Maxillofacial Deformity and Change in the Histochemical Characteristics of the Masseter Muscle after Unilateral Sectioning of the Facial Nerve in Growing Rabbits.

Abstract

Maxillofacial growth and histochemical effects of long-term unilateral chewing caused by unilateral facial paralysis on the masseter muscle were studied in growing rabbits. The sample for this study consisted of a total of 20 young male Polish rabbits. The experimental group was comprised of 15 rabbits, which were subjected to dissection of the right facial nerve on the 21st day after birth, while the remaining 5 animals served as a control. The duration of the experimental period was 6 months. In the experimental group, the rabbits with facial abnormality on the paralyzed right side performed unilateral chewing only on the surgically unmanipulated side. Morphometric analysis showed facial asymmetry, with the maxillofacial region severely atrophied on the side that had undergone surgery. Both 4 weeks and 6 months after the operation, type 1 fibers of the masseter muscle on the denervated side, i.e., the non-chewing side, were significantly fewer in number and smaller than those on the unmanipulated side. However, type 2 fibers did not differ in size between sides. NADH-tetrazolium-reductase (NADH-TR) of the masseter was less reactive on the denervated side, reflecting a lower oxidative capacity. These results indicate that the alteration of the functional activities contributed to selective disuse atrophy of the type 1 fibers and a conversion from type 1 to type 2A fibers on the non-chewing side. In other words, the larger diameter and greater proportion of type 1 fibers of the masseter was indicative of work hypertrophy on the unmanipulated, chewing side. This study reveals that facial nerve dissection changes the maxillofacial growth and the histochemical features of the portion of the masseter muscle innervated by the trigeminal nerve, but not by the facial nerve.