Introduction

Quadriceps performance following total knee arthroplasty (TKA) is a critical factor in patient satisfaction that can be significantly affected by implant design (Greene, 2008). The objective of this study was to compare quadriceps efficiency (QE) following TKA with a medial-pivot system (EVOLUTION®, MicroPort Orthopedics Inc., Arlington, TN, USA) to non-implanted control measurements.

Introduction: Many attempts have been made to describe the flexion axis of the knee based on landmarks or simple geometric representations of the anatomy. An alternative approach is to use kinematic data to describe the axis of motion of the joint. The helical axis is one kinematic parameter that can accomplish this. The purpose of this study was to compare the correlation between kinematic and anatomic axes of motion.

Methods: Six cadaver lower extremities were skeletonized except for the knee joint. Passive navigation markers were implanted, and CT scans obtained. The limbs were then placed in an open-chain lower extremity rig that allows full range of knee motion. Threedimensional kinematic data were recorded using a camera and the helical axis of motion was calculated. Anatomic landmarks were placed on CT derived CAD models of the extremities consisting of spherical and cylindrical fits of the femoral condyles and a trans-epicondylar axis. Data for the normal knee was processed, by comparison of the helical axis to the landmark axes over varying ranges of flexion and the variation in helical axis direction within that range was also calculated.

Results: The flexion range with the minimum variation of anatomic parameters to the helical axis was 30–100°. Helical axis variation in this range was 5.489 ± 1.173, while variation between the helical axis and those axis defined by spherical, TEA, and cylindrical landmarks were 5.115 ± 2.129°, 3.127 ± 2.029°, and 5.111 ± 1.710°, respectively. A students t-test was performed on each data set with the null hypothesis that the angular difference between the anatomically defined axes and the helical axis is zero. All axes were found to be significantly different from the average helical axis in the range of 30–100° (P= 0.002, 0.013, and 0.001, respectively). The tightest variation in the helical axis occurred at 40–50° of flexion 2.89 ± 0.722.

Conclusion/Discussion/Summary: None of the anatomic landmarks considered in this study represent a consistently valid approximation of the kinematic flexion axis of the knee. The TEA represents the closest approximation of the three with a 95% CI between 0.998 and 5.256°. The range of 30–100° represented the tightest variation over the largest range of flexion. Extension was defined at approximately 30° based on kinematic profiles of internal/external rotation which show a “screw-home” tendency beginning at 30° through extension. This behavior is consistent with an increase in helical axis variation in ranges that were less than 30° of flexion. In a previous open-chain model, both compartments of the joint were spinning around 45 degrees of flexion, which is consistent with the smallest helical axis variation observed in the 40–50° range.

Large diameter metal-metal bearings possess several clinical advantages over small bearings including greater joint stability, improved range of motion, and lower wear due to improved lubrication. Simulator wear tests were conducted to assess the effects of thermal processing on the wear behavior of large diameter metal-metal hip bearings.

Three groups of high carbon, cast 54 mm hip bearings with different thermal processing histories were tested. Two groups of bearings were manufactured to identical specifications and subjected to either no heat treatment(as-cast) or to typicall thermal processing prior to testing. The third group was comprised of commercially available as castbearing systems. Wear tests were performed on a Shore Western orbital bearing wear test machine. A simulated gait profile (triple-peak Paulprofile) with a maximum force of 2000N was applied to the bearings at a frequency of 1 Hz. The bearings were tested in the inverted position (headabove, shell below).

The general wear behavior of all three groups of bearing couples was similar to that previously reported for metal-metal bearings. All couples exhibited a run-in wear phase followed by a low-wear steady-state phase. For all bearing couples tested the heads demonstrated more wear than the shells. The appearance of the worn surfaces of all the bearing couples tested in this study were consistent with that of previously reported in-vitro wear testing as well as metal-metal hip bearing retrieval studies. There was no statistical difference among the three groups tested in the run-in or steady-state wear rates, although the heat treated bearings tended to wear less on average.

The results of this study indicate that thermal processing has no adverse effect on the wear of large diameter metal-metal hip bearings.