Snapshot A 21-year-old man presents to the emergency department for severe pain in the right knee. A few hours prior to presentation, an opposing football player hits his leg from his right side. Afterwards, he felt a "pop" sound that was followed by severe knee pain and a sensation of knee instability. On physical exam, there is anterior translation of the proximal tibia when pulled as the patient has the knee flexed at 90° and supine. Ice is applied to the knee and ibuprofen is prescribed. Orthopedic surgery is consulted to evaluate if ligamental reconstruction is needed. Introduction Clinical definition injury in the form of tear or rupture affecting the anterior cruciate ligament (ACL) in the knee Associated conditions unhappy triad which consists of injury to the ACL medial collateral ligament medial meniscus Prognosis The majority of athletes are able to return to their sports activities with ACL reconstruction Epidemiology Incidence non-contact ACL injuries are more common Demographics more common in female athletes Risk factors sports (e.g., football, soccer, skiers, and basketball) motor vehicle accidents ETIOLOGY Pathoanatomy Pathology contact mechanisms injury that causes hyperextension or valgus deformation of the knee e.g., football injury high-speed motor vehicle accident non-contact mechanisms changing direction, pivoting, or landing that leads to rotation or valgus stress of the knee after sudden deceleration Normal anatomy the ACL course originates in the posteromedial aspect of the lateral femoral condyle reaches the anteromedial aspect of the tibia the ACL plays an important role in knee stability it controls anterior translation of the tibia it also restrains against tibial rotation, varus, and valgus stress Presentation Symptoms feeling a "pop" in the knee thereafter, there is acute swelling and pain instability or "giving out" knee Physical exam anterior drawer test the proximal tibia is anteriorly pulled while the patient is supine and the knee is flexed at 90 degrees if there is anterior translation then the test is positive Lachman test the proximal tibia is anteriorly pulled with one hand, while the other hand stabilizes the distal femur while the knee is flexed at 30 degrees hemarthrosis Imaging Magnetic resonance imaging (MRI) indication an imaging modality that can confirm the diagnosis if reconstruction is a possibility or to identify concomitant knee pathology Studies Making the diagnosis can be made based on typical clinical presentation alone can be confirmed by MRI or knee arthroscopy Differential Meniscal tear differentiating factor patients can have a positive Thessaly test McMurray test Apley test Posterior collateral ligament (PCL) injury differentiating factor patients can have a positive posterior drawer test Treatment Conservative rest, ice, compression, and elevation (RICE) therapy indication management to reduce pain, edema, and hemarthrosis in the acute stage of the injury nonsteroidal anti-inflammatory drugs are typically added Operative surgical reconstruction indications performed in young and active patients with high demand sports or jobs significant knee instability such as injuries affecting multiple knee structures (e.g., unhappy triad) Complications Osteoarthritis Arthrosis Sports disability