Biochemical and morphological studies on rat skeletal muscles following prolonged immobilization of the knee joint by external fixation and plaster cast: A comparative study

The present report describes biochemical and morphological changes induced in hind-limb muscles of rats by immobilization with either plaster of Paris (POP) or external fixation (EF). The study lasted 4-6 weeks. Both methods of immobilization caused some loss in body weight of the experimental animals. Wet weights of the quadriceps, gastrocnemius, plantaris, and soleus muscles were measured, and weight loss was found in all of these muscles throughout the experimental period. In almost all muscles, EF caused a greater loss in muscle weight than POP immobilization. The plantaris muscle lost the least of its wet weight with POP immobilization for 4 weeks (22%), while the highest weight loss was recorded for the quadriceps muscle immobilized by EF for 6 weeks (59%). These findings were accompanied by significant alterations in the activity of acid phosphatase and less significant alterations in the activity of creatine phosphokinase as markers of muscle enzyme activity. In both methods of immobilization, the level of acid phosphatase activity rose by 34%, whereas creatine phosphokinase activity decreased by 9% in animals with EF and by 7% in those with POP immobilization. Morphological examination revealed loss of the characteristic structural patterns of the involved muscles, as manifested by the development of local hyaline degeneration and fibrosis. These pathological changes were more severe in the EF group than in the POP group. These findings suggest that immobilization causes true damage to muscles of the involved limb. EF, being a more rigid and more efficient method of immobilization, was associated with a more severe degree of muscle damage and degeneration.