The Science of PCL Injuries
Posterior cruciate ligament (PCL) injuries involve damage to the ligament that prevents excessive backward movement of your tibia relative to your femur. The PCL is the strongest ligament in your knee and has a better blood supply than the ACL, giving it superior healing potential when injured. PCL injuries are graded from I to III based on severity: Grade I involves stretching with microscopic tears, Grade II involves partial tearing with some functional loss, and Grade III represents complete rupture. Unlike ACL injuries, isolated PCL tears have an excellent capacity for healing due to their rich vascular supply. The mechanism of injury typically involves a posteriorly directed force on the proximal tibia, such as falling onto a bent knee or dashboard injuries in motor vehicle accidents. Sports injuries often occur during hyperflexion or hyperextension movements, particularly in contact sports. The PCL works in conjunction with other knee structures to provide stability. When injured, the quadriceps muscle becomes crucial as it can partially compensate for PCL function by preventing excessive posterior tibial translation during functional activities.
Contributing Factors
Your PCL serves as the primary restraint against posterior translation of the tibia, particularly when your knee is flexed beyond 30 degrees. Unlike the ACL, which is tight in extension, the PCL becomes increasingly tight as the knee flexes, with maximum tension occurring at 90 degrees of flexion.
The quadriceps muscle group, particularly the vastus medialis and vastus lateralis, can provide some functional replacement for a damaged PCL by preventing the femur from sliding forward on the tibia. This is why quadriceps strengthening is the cornerstone of PCL rehabilitation.
During normal gait, the PCL experiences loads of 0.5-1 times body weight, but during activities like squatting or climbing stairs, these forces can increase to 2-3 times body weight. Understanding these loading patterns is crucial for designing appropriate rehabilitation programs.
The posterior drawer test demonstrates the primary dysfunction in PCL injuries - when you sit with your knee bent and someone pushes your shin backward, there's excessive movement compared to the uninjured side. However, this instability may not translate to significant functional problems in daily activities.