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 . 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 . 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 ). specific details of several embodiments of the technology are described below with reference to fig2 a - 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 fig2 a - 6 . fig2 a 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 fig2 a - 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 . the implant 100 of fig2 a 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 . fig2 b is a schematic view of the implant 100 of fig2 a 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 . 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 . 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 fig2 a 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 . 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 . 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 fig2 a 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 . 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 . fig3 a 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 fig2 a 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 fig2 a 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 . fig3 b , for example , is a schematic , cross - sectional view of the implant 200 taken along line 3 b - 3 b in fig3 a . as best seen in fig3 b , the stem 230 of the implant 200 comprises a generally concentric radial modulus gradient ( as shown by the arrows a ). fig3 c 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 . fig4 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 fig2 a 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 . 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 fig2 a and 2b . in this embodiment , however , as shown by the shading in fig4 , the gradient at an external portion 336 of the stem portion 330 decreases more gradually than at an interior region 338 . fig5 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 fig5 ) 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 . fig6 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 . 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 . 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 .
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 .