The Posterior Cruciate Ligament is the primary restraint to posterior drawer, and secondary restraint to external rotation. Isolated sectioning of the PCL results in an increased posterior translation of the knee under a posterior tibial load. This increase in laxity is relatively small at full extension and most pronounced at 90 degree of flexion. Only small rotatory or valgus/ varus laxity results from isolated PCL injury. Up to 60% of PCL injuries involve disruption of the posterolateral structures. When both the PCL and posterolateral structures are cut, posterior laxity is significantly increased. Tears of the PCL do not appear to be as common as of the ACL, due partly to the greater strength of the PCL. However, the condition is under-diagnosed. PCL injuries are often associated with meniscal and chondral injury. The incidence of significant chondral damage with isolated PCL injury was not thought to be as high as with ACL injury, but a recent study showed chondral damage in 52% of those with PCL tears, with lesions of grade III or more found in 16 %.
Clinical Features
The mechanism of PCL injury is usually a direct blow to the anterior tibia with the knee in a flexed position. This can be from contact with an opponent, equipment or falling onto the hyperflexed knee. Hyperextension may also result in an injury to the PCL and posterior capsule. The patient complains of poorly defined pain, mainly posterior, sometimes involving the calf. On examination, there is usually minimal swelling as the PCL is an extrasynovial structure. The posterior drawer test is the most sensitive test for PCL deficiency. This is performed in neutral, internal, and external rotation. A posterior sag of the tibia, and pain and laxity on a reverse Lachman's test may be present. PCL rupture is particularly disabling for downhill skiers, who rely on this ligament for stability in the tucked up position adopted in racing.
Types of PCL Tears
PCL tears are graded I, II and III on the position of the medial tibial plateau relative to the medial femoral condyle at 90 degree of knee flexion (the posterior drawer position). The tibia normally lies approximately 1 cm (0.4 in.) anterior to the femoral condyles in the resting position.
- In grade I injuries the tibia continues to lie anteriorly to the femoral condyles but is slightly diminished.
- In grade II injuries the tibia is flush with the condyles.
- When the tibia no longer has a medial step and can be pushed beyond the medial femoral condyle, it is classified as a grade III injury.
It is important to distinguish between isolated PCL injury and a combined PCL and posterolateral corner injury.
- In isolated PCL tears, there is a decrease in tibial translation in internal rotation due primarily to the influence of the MCL.
- X-ray should be performed to exclude a bony avulsion from the tibial insertion of the PCL (best seen on lateral tibia radiographs).
- If a fracture is present, acute surgical repair is undertaken.
- Stress radiographs provide a non-invasive measure of sagittal translation compared to the uninjured knee.
- It is considered that more than 7G8 mm of posterior translation is indicative of a PCL tear.
- MRI has a high predictive accuracy in the diagnosis of the acute PCL injury but a lesser accuracy in chronic injuries.
- If an injury to the posterolateral corner is suspected, MRI can be helpful but to view this region properly usually requires a specific imaging protocol.
- When the MRI requisition states that injury to the posterolateral corner is suspected, the radiologist can optimize the imaging protocol.
Treatment
- PCL rupture can generally be managed conservatively with a comprehensive rehabilitation program.
- More severe injuries (grade III) should be immobilized in extension for the first two weeks. Results show that patients with isolated PCL tears have a good functional result despite ongoing laxity after an appropriate rehabilitation program.
- Regardless of the amount of laxity, half of the patients in one large study returned to sport at the same or higher level, one-third at a lower level and one-sixth did not return to the same sport.
- Surgical reconstruction is indicated when the PCL injury occurs in combination with other posterolateral structures or where significant rotatory instability is present.
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