Abstract:
Implants are made of materials having asymmetric modulus gradients. For example, an implant, such as a hip implant, is made of a material having a stiffness gradient between a proximal portion near a hip joint and a distal portion extending downward into the marrow of the femur. Among other benefits, the asymmetric modulus gradient mitigates problems associated with stress shielding and does not excessively wear or deteriorate the proximal portion of the implant.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to pending U.S. Provisional Application No. 61/303,846, filed Feb. 12, 2010, and pending U.S. Provisional Application No. 61/305,471, filed Feb. 17, 2010, both of which are incorporated herein by reference in their entireties. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure is directed generally to load bearing implants with engineered gradient stiffness and associated systems and methods. 
       BACKGROUND 
       [0003]    Bone and joint implants have improved the lives of many people who suffer from injury or disease by restoring mobility and even athleticism to patients. As the medical science advances and matures, however, problems have arisen with conventional implants. Natural bones are rigid but flexible. As a person or animal moves about, their bones experience natural mechanical stresses due to muscular loading and impacts that cause the bones to maintain a healthy density and even remodel. In the absence of natural mechanical stresses, bones tend to lose density. This phenomenon, known as Wolffs law, is a well-known scientific principle. Conventional implants are generally more rigid than bone, and are made of a solid material such as commercially pure titanium (“CPTi”) that can prevent these natural stresses from reaching the bone. This is known as stress shielding. 
         [0004]      FIG. 1 , for example, is a schematic view of an implant  10  made of a fully dense material according to the prior art. Fully dense CPTi is approximately 1% porous or less, and has a high stiffness of approximately 110 GPa. Because the implant  10  is made of uniformly dense material, the modulus of elasticity or stiffness of the implant  10  is generally uniform throughout the implant  10 . Thus, this implant  10  is much more rigid than the surrounding bone  14  and can cause stress shielding in the bone  14 . 
         [0005]    Another problem with conventional implants (such as the implant  10  of  FIG. 1 ) is deterioration of the implant over time due to debris build-up and other factors. In an attempt to solve the stress shielding problem, some conventional designs include implants with low modulus of elasticity so that the implant will flex more like a natural bone and allow the bone to experience normal stress. Accordingly, there is a need in the art for an improved implant that overcomes these issues. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]      FIG. 1  is a schematic view of a conventional bone implant made of fully dense CPTi according to the prior art. 
           [0007]      FIG. 2A  is a schematic view of a bone implant configured in accordance with an embodiment of the present disclosure. 
           [0008]      FIG. 2B  is a schematic view of the bone implant of  FIG. 2A  positioned within a human femur in accordance with an embodiment of the present disclosure. 
           [0009]      FIG. 3A  is a schematic view of a bone implant having a radial modulus gradient configured in accordance with an embodiment of the present disclosure. 
           [0010]      FIG. 3B  is a schematic, cross-sectional view of the bone implant of  FIG. 3A  taken along line  3 B- 3 B in  FIG. 3A . 
           [0011]      FIG. 3C  is a schematic, cross-sectional view of a bone implant configured in accordance with another embodiment of the present disclosure. 
           [0012]      FIG. 4  is a schematic view of a bone implant having an engineered modulus gradient configured in accordance with still another embodiment of the present disclosure. 
           [0013]      FIG. 5  is a schematic view of a bone implant having an engineered modulus gradient configured in accordance with yet another embodiment of the present disclosure. 
           [0014]      FIG. 6  is a schematic view of a bone implant having a uniform porosity of approximately 73% porosity configured in accordance with an embodiment of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    The present disclosure describes load bearing implants with engineered gradient stiffness and associated systems and methods. Several embodiments of the load bearing implants described herein, for example, are directed to implants having optimized stiffness gradients and methods for designing the stiffness gradients in such implants. In one embodiment, for example, stiffness gradients for implants (e.g., hip stem implants) can be engineered using simulations (e.g., finite element analysis) to minimize bone loss due to stress shielding and also to maintain the shear stress at the bone/implant interface to be below a desired threshold value. 
         [0016]    Mechanical properties of load bearing implants should not adversely affect the biological function and processes of surrounding anatomical structures. Specifically, implants should not adversely affect the surrounding bone (in case of joint implant) and should not compromise the bone healing (in case of implants for bone defects). As noted previously, one problem with many conventional implants is that high stiffness implant materials and configurations prevent bones from receiving normal levels of mechanical stimulation. This often results in bone loss around the implant, which can lead to pain, difficulty in revision surgery, and possible implant failure. 
         [0017]    In contrast with conventional implants, the load bearing implants disclosed herein have non-uniform distribution of stiffness within the individual implants. This is expected to significantly reduce stress shielding while maintaining low levels of interface stress. Among other benefits, the implants and associated techniques for forming such implants disclosed herein are further expected to (a) extend the life of the implants and reduce the need for revision surgeries, (b) reduce long term pain associated with implants due to stress concentration, and (c) provide a more physiologically compatible substrate for large bone defects (e.g., plates, screws, and substrates for bone growth). 
         [0018]    Specific details of several embodiments of the technology are described below with reference to  FIGS. 2A-6 . Other details describing well-known structures and systems often associated with implants have not been set forth in the following disclosure to avoid unnecessarily obscuring the description of the various embodiments of the technology. Many of the details, dimensions, angles, and other features shown in the Figures are merely illustrative of particular embodiments of the technology. Accordingly, other embodiments can have other details, dimensions, angles, and features without departing from the spirit or scope of the present technology. A person of ordinary skill in the art, therefore, will accordingly understand that the technology may have other embodiments with additional elements, or the technology may have other embodiments without several of the features shown and described below with reference to  FIGS. 2A-6 . 
         [0019]      FIG. 2A  is a schematic view of a load bearing bone (i.e., hip) implant  100  configured in accordance with an embodiment of the present disclosure. While the hip implant  100  is used to describe various aspects of the present technology in this disclosure with reference to  FIGS. 2A-6 , it will be appreciated that the discussion herein is equally applicable to replacement implants for other body parts, such as shoulder, knee, and ankle implants, as well as a variety of other load bearing implants. Various aspects of the disclosure may also be used with substrates for large bone defects (e.g., due to injury, trauma, or disease). In still further embodiments, the implants described herein may be used for animal subjects. 
         [0020]    The implant  100  of  FIG. 2A  includes a ball portion  110 , a neck portion  120 , and a stem portion  130 . The ball portion  110  comprises a generally spherical ball configured to engage a pelvis bone of the patient (not shown) in a ball-and-socket joint (not shown). The neck portion  120  is a narrow region between the ball portion  110  and the stem portion  130 . In the illustrated embodiment, the neck portion  120  is narrower at the ball portion  110  than at the stem portion  130 . In other embodiments, however, the neck portion  120  may have other arrangements. The neck portion  120  interfaces with the stem portion  130  at an interface region  122  that is angled from a horizontal plane by an angle θ. The stem portion  130  comprises an elongated member extending from the neck portion  120  slightly laterally outwardly and downwardly from the neck portion  120 . The stem  130  has a proximal portion  131  at the interface region  122 , and a distal portion  132  at an extreme end of the stem  130 . In the illustrated embodiment, the ball portion  110 , the neck portion  120 , and the stem portion  130  are integral components composed of the same material. For example, in some embodiments, the implant  100  can be made of commercially pure titanium (“CPTi”), titanium aluminum vanadium (“Ti 6 Al 4 V”), or another suitable material. In other embodiments, however, the components of the implant  100  are not all composed of the same material. 
         [0021]      FIG. 2B  is a schematic view of the implant  100  of  FIG. 2A  with the implant  100  inserted into the interior region of a human femur  140  in accordance with an embodiment of the present disclosure. The stem portion  130 , for example, is configured to be inserted into a central or marrow region of the femur  140 . The stem portion  130  can extend completely or approximately completely into the femur  140 , with a small portion of the stem portion  130  protruding from the femur  140 . The greater trochanter  142  of the femur  140  can extend above the interface region  122  between the neck portion  120  and the stem portion  130 . 
         [0022]    In some embodiments, the implant  100  has a varying modulus of elasticity as a function of a spatial parameter of the implant  100 . The implant  100 , for example, can have a modulus gradient, with the modulus of elasticity of any given point defined at least in part by a dimensional parameter of that point. For example, the implant  100  has an axial modulus gradient and the modulus is higher at the interface region  122  and decreases as a function of distance from the interface region  122 , such as along gradient lines  134 . The gradient can be expressed parametrically with a distance from the interface region  122  or from another reference point as the parameter by which the modulus is varied. In some embodiments, the gradient lines  134  are approximately equally spaced and mimic the profile of the implant  130 . In some embodiments, the modulus at a proximal portion  131  is anywhere between 110 and 9.9 GPa (e.g. the same modulus as fully dense CPTi and 70% porous CPTi, respectively), and the modulus at a distal portion  132  vary from the values set forth above. In other embodiments, however, the modulus values at the proximal portion  131  and/or distal portion  132  can be different. 
         [0023]    In accordance with the present disclosure, there are many ways by which the modulus of elasticity of bone implants can be varied at different positions throughout the implants. One such technique, for example, is varying the porosity of the implants. The stiffness of an implant is inversely related to the porosity level. For example, implants having a low porosity (i.e. a more dense material) have a relatively high modulus of elasticity or stiffness. Likewise, implants with greater porosity have a relatively lower modulus of elasticity or stiffness. The implant  100  of  FIGS. 2A and 2B  can have varying porosity levels of anywhere between 0% porous (fully dense) and 90% porous. At 0% porosity, the implant  100  has a modulus of elasticity of approximately 110 GPa; at 90% porosity, the modulus of the implant  100  is approximately 1.1 GPa. In some embodiments, the modulus of elasticity varies linearly between 110 GPa and 1.1 GPa as the porosity varies between 0% and 90% porosity. In other embodiments, however, the porosity may be different. 
         [0024]    In selected embodiments, the mechanism by which varying porosity levels are formed in the implant  100  include the Electron Beam Melting (“EBM”) method, the Laser Engineered Net Shaping (“LENS”®) method, or another suitable method. These methods are described in more detail in U.S. Provisional Application Nos. 61/303,846 and 61/305,471. As provided above, both of these applications are incorporated herein by reference in their entireties. 
         [0025]    Conventional implants having a low porosity and high modulus may be prone to stress shielding. The inventors in the present application have discovered that a desirable porosity that minimizes the potential difficulties can vary as a function of dimensional and material parameters of the implant. For example, the inventors in the present application have discovered that implants with high stiffness proximally and decreasing stiffness distally (such as the implant  100  of  FIGS. 2A and 2B ) provide significant improvements in bone stimulation (measured in terms of strain energy density) relative to a conventional fully-dense titanium (Ti) implant and an optimized uniformly porous implant. 
         [0026]    Another feature of the implant  100  is that the implant  100  has been numerically designed and optimized (e.g., using finite element analysis) to determine a desirable porosity and gradient configuration for a given implant size, material, and position in the body. The inventors have further discovered that such engineered implants outperform fully dense or uniform porosity implants in bone adaptation studies that simulate bone loss following implantation. Implants having the modulus gradients discussed herein allow the bone to experience natural mechanical stresses that stimulate healthy bones. 
         [0027]      FIG. 3A  is a schematic view of a load bearing implant  200  configured in accordance with another embodiment of the disclosure. The implant  200  can have a number of features generally similar to the implant  100  described above with reference to  FIGS. 2A and 2B . For example, the implant  200  includes a ball portion  210 , a neck portion  220 , and a stem portion  230 . The stem portion  230  has a proximal portion  231  at an interface region  222 , and a distal portion  232  at an extreme end of the stem  230 . The implant  200  differs from the implant  100  of  FIGS. 2A and 2B  in that the implant  200  comprises a radial modulus gradient rather than an axial modulus gradient. The radial modulus gradient, for example, is a generally concentric radial modulus in which peripheral regions  236  of the implant  200  are more rigid (higher modulus of elasticity) than an inner or center region  238  of the implant  200 . In other embodiments, however, this arrangement may be reversed and the center region  238  may be more rigid than the peripheral regions  236 . 
         [0028]      FIG. 3B , for example, is a schematic, cross-sectional view of the implant  200  taken along line  3 B- 3 B in  FIG. 3A . As best seen in  FIG. 3B , the stem  230  of the implant  200  comprises a generally concentric radial modulus gradient (as shown by the arrows A).  FIG. 3C  is a schematic, cross-sectional view of the implant  200  illustrating still another embodiment in which the modulus gradient comprises a unilateral gradient (as shown by the arrows A), with a laterally interior side  236   a  having a higher (or lower) stiffness than a laterally exterior region  236   b . In other embodiments, the unilateral gradient is oriented in other directions, such as front to back, exterior to interior, or any other suitable orientation. In still other embodiments, the radial modulus gradient of the implant  200  may have other arrangements. 
         [0029]      FIG. 4  is a schematic view of a portion of a load bearing implant  300  with an engineered modulus gradient configured in accordance with still another embodiment of the present disclosure. The implant  300  can have a number of features generally similar to the implant  100  described above with reference to  FIGS. 2A and 2B . For example, the implant  300  includes a ball portion  310 , a neck portion  320 , and a stem portion  330 . The implant  300  also includes an interface region  222  between the stem portion  330  and the neck portion  320 . 
         [0030]    The implant  300  can have a localized modulus gradient along the interface region  322 , where external portions  336  of the stem portion  330  are more rigid than interior portions  338  of the stem portion  330 . More specifically, at the interface region  322 , the stem portion  330  can have a modulus at least generally equal to the modulus of the neck region  320 . The modulus gradually decreases as a function of distance from the interface region  322 , similar to the arrangement described above with respect to  FIGS. 2A and 2B . In this embodiment, however, as shown by the shading in  FIG. 4 , the gradient at an external portion  336  of the stem portion  330  decreases more gradually than at an interior region  338 .  FIG. 5  is a schematic view of portion of the load bearing implant  300  illustrating a related embodiment in which the engineered gradient is unilateral or asymmetric in the sense that the engineered gradient on a first side  336   a  of the stem portion  330  (as shown by the shading in  FIG. 5 ) decreases more gradually than the gradient at the center region  338 , and decreases still more gradually at a second side  336   b  of the stem portion  330  opposite the first side  336   a.    
         [0031]      FIG. 6  is a schematic view of an implant  500  configured in accordance with a particular embodiment of the present disclosure in which the implant  500  is made of a uniformly porous material. In particular, to meet these design considerations, the implant  500  in the illustrated embodiment is composed of CPTi and has a generally uniform porosity of approximately 73%. The implant  500  has a modulus of elasticity of approximately 30.5 GPa. The relationship between porosity and modulus is a function of the material of the implant. The inventors in the present application have discovered that this particular configuration can effectively reduce stress shielding and promote healthy bone maintenance. 
         [0032]    In selected embodiments, the implants  200 ,  300 ,  400 , and  500  can have compound gradients that combine any two or more of the modulus gradients described herein. For example, an implant configured in accordance with embodiments of this disclosure may have an axial and a radial modulus gradient, an axial and an engineered gradient, or any other suitable combination of the gradients mentioned herein. In some embodiments, the gradients are non-linear gradients. The ranges of modulus of elasticity given for the embodiments described above are not limiting, and are merely used to illustrate certain features of the disclosed technology. 
         [0033]    From the foregoing it will be appreciated that, although specific embodiments of the technology have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the technology. For example, the modulus of elasticity of the implants can be varied using a variety of techniques, including by varying the porosity of the implants. Further, certain aspects of the new technology described in the context of particular embodiments may be combined or eliminated in other embodiments. For example, in the embodiments illustrated above, various combinations of modulus gradients may be combined into a single implant. Moreover, while advantages associated with certain embodiments of the technology have been described in the context of those embodiments, other embodiments may also exhibit such advantages, and not all embodiments need necessarily exhibit such advantages to fall within the scope of the technology. Accordingly, the disclosure and associated technology can encompass other embodiments not expressly shown or described herein. Thus, the disclosure is not limited except as by the appended claims.