Overview Snapshot A 27-year-old woman presents to the emergency department for severe pain in the right knee. A few hours prior to presentation, an opposing soccer player strikes her leg from her right side while her foot was planted to the ground. Afterwards, she felt a "pop" sound that was followed by severe knee pain and swelling. After the swelling improved, she feels that her right knee is unstable. On physical exam, the patient has a positive Lachman test. The patient is asked to rest, place ice and compress the knee, and elevate the affected leg. Given her sport demands, a conversation is made on the necessity of surgical reconstruction of the anterior cruciate ligament. Introduction Clinical definition injury in the form of tear or rupture affecting the anterior cruciate ligament (ACL) in the knee 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 Pathoanatomy 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 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 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 Presentation Symptoms feeling a "pop" in the knee thereafter, there is acute swelling and pain unstable 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 trauma is typically due to a motor vehicle accident leading to "dashboard injury" falling on a flexed knee while the foot is plantarflexed direct trauma to the proximal tibia 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