Effects of Reduced Masticatory Function on Muscle Fibers of the Masseter Muscle

Abstract

Much animal research has indicated that reduced masticatory function induces muscle atrophy and morphological changes during the growth period. Changes in masseter muscle fibers alter reducing masseter activity, however, remain unknown. The purpose of this study was to clarify changes in masseter muscle fibers at the histological level after reducing masticatory function as a result of an injection of botulinum toxin type A (BTXA). Thirty 30-day-old rats were randomly divided into 2 groups. Group I received an injection of 25 U/ml BTXA in the bilateral masseter muscles. In group Ⅱ (control), the bilateral masseter muscles received a 0.9% normal saline injection. After 45 days, the rats were perfused with 4% formaldehyde and sacrificed. An immunohistochemical analysis of the superficial masseter muscles was performed. Data were analyzed with Student's t-test. The fiber size in group Ⅰ was smaller than that in group Ⅱ, and there was a smaller percentage of type Ⅱ a fibers in the study group. Within the limits of this study, we determined that a reduction of muscle fiber size and a transition of muscle fiber subtypes from type Ⅱa to Ⅱx or Ⅱb fibers may occur because of reduced masticatory function.