Abstract
Background
The constraint of total knee replacement (TKR) implants is not simply defined and many of the factors that influence it are not well understood. Variability in the constraint of different TKR implants designed for the same indication (e.g. cruciate-retaining, or posterior-stabilized) have been previously demonstrated, but these differences among implants have yet to be simply quantified. Furthermore, the relative importance of several variables on the implant constraint remains unknown. The purpose of this study was to quantify the differences in constraint that exist between different implant designs, and to examine the effects of axial load and flexion angle on the constraint of current cruciate-retaining (CR) TKR components.
Methods
Four contemporary CR TKR designs underwent laxity testing using a multi-axis mechanical test machine. Implants were tested at flexion angles of 0°, 20°, 90° and maximum flexion and axial loads of 712 N (1 BW) and 1424 N (2 BW). Friction-free motion in all secondary degrees of freedom was allowed. Force-displacement curves were generated for each testing condition in both anterior-posterior (AP) and rotational tests. AP constraint (N/mm) and rotational constraint (Nm/deg) were then calculated.
Results
Differences in the AP and rotational constraint of the various implants were identified. The rotating platform implant was the most AP constrained and least rotationally constrained of the implants studied. Among the fixed bearing implants, NexGen CR had less AP and rotational constraint than the Genesis II and PFC Sigma. Doubling the applied axial load during the laxity tests resulted in a significant increase in both AP (p<0.0001) and rotational (p<0.0001) constraint. AP and rotational constraint decreased with increasing flexion angles.
Conclusion
The constraints of four contemporary CR TKR designs were quantitatively compared and ranked according to their calculated constraints. Differences in the constraint of these designs were identified. This quantitative analysis is an important step towards the creation of a clinically meaningful constraint index that would aid surgeons in choosing the optimal implants for their TKR patients. Additionally, the analysis of the effects of axial load and flexion angle on constraint has implications which may influence how in vitro testing of implants is conducted as well as clinical implications in terms of how the constraint envelope of a given implant may influence the feel and function that a TKR patient experiences.
