Histologic changes in rat masticatory muscles subsequent to experimental increase of the occlusal vertical dimension

Histologic changes in various masticatory muscles were studied in Wistar rats from 12 hours to 84 days after an experimental increase of the occlusal vertical dimension. The increases applied (1 to 2 mm) were based on the amount of interocclusal space (1 mm) in the rat. A sequence of tissue reaction from early acute inflammatory destruction of myofibers to healing with myofiber regeneration was most pronounced in the deep masseter muscle of the 2 mm group (12 hours to 21 days). A similar sequence was also observed to a lesser degree in the superficial masseter and anterior temporal muscles, while no changes were detectable in the posterior temporal, medial pterygoid, and anterior digastric muscles of this group. Variable severity of tissue response in different masticatory muscles might be attributable to the amount of stretching of myofibers as a result of an increased occlusal vertical dimension. On the other hand, degenerative atrophy of myofibers in the lateral pterygoid muscle occurred within 2 days with no sign of inflammation, and these condensed myofibers persisted as late as 84 days. Based on these findings, it was suggested that displacement of the mandibular condyle would exert an important influence on the prognosis after an increase in the occlusal vertical dimension. Only a transient acute inflammation occurred in the deep and superficial masseter muscles in the 1 mm group. Except for a small number of condensed myofibers in the lateral pterygoid muscle, degenerative changes of myofibers were not revealed. Therefore, increasing the occlusal vertical dimension within the extent of the interocclusal space might be regarded as appropriate for prosthodontic practices.