Osseointegration augment for orthopaedic implants

Abstract

An osseointegration augment for an orthopaedic implant comprises an endosteal fixation sleeve and a periosteal fixation collar. The endosteal fixation sleeve surrounds an intramedullary rod and is configured for insertion into a medullary canal, defining an external endosteal osseointegration surface. The periosteal fixation collar surrounds the intramedullary rod and bridges the endosteal fixation sleeve and a resected bone surface, defining an external periosteal osseointegration surface and a bone contacting face for physiological load distribution. The endosteal fixation sleeve and periosteal fixation collar define opposing flat contacting faces, facilitating stable, non-coaxial independent positioning. The endosteal fixation sleeve is spaced from the intramedullary rod, forming an internal gap filled with bone cement. This configuration optimises endosteal and periosteal osseointegration, improves implant stability, and enables intraoperative adjustability to accommodate anatomical variations.

Description

Osseointegration Augment for Orthopaedic ImplantsField of the Invention

[0001] The present invention relates to orthopaedic implants, and more particularly to an implant system comprising an endosteal fixation sleeve and a periosteal fixation collar for use in bone fixation and osseointegration. The invention is applicable to limb salvage procedures, joint reconstruction, and other orthopaedic applications requiring secure fixation of an implant within a resected bone.Background of the Invention

[0002] Orthopaedic implants are used to replace or reinforce damaged bone structures, ensuring mechanical stability and functional restoration. To enhance fixation, implants often incorporate osseointegration surfaces that facilitate direct bone attachment. These surfaces promote biological integration between the implant and bone tissue, reducing the risk of loosening and improving long-term stability.

[0003] In limb salvage and joint reconstruction procedures, implants must provide both intramedullary fixation within the medullary canal and cortical fixation at the resection interface. Achieving stable fixation in both regions presents technical challenges, particularly in cases where the alignment of the implant components does not coincide with the natural anatomical structure. Independent optimisation of fixation at different osseointegration zones is often required to accommodate anatomical variations, maintain mechanical integrity, and achieve reliable load transfer.

[0004] D1 (US 2014 / 0277528 A1 ) describes an osseointegration augment comprising a sleeve and a collar structure for use in orthopaedic implants. The augment in D1 provides an osseointegration surface and is designed to be secured within the medullary canal while interfacing with a resected bone surface. However, the system in D1 is limited in its ability to provide independent positioning of fixation components, which may constrain the optimisation of both endosteal and periosteal osseointegration zones. Furthermore, the fixation approach in D1 does not allow forcontrolled spacing between the implant stem and the surrounding fixation sleeve, which may affect cement distribution and overall stability.

[0005] There remains a need for an improved implant configuration that facilitates independent positioning of fixation components while ensuring controlled cement fixation, optimised osseointegration, and stable load distribution across the implantbone interface.Summary of the Disclosure

[0006] The described implant comprises a stem defining an intramedullary rod and an augment including an endosteal fixation sleeve and a periosteal fixation collar. The endosteal fixation sleeve surrounds the intramedullary rod and is configured for insertion into the medullary canal. The sleeve defines an external endosteal osseointegration surface to promote integration with the surrounding bone. The periosteal fixation collar also surrounds the intramedullary rod and bridges the endosteal fixation sleeve and the resection surface of the bone. The periosteal fixation collar defines an external periosteal osseointegration surface and a bone contacting face configured for direct contact and physiological load transfer against the resected bone end surface.

[0007] The periosteal fixation collar is fixed to the stem, whereas the endosteal fixation sleeve is not directly fixed to the intramedullary rod. The endosteal fixation sleeve and the periosteal fixation collar define opposing flat contacting faces to facilitate stable non-coaxial independent positioning of the endosteal fixation sleeve relative to the periosteal fixation collar. The endosteal fixation sleeve is sized and spaced apart from the intramedullary rod to define an internal gap, which is filled with bone cement in use.

[0008] This configuration allows for independent optimisation of endosteal and periosteal osseointegration zones while maintaining a stable mechanical interface. The spacing between the endosteal fixation sleeve and the intramedullary rod enables controlled cementation, ensuring uniform cement distribution and providing stress shielding effects. The engagement between the opposing flat contacting faces provides stability while permitting intraoperative adjustability to accommodateanatomical variations. By allowing independent positioning of the fixation components, the implant facilitates improved load transfer, enhanced osseointegration, and increased long-term fixation reliability.

[0009] In some embodiments, the endosteal fixation sleeve may define an interior diameter greater than an exterior diameter of the intramedullary rod. This configuration may create a controlled spacing between the sleeve and the rod, allowing for improved cement fixation while preventing direct contact between these components. Preferably, the endosteal fixation sleeve does not touch the intramedullary rod once inserted, ensuring the presence of an internal gap for uniform cement distribution and minimising stress shielding effects.

[0010] The opposing flat contacting faces of the endosteal fixation sleeve and the periosteal fixation collar may press against each other along a longitudinal axis defined by the intramedullary rod. This interface may facilitate stable engagement between the components while allowing for independent non-coaxial positioning. The periosteal fixation collar may define an interior diameter smaller than that of the endosteal fixation sleeve, enabling a secure fit around the intramedullary rod while maintaining a non-coaxial relationship with the endosteal fixation sleeve.

[0011] The periosteal fixation collar may engage a fixing collar of the intramedullary rod to provide axial stability and prevent unintended displacement during physiological loading. In some embodiments, the periosteal fixation collar may be fixed to the intramedullary rod using a side set screw, allowing for secure mechanical fixation and intraoperative adjustability. The side set screw may enable the collar to be locked in place once optimal positioning is achieved.

[0012] The external endosteal osseointegration surface may be frustoconical, which may provide a wedging effect for improved primary fixation within the medullary canal. Additionally, the external endosteal osseointegration surface may define longitudinal formations such as splines or grooves to resist torsional forces, thereby improving rotational stability and enhancing long-term fixation.

[0013] The periosteal fixation collar may be configured as a rectangular torus, which may provide an optimised load distribution profile when engaging the resected bonesurface. The external periosteal osseointegration surface may be circular in crosssection to facilitate uniform engagement with the surrounding periosteum. Alternatively, the external periosteal osseointegration surface may be elliptical, which may better conform to the natural anatomical variations of the bone. In some embodiments, the external periosteal osseointegration surface may be customised with a patient-specific geometry to optimise fit and enhance osseointegration.

[0014] The endosteal fixation sleeve may be adapted as a tibial component of a knee implant, providing secure fixation within the tibial medullary canal. In such embodiments, the endosteal fixation sleeve may define keel slots to accommodate the keels of a tibial bearing block, improving stability and resisting shear forces. Alternatively, the endosteal fixation sleeve may be configured as a femoral component of a knee implant, facilitating secure implantation within the femoral medullary canal while maintaining independent engagement with the periosteal fixation collar.

[0015] In some embodiments, the endosteal fixation sleeve may be porous between its inner and outer surfaces to promote bone ingrowth and enhance biological fixation. Alternatively, the endosteal fixation sleeve may define an internal barrier that prevents porosity between the inner and outer surfaces, ensuring that bone cement does not infiltrate the porous structure. This configuration may provide a combination of mechanical cement fixation and long-term biological integration.

[0016] The implant may comprise a plurality of endosteal fixation sleeves of different sizes or shapes, allowing for intraoperative selection based on patient-specific anatomical requirements. Similarly, a plurality of periosteal fixation collars of different sizes or shapes may be provided to optimise engagement with varying cortical bone geometries, ensuring a stable and adaptable implant configuration.

[0017] Other aspects of the invention are also disclosed.Brief Description of the Drawings

[0018] Notwithstanding any other forms which may fall within the scope of the present invention, preferred embodiments of the disclosure will now be described, by way of example only, with reference to the accompanying drawings in which:

[0019] Figure 1 illustrates an exploded view of an osseointegration augment configured for interfacing with a stem of an implant, such as a knee implant.

[0020] Figure 2 depicts a side view of the osseointegration augment in an assembled state, interfacing with a resected bone.

[0021] Figures 3 and 4 provide additional bottom perspective views of the osseointegration augment.Description of Embodiments

[0022] With reference to Figure 1 , an implant 150 comprises a stem 101 and an augment 100. The stem 101 defines an intramedullary rod 120 for insertion into a medullary canal 104 of a resected bone 105, such as a femur or tibia. The augment 100 comprises an endosteal fixation sleeve 102 and a periosteal fixation collar 103. The endosteal fixation sleeve 102 surrounds the intramedullary rod 120 and is configured for insertion into the medullary canal 104 in use. The endosteal fixation sleeve 102 defines an external endosteal osseointegration surface 106, which is structured to promote integration with the endosteal surface of the surrounding bone.

[0023] The periosteal fixation collar 103 surrounds the intramedullary rod 120 and is configured to bridge the endosteal fixation sleeve 102 and a resection surface 121 of the bone 105. The periosteal fixation collar 103 defines an external periosteal osseointegration surface 108, which is structured to facilitate osseointegration with the periosteum of the bone 105. The periosteal fixation collar 103 further defines a bone contacting face 109, which is configured for direct contact with and physiological loading against the resected bone end surface 121 .

[0024] The periosteal fixation collar 103 is fixed to the stem 101 , while the endosteal fixation sleeve 102 is not directly fixed to the intramedullary rod 120. The endosteal fixation sleeve 102 and the periosteal fixation collar 103 define opposing flat contacting faces 122 and 123, which facilitate stable non-coaxial independent positioning of the endosteal fixation sleeve 102 with respect to the periosteal fixation collar 103. This arrangement enables the endosteal fixation sleeve 102 to be adjusted independently of the periosteal fixation collar 103, thereby optimising the positioningof the endosteal and periosteal osseointegration zones based on anatomical and biomechanical considerations.The endosteal fixation sleeve 102 is sized and spaced apart from the intramedullary rod 120 to define an internal gap 124 between the endosteal fixation sleeve 102 and the intramedullary rod 120. The internal gap 124 is configured to receive bone cement 125, which secures the endosteal fixation sleeve 102 within the medullary canal 104 while maintaining the ability to independently position the periosteal fixation collar 103 relative to the stem 101 . This structure allows for controlled load distribution between the stem 101 and the surrounding bone while ensuring stable long-term fixation of the implant 150.In some embodiments, the endosteal fixation sleeve 102 may define an interior diameter greater than an exterior diameter of the intramedullary rod 120. This dimensional relationship provides a controlled clearance between the endosteal fixation sleeve 102 and the intramedullary rod 120, allowing for the formation of the internal gap 124. The internal gap 124 facilitates the injection and uniform distribution of bone cement 125, ensuring secure fixation of the endosteal fixation sleeve 102 within the medullary canal 104 while maintaining independent positioning relative to the periosteal fixation collar 103. The clearance also prevents direct contact between the endosteal fixation sleeve 102 and the intramedullary rod 120, which may reduce stress shielding and allow for more even load distribution across the cement interface. In some embodiments, the endosteal fixation sleeve 102 may be configured such that it does not touch the intramedullary rod 120 once inserted. This is achieved by the interior diameter of the endosteal fixation sleeve 102 being greater than the exterior diameter of the intramedullary rod 120, thereby maintaining the internal gap 124 between these components. The internal gap 124 may be filled with bone cement 125, which provides mechanical fixation while preventing direct contact between the endosteal fixation sleeve 102 and the intramedullary rod 120. This configuration may reduce stress concentrations on the intramedullary rod 120, improving load distribution and minimising stress shielding effects that could otherwise compromise bone integration. Additionally, the separation between the endosteal fixation sleeve102 and the intramedullary rod 120 allows for more precise alignment and independent positioning of the endosteal fixation sleeve 102 relative to the periosteal fixation collar 103.

[0025] The opposing flat contacting faces 122 and 123 of the endosteal fixation sleeve 102 and the periosteal fixation collar 103 may press against each other along the longitudinal axis defined by the intramedullary rod 120. This configuration provides a stable mechanical interface, allowing the endosteal fixation sleeve 102 to be positioned independently while ensuring structural integrity. By maintaining this engagement along the longitudinal axis, the interface may facilitate controlled load transfer between the augment components, reducing stress concentrations and enhancing fixation reliability.The periosteal fixation collar 103 may define an interior diameter smaller than that of the endosteal fixation sleeve 102. This relationship allows the periosteal fixation collar 103 to seat securely against the endosteal fixation sleeve 102 while maintain ing non-coaxial contact with the endosteal fixation sleeve 102.

[0026] The periosteal fixation collar 103 may engage a fixing collar 1 10 of the intramedullary rod 120 to ensure secure attachment to the stem 101. The fixing collar 1 10 may be an integral or modular feature of the stem 101 , providing a defined interface for coupling with the periosteal fixation collar 103. This engagement may allow the periosteal fixation collar 103 to maintain a fixed position relative to the intramedullary rod 120 while permitting independent positioning of the endosteal fixation sleeve 102. By securing the periosteal fixation collar 103 at a designated location on the intramedullary rod 120, this configuration may enhance the stability of the implant 150 and optimise the alignment of the external periosteal osseointegration surface 108 with the resected bone end surface 121.To further secure the periosteal fixation collar 103 to the intramedullary rod 120, a side set screw 1 1 1 may be used. The side set screw 1 1 1 may engage a threaded bore in the fixing collar 1 10, allowing for mechanical fixation of the periosteal fixation collar 103 to the intramedullary rod 120. This fixation method may prevent axial and rotational movement of the periosteal fixation collar 103, ensuring that it remains ina stable position during implantation and under physiological loading. The use of a set screw 1 1 1 may also facilitate intraoperative adjustments, allowing the periosteal fixation collar 103 to be positioned precisely before being locked in place. By securi ng the periosteal fixation collar 103 independently from the endosteal fixation sleeve 102, this arrangement maintains the ability to optimise endosteal and periosteal osseointegration zones separately.

[0027] The external endosteal osseointegration surface 106 of the endosteal fixation sleeve 102 may be frustoconical, tapering along its longitudinal axis to facilitate wedging engagement within the medullary canal 104. The conical profile may enhance primary fixation by creating a press-fit effect against the endosteal surface of the surrounding bone, reducing micromotion and promoting initial mechanical stability. The frustoconical shape may also aid in distributing axial loads more evenly along the implant length, minimising stress concentrations and improving long-term integration.

[0028] To further enhance stability, the external endosteal osseointegration surface 106 may define longitudinal formations 126, such as ridges, splines, or grooves, configured to resist torsional forces. These formations may interact with the surrounding bone to reduce rotational displacement under physiological loading, ensuring the endosteal fixation sleeve 102 remains securely positioned. The longitudinal formations 126 may also promote bone ingrowth by increasing surface area and providing microstructures that encourage osseointegration.

[0029] The periosteal fixation collar 103 may be shaped as a rectangular torus, providing a structurally robust interface between the endosteal fixation sleeve 102 and the resected bone end surface 121. The rectangular toroidal geometry may improve load transfer across the bone-implant junction while maintaining a secure engagement with the fixing collar 1 10 of the intramedullary rod 120. The rectangular profile may also assist in optimising the positioning of the external periosteal osseointegration surface 108 relative to the periosteum.

[0030] The external periosteal osseointegration surface 108 may define various cross-sectional geometries to accommodate different anatomical and biomechanicalrequirements. In some configurations, the external periosteal osseointegration surface 108 may be circular, facilitating uniform load distribution around the implant perimeter and simplifying manufacturing and implantation. Alternatively, the external periosteal osseointegration surface 108 may define an elliptical cross-section, which may better match natural variations in cortical bone geometry and enhance contact area for improved osseointegration.For applications requiring custom anatomical adaptation, the external periosteal osseointegration surface 108 may be tailored to define a patient-specific geometry. This configuration may be generated based on preoperative imaging data, ensuring an optimised fit that maximises bone-implant contact while minimising the risk of stress shielding. A patient-specific geometry may also account for individual variations in cortical bone thickness and curvature, improving the overall biomechanical performance of the implant 150.

[0031] The endosteal fixation sleeve 102 may be configured as a tibial component of a knee implant, adapted for insertion into the medullary canal 104 of a resected tibia. In this configuration, the endosteal fixation sleeve 102 provides structural support for the tibial stem of the knee implant while facilitating endosteal osseointegration through its external endosteal osseointegration surface 106. The periosteal fixation collar 103 may provide a stable interface against the resected tibial plateau, ensuring controlled load transfer and minimising micromotion at the bone-implant interface.

[0032] For enhanced fixation, the endosteal fixation sleeve 102 may define keel slots 127, configured to accommodate the keels of a tibial bearing block or tibial baseplate. The keel slots 127 may provide additional resistance to torsional and shear forces, preventing implant rotation and improving long-term stability. The inclusion of keel slots 127 may also allow for compatibility with modular tibial implant systems, permitting intraoperative adjustments to optimise alignment and fixation.Alternatively, the endosteal fixation sleeve 102 may be configured as a femoral component of a knee implant, adapted for insertion into the medullary canal 104 of a resected femur. In this embodiment, the endosteal fixation sleeve 102 supports the femoral stem of the knee implant while maintaining an independent interface with theperiosteal fixation collar 103. The frustoconical shape of the external endosteal osseointegration surface 106 may assist in achieving a secure press-fit within the femoral canal, while longitudinal formations 126 may enhance torsional stability. The periosteal fixation collar 103 may engage the distal femoral resection surface, ensuring physiological load distribution and minimising implant micromotion.

[0033] The endosteal fixation sleeve 102 may be porous between its inner and outer surfaces to facilitate enhanced osseointegration and biological fixation. The porosity may be achieved through additive manufacturing techniques, such as selective laser sintering or electron beam melting, allowing for the formation of an interconnected porous lattice structure. The porous structure may mimic the trabecular architecture of natural bone, providing pathways for bone ingrowth while maintaining sufficient mechanical strength. The degree of porosity may be optimised to balance mechanical stability with permeability, ensuring that bone tissue can infiltrate and integrate with the implant while maintaining sufficient structural integrity for load-bearing applications.In some configurations, the endosteal fixation sleeve 102 may define an internal barrier 128 to prevent porosity from extending between the inner and outer surfaces. This internal barrier 128 may be formed as a solid core or a thin dense layer positioned within the endosteal fixation sleeve 102, restricting the passage of bone cement 125 into the porous structure. The internal barrier 128 may serve to isolate the porous external endosteal osseointegration surface 106 from cement infiltration, ensuring that osseointegration is not compromised by the presence of non-biological materials. By incorporating an internal barrier 128, the implant may provide the benefits of both mechanical cement fixation and long-term biological integration, optimising implant stability and longevity.

[0034] The implant 150 may comprise a plurality of endosteal fixation sleeves 102 of different sizes or shapes, allowing for selection based on patient-specific anatomical variations and surgical requirements. Variability in size may include differences in diameter, length, and taper angle of the endosteal fixation sleeve 102 to accommodate medullary canals 104 of varying dimensions. Different shapes mayinclude cylindrical, frustoconical, or anatomically contoured profiles designed to optimise engagement with the surrounding bone structure. Providing a range of endosteal fixation sleeves 102 may allow surgeons to select the most suitable component for achieving secure fixation while ensuring proper alignment of the intramedullary rod 120. This modularity may also facilitate intraoperative adjustments, reducing the need for extensive bone modification.Similarly, the implant 150 may comprise a plurality of periosteal fixation collars 103 of different sizes or shapes. The periosteal fixation collars 103 may vary in thickness, outer diameter, or cross-sectional geometry to accommodate different resection surface dimensions and cortical bone profiles. Available shapes may include circular, elliptical, or patient-specific geometries tailored for optimal bone contact and load distribution. The ability to select from multiple periosteal fixation collars 103 allows for precise adaptation to the bone resection surface 121 , ensuring stable fixation and minimising stress concentrations at the implant-bone interface. The modular nature of the periosteal fixation collars 103 may also enhance compatibility with various implant systems and surgical techniques, providing greater flexibility for patientspecific implantation.

[0035] In an exemplary method of use, the implant 150 is prepared for insertion into a resected bone 105, such as a femur or tibia, following standard surgical resection techniques. Initially, the appropriate size and shape of the endosteal fixation sleeve 102 are selected based on preoperative imaging and intraoperative assessment of the medullary canal 104. The endosteal fixation sleeve 102 is then inserted into the medullary canal 104, ensuring that the external endosteal osseointegration surface 106 contacts the endosteal surface of the bone. If the endosteal fixation sleeve 102 features a frustoconical design, it may be wedged into the canal to provide primary mechanical stability.

[0036] Once the endosteal fixation sleeve 102 is positioned, bone cement 125 may be introduced into the internal gap 124 defined between the endosteal fixation sleeve 102 and the intramedullary rod 120. The cement may be applied using standardinjection techniques, ensuring uniform distribution and minimising voids that could compromise fixation.

[0037] The periosteal fixation collar 103 is then selected to match the resected bone end surface 121 and the anatomy of the surrounding periosteum. The periosteal fixation collar 103 is placed over the exposed end of the intramedullary rod 120, engaging a fixing collar 1 10 if present. The periosteal fixation collar 103 may be secured using a side set screw 1 1 1 , preventing axial and rotational displacement during loading. The bone contacting face 109 of the periosteal fixation collar 103 is positioned against the resected bone end surface 121 , allowing for direct load transfer and promoting osseointegration through the external periosteal osseointegration surface 108.

[0038] If the endosteal fixation sleeve 102 includes keel slots 127, a tibial bearing block or femoral component may be fitted into these slots to complete the knee implant assembly. The modular nature of the implant 150 allows for adjustments to be made during surgery, accommodating patient-specific anatomical variations and ensuring optimal alignment.

[0039] After assembly, the implant 150 is subjected to physiological loading, where the opposing flat contacting faces 122 and 123 between the endosteal fixation sleeve 102 and periosteal fixation collar 103 facilitate stable, non-coaxial positioning. This independent adjustment of the endosteal and periosteal components enables customisation of the osseointegration zones, promoting effective biological fixation and long-term stability of the implant.

[0040] The implant 150 may be utilised in various orthopaedic procedures, including limb salvage surgeries, joint replacements, and fracture repair, providing a versatile solution for enhancing implant-bone integration and improving patient outcomes.

[0041] The described implant 150 provides several technical advantages in achieving stable fixation and optimised osseointegration. The independent positioning of the endosteal fixation sleeve 102 and the periosteal fixation collar 103 enables non - coaxial alignment, allowing for separate optimisation of endosteal and periosteal osseointegration zones. The external endosteal osseointegration surface 106facilitates bone integration within the medullary canal 104, while the external periosteal osseointegration surface 108 promotes fixation at the cortical bone interface.

[0042] The presence of an internal gap 124 between the endosteal fixation sleeve 102 and the intramedullary rod 120 allows for controlled cementation, ensuring uniform cement distribution while preventing direct contact between these components. This configuration improves mechanical stability and reduces stress shielding effects. The engagement of opposing flat contacting faces 122 and 123 enhances structural integrity by maintaining a stable interface between the augment components while permitting non-coaxial alignment.

[0043] The periosteal fixation collar 103 preferably being secured to the intramedullary rod 120 provided axial and rotational stability through engagement with a fixing collar 1 10 and a side set screw 1 1 1. This arrangement prevents displacement under physiological loading and allows for intraoperative adjustability. The rectangular toroidal geometry of the periosteal fixation collar 103 in an embodiment may improve load transfer across the bone-implant interface, while variations in cross-sectional shape accommodate different anatomical structures.

[0044] The frustoconical profile of the external endosteal osseointegration surface 106 in an embodiment may enhance primary fixation through a wedge fit, while optional longitudinal formations 126 improve torsional stability. Porous configurations of the endosteal fixation sleeve 102 promote bone ingrowth, whereas an internal barrier 128 may be used to prevent cement infiltration into the porous structure.The preferably availability of multiple endosteal fixation sleeves 102 and periosteal fixation collars 103 of different sizes and geometries allows for patient-specific adaptation, reducing the need for extensive bone modification. The modular design facilitates intraoperative flexibility, ensuring precise alignment and optimal biomechanical performance. The described structural features collectively contribute to improved long-term fixation, enhanced load distribution, and reduced risk of implant micromotion.

[0045] The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that specific details are not required in order to practise the invention. Thus, the foregoing descriptions of specific embodiments of the invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed as obviously many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the following claims and their equivalents define the scope of the invention.

Claims

Claims1. An implant comprising a stem and an augment, the stem defining an intramedullary rod and the augment comprising: an endosteal fixation sleeve surrounding the intramedullary rod, configured for medullary canal insertion in use and defining an external endosteal osseointegration surface; a periosteal fixation collar surrounding the intramedullary rod and configured for bridging the endosteal fixation sleeve and a resection surface in use and defining an external periosteal osseointegration surface and a bone contacting face configured for contact and physiological loading against a resected bone end surface in use, wherein the periosteal fixation collar is fixed to the stem; the endosteal fixation sleeve is not directly fixed to the intramedullary rod and the endosteal fixation sleeve and the periosteal fixation collar define opposing flat contacting faces to facilitate stable non-coaxial independent positioning of the endosteal fixation sleeve with respect to the periosteal fixation collar; the endosteal fixation sleeve is sized and spaced apart from the intramedullary rod to define an internal gap between the endosteal fixation sleeve and the intramedullary rod; and the gap is filled with bone cement.

2. An implant as claimed in claim 1 , wherein the endosteal fixation sleeve defines an interior diameter greater than an exterior diameter of the intramedullary rod.

3. An implant as claimed in claim 1 , wherein the endosteal fixation sleeve does not touch the intramedullary rod once inserted.

4. An implant as claimed in claim 1 , wherein the opposing flat contacting faces press against each other along a longitudinal axis defined by the intramedullary rod.

5. An implant as claimed in claim 1 , wherein the periosteal fixation collar defines an interior diameter less than an interior diameter defined by the endosteal fixation sleeve.

6. An implant as claimed in claim 1 , wherein the periosteal fixation collar engages a fixing collar of the intramedullary rod.

7. An implant as claimed in claim 6, wherein the periosteal fixation collar is fixed to the intramedullary rod with a side set screw.

8. An implant as claimed in claim 1 , wherein the external endosteal osseointegration surface is frustoconical.

9. An implant as claimed in claim 1 , wherein the external endosteal osseointegration surface defines longitudinal formations configured for torsional stability.

10. An implant as claimed in claim 1 , wherein the periosteal fixation collar is a rectangular torus.1 1. An implant as claimed in claim 1 , wherein the external periosteal osseointegration surface defines a circular cross-section.

12. An implant as claimed in claim 1 , wherein the external periosteal osseointegration surface defines an elliptical cross-section.

13. An implant as claimed in claim 1 , wherein the external periosteal osseointegration surface defines patient-specific geometry.

14. An implant as claimed in claim 1 , wherein the endosteal fixation sleeve is a tibial component of a knee implant.

15. An implant as claimed in claim 14, wherein the endosteal fixation sleeve defines keel slots.

16. An implant as claimed in claim 1 , wherein the endosteal fixation sleeve is a femoral component of a knee implant.

17. An implant as claimed in claim 1 , wherein the endosteal fixation sleeve is porous between inner and outer surfaces.

18. An implant as claimed in claim 1 , wherein the endosteal fixation sleeve defines an internal barrier such that the endosteal fixation sleeve is not porous between inner and outer surfaces.

19. An implant as claimed in claim 1 , wherein the implant comprises a plurality of endosteal fixation sleeves of different sizes or shapes.

20. An implant as claimed in claim 1 , wherein the implant comprises a plurality of periosteal fixation collars of different sizes or shapes.

21. A method of implanting an orthopaedic implant, the method comprising: inserting an endosteal fixation sleeve into a medullary canal of a resected bone, the endosteal fixation sleeve surrounding an intramedullary rod of a stem and defining an external endosteal osseointegration surface; positioning a periosteal fixation collar around the intramedullary rod, the periosteal fixation collar bridging the endosteal fixation sleeve and a resection surface of the bone and defining an external periosteal osseointegration surface and a bone contacting face configured for contact and physiological loading against the resected bone end surface;maintaining a non-coaxial positioning of the endosteal fixation sleeve relative to the periosteal fixation collar by engaging opposing flat contacting faces between the endosteal fixation sleeve and the periosteal fixation collar; spacing the endosteal fixation sleeve apart from the intramedullary rod to define an internal gap between the endosteal fixation sleeve and the intramedullary rod; and filling the gap with bone cement to secure the endosteal fixation sleeve relative to the intramedullary rod.

22. A method as claimed in claim 21 , wherein the endosteal fixation sleeve has an interior diameter greater than an exterior diameter of the intramedullary rod.

23. A method as claimed in claim 21 , wherein the endosteal fixation sleeve is positioned such that it does not directly contact the intramedullary rod once inserted.

24. A method as claimed in claim 21 , wherein engaging the opposing flat contacting faces comprises pressing the faces together along a longitudinal axis defined by the intramedullary rod.

25. A method as claimed in claim 21 , wherein the periosteal fixation collar is secured to the intramedullary rod by engaging a fixing collar.

26. A method as claimed in claim 25, wherein the periosteal fixation collar is fixed to the intramedullary rod using a side set screw.

27. A method as claimed in claim 21 , wherein the endosteal fixation sleeve is inserted into the medullary canal such that its external endosteal osseointegration surface is frustoconical.

28. A method as claimed in claim 21 , wherein the external endosteal osseointegration surface comprises longitudinal formations configured for torsional stability.

29. A method as claimed in claim 21 , wherein the periosteal fixation collar is selected to be a rectangular torus.

30. A method as claimed in claim 21 , wherein the periosteal fixation collar is selected to define an external periosteal osseointegration surface having a circular, elliptical, or patient-specific cross-section.

31. A method as claimed in claim 21 , wherein the endosteal fixation sleeve is inserted as a tibial component of a knee implant.

32. A method as claimed in claim 31 , wherein the endosteal fixation sleeve defines keel slots for engagement with a tibial bearing block.

33. A method as claimed in claim 21 , wherein the endosteal fixation sleeve is inserted as a femoral component of a knee implant.

34. A method as claimed in claim 21 , wherein the endosteal fixation sleeve is porous between inner and outer surfaces to facilitate bone ingrowth.

35. A method as claimed in claim 21 , wherein the endosteal fixation sleeve defines an internal barrier preventing porosity between inner and outer surfaces to restrict bone cement infiltration.

36. A method as claimed in claim 21 , further comprising selecting an endosteal fixation sleeve from a plurality of available sleeves of different sizes or shapes.

37. A method as claimed in claim 21 , further comprising selecting a periosteal fixation collar from a plurality of available collars of different sizes or shapes.

38. An osseointegration augment configured for interfacing a stem of an implant, the osseointegration augment comprising: an endosteal fixation sleeve configured for insertion into a medullary canal in use and defining an external endosteal osseointegration surface; a periosteal fixation collar configured for bridging the endosteal fixation sleeve and a resection surface in use and defining an external periosteal osseointegration surface and a bone contacting face configured for contact and physiological loading against a resected bone end surface in use, wherein the endosteal fixation sleeve and the periosteal fixation collar define opposing flat contacting faces to facilitate stable non-coaxial independent positioning of the endosteal fixation sleeve with respect to the periosteal fixation collar; the endosteal fixation sleeve is sized and spaced apart from the stem to define an internal gap between the endosteal fixation sleeve and the stem; and the gap is configured to be filled with bone cement.

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