Total knee replacement wear during simulated gait with mechanical and anatomic alignments.

Total knee replacements (TKR) have historically been implanted perpendicular to the mechanical axis of the knee joint, with a commensurate external rotation of the femur in flexion relative to the posterior condylar axis (PCA). Although this mechanical alignment (MA) method has typically offered good long-term survivorship of implants, it may result in alignment of the implant that departs significantly from the native Joint Line (JL) in extension and flexion for a considerable portion of the patient population. There is a growing interest with surgeons to implant TKR components more closely aligned to the natural JL (Anatomic Alignment-AA) of the patient's knee joint to reduce the need for soft tissue releases during surgery, potentially improving knee function and patient satisfaction. Using a previously-validated finite element model of the lower extremity, implant- and alignment-specific loading conditions were developed and applied in a wear experiment via a six-degree-of-freedom joint simulator. MA was defined as 0° Joint Line (JL), 0° varus hip-knee-ankle (HKA) angle, and 3° external femoral rotation. AA was defined as 5° varus JL, 3° varus HKA, and 0° femoral rotation. The experiment returned wear rates of 3.76 ± 0.51 mg/million cycles (Mcyc) and 2.59 ± 2.11 mg/Mcyc for ATTUNE® cruciate-retaining (CR) fixed bearing (FB) in MA and AA, respectively. For ATTUNE posterior-stabilized (PS) FB in AA, the wear rate was 0.97 ± 1.11 mg/Mcyc. For ATTUNE CR rotating platform (RP), the wear rates were 0.23 ± 0.19 mg/Mcyc, 0.48 ± 1.02 mg/Mcyc in MA and AA respectively. Using a two-way ANOVA, it was determined that there was no significantly difference in the wear rates between AA and MA (p = 0.144) nor the wear rate of ATTUNE PS FB in AA significantly different from either ATTUNE CR FB or ATTUNE CR RP.

Validation of a new computational 6-DOF knee simulator during dynamic activities.

A new six-degree-of-freedom (6-DOF) joint simulator has recently been developed which facilitates testing of implants under more realistic loading conditions than has been possible previously. However, typical wear testing can be very time-consuming, taking weeks or months to complete. A validated computational model is an ideal complement to these types of long-running tests. In this study, a computational counterpart to the new 6-DOF joint simulator was developed and validated. Total knee replacement components were evaluated in both physical and computational simulations, and joint mechanics were compared between the experiment and the model. Kinematic comparisons were carried out for two total knee replacement designs, under loading conditions representative of three different activities of daily living: deep knee bend, gait, and stepdown. The model accurately reproduced the motions obtained in the physical simulator, and appropriately differentiated between activities and between implant designs. Root-mean-square differences in anterior-posterior translations and internal-external rotations were less than 1.7mm and 1.4°, respectively, for both implant designs and all three dynamic activities. Contact area, and peak and average contact pressure predicted by the model matched experimental measurements with a root-mean-square accuracy of 20mm2, 9MPa, and 1MPa, respectively. The computational model of the 6-DOF joint simulator will be a key tool in efficient evaluation of implant mechanics under loading conditions representative of the in vivo environment. These simulations may be used directly in comparison of devices, or may aid in facilitating optimal usage of the physical simulator through determining which activities and/or loading conditions best address specific clinical or design issues, for example, development of worst-case loading profiles for wear testing.