anterior cruciate ligament

ANATOMY: Attaches to the medial surface of the lateral femoral condyle (posterior) and the anterior intercondylar area of the tibia. Two bundles: anteromedial (tight in flexion) and posterolateral (tight in extension). Blood supply: middle genicular artery (from popliteal). Innervation: tibial nerve. Intra-articular but extrasynovial, which is why it heals poorly and typically requires reconstruction rather than repair.

FUNCTION: Primary restraint to anterior translation of the tibia relative to the femur (Anterior ACL, Anterior tibial translation). Secondary restraint to tibial internal rotation and varus/valgus stress. Mnemonic: ACL prevents the tibia from sliding Anterior; PCL prevents the tibia from sliding Posterior (and the femur from sliding anterior on the tibia, e.g., when walking downhill).

MECHANISM: Noncontact deceleration, sudden pivot/cutting, or hyperextension with a valgus force, with the foot planted (soccer, basketball, skiing, football). Female athletes have a higher risk (wider Q angle, smaller intercondylar notch, hormonal and neuromuscular factors).

CLINICAL PRESENTATION: Audible/felt "pop," immediate swelling within hours (acute hemarthrosis), a sense of instability or the knee "giving way," and inability to continue play.

EXAM: - Lachman test (most sensitive): knee at 20-30° flexion, anterior pull on the tibia with increased translation and a soft or absent endpoint. - Anterior drawer test: knee at 90° flexion, anterior tibial translation (less sensitive than Lachman). - Pivot shift test: most specific, but difficult to perform in an awake, guarded patient.

IMAGING: Plain radiographs rule out fracture (e.g., Segond fracture, an avulsion of the lateral tibial plateau, is nearly pathognomonic for ACL tear; tibial spine avulsion in children). MRI is the diagnostic study of choice (discontinuity of fibers, bone bruises on the lateral femoral condyle and posterolateral tibial plateau).

ASSOCIATED INJURIES: "Unhappy triad" (O'Donoghue triad): ACL + MCL + medial meniscus tear (classically from a lateral blow to the knee with the foot planted, causing valgus force). Note that lateral meniscus tears are actually more common with acute ACL injuries, whereas medial meniscus tears are more common in chronic ACL deficiency.

MANAGEMENT: Initial RICE, NSAIDs, and rehabilitation/prehabilitation. Reconstruction with autograft (bone-patellar tendon-bone, hamstring, or quadriceps tendon) is recommended for young, active patients or those with instability; nonoperative rehabilitation is an option for low-demand patients. Chronic ACL deficiency increases risk of meniscal injury and early osteoarthritis. Prevention programs (neuromuscular training) reduce incidence.

BOARD PEARLS: (1) Hemarthrosis within hours of knee trauma = ACL tear until proven otherwise. (2) Best initial physical exam test: Lachman. (3) Best imaging: MRI. (4) Unhappy triad = ACL, MCL, medial meniscus. (5) Anterior drawer = ACL; posterior drawer = PCL (dashboard injury or fall on a flexed knee). (6) Middle genicular artery supplies both cruciate ligaments.