posterior cruciate ligament

**Anatomy** - Attachments: originates on the lateral surface of the medial femoral condyle and inserts on the posterior intercondylar area of the tibia. - Direction: it runs posteriorly from the femur to the tibia, so it resists posterior displacement of the tibia (equivalently, anterior displacement of the femur). - Mnemonic: the PCL attaches to the Posterior tibia and prevents Posterior tibial translation. The ACL attaches to the Anterior tibia and prevents Anterior tibial translation. "ACL = Lateral femoral condyle (medial surface), anterior tibia"; "PCL = Medial femoral condyle (lateral surface), posterior tibia." Both cruciates cross in an X: the ACL goes from posterolateral femur to anterior tibia, and the PCL goes from anteromedial femur to posterior tibia. - Bundles: the larger anterolateral bundle is tight in flexion. The smaller posteromedial bundle is tight in extension. - Blood supply: middle genicular artery (branch of popliteal artery). - Innervation: tibial nerve. - Location: intra-articular but extrasynovial, so it is outside the synovial cavity. - Strength: it is roughly twice as strong as the ACL and is injured less often.

**Function** - Primary restraint to posterior tibial translation, which is greatest at about 90° of flexion. - Secondary restraint to external rotation of the tibia, along with the posterolateral corner. - It is also a central axis of knee rotation and helps with proprioception.

**Mechanism of injury** - Posteriorly directed force on the anterior proximal tibia with the knee flexed. Examples are the dashboard injury in a motor vehicle collision and a fall onto a flexed knee with a plantarflexed foot (common in athletes). - Hyperextension or a varus/valgus force can injure it, usually as part of a multiligament injury. - Less commonly, it is injured by hyperflexion. - Isolated PCL injuries are less common than ACL injuries. Combined injuries are common (posterolateral corner, ACL, MCL/LCL) and are often associated with knee dislocation.

**Clinical presentation** - Pain and swelling in the posterior knee, usually less dramatic than with ACL tears. Patients may report instability, especially going down stairs or downhill, and often have mild symptoms. - A "pop" is less commonly reported than with ACL tears.

**Physical exam** - Posterior drawer test: the most sensitive and specific exam, done with the knee at 90° flexion. Posterior translation of the tibia on the femur indicates PCL injury. - Posterior sag sign (Godfrey test): with the hips and knees flexed to 90°, the tibia sags posteriorly. - Quadriceps active test: contraction of the quadriceps pulls the posteriorly subluxed tibia anteriorly. - Compare with the ACL: anterior drawer and Lachman (ACL) vs posterior drawer (PCL). - Examine for a dislocated knee: check for popliteal artery injury (ankle-brachial index or CT angiography) and common peroneal nerve injury (foot drop).

**Imaging** - X-ray: to rule out avulsion fractures (posterior tibial avulsion) and other fractures. - MRI: the diagnostic test of choice (confirms the tear and evaluates associated injuries). - Stress radiographs can quantify posterior laxity.

**Management** - Isolated grade I-II injuries: nonoperative with RICE, bracing, quadriceps strengthening, and physical therapy. These generally do well. - Grade III, combined ligament injuries, avulsion fractures, or knee dislocation: surgical reconstruction. - Quadriceps strengthening is important because it dynamically offsets posterior tibial translation.

**Complications** - Chronic PCL deficiency leads to increased contact pressures in the patellofemoral and medial compartments and to early degenerative arthritis, especially medial compartment and patellofemoral. - Neurovascular injury with knee dislocation (popliteal artery, common peroneal nerve).

**Board tips** - Dashboard injury + posterior drawer sign = PCL tear. - ACL: most commonly injured knee ligament, with pivoting/deceleration injuries, a "pop," hemarthrosis, and a positive anterior drawer/Lachman. Unhappy triad: ACL, MCL, and medial meniscus. - PCL: injured by posterior force on a flexed knee. Remember the reversal of anterior and posterior in the injury mechanisms. - Always check the neurovascular status in suspected knee dislocation.