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 login to view 3 more bullets non-contact mechanisms login to view 1 more bullet Normal anatomy the ACL course login to view 2 more bullets the ACL plays an important role in knee stability login to view 2 more bullets 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 login to view 1 more bullet 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 login to view 3 more bullets 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 login to view 2 more bullets Operative surgical reconstruction indications login to view 4 more bullets Complications Osteoarthritis Arthrosis Sports disability