Device customized to fit a surface of a bone for reconstruction of bone defects

Abstract

The invention relates to a customized device made of an inert and stiff ceramic for reconstruction of bone, bridging bone defects or reshaping of bone, more specifically the combination of material characteristics and design to protect the bone fracture or the volume where new bone is to be formed from motions and external forces that may disturb the bone formation and bone healing. This contributes to an optimal situation for natural bone healing that enhances the clinical outcome. The design of the implant member (1) is based on the initial bone structure (2) and the intended treatment where at least a part of the inner surface (9) of the implant member (1) directed towards the bone follow the irregularities of the initial bone structure (2) to mechanically support the area around the fixation screws (6) used to connect the implant member to the initial bone structure (2).

Description

[0001] DEVICE CUSTOMIZED TO FIT A SURFACE OF A BONE FOR RECONSTRUCTION OF BONE DEFECTS

[0002] Field of the invention

[0003] The present invention relates to a device made of a ceramic material with a high stiffness and an inert chemical character for reconstruction of bone, bridging bone defects or reshaping of bone, more specifically a customised implant member intended to protect the bone formation and healing from external forces. The implant member is further designed to reduce the tensile stresses formed during the intended use through a combination of material characteristics and geometrical shape where the customisation includes bone supported fixation points. The implant member is further inert in the sense that the chemical composition of the material does not promote bone formation.

[0004] Background of the invention

[0005] The skeleton is to large part composed of bone, a hard tissue that provide several functions to the skeletons such as support a framework for the muscle and tissue attachment, posture to give the shape of the body, protection of internal organs and reduce the risk of impact, movement of the body as a whole and its individual parts, blood cell production and storage of minerals. With a loss of bone tissue caused by trauma, tumor or disease, several of the functions of the skeleton may be lost. The mandible, which has a central function for chewing, phonation and breathing as well as an aesthetic role, can be used as an example. Reconstructions can thus include healing of fractures, regeneration of bone tissue or a combination of these.

[0006] When this is applied on a larger bone defect in the mandible, the daily life results in motions when eating or talking that will disturb the bone formation. It is thus of interest to protect the region for bone formation from external forces through a stable fixation of the bone parts or to cover the bone defect, preferably in combination with a bone graft material that can enhance the bone forming capacity. An alloplastic procedure is often chosen following resection of malignant tumors with a poor prognosis. Such alloplastic replacement of the mandible may be temporary or definitive. Besides the mechanical stabilization, reconstruction plates are also used to avoid dislocation of the mandibular stumps, which can be an obstacle to later restoration of the mastication. The systems employed should ensure functional, mechanical and morphological rehabilitation. There is however some common complications that can occur when using metallic reconstruction plates such as plate exposure, infections, plate fracture and screw loosening. These complications can to some extent be related to the materials used and more specific the physical and chemical material characteristics and the material tissue interactions

[0007] Background art

[0008] There are several different application areas where the present invention related to a customised inert implant member with a high stiffness can be applied. One such area is reconstruction plates that can be used to stabilize various types of bone fractures such as mandibular defects during healing. The solutions available are standardized plates, bands that can be bent and customised or completely customized solutions. These implants are often made of titanium, a material with an excellent biocompatibility in relation to the bone tissue. The surface of these implants can be used as machined, blasted or printed with or without post treatment but it is often desired to have a relatively rough surface in connection with the bone. To initially stabilize the fracture or the bone defect during healing, the implants used for reconstructions are mechanically connected to the bone with fixation screws through the implant into the bone. Most of the load between the bone and the implant will thus initially be transferred by these fixation screws. With a successful bone implant integration an enhanced bone transfer capability can be obtained when a load transfer also can occur at the bone implant interface.

[0009] The titanium metal often used, has further a relatively low elastic modulus of around 115 GPa and the metal can be plastically deformed without fracture when exposed to high stresses. The standardized parts can then be customised to fit a specific patent by adjustment of the shape through a plastic deformation of the metal before or during surgery for bone reconstructions. The part is mechanically connected to the bone using fixation screws where a sufficient torque has to be applied to the fixation screws to secure the position of the reconstruction material and stabilize the bone. This will further introduce stresses around the screw connection but this is usually not a problem in metallic materials where the presence of high stresses can induce a plastic deformation instead of a critical fracture.

[0010] Several innovations in the field have instead of focusing on stresses been related to how to ensure that a stable fixation can be maintained over time. When the living bone is remodeled and the bone volume adjusted to the surrounding circumstances, there is a risk that a gap is formed between the metal implant and the bone, which would result in that the implant, can become unstable in relation to the bone. A solution to this has been to use a double threaded screw that can fixate both the implant and the bone even if there is no contact between the bone plate and the bone.

[0011] The interaction with the soft tissue is a general problem for metallic reconstruction materials in applications such as the jaw, where the soft tissue moves across the surface of the metallic support structures. Even if a metal such as titanium with an excellent bone tissue response is used, the soft tissue response will be different. The material chemistry and the rougher blasted, machined or additive manufactured surfaces are often designed to enhance the bone response. But in combination with a relative motion between the metal and the soft tissue, these surfaces may cause soft tissue penetration, causing infections followed by additional treatments and surgical procedures.

[0012] There are thus a couple of problems with metallic reconstruction materials used in clinical applications that is related to fixation screw loosening, fatigue fractures and soft tissue penetration. Some of these problems can be avoided with the invention presented when bone is to be healed, reshaped or regenerated, but it require a different approach with respect to design, materials and tissue response.

[0013] Summary of the invention

[0014] It is an object of the present invention to provide an improved customised implant member of an inert ceramic with a high stiffness, used to shield and stabilize the volume where new bone is intended to be protected from external forces and movements that can disturb the bone formation. The conventional approach when bone is to be regenerated is to use materials that promote bone formation. This is usually done through a suitable chemical composition or surface structures that encourage bone to grow along the material surface, osseocondutive or even with the ability to initiate bone formation directly on the material surface, osseoinductive. In these cases, the bone does not only accept the materials very well, the bone may even attach and bond to the material surface where the implant materials can osseointegrate. The invention presented uses a completely different approach where the the material used in the implant member does not actively interact with the bone tissue to stimulate bone formation through the chemical composition. The implant member will instead be composed of an inert material used to create an optimal environment for the natural bone healing to occur through shielding of the bone defect or fracture from external forces and motions.

[0015] The implant member can be made of an inert ceramic that belong to the most chemically stable materials available and has thus a very limited biological interaction with the bone tissue. The bone growth will thus not be influenced or guided by the implant member material but rather act as an external boundary for the bone where a thin periost is formed between the bone and the implant member. The bone will then grow from the original bone surface towards other bone surfaces or the surface of the implant member. The formation of the periost between the implant member and the bone can further be beneficial since it facilitates the removal of an implant member that was intended to be used temporarily during healing and reconstruction of the bone.

[0016] These inert materials are further very stiff with a high Youngs modulus and low fracture toughness, which results in that the materials have a brittle behavior. It is thus of importance to avoid the formation of high tensile stresses through a suitable design and choice of material characteristics. The shape of the implant member or device is preferably customised with a design that supports the intended treatment in an optimal way, defined by the shape of the bone structure in combination with the intended use and the expected loading conditions. According to the invention the shape of the bone structure can be obtained from computer tomography or similar techniques and used to generate a three dimensional model of the bone volume. The inner surface of the implant member directed towards the bone and the surface of the initial bone structure will then have the same shape. When installing the implant member at the intended location, the direct contact between the surface of implant member and the surface of the initial bone will contribute to an improved mechanical stabilization of the implant member along the bone surface and facilitate identification of the unique and correct position of the implant member. The customization results further in that the inner surface of the implant member is partly mechanically supported by the bone perpendicular to the inner surface of the implant member. These bone supported areas are preferably also used for fixation screws where the bone supported area around the location of the fixation screws used to securely fasten the implant member to one bone results in that a higher torque can be used for the fixation screws without increasing the tensile stresses as in the case without a mechanical support at the inner surface of the implant member. The location and angulation of the fixation screws used are preferably selected from suitable anatomical and practical locations, identified from the model of the bone structure. To further reduce the stresses in the device when the fixation screws are tightened, the holes prepared for the fixation screws are not only counter sunk, they are preferably designed with a shape that corresponds to the shape of the screw head to increase the contact area between the screw and the device. The overall reduction of the tensile and torsional stresses through the design and customization contributes further to that the reliability of the implant member can be maintained even if the implant member is made thinner. This is desired since it reduces the strain needed for the soft tissue to cover the implant member when fixated to the bone and also to maintain an acceptable anatomical situation.

[0017] Brief description of the drawings

[0018] In the following the invention will be described more in detail with reference to the accompanying drawings in which,

[0019] Figure 1 shows an example of a customised implant member located at the intended position with the aim to fixate a mandible defect where the fixation points are bone supported.

[0020] Figure 2 shows an example of a customised implant member according to the invention that is intended to be used for loaded applications such as bridging across a bone defect.

[0021] Figure 3 illustrates a different shape of the implant member that can be used for regeneration or reshaping of bone where the load bearing capabilities are less critical and the thickness of the implant member can be reduced when the fixation area is bone supported.

[0022] Figure 4 illustrates a cross section along a customised implant member used to fixate and stabilize a fracture. Small nail / edge like structures can be added at the inner surface, protruding towards the bone with the intention to increase the stability of the fixated implant member. The contact area between the screw head and the implant member can be designed to reduce tensile stresses which in combination with a mechanical support from the bone at the inner surface contribute to that a higher torque can be used when tightening the fixation screws. The design of the implant member contributes to an increased stability and enhances the conditions for new bone to be formed.

[0023] Figure 5 illustrates a situation where the device is supported by the bone tissue mainly in the area surrounding the fixation screw. A higher torque can then be used for the fixation screw which contributes to a more rigid fixation of the implant member to the bone. When the implant member is fixated, the peaks and valleys formed along the contact surface contribute to an interlocking of the implant member that improves the fixation stability. The shape of the hole for the fixation screw is designed to allow different angles to be used and still maintain a large contact area between the screw head and the implant member. Detailed description of the invention

[0024] A specific embodiment of the invention will now be described with reference to the accompanying drawings. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiment set forth herein.

[0025] The invention relates to a customised device made of an inert ceramic material with a high stiffness, an implant member, for reconstruction of bone, reshaping of bone, bridging of bone defects or fractures as illustrated in figure 1 to 5. The purpose of the implant member will be to protect the bone fracture or the volume where new bone is to be formed from motions and external forces that may disturb the bone formation and bone healing. The invention contributes to create an optimal situation for natural bone healing without the need to use an implant made of a material with a chemical composition that promote bone formation with osseoconductive or osseoinductive properties. This results further in a limited interaction between the bone tissue and the implant where a thin periost is formed between the bone and the surface of the implant member directed towards the bone, which facilitate the removal of the implant member when used as a temporary implant.

[0026] For an implant member to obtain these desired properties with a limited bone tissue interaction, the ability to transfer loads, shield and protect the bone growth from external forces would require certain material characteristics such as a chemically inert material with a high strength and high stiffness. However the combination of these material characteristics results in a material with a brittle behavior, which means that the material is non deformable and that fractures occur instantly when a critical tensile stress is reached. These materials are thus generally an inappropriate choice for applications related to bone reconstruction where they can be expected to be exposed to loaded situations and stresses. But if the design and the material are selected in a way that contributes to a reduction of the peak stresses formed, it would be possible to use also brittle materials in the field of bone reconstruction. Brittle materials have further a linear elastic behavior and tend to be less prone to fatigue.

[0027] In addition to the design, the critical stresses formed when a brittle material is exposed to a load will depend on the size of the defects present in the material. This means that the defect size needs to be reduced and that the material in the implant member should have a density above 97%, more preferably above 99% of the theoretical density to ensure a low degree of porosity that will act as defects. All surfaces expected to be exposed to high tensile stresses should further have a smooth surface to reduce the size of surface defects, which in combination with a high density can ensure a high mechanical performance.

[0028] To protect the desired bone formation from external forces, the stiffness of the implant member material should be above 125GPa, more preferably above 175GPa. The high stiffness increases the stability and reduces the motions transferred to the volume for the bone formation compared to if a material with a lower stiffness had been used. This contributes to create a more well protected and favorable situation for the bone formation with a reduction of disturbing micro motions. For a specific treatment, the implant member is installed between the bone and the soft tissue where the inner surface of the implant member is directed towards the bone tissue and the outer surface is directed towards the soft tissue. The shape of the customised ceramic device or implant member is based on digital data such as computer tomography data of the bone structure that can be used in a pure digital workflow or through a combination of manual modelling and a digital workflow. The surface of the implant member directed towards the bone is customised to have the same shape as the desired bone surface. When the implant member is installed at the intended location, the inner surface will be in direct contact and follow the shape of the bone, however, it is not necessary for the entire inner surface to be in direct contact with the bone. The outer side of the implant member can be designed more freely according to the intention with the treatment and purpose of the implant member. The average thickness of the implant member when measured perpendicular to the outer surface can then vary depending on the required loading capabilities, the anatomical situation and the intended treatment from around 1,5 to 6 mm in loaded applications and 0,5 to 1,5 mm in low loaded situations.

[0029] The implant member can be fixated to the bone through the use of fixation screws where the number of screws needed depends on the design and the expected loading situation. In low loaded situations there may be sufficient with one screw while several screws can be necessary to allow high loads to be transferred across a bone defect by the implant member. The shape, size and orientation of the holes for the fixation screws used to fasten the implant member to the bone are preferably defined in the digital model where suitable locations are selected with respect to practical, anatomical and mechanical considerations. To reduce the exposure of the fixation screw head above the external surface of the implant member, the screw hole is preferably counter sunk with a design and shape, corresponding to the shape of the screw head as illustrated more in detail in figures 4 and 5. Such a design of the hole for the fixation screw will further increase the contact area between the implant member and the screw, contributing to a reduction of the tensile stresses in the surrounding material. With a spherical shape of the fixation screw head, the contact area between the screw and the implant member can be preserved even if the angle of the screw is changed.

[0030] Even though the implant member material has a high strength, the high stiffness and brittle character of the material require a design that contributes to a reduction of the tensile stresses formed. In addition to the external mechanical loads, the main tensile stresses can be expected to be generated around the location of the fixation screws used to secure the position of the implant member to the bone. When the fixation screws are tighten to fixate the implant member to the bone, the tensile stresses are increasing with the distance between the location of the fixation screw and the surrounding points of contact between the implant member and the bone.

[0031] The tensile stresses formed at the inner side of the implant member will then be higher around the fixation point if this is not mechanically supported by the bone structure. Through the customisation of the implant member, the fixation points are located at anatomical and mechanical suitable locations where the inner surfaces around the fixation points are in direct contact with the bone structure. When the fixation screws are tightened the implant member will be mechanically supported by the bone and contribute to a reduction of the tensile stresses generated by the fixation screws. A higher torque can then be used to tighten the fixation screws, which improves the fixation stability of the implant member to the bone. The contribution to the stability of the implant member can further be divided in two parts, perpendicular and parallel to the inner surface of the implant member which is in direct contact with the bone. The option to use an increased torque to fixate the implant member at the bone supported fixation points contributes to a higher contact pressure perpendicular to the inner surface of the implant member and creates a closer contact between the implant member and the bone. This result further in that fine natural geometrical structures or waviness of the bone surface create peaks and valleys defined as the highest or lowest point in two or three dimensions when measured perpendicular to the inner surface of the implant member. The customization results in that the same peaks and valleys are also found at the inner surface of the implant member. When these two surfaces are in close contact, the surface irregularities, peaks and valleys will contribute to an interlocking and mechanical fixation also in the directions parallel to the inner surface of the implant member. Another contribution to improve the stability parallel to the inner surface can be obtained from two non parallel lines or curved lines where the implant member and the bone structure are in contact. The presence of peaks and valleys along these lines of contact will also give an interlocking that restricts the motion parallel to the inner surface of the implant member when kept in close contact with the bone. The geometrical dimensions of these peaks, valleys or waves will depend on the resolution of the digital data obtained from the computer tomography or similar imaging techniques used to generate a model of the bone structure.

[0032] The digital model of the bone structure is composed of a large number of small volume segments. For each segment is a certain amount of data from the imaging technique used is needed to define if a volume segment will consist of bone and contribute to the shape of the model or not. This data is often composed as a series of images where the size of the volume segment depends on the distance between the images and the resolution within the image. Through the use of a higher radiation dose, the size of the volume segment to be defined can generally be reduced and the resolution increased. When the shape of the bone surface that is used to design the inner surface of the implant member, the manufacturing technique used for the implant member should have the capability to produce the surface with the same resolution as the imaging technique.

[0033] To further increase the stability and reduce the potential motion of the implant member in the plane parallel to the inner surface is to introduce small nail or edge like features protruding out from the inner surface of the implant member towards the bone.

[0034] These nail or edge like structures should thus not protrude more than 2 mm from the inner surface, more preferably less than 1 mm. The nail and edge like structures can vary in size, number and location but are preferably located close to the fixation point. When the fixation screw is tightened, the nail or edge like structures should penetrate the bone and still allow the inner surface of the implant member to be in direct contact with the bone. These additional structures protruding into the bone will contribute to a more stable fixation of the implant member and reduce the relative motion between the implant member and the bone when exposed to external loads. The improved stability and protection of the volume for the intended bone formation will further contribute to an enhanced environment and better conditions for the bone formation. Additional design solutions to further reduce the tensile stresses are chamfering, fillets, smoothing and polishing of all edges around the fixation screw holes, on the inner and outer side. Also the external edges connecting the inner and outer surfaces of the implant member should have a suitable curvature and be smooth or polished to avoid the potential introduction of defects that can reduce the mechanical performance. A general reduction of the tensile stresses in the ceramic implant member contributes further to an increased reliability and that the material thickness can be reduced. This is desired since it reduces the anatomical deviations and the elongation needed for the soft tissue to enclose the implant member.

[0035] Suitable materials to be used for the customised implant member of the invention, having a high strength, high stiffness and an inert character are oxides (zirconia, alumina) with possible additions of various reinforcements or dopants (yttria, ceria, aluminates) or nitrides (silicon nitride). Suitable fabrication processes for the presented implant member are various casting methods (slip casting, gel casting), additive manufacturing, injection moulding, hard and soft machining. If the manufacturing process involves a sintering shrinkage, the size of the implant member has to be enlarged in order to compensate for the shrinkage. If the desired surface characteristics of the edges and the regions around the holes for the fixation screws were not reached directly from the manufacturing process used. A solution to improve the surface finish of the hole is to use a polishing tool with a shape that corresponds to the desired shape of the fixation hole from both the inner and outer surface to remove stair steps from additive manufacturing processes or milling tracks from machining since these surfaces can be difficult to access by conventional polishing techniques.

[0036] Figure 1 and 2 shows an example of a device, an implant member 1 with a beam like structure intended to bridge a bone defect, fracture or the like in the mandibular bone. The device in this case is designed with two opposite end regions la and lb, each region provided with a number of screw holes 3 for fixation of the device to healthy / stable portions of the mandibular bone structure 2. The holes for the fixation screws are made at locations defined in the digital planning to avoid anatomical and mechanical unpractical locations. The area around the fixation holes are further designed according to the fixation screw and the bone. The inner surface of the implant member directed towards the bone should follow a major part of the bone surface to mechanically support the area around the fixation points to allow a higher torque to be used for the fixation screws to ensure a stable connection to the bone.

[0037] Figure 3 illustrates an example of an implant member 1 with a sheet like structure where the holes 3 for fixation screws are located close to the front where the customisation gives a mechanical support at the inner surface around the fixation points from the bone. The tensile stresses generated by the fixation screws can then be reduced and the implant member can be made thinner. The edges are smooth or polished to reduce the size of the surface defects present, which will improve the mechanical performance of the implant member.

[0038] Figure 4 and 5 illustrates the contact area between the implant member 1 and the bone structure 2 in more detail. The shape of the screw hole opening 5 can been designed with respect to the shape of the fixation screw 6 head to increase the contact area between the fixation screw and the implant member. This reduces the exposure of the screw head towards the surrounding soft tissue 7 that is in contact with the outer surface 10 as well as the critical stresses formed in contact with the fixation screw. In addition to this, all edges are chamfered 8 to reduce stress concentrations and the area at the inner surface 9 surrounding the fixation screw hole 3 is mechanically supported by bone 2. To increase the loading capabilities of the implant member in the direction parallel to the inner surface, fine nail or edge 4 like structures can be introduced that protrudes from the inner surface 9 towards the bone. When tightening the fixation screws, theses structures can penetrate the bone and contribute to a mechanical interlocking between the implant member and the bone. The volume for bone formation 11 is then better protected and the conditions for bone formation improved.

[0039] Figure 5 illustrates an example in which only the region close to the fixation screw 6 is mechanically supported by the bone tissue 2. Outside this region, an internal cavity 11 is formed between the inner surface 9 of the implant member 1 and the bone structure 2. The tensile stresses formed in the implant member when the fixation screw is tightened are then reduced and allow a higher torque to be used when tightening the fixation screw 6. The reduced stresses related to the fixation screws allow other less loaded areas of the implant member to be made thinner without reducing the reliability. The thinner material in the implant member contributes further to that the soft tissue 7 more easily can cover the implant member. To allow the direction of the fixation screw to be varied with respect to the outer surface 10 of the device, the diameter of the fixation hole 3 can be increased towards the inner surface 9 to allow the most favorable direction of the fixation screws to be used. When tightening the screws, the surface structure with peaks 12 and valleys 13 along a surface area or lines will create a mechanical interlocking that will keep the implant member in the correct location and reduce the load transferred to the fixation screws in the radial direction of the screw 6.

[0040] The invention is not limited to the examples which have been described here but can be varied within the scope of the following claims.

Claims

AMENDED CLAIMS received by the International Bureau on 5 November 2024 (05.11 .2024)CLAIMS1. A customised implant member (1) for reconstruction of bone segmental defects, made of an inert ceramic material with holes (3) for fixation of the implant member (1) to the bone (2), wherein the implant member (1) has: an inner surface (9) intended to be directed towards the bone (2); and an outer surface (10) intended to be directed towards soft tissue (7), wherein at least a portion of the inner surface (9) of the implant member (1) adjacent to the holes for fixation to the bone is pre-shaped to follow surface irregularities of a corresponding surface portion of the bone (2), in such a way that when the portion of the inner surface (9) of the implant member (1) and the corresponding surface portion of the bone (2) are in contact, the surface irregularities contribute to an interlocking and mechanical fixation in the directions parallel to the inner surface (9) of the implant member (1).

2. An implant member (1) according to claim 1, where the material has a stiffness of more than 125GPa, more preferably a stiffness of more than 175GPa.

3. An implant member (1) according to claim 1 or 2 wherein nail / edge structures (4) are protruding from the inner surface (9) towards the bone (2) with a length of preferably less than 2mm, more preferably less than 1mm.

4. An implant member (1) according to claim 1 or 2 where the inner surface (9) has at least one peak (12) or valley (13) along at least two non parallel lines.

5. An implant member (1) according to claim 1 or 2 wherein the shape of the area intended to be in contact with the fixation screw (6) is designed according to the shape of the screw head.

6. An implant member (1) according to claim 1 or 2, wherein the diameter of the fixation hole (3) is enlarged towards the inner surface (9) of the implant member (1).

7. An implant member (1) according to claim 1 or 2, wherein the material of the implant member (1) is a ceramic material such as silicon nitride, alumina, zirconia, aluminates or composites based on these materials.

8. An implant member (1) according to claim 1 or 2, wherein the material has a density above 97% or more preferably above 99% of a theoretical density of the material.11AMENDED SHEET (ARTICLE 19)

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