Anterior cruciate ligament tear — CCFP MCQ

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ModerateMusculoskeletal and sportsAnterior cruciate ligament tearCCFP

A 22-year-old soccer player twists her knee. She heard a pop, developed swelling within 2 hours, and now has giving-way. Lachman test is positive. X-ray shows no fracture. What is the most likely diagnosis?

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Correct answer: E — Anterior cruciate ligament tear

Explanation lettering: E = shown as A · D = shown as B · B = shown as D · A = shown as E

This patient has an anterior cruciate ligament (ACL) tear. The clinical presentation is classic: a non-contact twisting mechanism with an audible pop, immediate swelling (hemarthrosis), and functional instability (giving-way). The positive Lachman test is the single most sensitive and specific physical examination finding, with 85% sensitivity and 94% specificity for ACL rupture. Together, these features form a highly discriminating diagnostic cluster (likelihood ratio > 38). The normal X-ray appropriately excludes fracture but does not exclude ligamentous injury. Distractors represent overuse or inflammatory conditions (B, C, E) or localized effusion (D) without acute traumatic mechanism or instability; none exhibit the combination of pop, rapid swelling, and positive Lachman test.

Reference: Benjaminse A, Gokeler A, van der Schans CP. Clinical diagnosis of an anterior cruciate ligament rupture: a meta-analysis. J Orthop Sports Phys Ther. 2006;36(5):267-88. https://pubmed.ncbi.nlm.nih.gov/16715828/