True Mechanical Locking Knee — SCE Rheumatology MCQ

Correct answer: C — Intra-articular interposition of a loose body or displaced meniscal fragment

The correct answer is C. Sudden loss of extension with an abrupt firm endpoint that persists during passive movement and after adequate analgesia defines true mechanical locking. This is most characteristically caused by interposition of a displaced meniscal fragment, particularly a bucket-handle tear, or an intra-articular loose body. Pain inhibition and reflex muscle spasm cause pseudo-locking and should improve with analgesia or relaxation; they do not produce a persistent firm passive endpoint. Ligament laxity causes instability or giving way rather than locking. Osteoarthritis can produce a gradual fixed flexion deformity, but this does not explain previously normal extension followed by an abrupt block. NICE advises against arthroscopic lavage or debridement for osteoarthritis itself, while recognising that separate mechanical pathology such as a meniscal tear or loose body may require specific assessment.

Reference: NICE. Osteoarthritis in over 16s: diagnosis and management (NG226), Evidence review N: Clinical and cost effectiveness of arthroscopic procedures, October 2022. https://www.nice.org.uk/guidance/ng226/evidence/n-clinical-and-cost-effectiveness-of-arthroscopic-procedures-for-the-management-of-osteoarthritis-pdf-405016302427