A 28-year-old professional basketball player presents to the clinic with persistent left knee pain and a sensation of the knee giving way that began two days ago. He reports that during a game, he planted his foot and twisted his torso to catch a pass, feeling a sharp pain along the inner aspect of his knee. He was able to finish the game but noted gradual swelling over the following 24 hours. On physical examination, there is a mild joint effusion and exquisite tenderness along the medial joint line. Passive flexion of the knee combined with external rotation of the foot followed by extension of the leg elicits a palpable and audible "thud" accompanied by sharp pain.
Which of the following is the most likely diagnosis?
The correct answer is:
C) Medial meniscus tear
The patient’s presentation is highly characteristic of a medial meniscus tear. The classic mechanism involves a twisting injury while the foot is planted. Unlike an ACL tear, which often presents with immediate, massive swelling due to hemarthrosis) meniscal tears typically result in a more gradual, inflammatory effusion that develops over several hours to a day. The physical exam maneuver described in the vignette is the McMurray test, which is specifically designed to trap the torn meniscal fragment between the femoral condyle and the tibial plateau, reproducing the patient's mechanical symptoms. Joint line tenderness is another highly sensitive finding for this pathology.
Answer choice A: Anterior cruciate ligament (ACL) tear, is incorrect. While both involve twisting injuries, an ACL tear typically causes an immediate "pop" and rapid, large-scale swelling. Furthermore, the McMurray test would not typically be positive in an isolated ACL injury. Instead, one would expect a positive Lachman or anterior drawer test.
Answer choice B: Medial collateral ligament (MCL) tear, is incorrect. MCL injuries are usually caused by a valgus stress (force applied to the lateral side of the knee). While they cause medial pain, the tenderness is usually located over the femoral or tibial attachments of the ligament rather than specifically along the joint line. Additionally, a valgus stress test would demonstrate laxity, which is not described here.
Answer choice D: Patellar subluxation, is incorrect. Patellar subluxation typically involves the patella shifting laterally out of the trochlear groove. It presents with anterior knee pain and a positive apprehension sign when the patella is pushed laterally, rather than joint line tenderness or mechanical clicking with rotation.
Answer choice E: Pes anserine bursitis, is incorrect. Pes anserine bursitis causes pain on the medial aspect of the knee, but the tenderness is located about 2-3 cm below the joint line at the insertion of the sartorius, gracilis, and semitendinosus tendons. It is an inflammatory condition that does not cause mechanical symptoms like locking or clicking.
Key Learning Point
Meniscal tears (medial more common than lateral) present with a history of twisting injury, delayed swelling, and mechanical symptoms such as locking, catching, or a sensation of the knee giving way. On physical examination, joint line tenderness and a positive McMurray test are the most reliable findings. While minor tears can be managed conservatively with physical therapy and NSAIDs, tears causing persistent mechanical symptoms or those in the "red-white" (vascular-avascular) zone may require arthroscopic repair or meniscectomy.
