Abstract
Introduction
Knee prostheses retrieved at revision often show patterns and severity of damage neither seen nor predicted from standard wear simulator testing. We hypothesized that this is because these implants are exposed to combinations of loads and motions that are more damaging than the simple loading profiles utilized in laboratory testing. We examined the magnitude, direction, and combination of forces and moments acting on the knee during various activities in order to guide the future development and testing of high-performance knee replacements.
Methods
In vivo data from five patients with instrumented tibial implants were obtained from an open database (www.orthoload.com). We determined the direction and magnitude of forces and moments that the knee experiences during the following common physiologic activities: stair descent, stair ascent, deep knee bend, one leg stance, and walking. In order to capture the loading pattern, we investigated the three component forces and moments acting on the knee at several high demand points for each of these activities. The e-tibia data were compared to the loading profiles used in conventional laboratory testing (ISO 14243-1).
Results
The vast majority of maximum forces and moments measured during these activities far exceeded those applied during laboratory testing, often by several-fold (Table 1). Analysis of loading profiles showed considerable differences in the loading patterns both between individuals and activities. At the point of peak axial force during level walking, there were four distinct loading patterns in five patients- none of which matched the laboratory testing pattern. The comparison of the median loading pattern at the point of maximum axial force showed that each of the five activities generated distinct loading patterns, which all differed substantially from the loading pattern applied during conventional knee testing.
Discussion
Current routines for laboratory testing of total knee joint prostheses fail to develop forces and moments of the magnitude present within knee prostheses and surrounding interfaces during physiologic activities. Moreover, the combinations of force and moment components generated during conventional testing differ fundamentally from those occurring in vivo. These discrepancies may explain the differences between the wear patterns seen in components retrieved at revision versus those generated by laboratory simulators. Clearly, new testing protocols imposing more severe loading conditions and variable loading patterns are required to simulate service conditions generated by more active patients and more diverse activities after TKR.
