Differences in wear between load and displacement control tested total knee replacements

Two identical sets of posterior cruciate retaining total knee replacements were tested for wear on one contemporary knee joint simulator according to the current ISO standards. The first set of implants was tested in load control mode (ISO 14243-1), while the second set was tested in displacement control mode (ISO 14243-3). The type of control mode alludes to the feedback signal of anterior–posterior and internal–external degrees of freedom. Resulting wear scars and rates between the different control mode tests were compared. Significant differences in wear rates were found: the wear rates in displacement control were approximately half of the wear rates in load control mode. This result was mirrored in the wear scars, which were significantly different in size on the medial plateau. Comparing wear rate with wear scar size (area) yielded a positive correlation. Analyzing the motion data of the two tests revealed similar ranges of movement in anterior–posterior translation and internal–external rotation, however the time-scale of the rotational movements differed between the two control types, explaining the differences in wear. This study demonstrated that substantial variability in wear outcome is possible depending on the type of control mode. Future revisions of testing directions should incorporate more implant specific test parameters to produce clinically relevant wear results.