Mechanical effects of immobilization on the Achilles' tendon

Objective: To measure the biomechanical effects of immobilization on the Achilles' tendon. Design: Experimental, controlled study. Setting: Physiatry research laboratory. Animals: Twenty adult rabbits. Intervention: One hindlimb immobilized in a cast for 4 weeks (n=10) or 8 weeks (n=10). The contralateral legs (n=20) served as controls. Main Outcome Measures: Cross-sectional Achilles' tendon area, mode of failure, mean failure load, and tendon stiffness. Results: The Achilles' tendon cross-sectional area did not change. Achilles' tendon- calcaneus units failed at insertion at 4 weeks and in controls. Calcaneus fracture was the most prevalent mode of failure at 8 weeks. The mean load to failure of both immobilized groups (4wk: 187.5+/-45.7N; 8wk: 162.6+/-39.3N) was significantly smaller than that of the control group (549.2+/-93.7N, both P<.005). The mean tendon stiffness of both immobilized groups (4wk: 64.6+/-24.8N/mm; 8wk: 53.9+/-19.9N/mm) was significantly lower than that of the control group (125.1+/-26.5N/mm, both P<.005). Conclusions: Immobility for up to 8 weeks caused decreased stiffness but no atrophy or rupture of the Achilles' tendon, suggesting that immobility does not constitute a risk factor for Achilles' tendon midsubstance rupture. Clinically, graded reloading is required after immobilization to restore the tendon insertion and to reverse calcaneal disuse osteoporosis.