WO2011146509A2

WO2011146509A2 - Liner deformation compensation in an acetabular assembly

Info

Publication number: WO2011146509A2
Application number: PCT/US2011/036842
Authority: WO
Inventors: Stanley Tsai; Kevin Weaver
Original assignee: Smith & Nephew Inc.
Priority date: 2010-05-17
Filing date: 2011-05-17
Publication date: 2011-11-24
Also published as: WO2011146509A3

Abstract

A method of forming multi-part components of different materials in order to yield a stiffness similar to current acetabular assembly designs is disclosed. This method can be used to minimize the required thickness of the components while still maintaining the required stiffness of the overall construct.

Description

LINER DEFORMATION COMPENSATION IN AN ACETABULAR ASSEMBLY

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application No. 61/345,448, filed on May 17, 2010, the disclosure of which is incorporated by reference.

TECHNICAL FIELD

[0002] The present invention relates generally to orthopedic prosthetic devices, and more particularly to an acetabular implant assembly.

BACKGROUND OF THE INVENTION

[0003] The common components of a conventional total hip arthroplasty typically include a femoral head articulating against a liner of an acetabular shell or cup. Generally, the material of the femoral head usually comprises either cobalt chrome (CoCr) or ceramic, and the material of the liner usually comprises polyethylene or ultrahigh molecular weight polyethylene (UHMWPE) liner. The material of the acetabular shell that typically backs the liner is usually a metal. The liner and acetabular shell mated to one another often by use of a locking mechanism. The metallic acetabular shell allows for the incorporation of a screw hole or a roughened or porous surface to aid with fixation of the shell to the patient's bone.

[0004] However, it is now generally believed that the wear particulates generated by the UHMWPE material upon use in vivo may lead to osteolysis of the surrounding bone, ultimately requiring revision in some patients. In response to the potentially deleterious effects of UHMWPE, metal-on-metal and ceramic-on-ceramic hard bearing articulation technologies have emerged as alternatives to UHMWPE liners. In the case of metal-on- metal implants, the acetabular component is either manufactured as a 1 -piece cup from metallic material and the femoral head also comprises a metal. Also, a typical metal-on- metal implant can include a metallic modular liner that is engaged in an acetabular shell similar to the conventional implant using an UHMWPE liner. For instance, the femoral head and acetabular liner/cup assembly are typically manufactured from CoCr, with the outside of the femoral head having a smaller diameter than the inside of the liner. Similarly, in a typical ceramic-on-ceramic hard bearing implant, the femoral head and acetabular liner usually comprise a ceramic while the acetabular cup typically comprises titanium.

[0005] While these hard bearing articulations have exhibited lower wear rates than UHMWPE bearings, they are more sensitive to factors such as implant surface finish and geometry. In particular, hard bearing components rely on the production of a fluid film of human synovial fluid between the femoral head and liner to reduce friction and wear. The lubrication provided is akin to a car hydroplaning on wet pavement. The thickness of the fluid film is dependent on factors such as fluid viscosity, speed of translation, load, elastic modulus of the bearing materials, and effective radius (i.e., dimensional mismatch between the femoral head and liner). Disruption of the fluid film by varying one of these factors can prevent the formation of a lubrication layer, thus increasing the wear of the bearing couple and diminishing the performance of the implant.

[0006] For instance, while the dimensions of the femoral head and liner can be controlled during production of the implants, the liner may become distorted upon impaction of the liner/cup into the patient's pelvic acetabular. Yew A, Jin ZM, Donn A, Morlock MM, Isaac G., Deformation of press-fitted metallic resurfacing cups. Part 2: Finite element simulation. Proc Inst Mech Eng H. 2006 Feb 220(2):311-9; and Hogg MC, et al., "Impaction of a press-fit acetabular cup using a dynamic finite element method," Proceedings 55th ORS, #2398, 2009. Accordingly, the liner deformation that may take place during use affects the effective radius between the femoral head and liner, thereby potentially disrupting the fluid film. In particular, if the effective radial clearance between the femoral head and liner is reduced due to liner deformation, such reduction may prevent the formation of a fluid film, and may in some cases cause contact between the rim of the liner and head. The degree of liner deformation is related to the density and geometry of the patient's pelvic bones, as well as the thickness and material properties of the acetabular component.

[0007] To account for the possible deformation of the acetabular component, there have been attempts to compensate through modification of the implant geometry to preserve the desired radial ratio between the head and acetabular component. One method to minimize liner deformation is to use a thicker acetabular component. This approach, however, can negatively impact the patient because as the thickness of the acetabular component increases, the size mismatch between the femoral head and the outer diameter of the acetabular component increases, requiring the surgeon to remove more of the patient's bone for proper implantation. Because the surgeon's preference is to maximize preservation of the patient's host bone, it is desirable to reduce the thickness of the acetabular component as much as possible.

[0008] Another compensation attempt is to increase the radial mismatch between the components beyond what may yield an optimal fluid film thickness, with the expectation that deformation may reduce the clearance in vivo to a more optimal range. One downside of this approach is that the components may not form an ideal fluid film layer in cases where there is little to no deformation of the acetabular component. Yet another method is to create "dual-radius" designs that utilize articular surfaces that have a larger radius toward the rim of the liner compared to the apex. One example of the "dual- radius" design is described in U.S. Patent No. 6,059,830 and Meng, QE, et al., "Elastohydrodynamic Lubrication Analysis of a MoM Hip Implant Design with an Aspheric Head", Proceedings 56th ORS, #2246, 2010. While the larger radius at the rim accounts for the possible closure of the acetabular component upon implantation, the "dual-radius" products are technically more difficult to manufacture and also may not create an ideal fluid film should deformation not occur. Other attempts to compensate for the liner deformation include creating an effectively thicker acetabular component by reducing the coverage arc of the acetabular component through increasing the wall thickness at the rim. While this may effectively stiffen the acetabular component, the reduction in coverage may lead to rim loading in cases where the cup is implanted more vertically than intended, leading to a high contact stress and high wear.

[0009] In view of the above, there exists a need for an acetabular assembly having an improved liner compensated for deformation during use without increasing the thickness and method to compensate for liner deformation in an acetabular implant assembly.

SUMMARY OF THE INVENTION

[0010] The present disclosure provides a method of forming an acetabular implant comprising an acetabular shell and modular acetabular liner, where the thicknesses of both components have been modified according to the elastic moduli of the components such that the effective ring stiffness of the construct is similar to current metal backed acetabular implants to minimize the deformation of the construct.

[0011] According to one aspect of the present disclosure, there is provided a method of forming a medical implant comprising: selecting a first value; determining a desired thickness for at least one of a liner component and a shell component of an acetabular assembly design by adjusting the thickness of the respective component until a selected thickness provides a second value that approximately matches an optimal value based on said first value; wherein said second value corresponds to the stiffness of said acetabular assembly design; wherein said liner component comprises a first material and said shell component comprises a second material; wherein said second value is based, at least in part, on the thickness of said liner component, the thickness of said shell component, elastic modulus of said first material and elastic modulus of said second material; and forming an acetabular assembly based, at least in part, on said determined desired thickness.

[0012] In one embodiment, the first material is the same as said second material. In another embodiment, the first material of said liner component comprises a diffusion hardened zone that is in contact with a substrate; a substantially defect-free ceramic layer overlaying said diffusion hardened zone, wherein said ceramic layer has a thickness of about 0.1 to 25 microns and said diffusion hardened zone has a thickness of greater than 2 microns. In another embodiment, the second material of said shell component comprises cobalt chrome. In yet another embodiment, the optimal value is within 10% of said first value. In another embodiment, said second material of said shell component comprises a material having an elastic modulus of at least 239.9 GPa.

[0013] In another embodiment, the second value is determined by approximating the moment of inertia of the respective liner and shell components based on a first equation j _ ύ t ^_6Γβ j -_s ^_{6 momen}t ₀f _inertia, d is the average diameter, and t is the 8

thickness of the wall; and determining the stiffness of the respective acetabular design

EI

based on a second equation— ^Λ where I is the determined moment of inertia of the

r

respective component, E is the elastic modulus of the material of the respective component, and r is the radius of the respective component. In another embodiment, said second value is determined by finite element analysis. In another embodiment, said first value reflects a preferred stiffness value for an acetabular assembly.

[0014] According to another aspect of the present disclosure, there is provided a method of forming a medical implant comprising: determining a first value corresponding to the stiffness of a first acetabular assembly design having a first liner component and a first shell component, wherein said first liner component comprises a first material and said first shell component comprises a second material, wherein said first value is based, at least in part, on the thickness of said first liner component, the thickness of said first shell component, elastic modulus of said first material, and elastic modules of second material; selecting another material to replace at least one of said first material and said second material to form a second acetabular assembly design, said second design having a second liner component and a second shell component; determining a desired thickness for at least one of said second liner component and said second shell component by adjusting the thickness of the respective component until a selected thickness provides a second value corresponding to the stiffness of said second acetabular assembly design that approximately matches an optimal value based on said first value; wherein said second value is based, at least in part, on the thickness of the second liner component, the thickness of said second shell component, the elastic modulus of the material of said second liner component, and elastic modulus of the material of said second shell component; and forming an acetabular assembly based, at least in part, on the desired thickness and selected material.

[0015] In one embodiment, said second liner component comprises a diffusion hardened zone that is in contact with a substrate; a substantially defect-free ceramic layer overlaying said diffusion hardened zone, wherein said ceramic layer has a thickness of about 0.1 to 25 microns and said diffusion hardened zone has a thickness of greater than 2 microns. In another embodiment, the second shell component comprises cobalt chrome. In another embodiment, said second shell component comprises a material having an elastic modulus of at least 239.9 GPa.

[0016] In another embodiment, the second value is determined by: approximating the moment of inertia of the respective liner and shell components based on a first equation / = where I is the moment of inertia, d is the average diameter, and t is the

8

EI

based on a second equation— ' where I is the determined moment of inertia of the r

respective component, E is the elastic modulus of the material of the respective component, and r is the radius of the respective component. In another embodiment, said second value is determined by finite element analysis. In yet another embodiment, said optimal value is within 10% of said first value. In another embodiment, said first value is determined by: forming an acetabular assembly based on said first acetabular assembly design; and measuring the stiffness of said acetabular assembly based on said first acetabular assembly design. In another embodiment, said second value is determined by: forming an acetabular assembly based, at least in part, on a selected thickness; and measuring the stiffness of said acetabular assembly based, at least in part, on a selected thickness.

[0017] According to another aspect of the present disclosure, there is provided an acetabular assembly comprising: a liner component; and a shell component; wherein said liner component comprises a first material and said shell component comprises a second material; wherein the thickness of at least one of said liner component provides a stiffness value that approximately matches a predetermined value corresponding to the desired stiffness of the acetabular assembly, wherein said stiffness value is based, at least in part, on the thickness of said liner component, the thickness of said shell component, elastic modulus of said first material and elastic modulus of said second material.

[0018] In one embodiment, said first material of said liner component comprises a diffusion hardened zone that is in contact with a substrate; a substantially defect-free ceramic layer overlaying said diffusion hardened zone, wherein said ceramic layer has a thickness of about 0.1 to 25 microns and said diffusion hardened zone has a thickness of greater than 2 microns. In another embodiment, said stiffness value is determined by: an approximated moment of inertia of the respective liner and shell components based on a

Tad 't

first equation / = where I is the moment of inertia, d is the average diameter, and t is the thickness of the wall; and a second equation — " where I is the determined

r

moment of inertia of the respective component, E is the elastic modulus of the material of the respective component, and r is the radius of the respective component. In another embodiment, said stiffness value is determined by finite element analysis. In yet another embodiment, said stiffness value is within 10% of said predetermined value.

[0019] The foregoing has outlined rather broadly the features and technical advantages of the embodiments present disclosure in order that the detailed description of these embodiments that follows may be better understood. Additional features and advantages of the embodiments of the present disclosure will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the embodiments of the present invention and together with the written description serve to explain the principles, characteristics, and features of the invention. In the drawings:

[0021] FIG. 1 is an illustration of an exemplary acetabular assembly having a liner and a shell. [0022] FIG. 2 is an exemplary embodiment of the present disclosure to select an appropriate thicknesses of acetabular liner and shell.

[0023] FIG. 3 is another exemplary embodiment of the present disclosure to select an appropriate thicknesses of acetabular liner and shell.

[0024] FIG. 4 shows exemplary relationships, determined using one embodiment of the present disclosure, between stiffness and thickness of various acetabular designs having a 25:75 liner-to-shell thickness ratio.

[0025] FIG. 5 shows exemplary relationships, determined using one embodiment of the present disclosure, between stiffness and thickness of various acetabular designs having a 50:50 liner-to-shell thickness ratio.

[0026] FIG. 6 shows exemplary relationships, determined using one embodiment of the present disclosure, between stiffness and thickness of various acetabular designs having a 75:25 liner-to-shell thickness ratio.

[0027] FIG. 7 shows an exemplary relationship, determined using another embodiment of the present disclosure, between the deflection of an acetabular design under a load of IN and at various thicknesses; the acetabular design having a diffusion hardened liner and CoCr shell having a 50:50 liner-to-shell thickness ratio.

[0028] FIG. 8 shows exemplary relationships, determined using one embodiment of the present disclosure, between stiffness and liner-to-shell thickness ratio based on a 2 mm thick acetabular design.

[0029] Figure 9 shows exemplary relationships, determined using one embodiment of the present disclosure, between stiffness and liner-to-shell thickness ratio based on a 4 mm thick acetabular design.

[0030] Figure 10 shows exemplary relationships, determined using one embodiment of the present disclosure, between stiffness and liner-to-shell thickness ratio based on a 6 mm thick acetabular design.

DETAILED DESCRIPTION OF THE EMBODIMENTS [0031] FIGS. 1A and IB show perspective views and FIG. 1C shows a cross section of an acetabular assembly 100 comprising a liner 102 seated in an acetabular shell 104. Referring to FIG. 1C, the liner 102 has a thickness tj, radius T\, and elastic modulus Ej. Acetabular shell 104 has a thickness t₂, radius r₂, and elastic modulus E₂. The thickness, radius, and elastic modulus of the liner 102 and shell 104 affect the overall stiffness of the acetabular assembly 100. The overall stiffness of an acetabular assembly, such as assembly 100, having components of certain materials and dimensions may be approximated so that (1) the same stiffness may be achieved in another acetabular assembly having components of different materials or (2) to determine the thickness of the liner and/or shell to increase or decrease, by a desired amount, the overall stiffness of the acetabular assembly 100. For instance, a liner 102 that has a smaller thickness may still have an equivalent stiffness as a liner with a larger thickness if liner 102 is formed with material with a higher elastic modulus.

[0032] According to one aspect of the present disclosure, the overall stiffness of acetabular assembly 100 may be approximated by modeling each of the liner 102 and shell 104 in two-dimension as a ring with moment of inertia using the following equation: =— 8 CD

where d is the average diameter of the liner 102 or shell 104, respectively, and t is the thickness of the wall of the respective liner or shell component.

[0033] Based on the determined moment of inertia, I, the stiffness of the liner 102 or shell 104 can be approximated by the following equation: (2)

r

where E is the elastic modulus of the material of the respective liner 102 or shell 104 and r is the radius of the respective component.

[0034] The overall effective ring stiffness of the acetabular assembly 100 may be calculated by combining the determined stiffness of the liner 102 and shell 104. By determining the effective ring stiffness of either (1) currently produced components or (2) selecting a desired stiffness, the minimum thicknesses of components utilizing different materials yet still provide the same or similar stiffness may be determined. Preferably, the stiffness of the currently produced components or the selected stiffness already provides a desired effective radius between the femoral head component and the liner component, where this effective radius may be substantially maintained or duplicated with components having different materials according to the embodiments of the present disclosure. Accordingly, by using a different material with a higher elastic modulus for the liner or the shell, the overall acetabular assembly can be thinner while the necessary resistance to deformation can be maintained.

[0035] In another embodiment, the overall stiffness (N/m) or corresponding deformation (mm) of the acetabular assembly 100 may be approximated using Finite Element Analysis (FEA) with the assistance of computer software programs, such as Abaqus or other similar programs. In general, referring to FIG. 2, the software program creates a geometric model, such as a three-dimensional CAD model, of the liner and shell components of the acetabular assembly according to dimensional inputs at step 202. The mechanical properties of the liner and shell are defined at step 204. In the preferred embodiment, the mechanical properties and dimensional inputs preferably depicts an acetabular assembly design that provides a desired effective radius. A three-dimensional finite element mesh of the liner and shell components at step 206. The resulting deformation is determined by a FEA simulation, at step 208, that applies various displacements or loads (e.g., Newton) to the acetabular assembly and predict the stress or deformation experienced by the acetabular assembly under such applied load. In particular, the resulting stress or deformation (e.g., mm of deflection) is determined through solving numerical equations iteratively, such as running the FEA simulation at step 210. Based on the calculated deformation, the thickness may be adjusted until the equivalent or a desired deformation is reached. In one embodiment, adjustments may be made until selection of a minimum thickness of the either the liner and/or shell that yields an overall stiffness for the assembly that is at most about 10% of the original design. In another embodiment, the thickness of the assembly is selected to provide a maximum deformation of about 10% greater than the original design. In yet another embodiment, the new stiffness value may be between about 0-5% of the stiffness value of the original design. In another embodiment, the new stiffness value may be between about 6-10% of the stiffness value of the original design. Similarly, the software may be programmed to determine the thickness of a material to produce an equivalent stiffness to a desired stiffness or deformation. In another embodiment, the stiffness of an acetabular assembly may also be determined by actual measurement of the particular assembly.

[0036] FIG. 3 shows an exemplary embodiment to determine the thickness of the liner 102 and shell 104 to maintain the overall stiffness of acetabular assembly 100 at a desired amount or within a desired range. At step 302, the method begins with an initial acetabular assembly design with specified materials and dimensions for the liner and shell components. At step 304, the stiffness or deformation of the acetabular assembly design is determined, using the equations (1) and (2) or FEA as described above, or other similar means known to those skilled in the art. Alternatively, instead of determining a stiffness or deformation of a particular design, the process can begin with a desired stiffness or deformation value.

[0037] Referring to FIG. 3, at step 306, the material of either the shell and/or the liner is modified. At step 308, the stiffness or deformation of the acetabular assembly comprising components with new materials is determined, using the equations (1) and (2) or FEA as described above, or other similar means known to those skilled in the art. Step 310 compares the stiffness of the new acetabular assembly with the stiffness of the original design and determines whether they match. If the values of the new and original designs do not match, the thickness of either the shell and/or the liner is modified at step 312. The process loops back to step 308 and repeats steps 308 through 312 until the appropriate thickness that provides a matching stiffness for the new acetabular assembly design is selected, ending the process at step 314.

[0038] In other embodiments, instead of FEA or approximating with equations (1) and (2), the preferred stiffness value may be determined by measuring the stiffness of an already manufactured acetabular assembly that has the desired dimensions. One method is to measure the deflection of the components under a specified load. In another embodiment, after a new thickness of at least one component of a second acetabular design with a different material has been selected, the stiffness provided by the combination of new material and thickness may be determined by forming a prototype of the second acetabular design and measuring the stiffness of the prototype. In light of the stiffness measurement, the thickness may be adjusted again and a prototype formed according to that adjustment until the desired stiffness is reached.

[0039] Referring to FIG. 1, in one embodiment, the liner 102 is formed with a metallic substrate having a ceramic surface while the shell 104 is formed with CoCr. Alternatively, a material having an elastic modulus equivalent to CoCr or higher may be used in place of CoCr for the acetabular shell 104. The ceramic material provides superior damage resistance and minimal wear particle formation over UHMWPE while providing an increase in fluid film thickness as well as a decrease in contact stresses over conventional metallic implants, such as implants with CoCr acetabular liner and shell. In the preferred embodiment, the liner 102 comprises an improved diffusion hardened material described in detail in U.S. Patent No. 7,550,209 to Pawar et al. and co-pending U.S. Application Nos. 12/127,413 and 12/244,492, the disclosures of which are incorporated by reference.

[0040] While the disclosures of U.S. Patent No. 7,550,209 to Pawar et al. and copending U.S. Application Nos. 12/127,413 and 12/244,492 describing the diffusion hardened composition of the present disclosure and method of making same have been incorporated by reference, certain exemplary features of the diffusion hardened composition of the present disclosure are also set forth below. The diffusion hardened zone of the compositions of the present disclosure has a layered structure. The thickness of the diffusion hardened zone of the present disclosure is at least equal to that of the ceramic (oxide) layer formed on the surface of such an implant.

[0041] In one embodiment, the diffusion hardened composition of the present disclosure comprises a metallic substrate, such as a biocompatible alloy, having a ceramic surface. Examples of biocompatible alloys include alloys that are made from either zirconium or titanium or tantalum or niobium or hafnium or combination thereof, such as cobalt- chromium-molybdenum, titanium-aluminum-vanadium, nickel-titanium and zirconium- niobium. The ceramic surface may be formed by various processes known to those skilled in the art, such as air oxidation. Beneath the oxide layer is a hard, oxygen-rich diffusion layer called the diffusion hardened zone. According to one aspect of the present disclosure, the diffusion hardened zone may be defined as the region which has hardness at least 1.1 times of the substrate hardness.

[0042] The composition of the present disclosure may have a totality of the thickness of the ceramic (or oxide) layer and the diffusion hardened zone that is greater than 5 microns, and preferably greater than 10 microns. In some embodiments, the ceramic layer may or may not be present (it can range in thickness from 0 to 25 microns). Accordingly, the diffusion hardened zone of these embodiments may have a thickness of greater than 5 microns (and preferably greater than 10 microns) with no ceramic layer above it or an infinitesimally small ceramic layer above it. Where both layers are present, the ceramic layer is on the surface and is above the diffusion hardened zone. Examples of metal or metal alloy substrates and diffusion hardening species appropriate for the diffusion hardened composition of the present disclosure are described in U.S. Patent No. 7,550,209 and co-pending U.S. Application Nos. 12/127,413 and 12/244,492. These substrates may include zirconium, titanium, tantalum, hafnium, niobium, the alloy of these metals and any combination thereof. The diffusion hardening species may include oxygen, nitrogen, boron, carbon, and any combination thereof.

[0043] While the diffusion hardened zone is one of the two aforementioned layers, the diffusion hardened zone itself consists of at least two distinct layers (visible by metallographic analysis). The first layer of the diffusion hardened zone has a relatively high concentration of diffusion hardening species (higher than that of the bulk metal or metal alloy substrate, e.g., zirconium or zirconium alloy) and may be saturated with the diffusion hardening species. In embodiments involving zirconium as the substrate, the zirconium in the first layer is predominantly alpha phase zirconium (the first layer of the diffusion hardened zone is that layer which is closest to the ceramic layer, or, where the ceramic layer is absent, the first layer is that layer which is nearest to the surface of the composition). The second layer is below the first layer and has a lower content of diffusion hardening species than the first layer. The diffusion hardened zone has a diffusion hardening species concentration profile such that, in one or more cross-sections of the diffusion hardened zone, the concentration of diffusion hardening species decreases as either an error function, an exponential function, a near uniform distribution, or sequential combinations thereof. The layered structure of the diffusion hardened zone can be detected by metallographic analytical techniques known to those of ordinary skill in the art. These include, but are not limited to, anodization, heat tinting, x-ray diffraction, Auger spectroscopy, depth profiling, etc.

[0044] The oxygen concentration at the interface (between the oxide and diffusion hardened zone) is approximately equal to the solubility limit of oxygen in alpha zirconium which is approximately 9 % (w/w) or 30 atomic %. The diffusion hardened composition of the present disclosure has an oxygen concentration profile of greater than 15 microns. The depth of hardening in the micro-hardness profiles of the diffusion hardened composition of the present disclosure can follow an exponential, error function type of profile or a combination of uniform and error/exponential function. Further, higher micro-hardness of the diffusion hardened composition of the present disclosure e.g., about 500 to about 1000 Knoop lOg, can extend further into the substrate, e.g., about 5 to 55 microns from the interface between the oxide and diffusion hardened zone.

[0045] In another embodiment, the diffusion hardened composition of the present disclosure has a diffusion hardened zone that is characterized by at least three layers. The first layer is beneath the oxide layer, the second layer is beneath the first layer and the third layer is beneath the second layer. The thickness of the first layer is greater than the second layer and thickness of second layer may be greater than the third layer. In another embodiment, the layers of the diffusion hardened zone may have similar thicknesses. In one aspect of this disclosure, the oxide layer is preferentially retained on the surface of the substrate. In one embodiment, the monoclinic content of the diffusion hardened composition of the present disclosure is typically greater than 96 % (v/v), and preferably between about 97 and 98%.

[0046] For embodiments with substrates comprising Zr-Nb-based alloys, the diffusion hardened zone can be 70 micron or greater. As stated previously, the diffusion hardened zone may comprise more than one layer and is underneath the ceramic layer. In one embodiment, the Zr-2.5Nb comprises two phases, alpha (hexagonal) and beta (cubic). The diffusion hardened zone is predominantly alpha phase (hexagonal). A minor amount of beta (cubic) phase (less than 7% (v/v)) can be present in the first layer of diffusion hardened zone. The first layer is predominantly alpha phase and the volume fraction of beta phase gradually increases in the diffusion layer towards the substrate. If the zirconium alloy is predominantly single phase (alpha) then the beta phase in the diffusion hardened zone will be significantly less than it is in the substrate.

[0047] In embodiments with an oxide layer, the oxide layer of the diffusion hardened composition of the present disclosure is substantially defect-free. Typically, the defects in the oxide layer can be broadly classified as pores and cracks. The pores can be circular or elongated and may be on the surface or at the interface. The cracks can be perpendicular to the oxide metal interface, and/or may be parallel to the oxide metal interface. Another type of defect that is anticipated in this disclosure is the wavy oxide metal interface and delaminated or spalled oxide. The defects in the ceramic layer may be evaluated on a cross-sectional metallographic sample at 1000X magnification with field of view of approximately 100 x 80 microns.

[0048] In another embodiment, the ceramic layer of the diffusion hardened composition of the present disclosure comprises a distinct secondary phase through the entire thickness of the ceramic layer. In one embodiment of the composition of the present disclosure, when the ceramic layer is retained on the surface, the composition comprises a metallic hardened surface formed on top of the ceramic layer along with the diffusion hardened zone formed underneath the ceramic layer. This layer may or may not be retained on the final medical implant.

[0049] The diffusion hardened composition of present disclosure can be produced by employing three processes. All processes can be performed in a single or multiple steps. The processes are (1) ceramic layer formation (i.e., oxidation, nitridation, boridation, carburization, or any combination thereof), (2) diffusion hardening, and optionally, (3) ceramic layer formation. If ceramic layer is retained on the surface during the diffusion hardening, process 1 and 2 may be sufficient. If the final application is such that a ceramic layer is not required on the surface, temperature and time are chosen in such a way that process 2 will dissolve the ceramic layer completely. Alternatively, the surface ceramic layer may be removed by mechanical, chemical or electrochemical means. As mentioned, when the ceramic layer is retained on the surface during process 2, there can be formation of a metallic hardened layer on the oxide layer. This metallic hardened layer may or may not be removed for the final product. If the ceramic layer is completely dissolved into the substrate and re-formation of the ceramic layer is desired then a diffusion profile is obtained which will produce a high integrity and defect-free ceramic layer during the ceramic layer formation process. This diffusion profile can be an exponential function, an error function, a uniform, or any sequential combination thereof. It should be noted that some of these functions may also be attributed to be linear or higher order polynomials. It should be noted that the combination of diffusion profile and retained oxide is obtained through careful control of time, temperature and pressure during ceramic layer formation process and diffusion hardening process. It should be understood that variations by way of substitutions and alterations from these general processes described above which do not depart from the spirit and scope of the invention are understood to be within the scope of the invention. In this way, the general processes described are merely illustrative and not exhaustive.

[0050] The embodiments of the present disclosure are applicable to non-modular acetabular assembly designs having various thicknesses, dimensions, and materials. For example, in one embodiment, the liner and/or shell can be made from metal, a synthetic plastics material, such as polyethylene or XPLE, a ceramic material, a metal substrate having a ceramic surface, or any other suitable materials.

[0051] The following examples are given for the purpose of illustrating various embodiments of the present disclosure. The examples are not intended to limit the scope of the invention. Various modifications and embodiments can be made without departing from the scope and spirit of the invention.

EXAMPLES

[0052] The stiffness of various acetabular assemblies having liner and shell components with different thickness and elastic modulus was calculated using equations (1) and (2) above. FIGS. 4-6 show the results of the stiffness calculation as compared to overall thickness of a construct or acetabular assembly. The reference acetabular assembly was a 3 mm thick monoblock CoCr cup similar to a currently marketed device. The reference construct was compared to acetabular assemblies having a "25:75" Liner-Shell

Thickness Ratio. For instance, an acetabular assembly with an overall thickness of 6 mm and a 25:75 liner to shell thickness ratio has a 1.5 mm liner and a 4.5 mm shell, where as a construct with an overall thickness of 4 mm would have a 1 mm liner and a 3 mm shell.

The radius value of the liner used for the stiffness calculation was 50 mm and the radius value of the shell is dependent upon the liner to shell thickness ratio and overall thickness of the acetabular assembly. For instance, an acetabular assembly having an overall thickness of 6 mm and a 50:50 liner to shell thickness ratio would have a 50 mm radius value for the liner and a 62 mm radius value for the shell.

[0053] The first acetabular assembly has a CoCr liner and a titanium shell, specifically Ti-6A1-4V. The second acetabular assembly has a ceramic liner, specifically the diffusion hardened material of U.S. Patent No. 7,550,209, and a titanium shell similar to the first construct. The third acetabular has a diffusion hardened liner similar to the second construct and a CoCr shell. The elastic modulus values used in the calculation were 239.9 GPa for CoCr, 113.8 GPa for Ti-6A1-4V, and 97.9 GPa for the diffusion hardened material of U.S. Patent No. 7,550,209.

[0054] As shown, FIG. 4 allows for the determination of the overall thickness of constructs with various liner-shell material configurations to achieve a desired stiffness of the reference non-modular CoCr acetabular assembly of 3 mm. For instance, to achieve the same stiffness as a 3 mm non-modular CoCr acetabular assembly, an acetabular assembly with a diffusion hardened liner and a titanium shell with a 25:75 thickness ratio needs to be about 6.6 mm thick. In comparison, a diffusion hardened liner with a CoCr shell, at the same liner-to-shell, achieves the same stiffness without significant increase in the overall thickness of the acetabular assembly. CoCr has a higher elastic modulus than titanium, which can assist the diffusion hardened liner from deformation, thereby allowing for the overall construct to be thinner.

[0055] Referring to FIG. 5, similar to FIG. 4, various stiffness values were calculated for acetabular constructs of the same material configurations as in FIG. 4 at different thicknesses. The difference between the acetabular assemblies of FIG. 4 and FIG. 5 is that instead of a 25:75 liner-to-thickness ratio, the acetabular constructs of FIG. 5 has a 50:50 liner-to-thickness ratio. For instance, an acetabular assembly with an overall thickness of 6 mm and a 50:50 liner to shell thickness ratio has a 3 mm liner and a 3 mm shell, where as a construct with an overall thickness of 4 mm would have a 2 mm liner and a 2 mm shell.

[0056] Referring to FIG. 6, similar to FIGS. 4 and 5, various stiffness values were calculated for acetabular constructs of the same material configurations as in FIGS. 4 and 5 at different thicknesses. The difference here is the liner-to-thickness ratio where the acetabular constructs of FIG. 5 has a 75:25 liner-to-thickness ratio. For instance, an acetabular assembly with an overall thickness of 6 mm and a 75:25 liner to shell thickness ratio has a 4.5 mm liner and a 1.5 mm shell, where as a construct with an overall thickness of 4 mm would have a 3 mm liner and a 1 mm shell.

[0057] Similar to FIG. 4, as shown, FIGS. 5 and 6 allow for the determination of the overall thickness of constructs with various liner-shell material configurations to achieve a desired stiffness of the reference non-modular CoCr acetabular assembly of 3 mm. For instance, to achieve the same stiffness as a 3 mm non-modular CoCr acetabular assembly, an acetabular assembly with a diffusion hardened liner and a titanium shell with a 50:50 ratio should be about 7 mm thick, and the same assembly with a 75:25 ratio should to be about 7.3 mm. In comparison, a diffusion hardened liner with a CoCr shell with a 50:50 ratio achieves the same stiffness without significant increase in the overall thickness of the acetabular assembly. CoCr has a higher elastic modulus than titanium, which can assist the diffusion hardened liner from deformation, thereby allowing for the overall construct to be thinner.

[0058] FIG. 7 shows the deflection as a function of a shell or liner thickness for an acetabular assembly having a diffusion hardened liner and a CoCr shell and a 50:50 liner-to-thickness ratio. Specifically, a data point at a "2" shell or liner thickness indicates that the overall thickness of the acetabular assembly is 4 mm and each of the shell or liner has a thickness of 2 mm. Other values such as radius and elastic modulus are similar to the other examples. In particular, the liner radius value is 50 mm and the shell radius value is determined based on the liner to shell thickness ratio and overall thickness. The deflection values as a function of construct thickness under 1 Newton of force is determined using the FEA program, specifically Abaqus. However, other programs may be used to determine these values. The deflection vs. thickness relationship is compared to the reference deflection of an acetabular assembly having a 6 mm overall thickness, and a 3 mm diffusion hardened liner and a 3 mm titanium shell, i.e., a 50:50 liner-to-shell thickness ratio. The characteristics of the diffusion hardened material, titanium, and CoCr are as described for FIGS. 4-6. As shown by FIG. 7, the results from an FEA program indicate that the same stiffness of the reference acetabular assembly can be achieved with an acetabular assembly having a diffusion hardened liner and a CoCr shell and a 50:50 liner-to-thickness ratio where the two lines intersect, which is at 2.6 mm thickness for one component or 5.2 mm thickness for the overall acetabular construct.

[0059] FIGS. 8-10 show graphs that describe the relationship between the stiffness of the construct as a function of the liner-to-shell thickness ratio. The values in the graphs are based on the calculations based on equations (1) and (2) as described above. In particular, FIG. 8 shows the stiffness as a function of the liner to shell thickness ratio for the acetabular assemblies of FIGS. 4-6 having an overall thickness of 2 mm and having the various material configurations, e.g., CoCr liner with a titanium shell, diffusion hardened liner with a titanium shell, and a diffusion hardened liner and CoCr shell. Similarly, FIGS. 9-10 show the stiffness as a function of the liner to shell thickness ratio for the acetabular assemblies of FIGS. 4-6 having an overall thickness of 4 mm and 6 mm, respectively.

[0060] Referring to FIGS. 8-10, the graphs show the stiffness of the assembly with the diffusion hardened liner and CoCr shell decreasing as the liner-to-shell thickness ratio increases. Conversely, the stiffness of the acetabular assembly with the CoCr liner and titanium shell increasing as the liner-to-shell thickness ratio increases. In particular, there is an intersection between the graphs for these two constructs. On the other hand, the acetabular assembly with a diffusion hardened liner and titanium shell has a significantly lower stiffness than the other two constructs. The graphs of FIGS. 8-10 showing these relationships indicate that an equivalent or near equivalent stiffness to a desired design or stiffness may be achieved by modifying the shell and/or liner material and liner-to-shell ratio.

[0061] Accordingly, the embodiments of the present disclosure allows for maintaining the desired effective radius between the femoral head and liner by compensating for deformation of the liner during use. For instance, one embodiment involves using a material for the shell that has a higher elastic modulus to provide a more rigid backing that compensates for the deformation of the liner material without requiring an increase in the thickness of the components. As such, the embodiments of the present disclosure allows for minimal disruption to the fluid film during use by compensating for deformation that may occur. [0062] Although the embodiments of the present disclosure and their advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the present disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.

Claims

CLAIMS What is claimed is:

1. A method of forming a medical implant comprising:

selecting a first value;

determining a desired thickness for at least one of a liner component and a shell component of an acetabular assembly design by adjusting the thickness of the respective component until a selected thickness provides a second value that approximately matches an optimal value based on said first value;

wherein said second value corresponds to the stiffness of said acetabular assembly design;

wherein said liner component comprises a first material and said shell component comprises a second material;

wherein said second value is based, at least in part, on the thickness of said liner component, the thickness of said shell component, the elastic modulus of said first material, and the elastic modulus of said second material; and

forming an acetabular assembly based, at least in part, on said determined desired thickness.

2. The method of claim 1 wherein said first material of said liner component comprises

a diffusion hardened zone that is in contact with a substrate; a substantially defect-free ceramic layer overlaying said diffusion hardened zone,

wherein said ceramic layer has a thickness of about 0.1 to 25 microns and said diffusion hardened zone has a thickness of greater than 2 microns.

3. The method of claim 1 wherein said optimal value is within 10% of said first value.

4. The method of claim 1 wherein said second material of said shell component comprises a material having an elastic modulus of at least 239.9 GPa.

5. The method of claim 1 wherein said first value reflects a preferred stiffness value for an acetabular assembly.

6. The method of claim 1 wherein said second value is determined by:

approximating the moment of inertia of the respective liner and shell components based on a first equation / = where I is the moment of inertia, d is the average

8

diameter, and t is the thickness of the wall;

determining the stiffness of the respective acetabular design based on a second EI

equation— " where I is the determined moment of inertia of the respective component, E r

is the elastic modulus of the material of the respective component, and r is the radius of the respective component.

7. The method of claim 1 wherein said second value is determined finite element analysis.

8. A method of forming a medical implant comprising:

determining a first value corresponding to the stiffness of a first acetabular assembly design having a first liner component and a first shell component,

wherein said first liner component comprises a first material and said first shell component comprises a second material,

wherein said first value is based, at least in part, on the thickness of said first liner component, the thickness of said first shell component, the elastic modulus of said first material, and the elastic modulus of said second material;

selecting another material to replace at least one of said first material and said second material to form a second acetabular assembly design, said second design having a second liner component and a second shell component;

determining a desired thickness for at least one of said second liner component and said second shell component by adjusting the thickness of the respective component until a selected thickness provides a second value corresponding to the stiffness of said second acetabular assembly design that approximately matches an optimal value based on said first value;

wherein said second value is based, at least in part, on the thickness of the second liner component, the thickness of the second shell component, the elastic modulus of the material of said second liner, and the elastic modulus of the material of said shell components; and

forming an acetabular assembly based, at least in part, on the desired thickness and selected material.

9. The method of claim 8 wherein said second liner component comprises

10. The method of claim 8 wherein said second shell component comprises a material having an elastic modulus of at least 239.9 GPa.

11. The method of claim 8 wherein at least one of said first value and said second value is determined by:

approximating the moment of inertia of the respective liner and shell components based on a first equation / = ^ where I is the moment of inertia, d is the average

8

diameter, and t is the thickness of the wall;

12. The method of claim 8 wherein at least one of said first value and said second value is determined by finite element analysis.

13. The method of claim 8 wherein said first value is determined by:

forming an acetabular assembly based on said first acetabular assembly design; and

measuring the stiffness of said acetabular assembly based on said first acetabular assembly design.

14. The method of claim 8 wherein said second value is determined by:

forming an acetabular assembly based, at least in part, on a selected thickness; and

measuring the stiffness of said acetabular assembly based, at least in part, on a selected thickness.

15. The method of claim 8 wherein said optimal value is within 10% of said first value.

16. An acetabular assembly comprising:

a liner component;

and a shell component;

wherein the thickness of at least one of said liner component provides a stiffness value that approximately matches a predetermined value corresponding to a desired stiffness of the acetabular assembly,

wherein said stiffness value is based, at least in part, on the thickness of said liner component, the thickness of said shell component, elastic modulus of said first material and elastic modulus of said second material.

17. The acetabular assembly of claim 16 wherein said stiffness value is determined by:

an approximated moment of inertia of the respective liner and shell components

Tvd^t

based on a first equation / = where I is the moment of inertia, d is the average

8

diameter, and t is the thickness of the wall; and

EI

a second equation— ^Λ where I is the determined moment of inertia of the

r

respective component, E is the elastic modulus of the material of the respective component, and r is the radius of the respective component.

18. The acetabular assembly of claim 15 wherein said stiffness value is determined by finite element analysis.