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 mug 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.