A 24-year-old female soccer player is brought to the urgent care clinic after sustaining a right knee injury during a match earlier this afternoon. She reports that while she was attempting to pivot to avoid an opponent, she felt a sudden pop in her knee, followed by immediate pain and rapid swelling. She was unable to continue playing and had difficulty bearing weight on the right leg. On physical examination, there is a large joint effusion. With the patient’s knee flexed to 30°, the clinician stabilizes the distal femur with one hand and pulls the proximal tibia forward with the other, noting significant anterior translation of the tibia compared to the left side. There is no laxity with valgus or varus stress. Which of the following is the most likely diagnosis?
The correct answer is:
A) Anterior cruciate ligament tear
The patient’s presentation is classic for an acute anterior cruciate ligament (ACL) tear. The mechanism of injury involving a non-contact pivoting or deceleration maneuver is the most common cause. The "pop" heard or felt by the patient, followed by the rapid onset of a large effusion indicating a hemarthrosis, is highly suggestive of an ACL injury. On physical examination, the Lachman test (anterior translation of the tibia at 30° of flexion) is the most sensitive maneuver for detecting an ACL tear, surpassing the anterior drawer test.
Answer choice B: Medial collateral ligament (MCL) tear, is incorrect. An MCL injury typically occurs due to a valgus stress, such as a blow to the lateral aspect of the knee. While it causes localized pain and swelling, it does not typically cause a large intra-articular effusion and would be diagnosed by laxity during a valgus stress test at 30° of flexion.
Answer choice C: Medial meniscus tear, is incorrect. While often associated with ACL tears as part of the "unhappy triad", an isolated meniscus tear usually presents with a more delayed swelling (over 24 hours), joint line tenderness, and mechanical symptoms such as locking or catching. The McMurray test would be expected to be positive.
Answer choice D: Patellar tendon rupture, is incorrect. This injury is characterized by the inability to actively extend the knee or perform a straight leg raise. Physical exam would reveal a high-riding patella and a palpable defect below the patella, rather than the ligamentous laxity seen in this patient.
Answer choice E: Posterior cruciate ligament (PCL) tear, is incorrect. PCL injuries typically occur from a dashboard injury involving direct anterior force on the tibia or extreme hyperextension. The physical exam would show a positive posterior drawer sign or a sag sign, where the tibia sits posteriorly relative to the femur.
Key Learning Point
An ACL tear is characterized by a pop during a pivoting injury followed by rapid hemarthrosis. The Lachman test is the most sensitive physical exam finding. Diagnosis is confirmed via MRI. In active individuals, management often involves surgical reconstruction using a tendon graft (e.g., patellar tendon or hamstring) to restore knee stability and prevent future meniscal damage.
