Strut structure

Abstract

An expandable strut lattice structure for a structural component, the structure comprising extendable struts.

Description

[0001] STRUT STRUCTURE

[0002] TECHNICAU FIELD

[0003] The invention relates generally to structures for structural components, and additive manufactured structures for structural components.

[0004] BACKGROUND OF THE INVENTION

[0005] There are many situations where there is a need to replace, augment or support sections of a structural component. In this context, the expression "structural components" is used to include any component that performs a dynamic or static load bearing function. Notable examples in that regard include biological structural components, such as a bone or a joint, or pre-existing construction components. However, existing devices that can replace, augment or support sections of a structural component (e.g. an orthopaedic implant for a bone) are defective in that they lack morphological adaptability, especially in combination with load bearing characteristics.

[0006] In the case of biological structural components, for instance a bone or a joint, conventional orthopaedic implants pose significant limitations where patient growth is a crucial factor, especially in paediatric patients. Traditional implants lack the capability to adjust their dimensions and adapt to the changing anatomical requirements of a developing musculoskeletal system. Consequently, paediatric patients often face the need for multiple revision surgeries to replace implants that become inadequate as they outgrow their original sizes. While there exist expandable prosthesis, they require multiple surgery and adjustment sessions to accommodate bone growth.

[0007] Other structural components, for example pre-existing construction components, may also require replacement, augmentation or additional support after their installation. Nevertheless, existing devices in that regard lack customisability. There remains therefore an opportunity to address and ameliorate limitations associated with conventional means to replace, augment, or support sections of a structural component.

[0008] SUMMARY OF THE INVENTION

[0009] The present invention provides an expandable strut lattice structure for a structural component, the lattice structure comprising extendable struts. By "structural component" is meant herein a component providing a static or dynamic load bearing function.

[0010] In some embodiments, the structural component is an orthopaedic implant. Accordingly, the present invention also provides an expandable strut lattice structure for an orthopaedic implant, the structure comprising extendable struts.

[0011] By having extendable struts, the structure of the invention can expand, for example when subject to external forces. This is particularly advantageous when the structure is used in an implant, for example a bone implant, which can extend as the patient grows.

[0012] In some embodiments, the extendable struts are telescopic struts. In this configuration, the struts can be made to extend with a high degree of precision along their axis, providing for a structure that can extend with strict spatial tolerance.

[0013] In some embodiments, the expandable strut lattice structure is made by additive manufacturing. The use of additive manufacturing affords the conception of any arbitrary lattice design, allowing to produce highly customized expandable strut lattice structures. This is particularly advantageous for structures used in orthopaedic implants, for example, which design can be readily customized for a specific patient.

[0014] Accordingly, the present invention also provides a method of fabricating an expandable lattice structure, the structure comprising extendable struts, and the method comprising providing a powder feedstock and processing said powder feedstock by additive manufacturing to form said expandable strut lattice structure. The expandable lattice structure obtained with the method may be an expandable lattice structure of the kind described herein.

[0015] In some embodiments, the expandable strut lattice structure is an additively manufactured expandable strut lattice structure.

[0016] It is believed that a structure of the kind described herein manufactured by additive manufacturing is unique in its own rights. Accordingly, the present invention also provides an additive manufactured expandable strut lattice structure, the structure comprising extendable struts. Key applications include paediatric orthopaedic implants and pre-existing structural reinforcement.

[0017] BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Embodiments of the invention will be now described with reference to the following nonlimiting drawings, in which:

[0019] Figure 1 shows a schematic of a lattice structure expandable along X- and Y-axes, and fixed along the Y-axis,

[0020] Figure 2 shows a schematic cross-section of an embodiment telescopic strut having sections that can engage through a linear ratchet locking mechanism,

[0021] Figure 3 shows a schematic cross-section of an embodiment telescopic strut with sections that can engage through a tapered end,

[0022] Figure 4 shows a schematic cross-section of an embodiment telescopic strut having a ball joint at one end, and

[0023] Figure 5 shows an example 3x3x3 TieAUV cubic telescopic lattice fabricated through laser powder bed fusion (EPBF). DETAILED DESCRIPTION OF THE INVENTION

[0024] The present invention provides an expandable strut lattice structure.

[0025] By "lattice structure" is meant herein a three-dimensional framework of interconnected unit cell elements. By being a "strut" lattice structure, each unit cell element is a rod-like element (i.e. a “strut”). In the structure of the invention, individual struts are connected, at their respective ends, in different orientations to form different unit cells of the lattice.

[0026] By being "expandable", the lattice structure can extend along at least one spatial direction. In some instances, the lattice structure may be capable to extend as well as contract along at least one direction, which may or may not be the same direction.

[0027] In some embodiments, the lattice structure can extend along one direction. Those instances are particularly useful, for example, for use in implants intended to grow linearly with a bone.

[0028] In some embodiments, the lattice structure is capable to extend as well as contract along at least one direction. Those instances are particularly useful for orthopaedic implants that must dynamically accommodate complex movements along one or more directions, for example in joint replacements.

[0029] The lattice structure may have any lattice configuration known to the skilled person. The configuration of the lattice structure would depend on the way the rods are connected.

[0030] In some embodiments, the lattice structure is a period lattice structure. In those instances, the lattice structure is characterised by having the same repeating unit cells throughout its volume. In some embodiments, the lattice structure is a stochastic lattice structure. In those instances, the lattice structure is made of unit cells having different dimensions and being connected randomly throughout the structure. When in a stochastic configuration, the lattice structure of the invention is particularly useful in biomedical applications since the structure can be designed to closely resemble the geometric and biomechanical characteristics of a patient's bone or joint part, making them excellent candidates for reconnecting bone structures inside the body.

[0031] Consequent to the configuration of the struts throughout the lattice structure, the structure may be a homogeneous lattice structure or a heterogeneous lattice structure.

[0032] A homogeneous lattice structure has uniform properties (for example mechanical properties or geometric properties such as unit cell density, strut dimensions, and size) across its entire volume.

[0033] Conversely, a heterogeneous lattice structure is characterised by at least one property changing across the structure. Heterogeneous lattice structures affords region- specific tailoring of the lattice properties. For example, a finite element analysis (FEA) may inform on the spatial distribution of mechanical stressors for a specific use of the structure, leading to a non-homogeneous distribution of the lattice density for spatial-specific mechanical strength. For example, structures in accordance with the invention may be characterized as having an elastic modulus in one direction and at least a second elastic modulus in another direction.

[0034] For instance, cancellous bone has anisotropic properties and it could be appropriate to use a lattice structure with anisotropic properties, as disclosed herein, as a bone growth and support scaffold. The lattice structure may also be used in applications where a scaffold providing growth and support for bone with different mechanical properties in at least two directions. In another example, it may be desirable for an implant to load bone in one or more directions of reduced stiffness, and not in one or more directions of shielding or increased stiffness to aid preferential bone growth in the direction(s) of loading. The expandable strut lattice structure may have any strut-based unit cell topology. For instance, the structure may have unit cell topology selected from cubic, body-centred cubic (BCC), face-centred-cubic (FCC), BCC with z-struts (BCCZ), FCC with z-struts (FCCZ), octet-truss, and diamond.

[0035] In some embodiments, the extendable lattice structure is an anisotropic lattice structure characterised by an elongated modified rhombic dodecahedron lattice that is elongated in the X and Y axes relative to the Z axis. The elongated modified rhombic dodecahedron lattice is presented as a single cell defined by struts, with additional struts extending away from the cell to show portions of the repeating structure. The unit cell may be repeated in a lattice structure to achieve an open cell structure of the desired volume and mechanical properties.

[0036] In some embodiments, the extendable lattice structure is characterised by a grouping of three elongated modified rhombic dodecahedron cells that have been elongated in the X and Y axes relative to the Z axis. The grouping is presented as three-unit cells defined by struts, in a stacked arrangement, with additional struts extending away from the cell to show portions of the repeating structure.

[0037] The expandable lattice structure may be defined by its volumetric density at a given extension configuration. By "volumetric density" is meant herein the percentage ratio between the total volume of the material elements forming the lattice (i.e. struts, strut joints etc.) and the volume of voids for a given three-dimensional structure. For example, a lattice structure with 30% volumetric density would have 30% of its unit volume occupied by solid material and 70% voids, etc.

[0038] In some embodiments, when in a most collapsed configuration the expandable lattice structure has a volumetric density of at least about 10%, at least about 25%, at least about 50%, at least about 75%, or at least about 90%. In some embodiments, when in a most expanded configuration, the expandable lattice structure has a volumetric density of at least about 10%, at least about 25%, at least about 50%, at least about 75%, or at least about 90%.

[0039] The struts forming the structure may be made of any material that is fit for the structure's purpose.

[0040] In some embodiments, the struts are made of metal.

[0041] In some embodiments, the metal comprises steel, titanium, aluminium, magnesium, nickel, iron, copper, gold, silver, bismuth, manganese, zinc, chromium, molybdenum, platinum, zirconium, iridium, yttrium, vanadium, niobium, tantalum, tungsten, cobalt, tin, lead, gallium, or an alloy of one or more thereof.

[0042] When the lattice structure is for an orthopaedic implant, the struts may be made of any biocompatible material that is fit for that purpose. In some embodiments, the struts are made of a biocompatible material selected from a biocompatible metal, including biocompatible alloys, a biocompatible ceramic, a biocompatible polymer, or a composite thereof.

[0043] Examples of suitable materials in that regard include cobalt-chromium-molybdenum (CCM) alloys, titanium, titanium alloys, stainless steel, aluminum, zirconium oxide, silicon nitride, an allograft, an autograft, a metal-allograft composite, polyaryl ether ketone (PAEK), poly ether ether ketone (PEEK), poly ether ketone ketone (PEKK), poly etherketone (PEK), poly etherketone ether-ketone-ketone (PEK-EKK), polytetrafluoroethylene (PTFE), and a fiber-reinforced polymer, such as a carbon fiber-reinforced polymer.

[0044] In some embodiments, the structure is made of titanium alloy. The expression “titanium alloy” used herein encompasses alloys in which the major element is titanium. The expression may also encompass proprietary titanium alloy systems as well as titanium alloys based on titanium powder metallurgy and titanium compounds, and may also encompass alloy systems such as titanium gum metal. The expression may encompass, for example, alpha titanium alloys, near alpha titanium alloys, alpha-beta titanium alloys, beta titanium alloys and titanium alloys strengthened by small additions of oxygen, nitrogen, carbon and iron. Examples of typical titanium alloys used herein include, for example, Ti- 1.50, Ti-0.2O, Ti-0.3O, Ti-0.20-0.2Pd, Ti-3A1-2.5V, Ti-6A1-4V, Ti-6A1-4V ELI (Extra Low Interstitials) and Ti-6Al-4V-0.06Pd. In some embodiments, the titanium alloy is an alpha titanium alloy (e.g. Ti-5Al-2Sn-ELI, Ti-8Al-lMo-lV). In some embodiments, the titanium alloy is a nearalpha titanium alloy (e.g. Ti-6Al-2Sn-4Zr-2Mo, Ti-5Al-5Sn-2Zr-2Mo, IMI 685, Ti 1100). In some embodiments, the titanium alloy is an alpha-beta titanium alloy (e.g. Ti-6A1-4V, Ti- 6A1-4V ELI, Ti-6Al-6V-2Sn, Ti-6Al-7Nb). In some embodiments, the titanium alloy is a beta-near beta titanium alloy (e.g. Ti-10V-2Fe-3Al, Ti-29Nb-13Ta-4.6Zr, Ti-13V-l lCr- 3A1, Ti-8Mo-8V-2Fe-3Al, Beta C, Ti-15-3).

[0045] In some embodiments, the structure is made of stainless steel.

[0046] In some embodiments, the structure is made of cobalt-chrome.

[0047] The lattice structure comprises extendable struts.

[0048] The expression “the lattice structure comprises extendable struts” will be understood to mean that the extendable struts are the constituting elements of unit cells forming the lattice itself. This is shown for example in the embodiment lattice structures of Figures 1 and 5, showing extendable lattice structures having extendable struts as constituting elements of the unit cells forming the lattice itself. This is opposed to, for example, a conventional (e.g. rigid) lattice structure with externally attached extendable elongating rod-like elements that are not constituting elements of the lattice unit cells.

[0049] Accordingly, the present invention may alternatively be said to relate to an expandable strut lattice structure for a structural component, the structure comprising extendable struts, said extendable struts being constituting elements of unit cells characterizing said the lattice. By the lattice structure comprising extendable struts, the lattice structure as a whole can extend, expand, and change shape, as shown in Figures 1 and 5.

[0050] By being "extendable", each extendable strut can extend along at least one direction, typically a main axial direction. In some instances, the lattice structure may be capable to extend as well as contract along at least one direction, typically the main axial direction of the strut.

[0051] By the struts being extendable, the lattice structure can change shape at least by virtue of at least a portion of the struts extending. As a result, the lattice structure of the invention is a deploy able structure along multiple directional axis. This is different, for example, from conventional structures based on flexible 2D honeycomb lattices, which may change shape along a single direction and solely due to the contraction / expansion of their honeycomb cell units along that direction, which structural components remain nevertheless unchanged.

[0052] Further, by the struts being extendable, the lattice structure of the invention can advantageously expand beyond the dimension of each of its constituent unit cells. This is advantageous over conventional structures made of two or more overlapping fixed lattice structures, which extension length is limited to the dimension of each of its constituent unit cells.

[0053] Any strut configuration making a strut extendable may be used in the lattice structure.

[0054] In some embodiments, the extendable struts are telescopic struts. By being "telescopic", a strut is made by two or more nested strut sections that can slide out from one another, resulting in the strut lengthening (such as an extendable antenna, a telescopic lens, etc.).

[0055] The strut sections of a telescopic strut may have any configuration enabling the required telescopic / sliding movement. For example, the strut sections may be made of two or more hollow cylinders of progressively smaller diameter that are nested within one another, with each section capable of sliding out of its corresponding nesting section. An example in that regard is shown in the schematic of Figure 1.

[0056] In alternative configurations, the strut sections may be in the form of planar beams nested within one another that can slide out from one another along their main length axis, for example by having a sliding rail arrangement.

[0057] The expandable lattice structure may comprise unit cells having any dimensions conducive to obtain a structure fit for its intended purpose.

[0058] In some embodiments, the expandable lattice structure comprises unit cells having a unit dimension range of about 5 mm to 8 mm.

[0059] In some embodiments, the expandable lattice structure comprises struts having a width from about 1 mm to about 4 mm.

[0060] In some embodiments, the expandable lattice structure comprises struts that, in a fully retracted configuration, each has a length of from about 3 mm to about 20 mm.

[0061] In some embodiments, the expandable lattice structure comprises telescopic struts with a gap between adjacent telescopic sections of from about 100 pm to about 200 pm, when the struts are unextended. By the term "gap" is meant herein the clearance between the internal dimension (e.g. the internal diameter) of a telescopic section and the external dimension (e.g. external diameter) the corresponding nested section.

[0062] It was observed that telescopic struts with a gap between adjacent telescopic sections of from about 100 pm to about 200 pm provides for a particularly effective telescopic operation. This is particularly beneficial for expandable lattice structures for use as or as part of an orthopaedic implant. In those instances, the implant can advantageously provide excellent extendibility characteristics under all configurations.

[0063] The structures of the invention may be provided with a mechanism to lock expansion / contraction following activation along any of the expansion / contraction directional axes. This can advantageously afford the possibility to lock the structure into place after extension along any direction in intermittent stages of expansion. This is different that, for example, conventional structures made of overlapping components that can move relative to one another but cannot interlock to control the degree of expansion or contraction.

[0064] Accordingly, in some embodiments the structure comprises one or more telescopic sections having a locking mechanism that prevents strut retraction as the strut extends. These configurations are particularly advantageous in structures intended for use in a bone implant that can grow with the patient. In those cases, the structure can effectively accommodate natural bone growth while maintaining biomechanical support.

[0065] Any locking mechanism allowing strut elongation while preventing retraction may be adopted. Said locking mechanism may be integral to the strut sections.

[0066] Examples of suitable locking mechanisms in that regard include a ratchet locking mechanism (for example a linear ratchet locking mechanism), screw coupling, or a push and release mechanism.

[0067] An example of a linear ratchet locking mechanism in a telescopic strut suitable for use in the structure of the invention is shown in Figure 2. Strut 1 is made of three telescopic sections including internal section 2 nested within mid-section 3, itself nested within external section 4. Consecutive sections are separated by gap 5 (which may be a gap of from about 100 pm to about 200 pm). The Figure shows the strut in an un-extended configuration. Upon extension, internal section 2 slides out of nesting mid- section 3, which itself slides out of external section 4. Sliding movements occur along the main length axis of the cylindrical assembly. When each section is fully extended, complementary elements 6 and 7 of a linear ratchet locking mechanism engage, locking the corresponding section(s) in an extended configuration, preventing retraction.

[0068] In some embodiments, expandable the lattice structure comprises extendable struts along an X-axis and a Y-axis, and non-extendable struts along a Z-axis. An example in that regard is shown in the schematic of Figure 1. The schematic shows a lattice with a cubic unit cell topology. The primary loading bearing struts along the Z-axis are not telescopic, as they are designed to withstand a large concentration of force. The struts along the X-axis and Z-axis are telescopic, as they will withstand primarily bending moments, consequentially the lattice is collapsible to a third of its extended dimensions along the X and Y-axes.

[0069] In some embodiments, the expandable lattice structure comprises extendable struts along an X-axis, and non-extendable struts along a Y-axis and a Z-axis.

[0070] In some embodiments, the expandable lattice structure comprises extendable struts along all X-, Y-, and Z-axis.

[0071] It will be appreciated that expansion / contraction along each directional axis may be independent from expansion / contraction along any of the other axes.

[0072] Instances where the expandable lattice structure comprises extendable struts along at least two of X-, Y-, and Z-axis are advantageous over conventional designs, particularly when used as implants. In those cases, flexibility / extendibility along multiple axes afford free movement within the body, with no need for external activation.

[0073] In some embodiments, the telescopic struts include telescopic sections that comprise blocking features that limit the sliding range of a section. An example of one such configuration is shown in the schematics of Figures 2 and 3. The Figures shows a schematic cross-section of embodiment telescopic struts in which the telescopic sections have inward protrusions that can interlock with corresponding protrusions on a successive telescopic section, thus limiting the sliding range of that section. The configuration is particularly useful to prevent separation of adjacent sections after they fully extend.

[0074] In Figure 3, strut 1 is made of three telescopic sections including internal section 2 nested within mid-section 3, itself nested within external section 4. Consecutive sections are separated by gap 5 (which may be a gap of from about 100 pm to about 200 pm). The Figure shows the strut in an un-extended configuration. Upon extension, internal section 2 slides out of nesting mid-section 3, which itself slides out of external section 4. Sliding movements occur along the main length axis of the cylindrical assembly. When each section is fully extended, complementary tapered surfaces 8 and 9 engage, locking the corresponding section(s) in an extended configuration while preventing further extension.

[0075] In some embodiments, the telescopic struts include a joint mechanism. Said joint mechanism may be located, for example, at an end of a telescopic strut. Use of a joint mechanism in the telescopic struts increases the degrees of freedom of the struts by adding off-axis motion along one or more directions depending on the type of joint mechanism. Examples of suitable joint mechanisms in that regard include hinge joints, ball-and-socket joints (also referred herein as “ball joints”), pivot joints, and plane joints.

[0076] An example of telescopic strut having a ball joint mechanism is shown in Figure 4. The telescopic strut in this figure is of the same kind of those shown in Figures 2-3, additionally comprising a ball joint at one end. The ball joint is made of ball 10 and socket 11 , providing additional rotational and angular movement as indicated by the arrows in the Figure.

[0077] The provision of a joint, such as a ball joint, provides the strut with increased resistance to torsion, ensuring greater stability and durability under rotational forces. The provision of a joint, such as a ball joint, also allows for a wider range of strut movements, enabling more flexibility and adaptability in dynamic applications. Additionally, the ball joint facilitates strut extension and compliance beyond the traditional X, Y, and Z axes, accommodating multi-directional forces and complex motion patterns that rigid systems cannot handle.

[0078] Furthermore, the addition of a joint, such as a ball joint, can improve load distribution across the structure, reducing stress concentrations and extending the lifespan of the components. The addition of a joint, such as a ball joint, also enables smoother transitions between movements, minimizing wear and tear while enhancing overall performance.

[0079] The proposed design opens up possibilities for applications in fields such as robotics, automotive suspension systems, and aerospace engineering, where precision, adaptability, and resilience are critical. By allowing for off-axis motion and improved energy absorption, the ball joint contributes to a more efficient and robust mechanical system capable of operating in demanding environments.

[0080] The lattice structure of the invention maybe fabricated by any means known to the skilled person.

[0081] In some embodiments, the structure is fabricated by additive manufacturing.

[0082] The expression "additive manufacturing" is used herein to indicate a process of joining materials to make physical objects from 3D model data, usually layer upon layer, as opposed to subtractive manufacturing methodologies. Additive manufacturing embraces additive processes where successive layers of material are laid down in different shapes using a heat source. This is in comparison to traditional machining techniques, which mostly rely on the removal of material by methods such as cutting, machining, or milling.

[0083] Additive manufacturing is sometimes known as “3D printing”, “additive layer manufacturing” (ALM) or “rapid prototyping”.

[0084] The use of additive manufacturing to fabricate the extendable lattice structure described herein advantageously enables the production of any arbitrary strut and structure configuration in a single production step. This is particularly advantageous over conventional multiple-component fabrication processes, which require post-fabrication assembly to produce the final component. Further, additive manufacturing affords fabrication of lattice structures with tailored architectures with a high level of quality, accuracy, and reliability. Besides providing an exceptional degree of control over the mechanical properties, such manufacturing processes are capable of building graded cellular structures. When used as or as part of an orthopaedic implant, those structures can be an asset for bone-replacement implants since the internal skeleton of the prosthesis can be designed to ease osseous integration as well as to match the local mechanical properties of the bone. By properly selecting topology, size, and relative density of the unit cell, it is thus possible to fabricate implants with mechanical properties mimicking those of the host bone, manufacture 3D structures with interconnected porosity and pore sizes suitable to bone ingrowth and vascularization, and customize implants by using CT scan data of a patient's own bone.

[0085] Additive manufacturing techniques suitable for use in the invention embrace all those known to a skilled person for the additive manufacturing of metal components that require the use of a metal powder feedstock.

[0086] For example, suitable additive manufacturing techniques may encompass those in which a heat source (such as a laser beam) is used to form a spatially confined melt pool on either a metallic substrate or on a previously deposited metal layer. A metal powder feedstock is fed into the melt pool using a carrier gas, where it melts to form a deposit that is fusion bonded to the substrate or on the previously deposited metal layer.

[0087] Other suitable additive manufacturing techniques may embrace those in which a metal powder feedstock is first spread on a metallic substrate or on a previously deposited metal layer. The feedstock is then locally melted using a heat source (such as a laser or electron beam) under a process gas, causing the feedstock particulate to coalesce and fuse to the substrate or on the previously deposited metal layer.

[0088] Further suitable additive manufacturing techniques may include processes in which a print head deposits a liquid binding agent onto selected areas of subsequent layers of metal particles, thereby building a three-dimensional object of metal particles joined by the binder. The object is subsequently consolidated by sintering to form the final metal article.

[0089] Accordingly, additive manufacturing suitable for use in the invention comprises selective laser sintering (SLS), laser powder bed fusion (LPBF), direct metal laser sintering (DMLS), direct metal laser melting (DMLM), binder jet printing (BJP), electron beam powder bed fusion (EB-PBF), laser engineered net shaping (LENS), direct laser forming (DLF), or directed energy deposition (DED).

[0090] In some embodiments, the additive manufacturing is laser powder bed fusion (LPBF). In a typical procedure, the process begins with a thin layer (typically 30 to 50 microns thick) of feedstock powder (e.g. titanium feedstock powder) spread across a build platform. A high- powered laser beam is precisely directed by a computer-controlled system to scan and melt the powdered material according to the specific cross-section of the lattice model being produced. Once the laser moves away, the melted material quickly solidifies (typically according to a cooling rate of 1,000 to 10,000 °C / second), forming a solid layer. The build platform is then lowered by a small increment, and a new layer of feedstock powder is spread over the previously solidified layer. The laser scans the new layer, and the process repeats, building the object layer by layer.

[0091] A skilled person would be aware of specific devices and operational parameters for the additive manufacturing of a metal component given a metal feedstock of the kind described herein.

[0092] As an example, LPBF operational parameters used to fabricate a TieALV telescopic lattice specimen are a laser power of 100 W, a laser spot size of 80 pm, a scan speed of 750 mm / s, and a layer thickness of 30 mm. A hatch-infill scan strategy was also used, which involves two sets of border scans (inset / offset) from the outer / inner diameter by a 60 pm offset to compensate for the width of the melt pool. The area between the sets of border scans was then filled with a hatching pattern that rotates 90° between subsequent layers. The spacing between the border scans and that between adjoining hatches were both set to 80 pm. It is believed that a structure of the kind described herein manufactured by additive manufacturing is unique in its own rights. Accordingly, the present invention also provides an additive manufactured expandable strut lattice structure, the structure comprising extendable struts. The lattice structure may be a structure of the kind described herein.

[0093] The present invention also provides a method of fabricating an expandable lattice structure, the structure comprising extendable struts, and the method comprising providing a powder feedstock and processing said powder feedstock by additive manufacturing to form said structure. The expandable lattice structure obtained with the method may be an expandable lattice structure of the kind described herein.

[0094] The powder feedstock for use in the method may be any powder feedstock that can be processed in an additive manufacturing procedure.

[0095] In some embodiments, the powder feedstock is metal powder feedstock. Said metal powder feedstock comprises metal particles.

[0096] The term "metal" is used herein in accordance to its broadest meaning to encompass materials that (i) comprise at least one metal element, and (ii) can be processed by additive manufacturing. As such, the term will be understood to include elemental metals as well as metal alloys. By "metal alloy" is meant herein a combination comprising either (i) two or more metal elements or (ii) one metal element and one or more non-metal alloying element(s).

[0097] Accordingly, in some embodiments the metal powder feedstock used in accordance with the invention comprises metal particles of steel, titanium, aluminium, magnesium, nickel, iron, copper, gold, silver, bismuth, manganese, zinc, chromium, molybdenum, platinum, zirconium, iridium, yttrium, vanadium, niobium, tantalum, tungsten, cobalt, tin, lead, gallium, or an alloy of one or more thereof. For example, the metal powder feedstock may be stainless steel, cobalt-chrome, aluminium alloy, or titanium metal powder feedstock. In some embodiments, the metal powder feedstock comprises titanium particles. The titanium may be in the form of pure titanium or titanium alloy. The titanium alloy may be titanium alloy of the kind described herein. In its pure form, the titanium may encompass commercially pure titanium, for example 98 to 99.5% titanium.

[0098] In some embodiments, the metal powder feedstock comprises particles of an alloy selected from 1000, 2000, 3000, 4000, 5000, 6000, 7000 and 8000 series aluminium alloy.

[0099] In some embodiments, the metal powder feedstock comprises particles of AlSilOMg, Ti- 6A1-4V, or Inconel 718.

[0100] In some embodiments, the powder feedstock comprises metal particles of Ti-6A1-4V alloy.

[0101] In some embodiments, the metal powder feedstock comprises particles of high- strengthening-phase nickel superalloy.

[0102] The metal particles of the feedstock may be of any size provided they can be sintered / melted under typical operational conditions (e.g. pressure, temperature) of additive manufacturing.

[0103] In some embodiments, the metal particles have an average particle size of less than about 500 pm, less than about 250 pm, less than about 100 pm, less than about 50 pm, less than about 25 pm, or less than about 10 pm.

[0104] Typically, the average particle size range of the metal particles is selected in accordance with the type and conditions of equipment used for powder additive manufacturing. For example, the average particle size range of the metal particles can range from about 5 pm to about 20 pm, from about 45 pm to about 150 pm, from about 10 pm to about 60 pm, from about 45 pm to about 105 pm, from about 15 pm to about 45 pm, from about 20 pm to about 63 pm, or from about 25 pm to about 75 pm. In some embodiments, the powder feedstock comprises particles having an average size of about 45 pm. As used herein, the expression “average particle size” refers to the average size of particulate matter measured using optical or SEM images following a standard linear measurement.

[0105] The metal particles of the feedstock may be made by any means known to a skilled person. For example, the metal particles can be produced by solid-state reduction, atomization, electrolysis, mechanical shearing, or chemical processes. Those processes may include, for example, rotational granulation, fluidized bed granulation, high shear granulation, crushing granulation, melt granulation, spray granulation, and micro-emulsion granulation.

[0106] In some embodiments, the additive manufacturing comprises focusing a laser beam on the powder feedstock to a focal spot having a size of about 80 pm. In those instances, the lattice structure can be fabricated with high degree of dimensional control to a degree of tolerance that is suitable for use of the structure in a medical implant.

[0107] In some embodiments, the lattice structure is fabricated through laser powder bed fusion. For example, the lattice structure may be fabricated through laser powder bed fusion that comprises focusing a laser beam on the powder feedstock to a focal spot having a size of about 80 pm.

[0108] By way of example, a lattice structure having telescopic struts may be fabricated by additive manufacturing on a nest of single low contact (0 pm integration into lattice) tree supports situated at each segment of the sliding strut sections to minimise surface roughness and prevent each segment from melting into each other. Typically, lattices and other additively manufactured components are fabricated with higher contact supports (30-150 pm integration into the component) which decreases the surface quality. The tree supports designed to support the lattice structure can be extremely thin (i.e. 200 pm), which enables the lattice to be far stronger than the supports and facilitates manual removal with no need for post-fabrication CNC machining. The offset between the sliding struts is designed to prevent the struts from detaching or melting together. The offset also facilitates a smooth sliding mechanism between strut segments that requires minimal actuation. The lattice structure may be fabricated at an inclination angle (e.g. X = 45°, Y = 35.264°, Z = 0°), so the telescopic struts can be supported equally and successfully fabricated even if every strut within the geometric unit cell of the lattice is telescopic. If the telescopic struts were instead fabricated at a 0° incline, each segment of the sliding strut sections would melt together, and the underside of the struts would be non-uniform with a high surface roughness. The fabrication orientation (e.g. X = 45°, Y = 35.264°, Z = 0°) also affords fabrication of lattice struts that are axially isotropic. This advantageously ensures that the struts can exhibit the same manufacturability, microstructure and consequently mechanical properties along every axial direction.

[0109] The lattice structure may be fabricated with each segment of the sliding strut pointing downwards, from the thinnest strut section at the bottom to the thickest at the top. This orientation enables the tree supports to be added to the lowest manufacturable layer of each sliding strut, enabling the entire segment to be supported. If it was fabricated any other way, the segments would melt together.

[0110] To achieve the smallest geometries possible (strut inner / outer diameters) possible the laser scan strategy was designed with a zero-focus offset and no up-skins and down-skins. The innermost sliding strut segment (the thinnest strut) may have an internal channel diameter of 0.7 mm. The dimension may be selected to be the smallest channel diameter affording easy removal of residue trapped powder following fabrication. Trapped powder could prevent the sliding mechanisms from actuating and limits applicability of the structure.

[0111] Each segment of the telescopic struts can be designed in computer-aided design software and fabricated in a pre-actuated state where segments of the sliding struts are in their extended configuration to prevent them from melting together. This also provides the necessary spacing to (i) add the filleting architecture that keeps the sliding strut segments in the structure, preventing them from detaching when fully extended and (ii) add the mechanisms that can lock the extended struts in a static state once fully extended. Examples of telescopic struts fabricated using the procedure described herein are presented in the Example Section.

[0112] In some embodiments, the expandable lattice structure is or is part of an orthopaedic implant.

[0113] The use of a structure described herein in an orthopaedic implant is believed to be unique in its own rights. Accordingly, the present invention may also be said to provide an orthopaedic implant comprising an expandable strut lattice structure, the structure comprising extendable struts. Said structure may be an expandable lattice structure of the kind described herein.

[0114] Orthopaedic implants may be used to replace, augment or support sections of bone in human or animal bodies, such as for replacement of material between or within bones in the spine, long bones in the arms or legs, in the knee, hip, shoulder, finger or other joints and following removal of a portion of bone due to tumour treatment or injury. To that effect, orthopaedic implants are manufactured devices widely used in reconstructive surgery and designed to provide stability and support to musculoskeletal structures, for example to replace a joint, bone, or cartilage due to damage or deformity.

[0115] In some cases, especially for patients still in their growth stage, such as children, expandable prosthesis may be used. Those prosthesis are conventionally fixed in place during a first surgery session and require periodic adjustments as the patient grows during subsequent adjustment sessions. During each of those sessions, the implant can be manually extended as required and re-fixed in an extended configuration until the next session. Once the patient has completed his / her growth, or the bone reached maturity, the expandable implant can be replaced with a permanent one. That entire procedure is of course less than ideal in that it requires multiple sessions and further surgery to replace the initial implant.

[0116] When used in or as an orthopaedic implant, the expandable lattice structure can be placed within the intramedullary canal of a bone and anchored at the top and lower end of the bones. In other applications, the lattice structure may substitute a bone section, in which instances the extendable struts can advantageously lengthen as the patient (for example a child) grows. The expandable lattice structure described herein can be used for bone replacement, as an intercalary material, or forming part or all of an implant at least a surface of which is adapted to contact bone tissue, whether the implant is a bone implant, a joint replacement implant (e.g. total hip replacement, total knee replacement, etc.), as an intercalary material used to replace a vertebral disc in vertebral fusion, or the like. Examples of use of the proposed structure forming part or all of an implant include vertebral implants, a hip implant, a knee implant, an elbow implant, a shoulder implant, a wrist implant, an ankle implant, a tumour, a trauma, or a dental implant.

[0117] As discussed herein, the morphological parameters of the extendable lattice structure, including topology, porosity and pore size, can be tightly controlled to meet bone ingrowth requirements while still enabling the structure to be tailored to match the local mechanical properties of the host bone, for example with gradients of properties throughout the design.

[0118] Additional requirements for a successful use of the structure of the invention as orthopaedic implant typically include osteoconductivity, high porosity to facilitate transport of nutrients and metabolic wastes, sufficient mechanical strength to support physiological loading conditions, and appropriate biodegradability (if any). Those attributes may be controlled by the microarchitecture of the strut arrangements, in particular by nodal connectivity, porosity, pore size as well as pore topology. Pore topology describes geometric properties independent of cell size and shape, as well as invariant to stretching, bending and twisting.

[0119] In that regard, a skilled person will be mindful that the cell architecture affects functional characteristics such as elastic modulus, permeability, and diffusivity. The latter describes mass transport conditions that in turn influence cell phenotype, tissue ingrowth, and nutrient settings. Optimal trade-off cell topologies can be obtained through multi-objective topology optimization, which allows for determined cell geometry that has high vascularization and superior osteochondral ossification. Implants that are or include an extendable lattice structure of the kind described herein can be configured for osteointegration and stimulating adequately stressed new bone growth. The implants are particularly useful for use when it is desirable to have strong bone attachment and / or bone growth throughout the body of an implant. Whether bone growth is desired only for attachment or throughout the implant, the proposed structure can provide mechanical spacing, a scaffold to support new bone growth and a modulus of elasticity that allows new bone growth to be loaded with dynamic physiological forces. As a result, the implants can dynamically grow and move with the bone while affording formation of stronger and healthier bone.

[0120] Certain embodiments of the invention will be now described by reference to the following non-limiting examples.

[0121] EXAMPLES

[0122] EXAMPLE 1

[0123] Figure 5 shows an example 3x3x3 TieAUV simple cubic telescopic lattice fabricated through laser powder bed fusion.

[0124] The lattice length in the lattice can increase from a retracted length of 26 mm to an extended length of 42 mm. The struts may be 3-section telescopic struts of the kind shown in Figures 2 and 3. In those instances, each extendable telescopic strut is split into three segments that can be extended or withdrawn, each segment is spaced 130 pm from the other (i.e. gap between internal diameter of larger section and external diameter of corresponding nested section), and the minimum inner diameter of the smallest section is 700 pm to ensure powder removal from the internal profile.

[0125] The outer diameter for the inside, middle, and outside strut sections are 1.1 mm, 2.34 mm, and 3.58 mm, respectively. The wall thickness for each strut section is 200 jam.

[0126] EXAMPLE 2

[0127] An extendable strut lattice structure (e.g. Figure 5) with extendable telescopic struts was fabricated with laser powder bed fusion (LPBF).

[0128] The process began with a thin layer of titanium powder spread across the build platform (30- 50 microns). A high-powered laser beam was precisely directed by a computer-controlled system to scan and melt the powdered material according to the specific cross-section of the lattice model being produced. Once the laser moved away, the melted material quickly solidified, forming a solid layer. The build platform was then lowered by a small increment, and a new layer of powder was spread over the previously solidified layer. The laser scanned the new layer, and the process was repeated to build the object layer by layer.

[0129] Before LPBF fabrication, the lattices were rotated by 45° along the X axis, and then 35.3° along the Y axis. This allowed to support each telescopic mechanism equally at 35.3° inclines. Without these rotations, the as-printed telescopic hollow strut sections would likely be defective and fuse together preventing the mechanism from working. Following the rotation the telescopic struts were all extended so each strut segment could be easily supported with hollow tree supports, this enabled them to float without touching to prevent struts from fusing together. All manufactured lattices were cooled in the machine to room temperature before being removed from the build plate.

[0130] The LPBF operational parameters used to fabricate the above TieAUV telescopic lattice specimen were a laser power of 100 W, a laser spot size of 80 pm, a scan speed of 750 mm / s, and a layer thickness of 30 pm. A hatch-infill scan strategy was also used, which involves two sets of border scans (inset / offset) from the outer / inner diameter by a 60 pm offset to compensate for the width of the melt pool. The area between the sets of border scans was then filled with a hatching pattern that rotates 90° between subsequent layers. The spacing between the border scans and that between adjoining hatches were both set to 80 pm. EXAMPLE 3

[0131] A lattice structure with extendable struts having a ball joint (as depicted in Figure 4) was fabricated through laser additive manufacturing using Ti-6A1-4V powder, using operational parameters described in Example 2.

[0132] The ball component of the ball joint was fabricated with a diameter of 2 mm and provided with a 0.7 mm hole through it to enable powder removal.

[0133] The socket component of the ball joint was designed with a 35° rotational extrusion around the ball, forming a curved lip that securely retains the ball and prevents disassembly, as shown in Figure 4 (elements 10 and 11). This lip not only ensures structural integrity but also allows each strut connection to achieve 110° of free movement at each strut connection.

[0134] The socket was offset by 0.18 mm from the ball to prevent adherence between the ball and socket. Both the ball and socket were supported with the same tree support as the expandable struts, and the socket was printed downwards so its bottom lip could be well supported to prevent defects. The ball and socket were fabricated in a single integrated mechanism.

[0135] Throughout this specification and the claims which follow, unless the context requires otherwise, the word ‘comprise’, and variations such as ‘comprises’ and ‘comprising’, will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

[0136] The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS1. An expandable strut lattice structure for a structural component, the lattice structure comprising extendable struts.

2. The structure of claim 1, wherein the struts are telescopic struts.

3. The structure of claim 1 or 2, comprising telescopic struts with a gap between adjacent telescopic sections of from about 100 pm to about 200 pm, when the struts are unextended.

4. The structure of any one of claims 1-3, comprising one or more telescopic sections having a locking mechanism that prevents strut retraction as the strut extends.

5. The structure of any one of claims 1-4, wherein said locking mechanism comprises a linear ratchet, screw coupling, or a push and release mechanism.

6. The structure of any one of claims 1-5, wherein the struts are made of metal.

7. The structure of claim 6, wherein the metal comprises steel, titanium, aluminium, magnesium, nickel, iron, copper, gold, silver, bismuth, manganese, zinc, chromium, molybdenum, platinum, zirconium, iridium, yttrium, vanadium, niobium, tantalum, tungsten, cobalt, tin, lead, gallium, or an alloy of one or more thereof.

8. The structure of any one of claims 1-7, having a unit cell topology selected from cubic, body-centred cubic (BCC), face-centred-cubic (FCC), BCC with z-struts (BCCZ), FCC with z-struts (FCCZ), octet-truss, and diamond.

9. The structure of any one of claims 1-8, comprising extendable struts along an X-axis and a Y-axis, and non-extendable struts along a Z-axis.

10. The structure of any one of claims 1-9, comprising unit cells having a unit dimension range of about 5 mm to 8 mm.

11. The structure of any one of claims 1-10, comprising struts having a width from about 1 mm to about 4 mm.

12. The structure of any one of claims 1-11, being formed by additive manufacturing.

13. The structure of any one of claims 1-12, wherein the structural component is an orthopaedic implant.

14. An orthopaedic implant comprising the expandable strut lattice structure of any one of claims 1-12.

15. A method of fabricating an expandable lattice structure as defined in any one of claims 1-13, the method comprising providing a powder feedstock and processing said powder feedstock by additive manufacturing to form said structure.

16. The method of claim 15, wherein processing the powder feedstock by additive manufacturing comprises selective laser sintering (SLS), laser powder bed fusion (LPBF), direct metal laser sintering (DMLS), direct metal laser melting (DMLM), binder jet printing (BJP), electron beam powder bed fusion (EB-PBF), laser engineered net shaping (LENS), direct laser forming (DLF), or directed energy deposition (DED).

17. The method of claim 15 or 16, wherein the powder feedstock is metal powder feedstock.

18. The method of claim 17, wherein the metal powder feedstock comprises metal particles of steel, titanium, aluminium, magnesium, nickel, iron, copper, gold, silver, bismuth, manganese, zinc, chromium, molybdenum, platinum, zirconium, iridium, yttrium,vanadium, niobium, tantalum, tungsten, cobalt, tin, lead, gallium, or an alloy of one or more thereof.

19. The method of any one of claims 15-18, wherein the powder feedstock comprises particles having an average size of about 45 pm.

20. The method of any one of claims 15-19, wherein the powder feedstock comprises metal particles of Ti-6A1-4V alloy.

21. The method of any one of claims 15-20, wherein said additive manufacturing comprises focusing a laser beam on the powder feedstock to a focal spot having a size of about 80 pm.

22. The method of any one of claims 15-21, wherein the structure is or is part of an orthopaedic implant.

23. An additive manufactured expandable strut lattice structure, the structure comprising extendable struts.

24. The structure of claim 23, wherein the struts are telescopic struts.

25. The structure of claim 23 or 24, comprising telescopic struts with a gap between adjacent telescopic sections of from about 100 pm to about 200 pm, when the struts are unextended.

26. The structure of any one of claims 23-25, comprising one or more telescopic sections having a locking mechanism that prevents strut retraction as the strut extends.

27. The structure of any one of claims 23-26, wherein said locking mechanism comprises a linear ratchet, screw coupling, or a push and release mechanism.

28. The structure of any one of claims 23-27, wherein the struts are made of metal.

29. The structure of claim 28, wherein the metal comprises steel, titanium, aluminium, magnesium, nickel, iron, copper, gold, silver, bismuth, manganese, zinc, chromium, molybdenum, platinum, zirconium, iridium, yttrium, vanadium, niobium, tantalum, tungsten, cobalt, tin, lead, gallium, or an alloy of one or more thereof.

30. The structure of any one of claims 23-29, having a unit cell topology selected from cubic, body-centred cubic (BCC), face-centred-cubic (FCC), BCC with z-struts (BCCZ), FCC with z-struts (FCCZ), octet-truss, and diamond.

31. The structure of any one of claims 23-30, comprising extendable struts along an X- axis and a Y-axis, and non-extendable struts along a Z-axis.

32. The structure of any one of claims 23-31, comprising unit cells having a unit dimension range of about 5 mm to 8 mm.

33. The structure of any one of claims 23-32, comprising struts having a width from about 1 mm to about 4 mm.

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