Torsion locking interface for an orthopaedic implant taper interlock

Abstract

A torsion locking interface for an orthopaedic implant taper interlock includes a proximal segment with a frustopyramidal boss having four flat faces and a distal segment with a receiver. Opposing set screws engage the flat faces obliquely in a plane orthogonal to a rotational axis, providing torsional resistance in both rotational directions. This prevents unintended rotational displacement that could compromise the taper interlock due to bodily fluids reducing friction. The receiver may define a cylindrical interior for controlled rotation before fixation, and the frustopyramidal boss may have radiused edges to reduce stress concentrations and improve adjustability. In one embodiment, the set screws engage opposite faces, while in another, they engage adjacent orthogonal faces, each configuration optimising torsional resistance. The interface is particularly suited for modular knee and hip implants, maintaining internal or external rotational stability to reduce the risk of taper slippage and implant misalignment.

Description

Torsion Locking Interface for an Orthopaedic Implant Taper InterlockField of the Invention

[0001] The present invention relates to orthopaedic implants and, more particularly, to a torsion locking interface for an orthopaedic implant taper interlock. The invention is designed to enhance the stability of taper interlocks by preventing rotational slippage through the use of a torsional locking mechanism to improve implant security, particularly in modular knee and hip implant assemblies, by mitigating the risk of unintended rotational displacement that could compromise the integrity of the taper interlock.Background of the Invention

[0002] In orthopaedic implants, a taper interlock typically refers to a mechanism used to join two components together securely by means of a tapered (conical or cylindrical) fit. This design relies on the principle that a taper fitting into a corresponding taper can create a very tight and stable connection that is resistant to pulling apart or rotational forces. This kind of interlocking mechanism is often utilized in modular orthopedic implants, such as hip and knee replacements, where different components need to be securely connected.

[0003] The taper interlock helps in reducing the risk of component loosening over time, which is crucial for the longevity and stability of the implant within the body. It allows for the transmission of loads across the joint in a more natural manner and can also facilitate easier assembly and disassembly of the components during surgery, allowing for a better fit and customisation to the patient's anatomy.

[0004] Taper interlocks, despite their design to provide a secure and stable connection between orthopaedic implant components, can encounter issues leading to loosening after installation. During the index surgery or subsequent revision surgery, it's possible for these components to become contaminated with bodily fluids or fat, which can act as lubricants. This unintended lubrication can significantly reduce the friction that is essential for the taper interlock's stability, leading to the potential for inadvertent slippage. Such slippage within the taper interlock system isnot merely a minor inconvenience but can necessitate revision surgery to correct the problem, indicating the severity of the issue. A specific example of this can be seen in knee implants, where the slippage of a taper interlock between the intramedullary stem component and the condylar bearing component can adversely affect the internal or external rotation of the knee implant, leading to patellar maltracking , a condition where the kneecap does not move properly, which can cause significant pain, instability, and potential long-term damage to the knee joint, underscoring the critical nature of maintaining the integrity of taper interlocks in orthopaedic implants. In proximal femoral or hip joint reconstructions, slippage or relative rotational motion can lead to suboptimal anteversion angle between the femoral axis and neck leading to hip joint instability and dislocation.

[0005] The present invention seeks to provide a way to overcome or substantially ameliorate at least some of the deficiencies of the prior art, or to at least provide an alternative.

[0006] It is to be understood that, if any prior art information is referred to herein, such reference does not constitute an admission that the information forms part of the common general knowledge in the art, in Australia or any other country.Summary of the Disclosure

[0007] There is provided an orthopaedic implant comprising a taper interlock and a torsion locking interface to enhance the stability of the taper interlock. The torsion locking interface includes a proximal segment defining a frustopyramidal boss having four flat faces and a distal segment defining a receiver for rotatably receiving the frustopyramidal boss. A pair of opposing set screws in the distal segment engage respective faces of the frustopyramidal boss to prevent unintended rotational displacement.

[0008] The opposing set screws engage their respective faces obliquely in a plane orthogonal to a rotational axis defined between the segments and are orientated oppositely in the plane to provide respective opposing torsional resistance in both rotational directions. The set screws preferably engage the flat faces at non-central locations and the frustopyramidal boss preferably tapers away from a distal end ofthe proximal segment so that tightening of the set screws induces an axial force component along the rotational axis, drawing the frustopyramidal boss further into the receiver, enhancing engagement between an interior tapered profile of the distal segment and an exterior tapered profile of the proximal segment.

[0009] The torsion locking interface is particularly suitable for modular knee implant assemblies, preventing rotational slippage of the taper interlock after implantation. By securing the taper interlock against unintended rotational displacement, the interface helps maintain the intended internal or external rotational angle of the knee implant, reducing the risk of patellar maltracking and instability. Additionally, the interface is applicable to hip implants, where maintaining the correct anteversion angle between the femoral axis and neck is critical for preventing joint instability and dislocation.

[0010] The receiver may define a cylindrical interior having a diameter greater than the maximum cross-section of the frustopyramidal boss to facilitate assembly and controlled rotation before fixation, allowing precise intraoperative adjustment of the rotational orientation before securing the implant. Radiused edges at the intersections of the flat faces may be provided to reduce stress concentrations and improve rotation before final locking, further minimising potential for micromovements that could otherwise compromise implant stability.

[0011] The distal segment may include a through-hole for a manipulation tool to allow precise rotational adjustment before impaction. The through-hole may be axially offset from the receiver so as not to interfere with the frustopyramidal boss. The set screws are preferably accessible through side apertures in the distal segment for post-implantation adjustment.

[0012] By securing the taper interlock against unintended rotational displacement, the torsion locking interface minimises micromovements that could compromise the taper interlock due to the presence of fat, synovial fluid, or other bodily fluids acting as lubricants. This ensures long-term stability of the implant and reduces the risk of taper fretting, wear, and loosening, which could otherwise necessitate revision surgery.

[0013] The torsion locking interface may be adapted for use in various orthopaedic applications where maintaining rotational stability of the taper interlock is critical .

[0014] Other aspects of the invention are also disclosed.Brief Description of the Drawings

[0015] 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:

[0016] Figure 1 illustrates an orthopaedic implant in the form of a knee implant, where the femoral component incorporates a taper interlock.

[0017] Figure 2 depicts the taper interlock featuring a torsion locking interface.

[0018] Figure 3 provides an additional view of the taper interlock and the torsion locking interface, highlighting their interaction.

[0019] Figure 4 shows a cross-sectional view of a receiver of the torsion locking interface in a plane orthogonal to the rotational axis.

[0020] Figure 5 presents an embodiment where the set screws engage opposite orthogonal faces of the frustopyramidal boss.

[0021] Figure 6 presents an alternative embodiment where the set screws engage adjacent orthogonal faces of the frustopyramidal boss rather than opposite faces.Description of Embodiments

[0022] Referring to Figure 1 , an orthopaedic implant 100 is illustrated, in this example, a knee implant comprising an upper femoral component 101 articulating with a lower tibial component 102. The femoral component 101 incorporates a taper interlock 103, which facilitates the connection between a condylar bearing component 104 and an intramedullary stem 105.

[0023] With reference to Figure 2, the taper interlock 103 includes a proximal segment 106 that is received by a distal segment 107, where both segments 106 and 107 define tapered profiles that frictionally engage and lock together upon impaction. This allows for controlled rotational adjustment of the condylar bearing component 104with respect to the intramedullary stem 105, after which impaction secures the components in a fixed position.

[0024] To prevent rotational slippage at the taper interlock 103, the proximal segment 106 and the distal segment 107 incorporate a torsion locking interface 108, as shown in Figure 3. The torsion locking interface 108 comprises a frustopyramidal boss 109, which extends from an end of the proximal segment 106, defining four flat faces 1 10.

[0025] The distal segment 107 defines a receiver 1 18 for rotatably receiving the frustopyramidal boss 109. The distal segment 107 includes screw holes 1 15 for engaging opposing set screws 1 1 1 , which engage the flat faces 1 10 to restrict rotational movement between the segments 106 and 107.

[0026] Figures 4 - 6 show the torsion locking interface 108 in a plane orthogonal to a rotational axis 1 19 defined between the segments 106 and 107. In this view, the relative positioning and engagement of the opposing set screws 1 1 1 with the flat faces 1 10 of the frustopyramidal boss 109 are illustrated in detail. The figures demonstrate how the set screws 1 1 1 are positioned and angled to ensure an effective locking mechanism that enhances torsional resistance between the proximal segment 106 and the distal segment 107.

[0027] These figures show how the opposing set screws 1 1 1 engage their respective faces 1 10 obliquely in the plane orthogonal to the rotational axis 1 19, meaning that rather than applying force perpendicular to the faces 1 10, the set screws 1 1 1 make contact at an angle. Additionally, the set screws 1 1 1 are oriented oppositely in the plane orthogonal to the rotational axis 1 19, meaning that the force vectors exerted by each screw 1 1 1 are directed in opposite rotational directions. This opposing orientation of the set screws 1 1 1 provides opposing torsional resistance in both rotational directions of the proximal segment 106 with respect to the distal segment 107. Specifically, if an external force attempts to rotate the proximal segment 106 in one direction, one of the set screws 1 1 1 resists this movement by increasing frictional engagement, while the other set screw 1 1 1 prevents rotation in the opposite direction.

[0028] This arrangement ensures that the frustopyramidal boss 109 remains securely engaged within the receiver 1 18 of the distal segment 107, preventing unintendedrotational displacement while maintaining the structural integrity of the taper interlock 103. By securely locking the frustopyramidal boss 109 in position, the torsion locking interface 108 minimises the risk of micromovements that could lead to loosening or disengagement of the adjacent taper interlock 103.

[0029] In some embodiments, the set screws 11 1 may feature rounded or chamfered tips, which further optimise the engagement with the flat faces 110 by providing a point-contact interface that maximises frictional resistance while reducing the risk of material deformation.

[0030] Preferably, the set screws 11 1 engage the respective faces at non-central locations to enhance frictional resistance against rotation. Specifically, Figure 5 shows centre points 1 12 defined by respective flat faces 110 and contact points 113 where the tips of the set screws 11 1 contact the flat faces 110, which are offset from the centre points 112. As alluded to above, these contact points 113 are both located on the same side of the rotational axis 119, thereby providing respective opposing torsional resistance against both rotational directions of the frustopyramidal boss 109.

[0031] Figure 4 shows the screw holes 115 oriented along longitudinal axes 116 in the plane perpendicular to the rotational axis 119, which are oblique with respect to radii 1 14 extending from the rotational axis. Preferably, the oblique angle is greater than 5° or approximately 10° with respect to the respective radius 114. These oblique angles provide sufficient torque to resist rotational displacement of the proximal segment 106 relative to the distal segment 107, effectively locking the taper interlock 103 against unintended micromovements that could compromise its structural integrity.

[0032] Figure 5 shows an embodiment wherein the set screws 1 11 engage opposite faces 110 of the frustopyramidal boss 109. This configuration provides a balanced distribution of torsional resistance, effectively counteracting rotational forces applied in either direction. By engaging directly opposite faces, the set screws create a symmetrical locking effect, ensuring uniform resistance and stability across the interface. This arrangement is particularly advantageous for applications requiring high torsional rigidity while maintaining ease of assembly and adjustment.

[0033] Figure 6 illustrates an alternative embodiment wherein the set screws 1 1 1 engage adjacent orthogonal faces 1 10 of the frustopyramidal boss 109. This configuration introduces an additional stabilising effect by constraining rotational movement more tightly at a corner of the frustopyramidal boss 109. The engagement of orthogonal faces may also create a more compact locking mechanism, which may be beneficial in implants with space constraints or applications where rotational precision is critical.

[0034] In both embodiments, the opposing set screws engage their respective faces1 10 obliquely in the plane orthogonal to the rotational axis 1 19 defined between the segments 106 and 107 and are oriented oppositely in the plane to provide respective opposing torsional resistance in both rotational directions of the proximal segment 106 with respect to the distal segment 107.

[0035] Preferably, the torsion locking interface 108 employs only two set screws 1 1 1 , which need to be tightened during installation.

[0036] The frustopyramidal boss 109 preferably tapers away from a distal end of the proximal segment 106, such that the faces 1 10 are angled away from the distal end in a plane lying along the rotational axis 1 19. This ensures that when the set screws1 1 1 are tightened, an axial force component is induced along the rotational axis 1 19, pulling the frustopyramidal boss 109 further into the receiver 1 18 and enhancing the engagement of the taper interlock 103.

[0037] As shown in Figure 3, the receiver 1 18 preferably defines a cylindrical interior having a diameter greater than the maximum cross-section of the frustopyramidal boss 109. This configuration allows for ease of assembly and free rotation between the components 106 and 107 while ensuring a secure torsional locking engagement once the set screws 1 1 1 are tightened. The frustopyramidal boss 109 may comprise radiused edges at the intersections of the flat faces 1 10 to facilitate rotation when the set screws 1 1 1 are partially tightened and to prevent potential corner-edge contact point loading.

[0038] It should be noted that in an alternative embodiment, the proximal segment 106 defines the receiver 1 18, while the distal segment 107 defines the frustopyramidalboss 109. This alternative arrangement maintains the same functional principles of the torsion locking interface 108 while allowing for variations in implantation technique or prosthetic design.

[0039] The distal segment 107 may comprise a through-hole 1 17 configured to accommodate a manipulation tool for adjusting the rotational orientation of the distal segment with respect to the proximal segment before impaction. The through-hole 1 17 is preferably axially offset beneath the receiver 1 18 so that the manipulation tool can be inserted completely through the through-hole 1 17 without interfering with the frustopyramidal boss 109.

[0040] The torsion locking interface 108 is particularly suitable for modular knee implant assemblies, where it prevents rotational slippage of the proximal segment 106 relative to the distal segment 107 after implantation. The set screws 1 1 1 are preferably accessible through side apertures in the distal segment 107, allowing for post-implantation adjustment if necessary.

[0041] The frustopyramidal boss 109 designed with four flat faces 1 10 optimises torsional resistance while maintaining structural integrity and ease of assembly. This configuration ensures stable engagement with the opposing set screws 1 1 1 , providing uniform torsional resistance in both rotational directions. The defined flat surfaces enhance frictional locking without causing excessive stress concentrations or material deformation, improving long-term stability. Also, a four-face design balances effective locking with controlled adjustability, preventing overconstraint and ensuring precise alignment during surgical installation. This geometry also simplifies manufacturing, reducing complexity and cost while maintaining durability and consistent engagement throughout the implant’s lifespan.

[0042] Insertion of the knee implant 100 may involve inserting the intramedullary stem 105 into the intramedullary canal of the femoral bone with the taper interlock 103 disengaged. The condylar bearing component 104 can then be rotated with respect to the stem 105 to control the degree of internal or external rotation of the implant 100. At the desired position, the condylar bearing component 104 and the stem 105 may be impacted together along their longitudinal axes so that the Morse taperprofiles of the proximal and distal segments 106 and 107 frictionally engage. The set screws 1 1 1 may then be tightened with a hex wrench or similar tool to secure the frustopyramidal boss 109 against rotation. During revision surgery, the set screws 1 1 1 may be loosened to release the torsion locking interface 108, allowing for disassembly or reorientation of the implant components.

[0043] The terms "proximal" and "distal" are used herein for orientational convenience and are not intended to be limiting. The described configuration is equally applicable in the opposite orientation, for example, where the receiver 1 18 is defined by the proximal segment 106 and the frustopyramidal boss 109 is defined by the distal segment 107, without departing from the functional principles of the invention.

[0044] The foregoing description presents preferred embodiments of the invention but is not intended to be limiting. Variations and modifications may be made without departing from the principles described, as would be understood by a person skilled in the art.

Claims

Claims1. An orthopaedic implant comprising: a taper interlock; and a torsion locking interface, the torsion locking interface comprising: a proximal segment defining a frustopyramidal boss having four flat faces; a distal segment defining a receiver for rotatably receiving the frustopyramidal boss and having a pair of opposing set screws engaging respective faces of the frustopyramidal boss, wherein, in a plane orthogonal to a rotational axis defined between the segments, the opposing set screws engage their respective faces obliquely and are orientated oppositely in the plane to provide respective opposing torsional resistance in both rotational directions of the proximal segment with respect to the distal segment, thereby preventing unintended rotational displacement that could compromise the taper interlock.

2. The torsion locking interface of claim 1 , wherein the set screws engage their respective flat faces at non-central locations.

3. The torsion locking interface of claim 1 , wherein the distal segment defines an interior tapered profile configured for engaging an exterior tapered profile of the proximal segment.

4. The torsion locking interface of claim 1 , wherein the frustopyramidal boss tapers away from a distal end of the proximal segment such that the faces of the frustopyramidal boss are angled away from the distal end in a plane lying along the rotational axis and such that tightening of the set screws induces an axial force component along the rotational axis, drawing the frustopyramidal boss further into the receiver.

5. The torsion locking interface of claim 1 , wherein the receiver defines a cylindrical interior having a diameter greater than a maximum cross-section defined by the frustopyramidal boss.

6. The torsion locking interface of claim 1 , wherein the frustopyramidal boss includes radiused edges at the intersection between the flat faces.

7. The torsion locking interface of claim 1 , wherein the distal segment comprises a through-hole configured to accommodate a manipulation tool for adjusting rotational orientation of the distal segment with respect to the proximal segment.

8. The torsion locking interface of claim 7, wherein the through-hole is axially offset from the receiver such that the manipulation tool does not interfere with the frustopyramidal boss.

9. The torsion locking interface of claim 1 , wherein the proximal and distal segments are components of a modular knee implant assembly, and the torsion locking interface prevents rotational slippage of the proximal segment relative to the distal segment after implantation.

10. The torsion locking interface of claim 1 , wherein the set screws are accessible through side apertures in the distal segment to allow post-implantation adjustment.1 1 . The torsion locking interface of claim 1 , wherein the interface has only two set screws.

12. The torsion locking interface of claim 1 , wherein the set screws define rounded tips.

13. The torsion locking interface of claim 1 , wherein the set screws engage opposite faces of the frustopyramidal boss.

14. The torsion locking interface of claim 1 , wherein the set screws engage adjacent orthogonal faces of the frustopyramidal boss.

15. The torsion locking interface of claim 1 , wherein the set screws are orientated such that their longitudinal axes are oblique with respect to radii extending from the rotational axis and wherein the oblique angle is greater than 5°.

16. The torsion locking interface of claim 18, wherein the oblique angle is approximately 10° or more.

17. A method of assembling an orthopaedic implant comprising a taper interlock, the method comprising: inserting a proximal segment into a distal segment, the proximal segment defining a frustopyramidal boss having four flat faces and the distal segment defining a receiver for rotatably receiving the frustopyramidal boss; rotating the proximal segment relative to the distal segment to achieve a desired rotational orientation; tightening a pair of opposing set screws within the distal segment to engage respective faces of the frustopyramidal boss, wherein, in a plane orthogonal to a rotational axis defined between the segments, the opposing set screws engage their respective faces obliquely and are orientated oppositely in the plane to provide respective opposing torsional resistance in both rotational directions of the proximal segment with respect to the distal segment, thereby preventing unintended rotational displacement that could compromise the taper interlock.

18. The method of claim 17, wherein tightening of the set screws induces an axial force component along the rotational axis, drawing the frustopyramidal boss further into the receiver.

19. The method of claim 17, wherein the set screws engage the respective flat faces at non-central locations.

20. The method of claim 17, wherein the set screws are oriented such that their longitudinal axes are oblique with respect to radii extending from the rotational axis, the oblique angle being greater than 5°.

21. The method of claim 20, wherein the oblique angle is approximately 10° or more.

22. The method of claim 17, further comprising adjusting the rotational orientation of the proximal segment using a manipulation tool inserted through a through-hole in the distal segment.

23. The method of claim 22, wherein the through-hole is axially offset from the receiver such that the manipulation tool does not interfere with the frustopyramidal boss.

24. The method of claim 17, wherein the orthopaedic implant taper interlock is a component of a modular knee implant assembly.

25. The method of claim 17, further comprising loosening the set screws to release the torsion locking interface and permit disassembly or reorientation of the proximal segment with respect to the distal segment.

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