Quantitative and Temporal Differential Recovery of Articular and Muscular Limitations of 4 Knee

25 Joint contractures alter the mechanical properties of articular and muscular structures. 26 Reversibility of a contracture depends on the restoration of the elasticity of both structures. We 27 determined the differential contribution of articular and muscular structures to knee flexion 28 contractures during spontaneous recovery. 250 rats divided in 24 groups had 1 knee joint 29 surgically fixed in flexion for 6 different durations, from 1 to 32 weeks, creating joint 30 contractures of various severities. After the fixation was removed, the animals were left to 31 spontaneously recover for 1 to 48 weeks. After the recovery periods, animals were killed and we 32 measured the knee extension before and after division of the transarticular posterior muscles 33 using a motorized arthrometer. No articular limitation had developed in contracture of recent 34 onset (≤2 weeks of fixation; P >.05); muscular limitations were responsible for the majority of 35 the contracture (34±8° and 38±6°, respectively; both P <.05). Recovery for 1 and 8 weeks 36 reversed the muscular limitation of contractures of recent onset (1 and 2 weeks of fixation; 37 respectively). Long-lasting contractures (≥4 weeks of fixation) presented articular limitations, 38 irreversible in all 12 durations of recovery compared to controls (all 12 P <.05). Knee flexion 39 contractures of recent onset were primarily due to muscular structures, and they were reversible 40 during spontaneous recovery. Long-lasting contractures were primarily due to articular structures 41 and were irreversible. Comprehensive temporal and quantitative data on the differential 42 reversibility of mechanically significant alterations in articular and muscular structures represent 43 novel evidence on which to base clinical practice. 44