Factors affecting the region of most isometric femoral attachments. Part I: The posterior cruciate ligament.

We measured how the distance between selected tibial and femoral attachments of the PCL changes with knee flexion in six intact cadaver knee. The femoral location was the primary determinant of whether the distance increased, decreased, or remained nearly constant. The proximal-distal location of a fiber's femoral attachment had a stronger effect than had the anterior-posterior location. The tibial location had only a small statistically significant effect. These results suggest that the function of fibers within the PCL is determined primarily by their femoral attachment location. We determined all femoral attachments whose tibio-femoral distance changed 0, 2, 4, 6, 8, and 10 mm during flexion from 0 degrees to 90 degrees. No absolutely isometric point existed. Attachments whose separation distance changed less than 2 mm formed a bullet-shaped region whose base was against the roof of the intercondylar notch and whose nose pointed posteriorly and slightly distally. The axis of the "bullet" was near the proximal edge of the femoral insertion of the PCL. Along the axis, anterior attachments, located near the roof of the intercondylar notch, were more isometric than were posterior attachments, located near the cartilage. Attachments located distal to the axis moved away from the tibial insertion of the PCL when the knee was flexed. The more distal the femoral attachment, the larger the increase in tibiofemoral distance that occurred with flexion. The opposite was true of attachments proximal to the 2 mm region.