Strain redistribution in the canine femur resulting from hip implants of different stiffnesses

Bone remodeling adjacent to orthopedic implants has been attributed to bone strain changes. Although many animal studies have assessed bone remodeling near implants, the altered bone strains and even the strains in the intact bone prior to implantation have not been mapped extensively. Instead, bone changes are often correlated with implant stiffnesses. In this study, a benchtop loading system was developed using measurements from in vivo strain analysis to simulate physiologic loading of a canine femur. The effect on bone strains of three different stiffness canine hip implants with the same anatomic shape were compared by taking measurements from the proximal greyhound femur during loading. Peak compressive and tensile strains of the order of 200 to 400 microstrain were measured in the intact and implanted femora. The measurements indicate that during simulated in vivo loading, none of the implants substantially alter the normal strain state of the bone. If initial axial strains significantly affect the remodeling response of bone, the similarity of measurements with the different implants in place suggests that the same remodeling response would be expected to both the stiffest and least stiff implant, as has been noted in animal studies adjacent to the intermediate stiffness implant. It also suggests that this implant shape and initial bone implant interface condition can compensate for strain reductions expected near stiff straight-stemmed implants.