheral rim may extend along an anterior side of the central portion. The posterior part of the keel peripheral rim may extend along a posterior side of the central portion. The inferior side may be substantially parallel to the plate portion. The inferior side may be substantially straight. Advantageously, the peripheral rim may provide increased strength and stiffness to withstand insertion forces, as the keel is inserted into a keel slot in the tibia. This may, in turn, allow for a tighter fit between the keel slot and the keel, which means that there might be closer engagement between bone material and the porous structure of the keel, thereby promoting bone ingrowth into the porous structure of the keel. The porous structure may allow bone in-growth into the keel, once the tibial component has been secured in place. Furthermore, the peripheral rim may provide less abrasion to the bone material than the porous structure of the keel, also allowing for a tighter interference fit. This may cause improved initial fixation of the tibial component whilst allowing long term bone ingrowth, due to the combination of a non-porous rim and a porous structure. This may better secure the tibial component in place, and lead to better longevity of the implant and more comfort for the patient. Furthermore, better securing of the tibial component may reduce deterioration of the bone in contact with the tibial component, which might slow down or prevent the onset of loosening of the tibial component. The keel structure may also lead to increased strength and / or stiffness of the keel, which, in turn, may improve the strength and / or stiffness of the part of the plate portion from which the keel extends. The tibial component may be for partial knee arthroplasty. The keel peripheral rim may comprise a superior part which is connected to the plate portion. The superior part of the keel peripheral rim may provide a superior side of the keel. A posterior end of the superior part of the keel peripheral rim may be connected to the posterior part of the keel peripheral rim. An anterior end of the superior part of the keel peripheral rim may be connected to the anterior part of the keel peripheral rim. The superior part of the keel peripheral rim may extend along a superior side of the central portion. The keel may comprise one or more reinforcing region. Each of the one or more reinforcing region may divide regions of the porous structure of the central portion of the keel. That is, the central portion may be divided by the one or more reinforcing regions. Each reinforcing region may be substantially non-porous. Each reinforcing region may have a greater elastic modulus than the porous structure of the central portion. Each reinforcing region may extend in any one of the following directions: between the inferior part of the keel peripheral rim and the plate portion or the superior part of the keel peripheral rim; between the inferior part of the keel peripheral rim and the posterior part of the keel peripheral rim; between the inferior part of the keel peripheral rim and the anterior part of the keel peripheral rim; between the posterior part of the keel peripheral rim and the plate portion or the superior part of the keel peripheral rim; between the anterior part of the keel peripheral rim and the plate portion or the superior part of the keel peripheral rim; between the posterior part of the keel peripheral rim and the anterior part of the keel peripheral rim Advantageously, the one or more reinforcing region may be used to provide additional strength and / or stiffness to the keel. The size and / or position of each of the reinforcing region may be optimised to provide a required, desired or predetermined strength and / or stiffness while maintaining a maximum, or acceptable, amount of porous structure. Advantageously, bone ingrowth into the keel may be maximised while maintaining sufficient strength and / or stiffness of the keel. The keel may be trapezoidal in shape. The anterior side of the keel may extend away from the bo ne-interfacing side of the plate portion with a posterior direction component. The posterior side of the keel may extend from the bone-interfacing side of the plate portion with an anterior direction component. Advantageously, during insertion of the trapezoidal keel into a keel slot with a corresponding trapezoidal shape, the keel may self-centre in the keel slot to lead to better placement of the tibial component. The posterior side of the keel may comprise an overhanging portion to define an undercut between the overhanging portion and the bone-interfacing side of the plate portion. The undercut may be configured to engage with bone material when the keel is inserted into the keel slot. Advantageously, when a surgeon is performing knee arthroplasty from an anterior side, the overhanging portion of the posterior side of the keel can be located at a posterior end of the keel slot such that bone material is present in, or engages, the undercut between the overhanging portion and the plate portion. This may robustly secure the posterior side of the implant in place and prevent posterior lift-off of the implant during insertion into the knee. This may, in turn, enable better and more secure positioning of the tibial tray during total or partial knee arthroplasty. The overhanging portion of the posterior side of the keel may be substantially straight. The overhanging portion of the posterior side of the keel may extend away from the plate portion with a posterior direction component. An angle between the bone-interfacing side of the plate portion and the overhanging portion of the posterior side of the keel, measured on a posterior of the posterior side of the keel, may be between 45 and 90 degrees, for example between 45 and 80 degrees, between 45 and 70 degrees, or between 45 and 60 degrees. The overhanging portion may comprise the furthest point of the posterior side of the keel from the plate portion. The posterior side of the keel may comprise a fillet or chamfer at an intersection, or joining portion, between the plate portion and the overhanging portion of the posterior side of the keel. The undercut may be defined between the furthest point of the posterior side of the keel from the plate portion and the fillet or chamfer. Advantageously, by providing a straight overhanging portion, pushing the tibial component in the posterior direction, during surgery, may cause the tibial component to also move in an inferior direction to provide a more robust fit of the tibial component into the tibia. The anterior side of the keel may extend away from the plate portion with a posterior direction component. It will be understood that the posterior direction is the posterior direction when the keel is located in the keel slot, in use. The anterior side of the keel may comprise a contact portion configured to contact bone material when in the keel slot. The contact portion may extend in a direction away from the plate portion with a posterior direction component. That is, the contact portion of the anterior side of the keel may extend in the posterior direction away from the plate portion. The anterior side of the keel may comprise a fillet or chamfer at an intersection, or joining portion, between the plate portion and the contact portion of the anterior side of the keel. Advantageously, when the surgeon is performing knee arthroplasty from an anterior side, the anterior side, or the contact portion, of the keel might be slid along a corresponding anterior bone surface of the keel slot, which may cause the keel to move in a posterior direction to provide more secure engagement between the undercut of the posterior side of the keel and the posterior side of the keel slot. The anterior side of the keel, or the contact portion of the anterior side of the keel, may be substantially straight. The anterior side of the keel, or the contact portion of the anterior side of the keel, may extend from the plate portion at an angle, measured on an anterior of the anterior side, of between 90 and 160 degrees, for example between 100 and 160 degrees, between 110 and 160 degrees, between 120 and 160 degrees or between 130 and 160 degrees. Advantageously, this may allow a greater number of tibial components to be additively manufactured on a platen, as the tibial components can be manufactured with the plate portion substantially vertical and the anterior side of the keel facing downwards, without a requirement that support elements are used to support the keel. This is advantageous because support elements require removal in a later manufacturing step, and so this may make manufacturing more efficient. An angle between the overhanging portion of the posterior side of the keel and the boneinterfacing side of the plate portion, measured on a posterior of the posterior side of the keel, may be greater than an angle between the contact portion of the anterior side of the keel and the bone-interfacing side of the plate portion, measured on a posterior of the anterior side of the keel. Advantageously, this may allow for the undercut to be hooked under bone material on the posterior side, and then the anterior side slid down an anterior side of the keel slot to robustly engage the keel with the keel slot. The keel may have a thickness, in the medial to lateral direction, of less than 7 mm, for example less than 5 mm. The keel may have a thickness, in the medial to lateral direction, of greater than 1 mm. The keel may have a thickness, in the medial to lateral direction, of between 1 and 5 mm. The keel may become narrower with distance from the plate portion. Advantageously, this narrowing of the keel may help with insertion of the keel into the keel slot, in use. The tibial component may comprise one or more peg extending from the bone-interfacing side of the plate portion. The tibial component may comprise two pegs extending from the bone-interfacing side of the plate portion. The or each peg may be configured to be received in a respective hole, for example a reamed hole, in the tibia of the patient. The or each peg may extend away from the bone-interfacing side of the plate portion with a posterior direction component. The or each peg may comprise a porous, or lattice, structure. The or each peg may comprise a porous, or lattice, structure through an entire thickness, or width, of the peg. Advantageous, bone ingrowth may be achieved through the pegs. The or each peg may comprise a solid peripheral portion extending around the porous, or lattice structure, at a tip of the peg. The plate portion may comprise a second side which is opposite to the bone-interfacing side. The second side may be configured to interface with an articulating joint component. The tibial component may comprise one or more protrusion extending from the second side of the plate portion. One of the one or more protrusion may be located at a position on the second side corresponding to a position of the intersection, or joining portion, between the posterior side of the keel and the plate portion on the bone-interfacing side. One of the one or more protrusion may be located at a position on the second side corresponding to a position of the intersection, or joining portion, between the posterior side of the keel and the plate portion on the bone-interfacing side. Advantageously, the effects of any stress concentration caused by the side of the keel may be mitigated by the protrusions. The plate portion may comprise a porous structure. The plate portion may comprise a substantially non-porous peripheral portion, or rim, surrounding a central portion. The central portion may be, or comprise, the porous structure. The peripheral portion may be a solid band. A peripheral portion of the bone-interfacing side may be provided by the peripheral rim. A central portion of the bone-interfacing side may be provided by the central portion of the plate portion. Advantageously, the peripheral rim may provide increased stiffness and strength of the plate portion, may provide a relatively smooth edge, and may provide means for fixing the articulating joint component to the tibial component. The peripheral rim may have a thickness, where the keel is connected thereto, which is greater a thickness of the keel. The tibial component may be for partial knee arthroplasty. The keel may extend from the peripheral rim of the plate portion. Advantageously, this may provide a stronger joint between the keel and the plate portion, than if the keel extended from a porous structure. The peripheral rim of the plate portion may comprise: a side portion, which is one of a medial portion or a lateral portion, in use; an anterior portion; and a posterior portion; wherein the side portion is located between the anterior portion and the posterior portion, and wherein a thickness of the side portion is greater than a thickness of one or each of the anterior portion and the posterior portion. Because the peripheral rim is stiffer than the porous structure of the central portion, the majority of the force applied to the tibia through the tibial component is imparted by the peripheral rim because the porous structure is not sufficiently stiff to transmit as much force. Therefore, increasing the thickness of the bone-interfacing side may reduce the stress installed in the bone by the peripheral rim, because the force is applied over a greater area. Accordingly, bone deformation in this region may be reduced, which may reduce subsidence of the implant component. By reducing the stress in the bone imparted by the first portion of the peripheral rim, and so reducing deformation of the bone in this region, more stress may thus be transmitted through the porous structure to the bone material in contact with the porous structure, thereby producing strains in this bone material which better resemble strain observed in healthy, native bone material. This strain installed in the bone adjacent the porous structure may also better match strain in the porous structure, and so promote bone ingrowth into the porous structure, which may also increase the longevity of the implant. The implant component may also be advantageous in that the rim has increased strength at the first portion, for example to offer improved fatigue strength. The thickness of one or each of the anterior portion and the posterior portion may be less than 11 times the thickness of the side portion. Any or each of the side, anterior or posterior portions of the peripheral rim may have a length, along the perimeter of the central portion, which is at least 5%, or at least 10%, 20%, 25%, 30%, 35%, 40% or 45% of a length of the perimeter of the central portion. That is, the side portion of the peripheral rim may have a length, along the perimeter of the central portion, which is at least 5%, or at least 10%, 20%, 25%, 30%, 35%, 40% or 45% of a length of the perimeter of the central portion. The anterior portion of the peripheral rim may have a length, along the perimeter of the central portion, which is at least 5%, or at least 10, 20%, 25%, 30%, 35%, 40% or 45% of a length of the perimeter of the central portion. A third portion of the peripheral rim may have a length, along the perimeter of the central portion, which is at least 5%, or at least 10%, 20%, 25%, 30%, 35%, 40% or 45% of a length of the perimeter of the central portion. The side portion may be located between the second and third portions. The peripheral rim may comprise a first intermediate portion between, and adjacent e.g., and connected to, both of, the anterior portion and the side portion. The peripheral rim may comprise a second intermediate portion between, and adjacent e.g., and connected to, both of, the posterior portion and the side portion. A thickness of the first intermediate portion may vary with position between the side portion and the anterior portion. A thickness of the second intermediate portion may vary with position between the side portion and the posterior portion. At least a part of the thickness of the first intermediate portion may vary linearly with position between the side portion and the anterior portion. At least a part of the thickness of the second intermediate portion may vary linearly with position between the posterior portion and side portion. Intersections, or joining portions, between the first intermediate portion and each of the anterior portion and the side portion may be rounded, for example filleted. Intersections, or joining portions, between the second intermediate portion and each of the posterior portion and the side portion may be rounded, for example filleted. Advantageously, this may reduce stress concentrations in these regions, both in the component and in adjacent bone material. The side portion may have a substantially constant thickness. The anterior portion may have a substantially constant thickness. The posterior portion may have a substantially constant thickness. The tibial component may be for partial knee arthroplasty, and the keel may extend from a keel portion of the peripheral rim of the plate portion. The keel portion may have a thickness which is greater than a thickness of one or each of the anterior portion and the posterior portion. The keel portion may be located between the anterior portion and the posterior portion. The keel portion may be on an opposite side to the side portion. The keel portion may have a thickness which is less than the thickness of the side portion. The keel portion of the peripheral rim may have a length, along the perimeter of the central portion, which is at least 5%, or at least 10%, 20%, 25%, 30%, 35%, 40% or 45% of a length of the perimeter of the central portion. Advantageously, the keel portion of the peripheral rim may provide a solid base structure for the keel. The keel portion of the peripheral rim may have a thickness which is greater a thickness of the keel. The or each peg may extend from the porous structure of the plate portion. The keel portion of the peripheral rim may have a height, in a direction extending away from the keel, which is greater in a location of the posterior end and / or a location of the anterior end of the keel than away from the posterior and / or anterior end of the keel. The porous structure of the keel and / or the plate portion and / or of the one or more pegs may be, or comprise, a lattice structure. The lattice structure may comprise a plurality of struts connected together at nodes. The struts may have directional components in 3 Cartesian directions. The porous structure of the keel and / or the plate portion and / or the pegs may have an elastic modulus, e.g., a bulk elastic modulus, of between 0.1 and 5 GPa. The porous structure of the keel and / or the plate portion and / or the one or more pegs may have a porosity of between 5 and 30 %. The lattice structure of the keel and / or the plate portion and / or one or more pegs may have a strut density of between 2 and 7 struts per mm3. Each strut may have a thickness, or diameter, of between 100 and 400 microns. The component may be a unitary structure. The component may be metallic. The component may be made from titanium alloy, for example Ti64. The component may be manufactured using additive manufacturing, for example laser sintering. The plate portion may comprise a second side which is configured to interface with an articulating joint component, for example a plastic articulating joint component. The articulating joint component may be configured to be in articulating engagement with a femoral component, or with a femur, in use. A further aspect of the invention provides a tibial implant comprising the aforementioned tibial component and an articulating joint component attached to the second side of the plate portion of the tibial component. The articulating joint component may be a plastic articulating joint component. The articulating joint component may be configured to slidingly engage with an articulating surface of a femoral component or of a femur. The articulating joint component may be overmoulded onto the second side of the plate portion. A further aspect of the invention provides a kit of parts comprising: the aforementioned tibial component or the aforementioned tibial implant; a surgical cutting tool, the surgical cutting tool being for cutting a keel slot in a superior end of a tibia, the keel slot for receiving the keel of the tibial component, the surgical cutting tool comprising: an elongated body having a proximal end and a distal end, a length of the elongated body defining a reciprocating axis; and a cutting portion extending from a side of the elongated body, the cutting portion having a cutting profile with a distal side and a proximal side, the distal side being a side of the cutting profile furthest from the proximal end of the elongated body along the reciprocating axis and the proximal side being a side of the cutting profile nearest to the proximal end of the elongated body along the reciprocating axis, the distal side of the cutting profile having a shape corresponding to the shape of the posterior side of the keel and the proximal side of the cutting profile having a shape corresponding to the shape of the anterior side of the keel. The kit of parts may further comprise a cutting template configured for use with the surgical cutting tool, the cutting template for attachment to a resected tibia and comprising a keel slot guide for receiving a cutting portion of the surgical cutting tool, in use, to produce a keel slot for receiving the keel of the tibial component A further aspect of the invention provides kit of parts comprising: the aforementioned tibial component, the aforementioned tibial implant or the aforementioned kit of parts; and a cutting template configured for use with the or a surgical cutting tool, the cutting template for attachment to a resected tibia and comprising a keel slot guide for receiving a cutting portion of the surgical cutting tool, in use, to produce a keel slot for receiving the keel of the tibial component A further aspect of the invention provides a surgical cutting tool comprising: an elongated body having a proximal end and a distal end, a length of the elongated body defining a reciprocating axis; and a cutting portion extending from a side of the elongated body, the cutting portion having a cutting profile with a distal side and a proximal side, the distal side being a side of the cutting profile furthest from the proximal end of the elongated body along the reciprocating axis and the proximal side being a side of the cutting profile nearest to the proximal end of the elongated body along the reciprocating axis, the distal side of the cutting profile having a shape corresponding to the shape of the posterior side of the keel of the aforementioned tibial component and the proximal side of the cutting profile having a shape corresponding to the shape of the anterior side of the keel of the aforementioned tibial component A further aspect of the invention provides a method of manufacturing any one of the aforementioned tibial components, the method comprising: providing a metal powder; selectively heating areas of the metal powder to fuse the metal powder together to form the tibial component. Advantageously there may be less material wastage by using this additive manufacturing method than when a tibial component is produced using a subtractive manufacturing method. The additive manufacturing method may also be more efficient due to the ability to selectively heat specific, small areas of metal powder. The keel of the tibial component may comprise an anterior side which is at a second end of the keel, the second end being the opposite end to the posterior side in the length direction of the keel, the anterior side of the keel comprising a contact portion configured to contact bone material when in the keel slot, the contact portion extending in a direction away from the plate portion and towards the posterior side of the keel. The method may comprise additively manufacturing the tibial component on a horizontal platen, with the plate portion of the tibial component orientated vertically, with no support structure being formed between the anterior side of the keel and the platen. Advantageously, more tibial components can be produced during the additive manufacturing process because the tibial components are manufactured on their ends. Further advantageously, a second manufacturing step is omitted because support structures need not be removed. The method may comprise additively manufacturing a plurality of tibial components on the platen. A further aspect of the invention provides computer readable instructions which, when executed by an additive manufacturing machine, are configured to cause the additive manufacturing machine to perform the aforementioned method of manufacturing the tibial component. A further aspect of the invention provides method of performing knee arthroplasty, or a method of performing knee arthroplasty on a cadaveric specimen, the method comprising: attaching a keel cutting template to a resected tibia; inserting a cutting tool through the keel cutting template and operating the tool to cut a keel slot in the resected tibia, the keel slot having a profile which conforms to a profile of the keel of any one of the aforementioned tibial components; removing the template from the resected tibia; and inserting the keel of the tibial component into the keel slot. For the avoidance of doubt, any of the features described herein apply equally to any aspect of the invention. That is, features of each aspect of the invention may be combined with other aspects of the invention. More specifically, features of each aspect of the tibial component may be combined with other aspects of the tibial component, for example optional features of the tibial component may be applicable to any aspect of invention of the tibial component. It will be appreciated that anatomical directions used throughout, refer to anatomical directions when the tibial component is in-situ in the intended position and orientation in a patient. That is, posterior, anterior, superior, inferior, lateral and medial have their usual meanings, and refer to when the tibial component is in-situ in the intended position and orientation in a patient. A further aspect of the invention provides a surgical cutting tool, the surgical cutting tool being for cutting a keel slot in a superior end of a tibia, the surgical cutting tool comprising: an elongated body having a proximal end and a distal end, a length of the elongated body defining a reciprocating axis; and a cutting portion extending from a side of the elongated body, the cutting portion having a cutting profile with a distal side, which is a side of the cutting profile furthest from the proximal end of the elongated body along the reciprocating axis. The distal side of the cutting profile may comprise an overhanging portion which defines an undercut between the overhanging portion and the reciprocating axis. The distal side of the cutting profile may extend away from the elongated body with a direction component along the reciprocating axis and away from the proximal end of the elongated body. The surgical cutting tool may be configured such that when reciprocated along the reciprocating axis and used to cut the keel slot, the cutting tool produces the keel slot with a profile at one end corresponding to the distal side of the cutting profile. A further aspect of the invention provides a surgical cutting tool, the surgical cutting tool being for cutting a keel slot in a superior end of a tibia, the surgical cutting tool comprising: an elongated body having a proximal end and a distal end, a length of the elongated body defining a reciprocating axis; and a cutting portion extending from a side of the elongated body, the cutting portion having a cutting profile with a distal side, which is a side of the cutting profile furthest from the proximal end of the elongated body along the reciprocating axis, the distal side of the cutting profile comprising an overhanging portion which defines an undercut between the overhanging portion and the reciprocating axis; wherein the surgical cutting tool is configured such that when reciprocated along the reciprocating axis and used to cut the keel slot, the cutting tool produces the keel slot with a profile at a first end corresponding to the distal side of the cutting profile. Advantageously the cutting portion may be used to cut an overhanging shape in a keel slot, such that a keel of a tibial component, which has a corresponding overhang, can be inserted into the keel slot more securely to realise the aforementioned advantages with a tibial component with such a keel shape. The surgical cutting tool may be for use with or in a reciprocating surgical saw. The proximal end of the elongated body may be configured for attachment to the reciprocating surgical saw. The proximal end of the elongated body may be configured for being releasably securable in a chuck or jaws of the reciprocating surgical saw. The distal side of the cutting profile may extend away from the elongated body with a direction component along the reciprocating axis and away from the proximal end of the elongated body. The distal side of the cutting profile may correspond to a distal side of the cutting portion. The distal side of the cutting portion may comprise one or more cutting element. The one or more cutting element comprised on the distal side of the cutting portion may be a plurality of teeth, e.g., cutting teeth or saw teeth. The teeth may extend along a line, for example a line extending through tips of the teeth. The line may extend in a linear direction from the elongated body. The line may extend away from the elongated body with a direction component along the reciprocating axis away from the proximal end of the elongated body. The distal side of the cutting profile may extend in a substantially linear direction from the elongated body. The distal side of the cutting profile may extend at an angle of between 45 and 90 degrees relative to the reciprocating axis, on a distal side of the distal side of the cutting profile. The distal side of the cutting profile may extend in a direction away from the proximal end of the elongated body. The line extending through the tips of the teeth on the distal side of the cutting portion may extend at an angle of between 45 and 90 degrees relative to the reciprocating axis, on a distal side of the distal side of the cutting profile. The line extending through the tips of the teeth on the distal side of the cutting portion may extend in a direction away from the proximal end of the elongated body. The cutting profile may comprise a proximal side, which is a side of the cutting profile nearest to the proximal end of the elongated body. At least a part of, for example a majority of or all of, the proximal side of the cutting profile may extend in a direction away from the elongated body with a direction component along the reciprocating axis away from the proximal end of the elongated body. The surgical cutting tool may be configured such that when reciprocated along the reciprocating axis and used to cut the keel slot, the cutting tool produces the keel slot with a profile at a second end, opposite to the first end, corresponding to the proximal side of the cutting profile. The proximal side of the cutting profile may correspond to a proximal side of the cutting portion. The proximal side of the cutting portion may comprise one or more cutting element. The one or more cutting element comprised on the proximal side of the cutting portion may be a plurality of teeth, e.g., cutting teeth or saw teeth. The teeth may extend along a line, for example a line extending through tips of the teeth. The line may extend in a linear direction from the elongated body. The line may extend away from the elongated body with a direction component along the reciprocating axis away from the proximal end of the elongated body. The at least part of the proximal side of the cutting profile may be less inclined relative to the reciprocating axis, than the overhanging portion of the distal side of the cutting profile. The proximal side of the cutting profile may extend in a linear direction from the elongated body. The proximal side of the cutting profile may extend at an angle of between 90 and 160 degrees relative to the reciprocating axis in a direction away from the proximal end of the elongated body. The line extending through the tips of the teeth on the proximal side of the cutting portion may extend at an angle of between 90 and 160 degrees relative to the reciprocating axis and in a direction away from the proximal end of the elongated body. A third side of the cutting profile may extend between the distal side of the cutting profile and the proximal side of the cutting profile. The third side of the cutting profile may comprise one or more cutting element. The third side may extend in a linear direction which is substantially parallel to the reciprocating axis. The one or more cutting element comprised on the third side of the cutting portion may be a plurality of teeth e.g., cutting teeth or saw teeth. The cutting teeth may extend along a line, for example a line extending through tips of the teeth. The line may extend in a linear direction parallel to the reciprocating axis. The distal side of the cutting portion may comprise an abutment portion which is devoid of cutting elements and which is adjacent the elongated body. The proximal side of the cutting portion may comprise an abutment portion which is devoid of cutting elements and which is adjacent the elongated body. Advantageously, when the surgical cutting tool is used with a cutting template, the abutment portions on the distal and proximal sides of the cutting portion limit cutting movement. The abutment portion of the distal and / or proximal sides of the cutting portion may be configured to abut ends of a keel slot guide in a cutting template, in use, for example to limit cutting movement of the surgical cutting tool. An end of the distal side of the cutting portion, which is furthest from the elongated body, may comprise a leading tooth. The leading tooth may have triangular shape with an angle at a cutting tip of the leading tooth, a central intersection of the angle of the cutting tip extending away from the elongated body with a direction component along the reciprocating axis and away from the proximal end of the elongated body. Advantageously, the leading tooth may be especially effective at removing bone material during cutting of the keel slot. The central intersection of the angle of the cutting tip of the leading tooth may be less inclined, relative to the reciprocating axis, than the overhanging portion of the distal side of the cutting profile. The elongated body may comprise a head portion from which the cutting portion extends. The head portion may comprise an abutment surface at least partially surrounding the cutting portion. The abutment surface of the head portion may be configured to abut a surface of a cutting template, in use, for example to provide a cutting depth stop. Advantageously, the abutment surface may provide a depth stop for the cutting tool. The elongated body may comprise a shaft portion extending between the head portion and the proximal end of the elongated body. The head portion may be wider, in a thickness direction of the cutting portion, than the shaft portion. The head portion may be substantially aligned with a centreline of the shaft portion. The cutting portion may be substantially aligned with the centreline of the shaft portion. A further aspect of the invention provides a reciprocating surgical saw comprising any one of the aforementioned surgical cutting tools. The surgical cutting tool may be permanently attached to the reciprocating surgical saw. The surgical cutting tool may be releasably securable in a chuck, or jaws, of the reciprocating surgical saw. A further aspect of the invention provides a kit of parts comprising: any one of the aforementioned surgical cutting tools, of the aforementioned reciprocating surgical saw; and a cutting template configured for use with the surgical cutting tool, the cutting template for attachment to a resected tibia and comprising a keel slot guide for receiving the cutting portion of the surgical cutting tool, in use. A further aspect of the invention provides a kit of parts comprising: any one of the aforementioned surgical cutting tools, the aforementioned reciprocating surgical saw, or the aforementioned kit of parts; and one or more tibial component for an orthopaedic implant for knee arthroplasty, the tibial component comprising: a plate portion which comprises a bone-interfacing side; and a keel extending from the bone-interfacing side of the plate portion, the keel having a length with a posterior side at one end of the length, the keel being configured to be inserted into a keel slot which extends in a posterior to anterior direction in a tibia of a patient; wherein the posterior side of the keel comprises an overhanging portion to define an undercut between the overhanging portion and the bone-interfacing side of the plate portion, the undercut being configured to engage with bone material when the keel is inserted into the keel slot produced by the surgical cutting tool. A further aspect of the invention provides a method of manufacturing any one of the aforementioned surgical cutting tools, the method comprising: providing a metal powder; selectively heating areas of the metal powder to fuse the metal powder together to form the surgical cutting tool. A further aspect of the invention provides computer readable instructions which, when executed by an additive manufacturing machine, are configured to cause the additive manufacturing machine to perform the aforementioned method of manufacturing the surgical cutting tool. For the avoidance of doubt, any of the features described herein apply equally to any aspect of the invention. That is, features of each aspect of the invention may be combined with other aspects of the invention. It will be appreciated that anatomical directions used throughout, refer to anatomical directions when the tibial component is in-situ in the intended position and orientation in a patient. That is, posterior, anterior, superior, inferior, lateral and medial have their usual meanings, and refer to when the tibial component is in-situ in the intended position and orientation in a patient Another aspect of the invention provides a computer program element comprising and / or describing and / or defining a three-dimensional design, e.g. of the tibial component of or the surgical cutting tool described above or an embodiment thereof. The three-dimensional design may be for use with a simulation means or an additive manufacturing means, system or device. Within the scope of this application it is expressly intended that the various aspects, embodiments, examples and alternatives set out in the preceding paragraphs, in the claims and / or in the following description and drawings, and in particular the individual features thereof, may betaken independently or in any combination. That is, all embodiments and / or features of any embodiment can be combined in any way and / or combination, unless such features are incompatible. For the avoidance of doubt, the terms “may”, “and / or”, “e.g.”, “for example” and any similar term as used herein should be interpreted as non-limiting such that any feature so-described need not be present. Indeed, any combination of optional features is expressly envisaged without departing from the scope of the invention, whether or not these are expressly claimed. The applicant reserves the right to change any originally filed claim or file any new claim accordingly, including the right to amend any originally filed claim to depend from and / or incorporate any feature of any other claim although not originally claimed in that manner. Embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is an isometric view of an inferior side of a tibial component of an orthopaedic implant for partial knee arthroplasty, according to an aspect of the invention; Figure 2 is an isometric view of a superior side of the tibial component shown in Figure 1; Figure 3 is a side view of an anterior side of the tibial component shown in Figure 1; Figure 4 is an isometric view of an inferior side of a tibial component of an orthopaedic implant for partial knee arthroplasty, according to another aspect of the invention; Figure 5 is an isometric view of an inferior side of a tibial component of an orthopaedic implant for partial knee arthroplasty, according to another aspect of the invention; Figure 6 is an isometric view of a superior side of the tibial component shown in Figure 5; Figure 7 is a plan view of a superior side of the tibial component shown in Figure 5; Figure 8 is a side view of a medial or lateral side of the tibial component shown in Figure 5; Figure 9 is an image of the tibial component shown schematically in Figure 5; Figure 10 is another image of the tibial component shown in Figure 9; Figure 11 is an isometric view of a superior side of a tibial component of an orthopaedic implant for partial knee arthroplasty, according to another aspect of the invention; Figure 12 is an isometric view of a superior side of a tibial component of an orthopaedic implant for partial knee arthroplasty, according to another aspect of the invention; Figure 13 is an image of a lattice structure used in the tibial components shown in Figures 1 to 10; Figure 14 is a schematic of a resected tibia prepared to receive the tibial component of any of Figures 5 to 10; Figure 15 is an isometric view of a surgical cutting saw according to another aspect of the invention; Figure 16 is a side view of the surgical cutting tool shown in Figure 15; Figure 17 is a plan view of the surgical cutting tool shown in Figure 15; Figure 18 is a schematic of an example of a cutting portion of the surgical cutting tool shown in Figure 13; Figure 19 is a schematic of the surgical cutting tool shown in Figure 15 used with a cutting template; Figure 20 is another schematic of the surgical cutting tool shown in Figure 15 used with the cutting template; and Figure 21 is a schematic showing the surgical cutting tool shown in Figure 15 being used to cut a keel slot in a tibia. Referring to Figures 1, 2 and 3, there is shown a first embodiment of a tibial component 1 of an orthopaedic implant for partial knee arthroplasty. It will be appreciated that anatomical directions used to describe this tibial component 1, as well as those used to describe tibial components in subsequent embodiments of the invention, relate to anatomical directions when the tibial component is correctly installed in a patient. More specifically, “lateral”, “medial”, “posterior”, “anterior”, “superior” and “inferior” all have their usual anatomical meanings, and relate to the respective direction when the tibial component is located in the patient. The tibial component 1 has a plate portion 11 which has a bone interfacing side B. The tibial component 1 has a keel 12 extending from the bone interfacing side B of the plate portion 11. The keel 12 extends from the bone-interfacing side B in an inferior direction. The keel 12 is configured to be inserted into a keel slot which extends in a posterior and anterior direction in a tibia of a patient. The keel 12 has a length with a posterior side 121 at one end of the length, an anterior end 122 at the other end of the length and an inferior side 126 extending between the posterior and anterior sides 121, 122. In this example the inferior side 123 is substantially parallel to the plate portion 11 and is substantially straight. The keel 12 has a central portion 124 which is a porous structure. In this example the porous structure is a lattice structure, which is made up of a plurality of stuts connected together at nodes. The lattice structure is not shown in Figures 1 to 3 but is visible in the keel of another embodiment of a tibial component in Figure 10. The keel 12 has a substantially non-porous keel peripheral rim 123 extending around a perimeter of, and connected to, the central portion 124. A posterior part of the keel peripheral rim provides the posterior side 121 of the keel, an anterior part of the keel peripheral rim provides the anterior side 121 of the keel, and an inferior part of the keel peripheral rim 123 provides an inferior side 126 of the keel. A posterior end of the inferior part of the keel peripheral rim 123 is connected to the posterior part of the keel peripheral rim 123 and an anterior end of the inferior part of the keel peripheral rim 123 is connected to the anterior part of the keel peripheral rim 123. The keel peripheral rim 123 also has a superior part which is connected to the plate portion 11. A posterior end of the superior part of the keel peripheral rim 123 is connected to the posterior part of the keel peripheral rim 123 and an anterior end of the superior part of the keel peripheral rim 123 is connected to the anterior part of the keel peripheral rim 123. The superior part of the keel peripheral rim 123 extends along, and is connect to, a superior side of the central portion 124. The inferior part of the keel peripheral rim 123 extends along, and is connected to, an inferior side of the central portion 124. The anterior part of the keel peripheral rim 123 extends along, and is connected to, an anterior side of the central portion 124. The posterior part of the keel peripheral rim 123 extends along, and is connected to, a posterior side of the central portion 124. Therefore the lattice structure of the central portion 124 of the keel 12 is completely surrounded by the keel peripheral rim 123. The keel peripheral rim 123 provides additional strength and stiffness to the keel 12, beyond that which would be achieved if the keel consisted of only a lattice structure. This is such that the keel 12 can be inserted into a keel slot in a tibia with a tighter fit between the bone material and the keel 12. This provides close contact with bone material and so promotes bone ingrowth into the lattice structure of the central portion 124. This also increases the strength and / or stiffness of the keel 12, which, in turn, offers greater strength and / or stiffness to the part of the plate portion 11 from which it extends. In this example the keel 12 has two reinforcing regions 125a, 125b. Each reinforcing region 125a, 125b divides regions of the lattice structure of the central portion 124 of the keel 12. In this example each reinforcing region 125a, 125b is substantially non-porous. However, it will be appreciated that each reinforcing region 125a, 125b may, instead, be provided by porous or lattice structure having a greater elastic modulus than the lattice structure of the central portion 124 of the keel 12. Both of the reinforcing regions 125a, 125b extend between the inferior part of the keel peripheral rim 123 and the superior part of the keel peripheral rim 123. In this way, the reinforcing regions 125a, 125b provide stiffeners, or strengtheners, for the keel 12. It will be appreciated that different numbers and configurations of reinforcing regions may be provided. For example, in other embodiments reinforcing regions may extends between the inferior part of the keel peripheral rim 123 and the anterior part of the keel peripheral rim 123; the posterior part of the keel peripheral rim 123 and the superior part of the keel peripheral rim 123; the anterior part of the keel peripheral rim 123 and the superior part of the keel peripheral rim 123; or the posterior part of the keel peripheral rim 123 and the anterior part of the keel peripheral rim 123. The size, position and orientation of the reinforcing regions can be optimised based upon factors affecting mechanical loading and bone ingrowth. For example, the size, position and orientation of the reinforcing regions may be optimised to maximise the amount of lattice structure whilst providing the keel with sufficient strength and stiffness. In the example of Figures 1,2 and 3 the keel 12 is trapezoidal in shape. The anterior side 122 of the keel 12 extends away from the bone-interfacing side B of the plate portion 11 with a posterior direction component. The posterior side 121 of the keel 12 extends from the bone-interfacing side B of the plate portion 11 with an anterior direction component. When the trapezoidal keel 12 is inserted into a keel slot with a corresponding trapezoidal shape, the keel 12 may self-centre in the keel slot to lead to better placement of the tibial component. In this example, the keel 12 has a thickness, in the medial to lateral direction, of less than 5 mm, and becomes narrower with distance from the plate portion 11. This narrowing of the keel 12 may help with insertion of the keel 12 into the keel slot. The bone-interfacing side B of the plate portion 11 has a central portion and a peripheral portion, the peripheral portion at least partially surrounding the central portion 111. The plate portion has a porous structure, which in this example is a lattice structure. The lattice structure is not shown in Figures 1 to 3 but is visible in the plate portion of another embodiment of a tibial component in Figure 9. However, it will be appreciated that other porous structures may be used. The lattice structure provides the central portion 111 of the bone-interfacing side B. The plate portion 11 has a substantially non-porous peripheral rim 112 extending around a perimeter of, and connected to, the lattice structure of the central portion 111. The peripheral rim 112 provides the peripheral portion of the bone-interfacing side B. The peripheral rim 112 extends around the entirety of the central portion 111. The plate portion 11 has a height HD of between 2 and 5 mm. A superior side of the plate portion 11 provides an second side A for interfacing with an articulating joint component (not shown), for example a polymer articulating joint component. The second side A is on the opposite side of the plate portion 11 to the bone-interfacing side B. The peripheral rim 112 provides a peripheral portion of the second side. The lattice structure provides a central portion of the second side. In other examples, the central portion of the second side if provided by a solid layer, this solid layer being disposed on the lattice structure. The articulating joint component may be attached to the second side A via attachment to the peripheral rim 112 or via attachment to the lattice structure. The articulating joint component, in this example, is configured for articulation with an articulation surface of a femoral component (not shown) or of a femur (not shown). The peripheral portion of the bone-interfacing side B is divided into at least two portions, which, in this example, correspond to the peripheral rim 112 being divided into at least two portions. This is because, in this example, a thickness TD of the peripheral rim 112, is substantially constant across a height HD at any position around the peripheral rim 112. A first portion of the at least two portions of the peripheral rim 112 is a side portion P1, which is either a medial or lateral side of the tibial component, in use. The side portion P1 is located between a second portion, which is an anterior portion P2, in use, and a third portion, which is a posterior portion P3, in use, of the peripheral rim 112. The side portion P1 has a greater thickness TD than the anterior portion P2 and a greater thickness TD than the posterior portion P3. The thickness TD of each of the anterior and posterior portions P2, P3 is less than 11 times the thickness TD of the side portion P1. In this example, each of the side, anterior and posterior portions P1, P2, P3 has substantially uniform, or constant, thickness TD. In this example the peripheral rim 112 of the tibial component 1 has a first intermediate portion 11 located between, and adjacent, and connected to both of, the anterior portion P2 and the side portion P1. The peripheral rim 112 of the tibial component 1 has a second intermediate portion I2 located between, and adjacent, and connected to both of, the posterior portion P3 and the side portion P1. The thickness TD of the first intermediate portion 11 varies with position between the side portion P1 and the anterior portion P2, and the thickness TD of the second intermediate portion I2 varies with position between the side portion P1 and the posterior portion P3. A thickness TD of at least a part of the first intermediate portion 11 varies linearly with position between the side and anterior portions P1, P2. A thickness TD of at least a part of the second intermediate portion I2 varies linearly with position between the side and posterior portions P1, P3. Intersections, or joining portions, between the first intermediate portion 11 and each of the side and anterior portions P2, P3 are rounded. Intersections, or joining portions, between the second intermediate portion I2 and each of the side and posterior portions P1, P3 are rounded. This design of peripheral rim 112 may reduce stress concentrations where the intermediate portions 11, I2 meet the side, anterior and posterior portions P1, P2, P3 of the peripheral rim 112. As shown by the dashed arrowhead lines in Figure 3, the thickness TD is defined along or parallel to the bone-interfacing side. This may also be referred to as along or parallel to a plane of the plate portion 11, in this example. The thickness TD may also be defined as extending away from the central portion at any position around a perimeter of the central portion. That is, the thickness TD is in a direction which is normal to the perimeter of the central portion at each location on the perimeter. It will be appreciated that, using this definition, the direction normal to the perimeter refers to the direction normal to the perimeter on a macro scale, and does not account for surface details of the porous, or lattice, structure. In this example, the side portion P1 of the peripheral rim has a length which is around 20% of a length of the perimeter of the central portion. The thickness TD of the side portion depends upon the size of the component used, and is between 2 and 15 mm. The anterior and posterior portions P2, P3 have lengths which are around 10% of the length of the perimeter of the central portion. The side, anterior and posterior portions P1, P2, P3 are shaped to follow a corresponding outer shape of the peripheral portion. That is, the thickness TD of each of the side, anterior and posterior portion P1, P2, P3 is uniform along the respective length. The first and second intermediate portions 11, I2 have lengths which are around 5% of the length of the perimeter of the central portion. It will be appreciated that the lengths of any of these portions P1, P2, P3, 11, I2 may be different, and can be tailored to the specific requirements of the components, to achieve optimum strain distribution in the bone. The keel 12 extends from a keel portion P4 of the peripheral rim 112. The keel portion P4 of the peripheral rim 112 has a thickness TD which is greater than the thickness TD of the anterior and posterior portions P2, P3. The keel portion P4 of the peripheral rim 112 has a thickness TD which is less than the thickness TD of the side portion P1. In this example the keel portion P4 has a length which is around 25% of the length of the perimeter of the central portion. The thickness of the keel portion P4 is greater than a thickness of the keel 12. The keel portion P4 thereby provides a robust base for the keel 12 of the tibial component 1. In this example, the tibial component 1 has two bone-insertion pegs 13 extending from the bone-interfacing side B of the plate portion 11. The bone insertion pegs 13 are configured to be received in reamed holes in the tibia of the patient. Each peg 13 extends away from the bone-interfacing side B with a posterior direction component. Some or all of each peg 13 is provided with a porous structure, which in this case is a lattice structure, to allow or promote bone ingrowth into the respective peg. Tips of each peg 13 are provided with a solid peripheral rim extending around the porous structure (as visible in Figure 9 with reference to another embodiment of the tibial component, with reference numeral 131”). Referring now to Figure 4, there is shown another embodiment of a tibial component T for partial knee arthroplasty. Similar features of this tibial component T are denoted with the same reference numerals as the tibial component 1 of the first embodiment, with a succeeding prime (‘). The tibial component 1’ of this embodiment differs from the tibial component 1 of the first embodiment in that there are no reinforcing regions in the keel 12”. Therefore, all strengthening and stiffening is provided by the keel peripheral rim 123’, thereby allowing a greater volume of lattice structure in the central portion 124’, to allow more bone ingrowth into the keel 12”. Referring now to Figures 5-10, there is shown another embodiment of a tibial component 1” for partial knee arthroplasty. Similar features of this tibial component 1” are denoted with the same reference numerals as the tibial component 1 of the first embodiment, with a succeeding double-prime (“). The tibial component 1” of this embodiment differs from the tibial component 1 of the first embodiment in that the posterior side 121” of the keel 12” of this embodiment has an overhanging portion to define an undercut between the overhanging portion and the plate portion 11”. The undercut is configured to engage with bone material when the keel 12” is inserted into the keel slot in the tibia of the patient. The overhanging portion of the posterior side 121” of the keel 12” is substantially straight and extends away from the plate portion 11” with a posterior direction component. An angle between the bone-interfacing side of the plate portion 11” and the overhanging portion of the posterior side 121” of the keel 12”, is around 70°, measured on a posterior of the posterior side 121” of the keel 12”. The overhanging portion has the furthest point of the posterior side 121” of the keel 12” from the plate portion 11”. The posterior side 121” of the keel 12” has a fillet at an intersection, or joining portion, between the plate portion 11” and the posterior side 121” of the keel 12”. The undercut is defined between the furthest point of the posterior side 121” of the keel 12” from the plate portion 11” and the fillet The anterior side 122” of the keel 12” extends away from the plate portion 11” with a posterior direction component. The anterior side 122” of the keel 12” has a contact portion configured to contact bone material when in the keel slot. The contact portion extends in a direction away from the plate portion 11” with a posterior direction component. The contact portion of the anterior side 122” of the keel 12” is substantially straight and extends from the plate portion 11” at an angle of around 140°, measured on an anterior of the anterior side 122” of the keel 12. The anterior side 122” of the keel 12” has a fillet at an intersection, or joining portion, between the plate portion 11” and the contact portion of the anterior side 122” of the keel 12”. The keel 12” has an inferior side extending between the posterior side 121” and the anterior side 122”. The inferior side is substantially parallel to the plate portion 11” and is substantially straight. When the tibial component 1” is pushed in the posterior direction, during surgery, the overhanging portion causes the tibial component 1” to also move in an inferior direction to provide a more robust fit of the tibial component 1” into the tibia, and prevent lift off on the posterior side of the tibial component 1”. The contact portion of the anterior side 121” of the keel 12” is also slid along an corresponding anterior bone surface of the keel slot, which causes the keel 12” to move in a posterior direction to provide more secure engagement between the undercut of the posterior side 121” of the keel 12” and the posterior side of the keel slot. The tibial component 1” of this embodiment also differs from the tibial component 1 of the first embodiment in that there are three reinforcing regions 125a”-c”. Two of the reinforcing regions 125b”, 125c” extend between the inferior part of the keel peripheral rim 123” and the superior part of the keel peripheral rim 123”, as in the first embodiment, albeit in different places along the length of the keel 12”. The third reinforcing region 125a” extends between the inferior part of the keel peripheral rim 123” and the posterior part of the keel peripheral rim 123”. It will be appreciated, however, that various arrangements of reinforcing region are possible, as described with reference to the first embodiment. The tibial component 1” of this embodiment also differs from the tibial component 1 of the first embodiment in that the keel portion P4” of the peripheral rim 112” has a height HD, in a direction extending away from the keel 12”, which is greater in a location of the posterior end 121” of the keel 12” than away from the posterior end 121” of the keel 12”. This may reduce the effects of stress concentrations at the posterior end 121” of the keel 12”. In other examples, the keel portion P4” of the peripheral rim 112” also has a height HD”, in the direction extending away from the keel 12”, which is greater in a location of the anterior end 122” of the keel 12” than away from the anterior end 122” of the keel 12”, again, to reduce the effects of stress concentrations caused by the end of the keel 12”. It will be appreciated that height HD” is defined in a direction perpendicular to the bone-interfacing side, which may also be referred to as perpendicular to the plane of the plate portion 11”. Referring now to Figure 11, there is shown another embodiment of a tibial component T” for partial knee arthroplasty. Similar features of this tibial component T” are denoted with the same reference numerals as the tibial component 1” of the previous embodiment shown in Figures 5-10, with a triple-prime (“) instead of a double-prime (“). The tibial component T” of this embodiment differs from the tibial component 1” of the previous embodiment in that that there are no reinforcing regions in the keel 12”’. Therefore, all strengthening and stiffening is provided by the keel peripheral rim 123”’, thereby allowing a greater volume of lattice structure in the central portion 124’”, to potentially allow more bone ingrowth into the keel 12’”. Referring now to Figure 12, there is shown another embodiment of a tibial component 1”” for partial knee arthroplasty. Similar features of this tibial component 1”” are denoted with the same reference numerals as the tibial component T” of the previous embodiment shown in Figure 11, with a quadruple-prime (““) instead of a triple-prime The tibial component 1”” of this embodiment differs from the tibial component T” of the previous embodiment in that that the keel is fully solid, or non-porous. Therefore, the keel 12”” is much stronger and stiffer than the embodiments with a porous structure, but does not permit bone ingrowth into the keel. It will be appreciated that, whilst the embodiments of Figures 1-12 show the various keels extending from a plate portion which has a lattice central portion surrounded by a non-porous peripheral rim, the keels could be used with plate portions which are completely porous, or lattice, which are completely solid, or non-porous, or which have peripheral rims which do not extend around the entirety of the central portion. Referring now to Figure 13 there is shown an illustration of the lattice structure used in the various features in the various embodiments. More specifically, the lattice structure shown in Figure 13 is representative of the lattice structure of the central portions of the plate portions, the central portion of the keels and the pegs. The lattice structure is formed of a plurality of struts S connected together at nodes N. Most or each strut has direction components in all three Cartesian coordinates. In other words, most of the struts S do not extend within just one plane. The lattice structure has an elastic modulus of between 0.1 and 5 GPa. It will be appreciated that the elastic modulus refers to the bulk elastic modulus of the lattice structure, and not the elastic modulus of each strut S individually. The lattice structure has a porosity of between 5 and 30 %. The lattice structure has a strut density of between 2 and 7 struts per mm3. Each strut has a thickness, or diameter, of between 100 and 400 microns. In all embodiments the tibial component is unitary and metallic. The tibial component is produced via additive manufacturing, by providing metal powder and selectively heating areas of the metal powder to fuse the metal powder together into the unitary metal component which is the tibial component. This additive manufacturing may use a technique referred to as laser sintering. The tibial components are additively manufactured on a horizontal platen, with the plate portion of the tibial component orientated vertically and the anterior side of the keel facing towards platen, with no support structure being formed between the anterior side of the keel and the platen. With the plate portion aligned vertically, this means that more tibial components can be produced during the additive manufacturing process because the tibial components are manufactured on their ends. Furthermore, a second manufacturing step is omitted because support structures need not be removed. The additive manufacturing process is controlled by computer readable instructions which, when executed by an additive manufacturing machine, are configured to implement the manufacturing method to produce the tibial component. The tibial component, in this example, is produced using a titanium alloy, for example Ti64. The use of the tibial components 1”. T”, 1””, described with reference to Figures 5-12, will now be described with reference to Figure 14 which shows schematics of a resected tibia. Figure 14A shows a view of the resected tibia looking in an inferior direction, and Figure 14B shows a section view through the tibia in a plane along which the keel lies. As the tibia is prepared for partial knee arthroplasty in this example, there is a resected side TR and a non-resected side TN. The resected side TR has a keel slot 21 with a shape corresponding to the keel of the tibial component. The keel slot 21 has a posterior end 23 which is shaped to engage with the undercut, formed by the overhanging portion of the posterior side of the keel. The keel slot 21 has an anterior end 24 which is shapedcorrespondingly to the anterior side of the keel. The resected side TR of the tibia also has two holes 25, which have sizes, positions and inclinations corresponding to the pegs on the tibial component. When a surgeon, after preparing the tibia for receiving the tibial component and operating from the anterior side of the knee, inserts the keel into the keel slot, the undercut engages with the posterior end 23 of the keel slot to prevent the posterior side of the tibial component from lifting while the anterior side of the keel is pushed into the keel slot 21. The shape of the anterior side of the keel and the anterior end 24 of the keel slot 21 further enforce the engagement at the posterior end of the keel as the anterior end of the keel is pushed in an inferior direction, as the corresponding shapes also push the keel 12 in a posterior direction. Also, during this motion, the pegs 13 are received in the respective holes 25 in the tibia. When the tibial component 1 is in place on the tibia, the bone-interfacing side B of the plate portion 11 is in contact with an interfacing surface 22 of the resected side TR of the tibia. Referring now to Figures 15 to 17, there is shown a surgical cutting tool 3 used to produce the keel slot 21 shown in Figure 14. The surgical cutting tool has an elongated body 31 having a proximal end, which is at an end of a proximal portion 32, and a distal end, which is at an end of a head portion 33. A shaft portion 34 connects the head potion 33 to the proximal portion 32. The proximal, head and shaft portions 32, 33, 34 together define a length of the elongated body 31, the length defining a reciprocating axis RA. In this example, the proximal portion 32 has a cross-sectional shape which is a cross, with two lateral plates extending outwardly from, and perpendicular to, two longitudinal plates, the two longitudinal plates extending outwardly from, and perpendicular to, the lateral plates. The cross shape is configured to be received in a chuck of a reciprocating saw (not shown). The proximal portion 32 also has a limiter 35, which is in the form of a protrusion extending in an outward direction from an edge of each lateral and longitudinal plate. The limiter 35 is spaced from the proximal end of the elongated body 31, and is configured to abut a face of the chuck to receive a pushing force from the chuck, to drive the surgical cutting tool 3 along the reciprocating axis RA, in use. It will be appreciated, however, that the proximal portion 32 of the surgical cutting tool 3 is designed to fit into a specific chuck, and the design may be changed for fitting into a different types of chuck. A cutting portion 36 extends from a side of the elongated body 31. More specifically, the cutting portion 36 extends from a side of the head portion 33. The head portion 33 has an abutment surface surrounding the cutting portion 36, the abutment surface configured to abut a surface of a cutting template 4 (as shown in Figures 19 and 20) in use. This provides a cutting depth stop or gauge. The head portion 33 is wider, in a thickness direction of the cutting portion 36, than the shaft portion 34. The cutting portion 36 and the head portion 33 are aligned with a centreline of the shaft portion 34. The cutting portion 36 has a cutting profile with a distal side, which corresponds to a distal side 37 of the cutting portion 36. The distal side of the cutting profile is a side of the cutting profile furthest from the proximal end of the elongated body 31 along the reciprocating axis RA. The distal side of the cutting profile has an overhanging portion which defines an undercut between the overhanging portion and the reciprocating axis RA. That is, the distal side of the cutting profile extends away from the elongated body 31 with a direction component along the reciprocating axis RA away from the proximal end of the elongated body 31, substantially linearly. In this example, the distal side of the cutting profile extends at an angle of around 70° relative to the reciprocating axis RA, on a distal side of the distal side of the cutting profile. The cutting portion 36 is therefore used to cut the overhanging shape in a keel slot 21, such that the keel of any of the tibial components shown in Figures 5 to 12, which has a corresponding overhanging portion, can be inserted into the keel slot 21 more securely, as discussed previously. The cutting profile also has a proximal side, which corresponds to a proximal side 38 of the cutting portion 36. The proximal side of the cutting profile is a side of the cutting profile nearest to the proximal end of the elongated body 31. The proximal side of the cutting profile extends in a direction away from the elongated body 31, substantially linearly, with a direction component along the reciprocating axis RA away from the proximal end of the elongated body 31. The proximal side of the cutting profile is less inclined, relative to the reciprocating axis RA, than the overhanging portion of the distal side of the cutting profile. The proximal side of the cutting profile extends at an angle of around 140° relative to the reciprocating axis RA, on a proximal side of the proximal side of the cutting profile. The cutting profile has a third side extending between the distal side of the cutting profile and the proximal side of the cutting profile. The third side of the cutting profile corresponds to a third side 39 of the cutting portion 36. The third side extends in a linear direction which is substantially parallel to the reciprocating axis RA. The surgical cutting tool 3 is thereby configured such that, when reciprocated along the reciprocating axis RA and used to cut the keel slot 21, the cutting tool 3 produces the keel slot 21 with a profile at a first end corresponding to the distal side of the cutting profile, and with a profile at a second end, opposite to the first end, corresponding to the proximal side of the cutting profile. In the Figures, the cutting portion 36 is illustrated with no cutting elements, for ease of illustration of the cutting profile. However, each of the distal, proximal and third sides 37, 38, 39 of the cutting portion 36 has cutting elements located thereon. The cutting elements may be in the form of saw, or cutting, teeth, or may be in the form of abrasive elements, or both. Figure 18 shows an example cutting portion 36’ where the cutting elements are cutting teeth. Features present in Figures 15 to 17, which are also present in Figure 18, are shown with succeeding prime (‘) in Figure 18. Teeth on the distal side 37’ are located along a line L37’ (shown in dashed in Figure 18) extending through tips of the teeth. The line L37’ extends in a linear direction from the elongated body 31’ with a direction component in the reciprocating axis RA’ away from the proximal end of the elongated body 3T. The line L37’ extending through the tips of the teeth on the distal side 37’ of the cutting portion 36’ extend at an angle of around 70° relative to the reciprocating axis RA’, on a distal side of the distal side 37’ of the cutting profile 36’. Teeth on the proximal side 38’ are located along a line L38’ (shown in dashed in Figure 18) extending through tips of the teeth. The line L38’ extends in a linear direction from the elongated body 31’ with a direction component in the reciprocating axis RA’ away from the proximal end of the elongated body 3T. The line L38’ extending through the tips of the teeth on the proximal side 38’ of the cutting portion 36’ extend at an angle of around 140° relative to the reciprocating axis RA’, on a proximal side of the proximal side 38’ of the cutting profile 36’. Teeth on the third side 39’ are located along a line L39’ (shown in dashed in Figure 18) extending through tips of the teeth. The line L39’ extends in a linear direction parallel to the reciprocating axis RA’. In this example, the distal side 37’ of the cutting portion 36’ has an abutment portion 310’ which is devoid of cutting elements and which is adjacent the elongated body 31’. The proximal side 38’ of the cutting portion 36’ also has an abutment portion 311’ which is devoid of cutting elements and which is adjacent the elongated body 3T. When the surgical cutting tool 3’ is used with a cutting template 4, the abutment portions 310’, 311’ on the distal and proximal sides 37’, 38’ of the cutting portion 36’ are configured to limit cutting movement along the reciprocating axis RA’, by abutting ends of a keel slot guide 41 in the template 4. In this example an end of the distal side 37’ of the cutting portion 36’, which is furthest from the elongated body 3T, has a leading tooth 312’. The leading tooth 312’ has a triangular shape with an angle at a cutting tip of the leading tooth 312’, a central intersection Cl’ of the angle (shown in a dash-dot line) of the cutting tip 311’ extending away from the elongated body 31’ with a direction component along the reciprocating axis RA’ and away from the proximal end of the elongated body 3T. The central intersection Cl’ of the angle of the cutting tip of the leading tooth 312’ is less inclined, relative to the reciprocating axis RA’, than the overhanging portion of the distal side of the cutting profile. Referring now to Figures 19 and 20, a method of using the surgical cutting tool 3, 3’ with the cutting template 4 is described. The template 4 is in the form of a plate, which is configured to be attached to the resected side TR of the tibia. The template 4 is attached to the tibia via fixing means extending through fixations holes 42 of the template 4. The template 4 has the keel slot guide 41, which extends in a posterior to anterior direction, in use. An abutment surface surrounds the keel slot guide 41, for abutment with the abutment surface of the head portion 33 of the surgical cutting tool 3. The template 4 also has two hole cutting guides 43, which provide guides for reamers for cutting holes in the tibia, the holes in the tibia for receiving the pegs 13, 13’ of the tibial component 1, T.When the template 4 is fixed to the tibia, the surgeon reams the holes for the pegs 13, 13’, using the hole cutting guides 43, and cuts the keel slot 21, by inserting the surgical cutting tool 3, 3’ into the keel slot guide 41. The cutting tool 3, 3’ is reciprocated by the reciprocating saw to cut the keel slot 21. When the keel slot 21 is cut to the correct depth, the abutment surface of the head portion 33, 33’ of the surgical cutting tool 3, 3’ abuts the abutment surface of the template. When the cutting portion 36’ is as shown in Figure 18, and when the keel slot is cut to the correct length, the abutment portions 310’, 311’ abut posterior and anterior edges of the keel slot guide 41. An optimal cutting method is illustrated in Figure 21, wherein Figure 21A shows the orientation of the surgical cutting tool 3 when the surgeon starts to cut the keel slot 21. The surgeon orientates the surgical cutting tool 3 with the reciprocating axis RA having an inferior and posterior direction component. That is, the cutting tool 3 is orientated such that is operates in a partly inferior direction. The surgeon operates the saw as such until the distal end of the cutting tool 3, 3’ abuts the abutment surface of the template (which is not shown in Figure 21), and then moves the proximal end of the cutting tool in an inferior direction, and shown in Figure 21B. The overhanging shape of the cutting profile thereby prevents the cutting tool from lifting, and so creates a cleaner keel slot 21. The template 4 is subsequently removed from the tibia, and the keel 12”, 12’”, 12”” of the tibial component 1”, T”, 1”” is inserted into the keel slot 21, during the knee arthroplasty method described earlier. In examples, the surgical cutting tool is metallic and additively manufactured. The additive manufacturing method includes providing a metal powder and selectively heating areas of the metal powder to fuse the metal powder together to form the surgical cutting tool. The additive manufacture method is controlled by computer readable instructions which, when executed by an additive manufacturing machine, are configured to cause the additive manufacturing machine to perform the additive manufacturing method. It will be appreciated that the surgical cutting tool may be produced using other manufacturing methods instead, such as subtractive manufacturing methods. It will be appreciated by those skilled in the art that any number of combinations of the aforementioned features and / or those shown in the appended drawings provide clear advantages over the prior art and are therefore within the scope of the invention described herein.

Claims

1. A surgical cutting tool, the surgical cutting tool being for cutting a keel slot in a superior end of a tibia, the surgical cutting tool comprising:an elongated body having a proximal end and a distal end, a length of the elongated body defining a reciprocating axis; anda cutting portion extending from a side of the elongate body, the cutting portion having a cutting profile with a distal side, which is a side of the cutting profile furthest from the proximal end of the elongated body along the reciprocating axis, the distal side of the cutting profile comprising an overhanging portion which defines an undercut between the overhanging portion and the reciprocating axis;wherein the surgical cutting tool is configured such that when reciprocated along the reciprocating axis and used to cut the keel slot, the cutting tool produces the keel slot with a profile at a first end corresponding to the distal side of the cutting profile.

2. A surgical cutting tool according to claim 1, wherein the distal side of the cutting profile corresponds to a distal side of the cutting portion, the distal side of the cutting portion comprising one or more cutting element.

3. A surgical cutting tool according to claim 2, wherein the one or more cutting element comprised on the distal side of the cutting portion is a plurality of teeth.

4. A surgical cutting tool according to any preceding claim, wherein the distal side of the cutting profile extends in a substantially linear direction from the elongated body.

5. A surgical cutting tool according to claim 4, wherein the distal side of the cutting profile extends at an angle of between 45 and 90 degrees relative to the reciprocating axis, on a distal side of the distal side of the cutting profile, and in a direction away from the proximal end of the elongated body.

6. A surgical cutting tool according to any preceding claim, wherein the cutting profile comprises a proximal side, which is a side of the cutting portion nearest to the proximal end of the elongated body,wherein at least a part of the proximal side of the cutting profile extends in a direction away from the elongated body with a direction component along the reciprocating axis away from the proximal end of the elongated body; andwherein the surgical cutting tool is configured such that when reciprocated along the reciprocating axis and used to cut the keel slot, the cutting tool produces the keel slot with a profile at a second end, opposite to the first end, corresponding to the proximal side of the cutting profile.

7. A surgical cutting tool according to claim 6, wherein the proximal side of the cutting profile corresponds to a proximal side of the cutting portion, the proximal side of the cutting portion comprising one or more cutting element.

8. A surgical cutting tool according to claim 7, wherein the one or more cutting element comprised on the proximal side of the cutting portion is a plurality of teeth.

9. A surgical cutting tool according to claim 7 or claim 8, wherein the at least part of the proximal side of the cutting profile is less inclined relative to the reciprocating axis, than the overhanging portion of the distal side of the cutting profile.

10. A surgical cutting tool according to any of claim 7 to 9, wherein the proximal side of the cutting profile extends in a linear direction from the elongated body.

11. A surgical cutting tool according to claim 9 or claim 10, wherein the proximal side of the cutting profile extends at an angle of between 90 and 160 degrees relative to the reciprocating axis in a direction away from the proximal end of the elongated body.

12. A surgical cutting tool according to any of claims 7 to 11, wherein a third side of the cutting profile extends between the distal side of the cutting profile and the proximal side of the cutting profile, the third side of the cutting profile comprising one or more cutting element.

13. A surgical cutting tool according to claim 12, wherein the third side extends in a linear direction which is substantially parallel to the reciprocating axis.

14. A surgical cutting tool according to claim 13, wherein the one or more cutting element comprised on the third side of the cutting portion is a plurality of teeth.

15. A surgical cutting tool according to any preceding claim, wherein the distal side of the cutting profile corresponds to the or a distal side of the or a cutting portion and the or a proximal side of the cutting profile corresponds to the or a proximal side of the cutting portion, wherein the distal side of the cutting portion comprises an abutment portion which is devoid of cutting elements and which is adjacent the elongated body, and wherein the proximal side of the cutting portion comprises an abutment portion which is devoid of cutting elements and which is adjacent the elongated body.

16. A surgical cutting tool according to any preceding claim, the distal side of the cutting profile corresponds to the or a distal side of the or a cutting portion and wherein an end of the distal side of the cutting portion, which is furthest from the elongated body, comprises a leading tooth, the leading tooth having triangular shape with an angle at a cutting tip of the leading tooth, a central intersection of the angle of the cutting tip extending away from the elongated body with a direction component along the reciprocating axis and away from the proximal end of the elongated body.

17. A surgical cutting tool according to claim 16, wherein the central intersection of the angle of the cutting tip of the leading tooth is less inclined, relative to the reciprocating axis, than the overhanging portion of the distal side of the cutting profile.

18. A surgical cutting tool according to any preceding claim, wherein the elongated body comprises a head portion from which the cutting portion extends, the head portion comprising an abutment surface at least partially surrounding the cutting portion.

19. A surgical cutting tool according to claim 18, wherein the elongated body comprises a shaft portion extending between the head portion and the proximal end of the elongated body, wherein the head portion is wider, in a thickness direction of the cutting portion, than the shaft portion.

20. A surgical cutting tool according to any preceding claim, wherein the proximal end of the elongated body is configured for attachment to a reciprocating surgical saw.

21. A reciprocating surgical saw comprising a surgical cutting tool according to any preceding claim.

22. A kit of parts comprising:a surgical cutting tool according to any of claims 1 to 20 or a reciprocating surgical saw according to claim 21; anda cutting template configured for use with the surgical cutting tool, the cutting template for attachment to a resected tibia and comprising a keel slot guide for receiving the cutting portion of the surgical cutting tool, in use.

23. A kit of parts comprising:a surgical cutting tool according to any of claims 1 to 20, a reciprocating surgical saw according to claim 21, or a kit of parts according to claim 22; andone or more tibial component of an orthopaedic knee implant, the tibial component comprising:a plate portion which comprises a bone-interfacing side; anda keel extending from the bone-interfacing side of the plate portion, the keel having a length with a posterior side at one end of the length, the keel being configured to be inserted into a keel slot which extends in a posterior to anterior direction in a tibia of a patient;wherein the posterior side of the keel comprises an overhanging portion to define an undercut between the overhanging portion and the boneinterfacing side of the plate portion, the undercut being configured to engage with bone material when the keel is inserted into the keel slot produced by the surgical cutting tool.

24. A method of manufacturing the surgical cutting tool according to any of claims 1 to 20, the method comprising:providing a metal powder;selectively heating areas of the metal powder to fuse the metal powder together to form the surgical cutting tool.

25. Computer readable instructions which, when executed by an additive manufacturing machine, are configured to cause the additive manufacturing machine to perform a method according to claim 24.