Early Achilles tendon healing in sheep

Introduction The biomechanics of early tendon healing is important for designing post-injury training, but this has not been described in an animal model, similar to humans in size. We measured elastic and viscoelastic properties of a tendon regenerate in sheep, in a study designed to see the effects of exogenously applying the growth and differentiation factor CDMP-2. This is the first description of early tendon healing in sheep Achilles tendons. Materials and methods Twenty female sheep underwent Achilles tendon transection without suturing or immobilization. Two hours after the operation, 100 mu g of CDMP-2 or placebo was injected into the hematoma. The sheep were slaughtered after 3 weeks, and tendon regenerates tested for viscoelastic properties by cyclical loading, before a destructive tensile test. Thereafter, all specimens were examined by high resolution computerized tomography (CT), and histology. Results The tendon regenerate formed a sleeve, around the tendon stumps. Failure occurred between the regenerate sleeve and the tendon stumps. There was an unexpectedly large variation in force at failure. In the CDMP-2 group, force correlated with regenerate transverse area, but not in the controls. Thus, the variation in maximum stress was smaller in the CDMP-2 group (P = 0.009). Although the force at failure was only a tenth of normal, the capacity to store elastic energy was already near normal (hysteresis 16%). The mean transverse area, force at failure and stiffness were all about 30% larger in the CDMP-2 group, but this was not significant. There were no signs of bone or cartilage formation on CT or histology. Conclusions Results are compatible with a positive effect of CDMP-2, but the power was too low to demonstrate any such effect. Considering that spontaneous ruptures in humans are likely to have a more variable geometry than in this model, humans can also be expected to vary a lot in early mechanical characteristics. This emphasizes the importance of individualized rehabilitation programs. The low hysteresis suggests that the energy storing capacity is rather easy for the tissues to develop; possibly it is harder to create appropriate energy dissipation, in order to avoid re-rupture.