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Mechanical effects of immobilization on the Achilles' tendon
Fujio Matsumoto1, Guy Trudel, Hans K Uhthoff
1Bone and Joint Laboratory, University of Ottawa, Ottawa, ON, Canada.
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.