Degenerative changes in masseter and temporalis muscles in limited mouth opening and TMJ ankylosis

The masseter and temporalis muscles were studied ultrastructurally in eight cases suffering from restricted mouth opening and TMJ ankylosis. The changes varied from slight degeneration of muscle fibers, such as accumulation of lipid or lipofuscin deposits, Z-band streaming and derangement of myofilaments, in one case to moderate or severe changes of muscle atrophy and necrosis in the remaining seven cases. Nemaline bodies were also observed in one of these seven cases. In addition to these muscle changes neural degeneration especially of myelinated nerves were found in three of the cases studied. The present findings showed that in spite of the diversity of the underlying factors affecting the joint, there was many similarities in the changes observed within the muscle. It is postulated that these degenerative muscle changes probably play an important role in restricted mouth opening and TMJ ankylosis, and that some of these muscle pathology may be the result of neural involvement.