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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.

Valgus or varus malpositioning of the tibial component of a total knee implant may cause increased propensity for loosening or implant wear and eventually may lead to revision surgery. The aim of this study was to determine the effect of valgus/varus malalignment on tibio-femoral mechanics during surgical trial reduction and simulated gait loading. In seven cadaver legs, posterior cruciate sparing total knee replacements were implanted and tibial inserts representing a neutral alignment and 3 degrees and 5 degrees varus and valgus alignments were sequentially inserted. Each knee with each insert was loaded in a manner representative of a trial reduction performed during knee surgery and loaded in a physiological knee simulator. Simulated gait performed on the simulator demonstrated that internal/external and adduction/abduction rotations showed statistical changes with some of the angled inserts at different points in the walking cycle. Neither medial/lateral nor anterior/posterior translations changed statistically during simulated walking. The pressure distribution and total load in the medial and lateral compartments of the tibial component changed significantly with as little as a 3 degrees variation in angulation when loaded in a manner representative of a trial reduction or with a knee simulator. These results support the need for precise surgical reconstruction of the mechanical axis of the knee and proper alignment of the tibial component. These results further demonstrate that tibial contact pressures measured during a trial reduction method may be predictive of contact mechanics at the higher loading seen in the knee simulator.

The objectives of this study were to (a). create 3D reconstructions of two carpal bones from single CT data sets and animate these bones with experimental in vitro motion data collected during dynamic loading of the wrist joint, (b). develop a technique to calculate the minimum interbone distance between the two carpal bones, and (c). validate the interbone distance calculation process. This method utilized commercial software to create the animations and an in-house program to interface with three-dimensional CAD software to calculate the minimum distance between the irregular geometries of the bones. This interbone minimum distance provides quantitative information regarding the motion of the bones studied and may help to understand and quantify the effects of ligamentous injury.

The objective of this study was to determine the relative mechanical properties of the cement-bone interface due to tensile or shear loading. Mechanical tests were performed on cement-bone specimens in tensile (n = 51) or shear (n = 55) test jigs under the displacement control at 1 mm/min until complete failure. Before testing, the quantity of bone interdigitated with the cement was determined and served as a covariate in the study. The apparent strength of the cement-bone interface was significantly higher (p < 0.0001) for the interface when loaded in shear (2.25 MPa) when compared to tensile loading (1.35 MPa). Significantly higher energies to failure (p < 0.0001) and displacement before failure (p < 0.01) were also determined for the shear specimens. The post-yield softening response was not different for the two test directions. The data obtained herein suggests that cement-bone interfaces with equal amounts of tensile and shear stress would be more likely to fail under tensile loading.

An experimental investigation was performed to (1) determine the general mechanical behavior and in particular, the post-yield behavior of the cement-bone interface under tensile loading, (2) determine where interface failure occurs, and (3) determine if the mechanical properties of the interface could be related to the density of bone at the interface and/or the amount of cement-bone interdigitation. Seventy-one cement-bone test specimens were machined from human proximal femurs that had been broached and cemented using contemporary cementing techniques. The amount of cement-bone interdigitation was documented and the quantitative computed tomography equivalent mineral density (QCT density) of the bone with cement was measured. Specimens were loaded to failure in tension under displacement control and exhibited linear elastic behavior with some reduction in stiffness until the peak tensile stress was reached (1.28 +/- 0.79 MPa). A substantial amount of strain softening (negative tangent stiffness) with an exponential-type decay was found after the peak stress and continued until there was complete debonding of the specimens (at 0.93 +/- 0.44 mm displacement). Interfacial failure most often occurred at the extent of cement penetration into the bone (56% of specimens) or with small spicules of cement left in the bone (38% of specimens). The results showed that the post-yield tensile behavior contributes substantially to the energy required to cause failure of the cement-bone interface, but the post-yield behavior was not well correlated with the amount of interdigitation or density of bone. Linear regression analysis revealed a moderate (r2 = 0.499, p0.0001) positive relationship between the tensile strength of the cement-bone interface and the quantity of bone interdigitated with the cement.