Achilles tendon injury. Tendon elongation and soleus muscle fine structure in rabbit after different therapies.

Unilateral tenotomy of the Achilles tendon was carried out in 60 rabbits. The limb was then either mobilized directly or immobilized for 10 to 35 days using a plaster usually after tendon suture. In certain cases the plaster was removed early (on the 7th or 16th day) and the animals were than allowed to use this leg. Separation between tendon ends was apparent from steel markers, placed close to each cut end of the tendon and examined by X-ray. The separation curve was biphasic and both the first and the inactive phase reflected the degree of tension over the tendon suture. However, during the second separation phase, which began between the 17th and 21st day, the separation gradually reached the same level in all groups. Enzyme histochemistry and electron microscopy revealed severe degenerative changes in immobilized and in shortened muscles. Furthermore, a gradual shift in fibre type characteristics from type 1 slow-twitch fibres towards type 2 fast-twitch fibres occurred. Rapid recovery followed removal of the plaster. The findings indicated that both degenerative and regenerative processes and adaptive processes had been initiated in all experimental muscles when the tendon continuity was broken. The adaptive processes progressed gradually during the five-week post-operative period and might have been responsible for the second phase of the tendon end separation. The fibre adaptation, i.e. the transformation, may be accounted for by changes in structure of the myofibrils and composition of the myosin molecules.