Rotational constraint of posterior-stabilized total knee prostheses

The rotational constraint characteristics of 4 commercially available posterior-stabilized implants (Triathlon, NexGen, PFC, and Genesis II) were determined. The torques required to achieve up to +/-20 degrees of internal and external rotation were quantified at hyperextension, 0 degree, 15 degrees, 60 degrees, and 90 degrees of flexion. The effect of lubrication was tested by comparing the torque values generated in dry and serum environments. Results indicated that substantially higher torques were generated in the dry environment, compared with the serum environment. In extension, the principal design features influencing the torques were interaction of the box and post, as well as the insert anterior lip. In higher flexion angles, the amount of rollback, coupled with the posterior lip geometry, most heavily influenced the generation of torque. Comparison of the 4 designs showed that the Triathlon generated the least torques to +/-10 degrees of rotation, whereas the PFC and Genesis II were the most constrained.