Why Posterior Cruciate-Retaining and Substituting Total Knee Replacements Have Similar Ranges of Motion

Fluoroscopic studies of cruciate-retaining total knee replacements have shown paradoxical anterior slide in flexion1. Without rollback, the tibial component may impinge earlier on the posterior part of the femur. Cruciate-retaining and cruciate-sacrificing total knee replacements have been reported to have similar ranges of motion2. It is possible that the position and function of the tibial post that substitutes for the posterior cruciate ligament in the cruciate-sacrificing implant may not optimize rollback enough to create a clinically important difference between these two designs. Recent newer high-flexion knee designs have moved the post posteriorly and extended the available posterior condylar surface proximally for greater articulation in flexion. These designs may demonstrate a larger flexion difference in the future. With modern implant designs, which began with the cementless porous-coated cruciate-retaining components, less posterior condylar bone was resected than was the case with early condylar implants. Removal of less bone was a result of new instrumentation that was based on a measured resection. The maintenance of posterior bone created a larger posterior offset. Offset is measured by the distance between the apex of the posterior condyle surface and the cortex of the posterior metaphysis (Fig. 1).