In vitro response of the natural cadaver knee to the loading profiles specified in a standard for knee implant wear testing.

The purpose of this study was to examine how a natural knee responds to the inputs of a total knee replacement testing standard developed by the International Organization for Standardization (ISO). This load control standard prescribes forces to be used for wear testing of knee replacements independent of implant size or design. A parallel ISO standard provides wear testing inputs that are displacement based instead of force based. Eight fresh frozen cadaveric knees were potted and tested in a 6 degree of freedom knee simulator using the load-control standard. The resulting displacements during load-control testing were compared to the prescribed displacements of the ISO displacement standard. At half the tibial torque prescribed by the load standard there was three times more average internal tibial rotation (20.3 degrees) than is prescribed by the displacement standard (5.7 degrees). The AP motion resulting from load testing was much different than is specified by the displacement standard. All eight knees had anterior tibial translation with respect to the femur during swing phase while the displacement standard specifies posterior tibial displacement. The variation in these motions among knees and their difference from the ISO displacement standard may be one factor that explains why wear results of total knee replacements based on ISO load or displacement testing frequently do not agree with each other or with clinical retrievals.

Determining the degree of cortical bone asymmetry in bilateral, nonpathological, human femur pairs.

When testing the effects of a femoral component on cortical bone following total hip arthroplasty, the patient's implanted femur is often compared with his/her contralateral nonimplanted femur, with differences attributed to the femoral component. However, if normal anatomical differences exist between bilateral femurs, they need to be quantified in order to validate whether the differences between implanted and nonimplanted bilateral femurs are due to the implant or possibility due to intrinsic differences before implantation. This study quantified the geometric properties of cortical bone shape between seven pairs of bilateral, cadaveric, human femurs. The null hypothesis tested stated that the bilateral femurs would not be significantly different in cortical bone geometry. Digitized images of cortical bone cross-sections taken at percent biomechanical lengths (levels 1-8) were used to calculate bone geometry measurements. The paired t-test showed that the only significant difference was in the location of principal axes at the most proximal location, level 1 (p = 0.015). All other measurements and levels were not significant with percent differences less than 6.6%. In conclusion, the data supports attributing cortical bone shape differences between implanted and contralateral nonimplanted femurs in levels 2-8 to the presence of the implant when the significant differences are greater than 6.6%.