Decreases of occlusal vertical dimension induce changes in masticatory muscle fiber composition

ObjectivesTo evaluate the effects of artificially decreased occlusal vertical dimension on the fiber phenotypes of the masseter muscle in the rat.Materials and methodsThe experimental animals were divided into two groups: A decreased occlusal vertical dimension (DOVD) group and a sham-operated control group. Five rats from each group were sacrificed on days 0, 3, 7, and 14 after the operation. The extent of DOVD, the masseter muscle fiber composition, and fiber diameter were analyzed.ResultsThe average decrease in the vertical dimension was 1.1 ± 0.5 mm. The masseter muscle fiber composition of Type IIA and Type IIB muscle fibers shifted significantly in favor of Type IIA fibers on day 3 in the DOVD group. Toward the end of the observation period, shifting occurred back to the original composition. A significant decrease in the diameter of muscle fibers was seen on day 3 in the DOVD group.ConclusionIn this experiment, when the occlusal vertical dimension was artificially reduced, the masseter muscle atrophied and the muscle fiber type changed temporarily while adapting to the new position of the mandible.