Mechanical effects of immobilization on the Achilles' tendon.

Abstract

To measure the biomechanical effects of immobilization on the Achilles' tendon. Experimental, controlled study. Physiatry research laboratory. Twenty adult rabbits. One hindlimb immobilized in a cast for 4 weeks (n=10) or 8 weeks (n=10). The contralateral legs (n=20) served as controls. Cross-sectional Achilles' tendon area, mode of failure, mean failure load, and tendon stiffness. 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). 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.