lateral collateral ligament

ANATOMY: The LCL (fibular collateral ligament) is a cord-like, extracapsular ligament. It attaches proximally at the lateral femoral epicondyle and distally at the head of the fibula (conjoined with the biceps femoris tendon insertion). Unlike the MCL, it is NOT attached to the lateral meniscus and is separated from the joint capsule, so isolated LCL tears do not typically cause hemarthrosis or meniscal injury. It is a key component of the posterolateral corner (PLC), along with the popliteus tendon, popliteofibular ligament, and arcuate ligament complex.

FUNCTION: Primary restraint to varus stress at the knee (strongest at about 30 degrees of flexion). It also serves as a secondary restraint to external rotation and posterior tibial translation.

MECHANISM OF INJURY: Direct blow to the anteromedial knee or varus force with the knee in extension, as in sports (soccer, football, skiing) or trauma. It is much less common than MCL injury, because the lateral side of the knee is less exposed to contact. Isolated LCL injury is rare. Most cases involve other structures (PLC, ACL, PCL).

CLINICAL FEATURES AND EXAM: - Lateral knee pain and swelling and a sense of instability. - Varus stress test: apply a varus force with the knee at 0 degrees and 30 degrees flexion. Laxity at 30 degrees indicates an isolated LCL injury. Laxity at both 0 and 30 degrees suggests combined injury (PLC, cruciate ligaments). - Associated findings include a positive dial test (increased external rotation, suggesting PLC injury), a varus thrust gait, and a possible foot drop.

COMPLICATIONS AND ASSOCIATIONS: - Common peroneal (fibular) nerve injury: the nerve wraps around the fibular neck and is vulnerable with varus or PLC injuries. It causes foot drop (weak dorsiflexion and eversion) and sensory loss over the dorsum of the foot and lateral leg. - Associated ACL/PCL tears and fibular head avulsion fractures (arcuate sign). - Chronic instability can occur if the injury is missed.

IMAGING: Plain radiographs may show a fibular head avulsion (arcuate sign) or a Segond fracture (lateral tibial plateau avulsion, which is highly associated with ACL tear). MRI is the best test for ligament assessment.

MANAGEMENT: Grade I-II injuries are treated nonoperatively (RICE, bracing, rehabilitation). Grade III injuries and combined injuries (especially PLC) usually require surgical repair or reconstruction.

COMPARISON WITH MCL: The MCL (tibial collateral ligament) resists valgus stress, is attached to the medial meniscus, and is injured much more often. The "unhappy triad" (O'Donoghue triad) involves the MCL, ACL, and medial meniscus, typically from a lateral blow to the knee with a valgus force.

HIGH-YIELD BOARDS POINTS: LCL = varus stress. Fibular head attachment. Not attached to the lateral meniscus. Look for common peroneal nerve injury. Injury to the lateral side of the knee suggests a PLC injury.