The effects of increasing occlusal vertical dimension on the deep masseter of rat at different ages

Abstract Objective To investigate the influence of increasing the occlusal vertical dimension (iOVD) on the fibre-type distribution and ultrastructure of deep masseter of rat at different ages. Design A total of forty-eight male Wistar rats were divided into two groups according to age: ‘teenage’ group (n = 24, 1.5 months) and ‘young adult’ group (n = 24, 8 months). Both the teenage and the young adult rats were then randomly divided into the control group (n = 12) and the experimental group (n = 12). The occlusal vertical dimensions of the rats in the experimental groups were increased by placing composite resin on all maxillary molars. The fibre-type distribution and ultrastructure of the deep masseter were subsequently observed on day 7 and day 14 after iOVD. Results In the teenage experimental group, the proportion of type IIa fibres increased, while the proportion of type IIb and type IIx fibres decreased by day 7 after iOVD ( P P Conclusion The teenage rat alters masseter muscle structure to a slower phenotype earlier and to a greater degree than that of the young adult rat when increasing the occlusal vertical dimension.