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Background: Recent studies have shown that the posterior cruciate ligament (PCL) is composed of a continuum of fiber regions that display characteristic mechanical behavior under different motion and loading conditions.

Hypothesis: The anterior, central, and posterior fiber regions of the PCL differentially contribute to control of posterior translation of the tibia.

Study Design: Controlled laboratory study.

Methods: Nine intact, fresh-frozen cadaveric knees were instrumented with excursion wires implanted within the anterior, central, and posterior fiber regions of the PCL. In groups of 3, patterns of incremental posterior tibial translation using a 74-N posterior force were analyzed as a function of the variable linear separation distance between tibial and femoral fiber region attachment sites during posterior drawer testing at knee flexion angles of 20° and 90° before and after sequential fiber region section.

Results: At 20° of knee flexion, there was no statistical difference in the relatively small amount of posterior tibial translation, regardless of whether the anterior, central, or posterior fibers were alone transected (P = .350). At 90° of knee flexion, whether the posterior fibers were cut first, second, or third (order of section), the incremental difference in posterior tibial translation this produced was significantly different (P = .039). For the fiber regions combined, the third fiber region section resulted in a significantly larger incremental translation than did either the first or second section with the knee flexed 90° (P = .001). After transection of all fiber regions, significantly more total posterior tibial translation occurred at 90° versus 20° of flexion (P = .002).

Conclusions: This study shows that fiber regions within the PCL have unique characteristics and behave differently in response to posterior drawer forces.

Clinical Relevance: This study provides additional information on the complex mechanical behavior of the PCL and suggests that some partial tears (ie, those involving 1 or 2 fiber regions) may only result in minimal posterior translation during drawer testing at 90°.

A careful history and physical examination are the cornerstones of orthopaedic sports medicine. When evaluating a patient for ligamentous instability of the knee joint, an understanding of the contribution of anatomic structures to stability enhances a practitioner’s ability to achieve an accurate clinical diagnosis. This article reviews the various types of knee instability and the associated anatomic structures. Ultimately, information must be obtained from multiple tests to reach the final diagnosis. We describe in detail the pathologic and biomechanical basis of the tests for both tibiofemoral and patellofemoral instability of the knee joint and provide recommendations for performance and interpretation of these physical examinations.