A 14-year-old girl is brought to the emergency department for evaluation of acute right knee pain. The patient is a gymnast and was dismounting the balance beam when she landed awkwardly and collapsed to the ground. She heard an audible pop when she landed and has been unable to straighten her knee since. Temperature is 36.7 C (98 F), blood pressure is 120/70 mm Hg, pulse is 88/min, and respirations are 16/min. BMI is 17 kg/m². Cardiopulmonary examination is unremarkable. The right knee is in a flexed position and has decreased range of motion with significant swelling and tenderness over the medial side. The lateral aspect of the knee has a large, painful, immobile deformity. Palpation of the anterior aspect reveals a divot over the trochlea and minimal pain over the tibial tuberosity. Which of the following is the most likely diagnosis in this patient?

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11ec25c8_d971_e6be_bc81_bfa94010ccee_MD0003_00 This patient likely has a patellar dislocation, which usually occurs after quick, lateral movements on a flexed knee and is most common in young athletes (eg, soccer or ice hockey players, gymnasts), dancers, and military trainees. Patients may feel the knee giving way, followed by a popping noise and severe pain. The quadriceps muscles normally exert a lateral force on the patella, especially during quadriceps contraction; therefore, in cases of patellar dislocation, lateral displacement with associated tear of the medial patellofemoral ligament (which normally provides an opposing medial force) is most common. Superior and medial dislocations are rare. Risk factors for patellar dislocation include age <20, joint laxity, lower extremity malalignment, and patellar subluxation (excess lateral movement within the trochlea). Examination shows a flexed knee with reduced range of motion and lateral displacement of the patella (seen as a large, immobile deformity in this patient) out of the trochlea, which may be palpable as a depression at the anterior knee. Other findings can include hemarthrosis and tenderness along the medial patella (reflecting tear of the medial patellofemoral ligament). X-rays may not be needed for diagnosis in clinically apparent dislocation but are performed after reduction to rule out additional injuries. Patellar dislocation often reduces spontaneously. However, closed reduction is required for patients without spontaneous resolution. Follow-up care includes splinting and rehabilitation for muscle strengthening. (Choices A and F) Anterior cruciate ligament (ACL) tears are typically due to rapid deceleration or pivoting at the knee with the foot planted. The medial tibial spine (intercondylar tubercle) is the insertion of the ACL; avulsion fractures can occur in young patients and have a similar mechanism as ACL tears. Acute hemarthrosis is common in these injuries and may limit extension of the knee, but the patella remains seated in the trochlea. (Choice B) Medial collateral ligament tears are caused by severe valgus stress (eg, blow to the lateral knee) or twisting injury. Ligamentous laxity may be masked in the acute phase by swelling and muscle spasm, but range of motion is preserved, and the patella would not be displaced. (Choice C) Meniscal tears often occur from pivoting on a flexed knee while the foot is planted. Examination typically shows joint line tenderness, a small joint effusion, and crepitus, locking, or catching with range of motion. (Choice E) Patellar tendonitis ("jumper's knee") is a chronic overuse injury characterized by anterior knee pain that worsens with activity. Examination may show tenderness at the inferior pole of the patella, but range of motion and patellar position are normal. Educational objective: Patellar dislocation usually occurs after quick, lateral movements on a flexed knee. Lateral dislocation is most common. Risk factors include age <20, joint laxity, lower extremity malalignment, and patellar subluxation. Examination shows reduced range of motion and lateral displacement of the patella out of the trochlea. References: Acute traumatic patellar dislocation. (http://www.ncbi.nlm.nih.gov/pubmed/25592052)

A 14-Year-Old Girl Is Brought to the Emergency Department for Evaluation