A 22-year-old soccer player comes to the emergency department for evaluation of a knee injury. During practice earlier that day, the patient felt her knee buckle when planting her right foot to kick the ball. Other players on the field recalled hearing an audible pop as she fell to the ground. The patient was able to limp off the field with help and immediately placed ice on her right knee. Vital signs are normal. BMI is 22 kg/m². On physical examination, the right knee is grossly swollen with limited range of motion due to pain. She is guarding on examination and has increased anterior translation of the tibia on the femur as compared to the contralateral side. Which of the following is the most likely diagnosis in this patient?

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11ec25c8_d96b_f340_bc81_a72b908981ac_MD0003_00 Anterior cruciate ligament (ACL) tears are common in young athletes, especially women participating in sports requiring rapid direction changes or pivots on the lower extremity (eg, soccer, basketball, tennis). ACL tears can also occur following a blow to the knee or significant twisting force. Patients with ACL tears typically experience an acute "popping" sensation in the knee, followed by rapid onset of effusion due to hemarthrosis and a feeling of instability when bearing weight. Examination in an ACL tear shows laxity of anterior motion of the tibia relative to the femur. Two such maneuvers-the Lachman and anterior drawer tests-are highly sensitive (>90%) and specific for ACL injuries. The diagnosis can be confirmed on MRI. ACL tear (Choice B) Medial collateral ligament (MCL) tears may also occur in pivoting/twisting injuries or if the knee is struck from the lateral side with the foot planted. Examination shows medial knee tenderness and laxity when the foot is gently forced into abduction with the knee stationary (valgus stress test). However, MCL injuries are not usually associated with a significant joint effusion or hemarthrosis unless there is a concurrent ACL injury. Medial collateral ligament (Choice C) Meniscal tears are characterized by a subacute or chronic locking or popping sensation in the knee. However, immediate symptoms may be mild, effusions typically develop slowly, and rapid hemarthrosis formation is rare. (Choice D) Rupture of a popliteal (Baker) cyst may cause acute knee pain. However, this is more common in older adults with osteoarthritis of the knee. Patients may have swelling at the posterior knee and calf, but joint effusions and hemarthrosis are usually not present. (Choice E) Posterior cruciate ligament injuries are often caused by hyperextension or by a blow to the anterior aspect of the proximal tibia. They are less common than ACL tears and are usually associated with little pain or alteration in range of motion. (Choice F) Stress fractures of the tibial plateau may occur in female athletes, especially following abrupt initiation or changes in exercise regimen. However, stress fractures typically present with subacute pain and are much less common than ligament injuries. Educational objective: Anterior cruciate ligament injuries are common in sports requiring rapid direction changes or twisting movements of the lower extremity. They usually present with rapid onset of pain and swelling with hemarthrosis. Examination findings include laxity of anterior motion of the tibia relative to the femur. References: Anterior cruciate ligament rupture: differences between males and females. (http://www.ncbi.nlm.nih.gov/pubmed/23281470) Anterior cruciate ligament injuries: anatomy, physiology, biomechanics, and management. (http://www.ncbi.nlm.nih.gov/pubmed/22695402)

A 22-Year-Old Soccer Player Comes to the Emergency Department for Evaluation