ACL

**Anatomy** - Origin: medial aspect of the lateral femoral condyle (posteromedial surface). Insertion: anterior intercondylar area of the tibia. - Function: prevents anterior displacement of the tibia relative to the femur (equivalently, posterior displacement of the femur on the tibia). Also resists tibial internal rotation and hyperextension. - Two bundles: anteromedial (tight in flexion) and posterolateral (tight in extension). - Blood supply: middle genicular artery (branch of popliteal). Innervation: posterior articular branch of the tibial nerve. - Intra-articular but extrasynovial, which limits its healing capacity. This is why complete tears are usually reconstructed. - Memory aid: ACL = "Always Cruciate Lateral" (attaches to the lateral femoral condyle). PCL attaches to the medial femoral condyle and prevents posterior tibial translation.

**Mechanism and epidemiology** - Most injuries are non-contact: sudden deceleration, cutting, pivoting, or landing from a jump with a valgus load (soccer, basketball, skiing, football). - Female athletes have a 2-8x higher risk than male athletes. Contributing factors include a wider Q-angle, a smaller intercondylar notch, hormonal effects on ligament laxity, and neuromuscular and landing mechanics.

**Clinical presentation** - The patient hears or feels a "pop," then has immediate pain, inability to continue play, and rapid swelling within hours (acute hemarthrosis). Hemarthrosis after acute knee trauma is ACL tear until proven otherwise. - Later: instability or "giving way" with pivoting.

**Physical exam** - Lachman test: the most sensitive test. The knee is flexed 20-30°, the femur is stabilized, and the tibia is pulled anteriorly. Excess anterior translation with a soft or absent endpoint is positive. - Anterior drawer test: the knee is flexed 90° and the tibia is pulled anteriorly. It is less sensitive than Lachman. - Pivot shift test: the most specific test. It is difficult to perform in an acutely painful knee.

**Associated injuries** - "Unhappy (terrible) triad" (O'Donoghue): ACL + MCL + medial meniscus tear (classically from a lateral blow to the knee with a valgus force). Some sources describe the lateral meniscus as more commonly torn in acute ACL injuries. - Bone bruises of the lateral femoral condyle and posterolateral tibial plateau. - Segond fracture: avulsion fracture of the lateral tibial plateau, which is highly associated with ACL tear.

**Diagnosis** - Clinical exam plus MRI (the best test, which also evaluates menisci and cartilage). Radiographs are used to rule out fracture (e.g., Segond fracture, tibial spine avulsion).

**Management** - Initial: RICE, NSAIDs, protected weight bearing, and early rehabilitation to regain range of motion. - Surgical reconstruction (autograft, e.g., patellar tendon, hamstring, or quadriceps tendon) is indicated for young or active patients, athletes, those with instability, or those with concomitant meniscal or other ligament injuries. - Non-operative rehab may be considered in older, sedentary, or low-demand patients. - Complication of untreated ACL deficiency: recurrent instability, meniscal damage, and early osteoarthritis.

**Board pearls** - Positive anterior drawer/Lachman = ACL. Positive posterior drawer / posterior sag = PCL. - Valgus stress laxity = MCL. Varus stress laxity = LCL. - ACL tear after a non-contact pivot with a pop and hemarthrosis is a classic vignette.