Effect of botulinum toxin injection into human masseter muscle on the mandibular angle area

Abstract

Injection of botulinum toxin type (BoNT) into masseter muscle is a noninvasive treatment for the masseteric hypertrophy, which is safer than the surgical treatment. Most studies regarding the injection of BoNT reported its effect on reducing the masseter muscle using ultrasonography and computed tomography. Also, it was reported that localized masticatory muscle atrophy after injection of BoNT alters craniofacial growth and development in animal studies, such as decreased ramus height, increased gonial angle. However, most of those studies were performed on animals, thus, the effects of BoNT injection into masseter muscle on the craniofacial bony structure has not been fully elucidated in humans.

The aim of this study was to evaluate soft and hard tissue changes on the mandibular angle area after injection of BoNT in patients with massetric hypertrophy using three-dimensional cone-beam computed tomography (3D CBCT).

Twenty volunteers were randomly divided into two groups. Group I (n=10) received a single BoNT injection into the bilateral masseter muscle, while group II (n=10) received double BoNT injections, the second being administered 4 months after the first. Each injection was comprised of 25 U of BoNT.

3D CBCT were taken before injection and after the first injection 6 months later in two groups.

Thicknesses and cross-sectional areas of the masseter muscle were measured on the 6 cutting images of masseter muscle parallel to the mandibular plane with interval of 5 mm distance, which were referred to as C5, C10, C15, C20, C25, and C30. For bony changes of mandibular angle area, three reference points were used (Go inf, Go post, distal surface of mandibular second molar). Inter-gonial width (inter Go inf Rt-Lt, inter Go post Rt-Lt) were measured. The bone volume of mandibular angle formulated by 3 reference points were measured.

The results of the present study were as follows,

1. The thicknesses and cross-sectional areas were significantly reduced in both groups (P<0.001), but the amount of reduction was significantly greater in group II.

2. The intergonial width of mandibular angle area was not significantly changed in groups I and II (P>0.05).

3. The bone volume of mandibular gonial angle area was significantly reduced only in group II (P<0.001).

BoNT injection reduced thicknesses and cross sectional areas of masseter muscle in both groups, but the amount of reduction were significantly greater in group II than group I. (P<0.05). The bone volume of mandibular angle area reduced only in group II. These finding demonstrate that the administration of double BoNT injections might induce change in bone volume at the mandibular angle area.