Flexion contracture in knee OA
Two components, and they behave differently.
Structural Posterior femoral and tibial osteophytes tenting the capsule; posterior capsular fibrosis; posterior condylar bone loss with reduced condylar offset; meniscal extrusion or root insufficiency; adaptive hamstring/gastrocnemius shortening. Often a compensatory hip flexion contracture or equinus above and below.
Pain-mediated Capsular volume is maximal and intra-articular pressure minimal near 30°. Add synovitis with reflex hamstring spasm and arthrogenic quadriceps inhibition — an extensor lag that consolidates into fixed deformity.
Whatever persists under anaesthesia is capsule and bone. Preoperative contracture >15° is the strongest predictor of residual contracture at one year. Correct it on the table, not in physiotherapy.
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