Changes following denervation to the masseter muscle

BACKGROUND: Masseter muscle nerve is often injured in mandible osteotomy. What changes in food intake and masseter muscle will be brought after masseter muscle nerve injury? OBJECTIVE: This study was designed to selectively establish animal models of denervated masseter muscle and investigate the effects of severing masseter muscular nerve on masseter muscle and animal’s food intake. DESIGN, TIME AND SETTING: A randomized controlled animal experiment was performed at the Laboratory Animal Center, Nanfang Hospital, Southern Medical University from September to November 2005. MATERIALS: A total of 50 healthy, adult, SPF-grade, New Zealand rabbits, of both genders, were used to develop an animal model of selectively denervated masseter muscle. METHODS: Five rabbits were randomly selected as normal controls. According to various mutilation methods, the remaining animals were randomly divided into 3 experimental groups, with 15 rabbits in each group: masseter muscular neural stem denervated, masseter muscular neural superior branch-denervated, and masseter muscular neural inferior branch-denervated groups. Self-control comparison was performed on each animal. The right masseter muscle served as the experimental side, and the left masseter muscle served as the control side. In each group, 3 time points (2, 8, and 24 weeks post-surgery) were allotted for observation. MAIN OUTCOME MEASURES: At the pre-set time points, masseter muscular thickness was measured with a Logic 500 color Doppler ultrasonic diagnostic apparatus. Masseter muscle tissue was resected for hematoxylin eosin staining. Masseter muscular fiber diameter and area were measured with an optical microscope. Masseter muscle tissue was sectioned and nicotinamide adenine dinucleotide tetrazolium oxidoreductase (NADH-TR) and adenosine triphosphatase staining were performed. Following staining, the sections were quantitatively analyzed using an IBAS200 image analyzer. RESULTS: Post-surgery food intake: No abnormal food intake was found after surgery among the three groups. Intragroup comparison: In each experimental group, following denervation, masseter muscular thickness was decreased (P < 0.05) and masseter muscular fiber cross-section area was reduced in the experimental side compared to the control side (P < 0.05). Inter-group comparison: There was a significant difference in masseter muscular thickness and masseter fiber cross-section area among the three experimental groups (P < 0.05). There was no significant difference in masseter fiber type between the control side of the three experimental groups and the normal control group (P > 0.05). CONCLUSION: Following various methods of masseter muscle denervation, masseter muscle thickness and masseter fiber cross-section area was reduced by varying degrees. Denervated masseter muscle did not influence food intake of experimental animals or alter masseter muscle fiber ratio.