Achilles tendon injury. 3. Structure of rabbit soleus muscles after immobilization at different positions

This investigation was designed to study the short and long term effects of inactivity on the fine structure of skeletal muscle tissue. Ankles of rabbits were unilaterally immobilized for one of three weeks. The knees were semiflexed and the feet were either in plantar or in dorsal flexed positions. The soleus muscles were examined immediately after removal of the plaster or after six months of observation. The muscle fibres, when immobilized in the slackened state, showed serious degenerative changes and Type 1 fibres, i.e. fibres lightly stained for myofibrillar ATPase at pH 9.4, were often necrotic. The fibre type ratio was changed when examined immediately after removal of the casts so that the relative number of Type 2 fibres was increased. If stretched, essentially normal morphology was seen. The contralateral soleus muscles, which were excessively weight-bearing during the plaster period, were characterized by small, sometimes necrotic, Type 2 fibres and splitting of Type 1 fibres. Whatever the reason for the structural changes it seems correct to point out to the surgeon that too much emphasis on tendon aspects implies certain risks. Overeagerness to obtain a safe way of unloading the tendon during the necessary period of immobilization after suture may result in longstanding damage to the muscle structure and, as a sequel, decreased strength.