Three-Dimensional Evaluation of Masseter Muscle in Different Vertical Facial Patterns: A Cross-Sectional Study in Growing Children

The aim of the present study was to analyze the anatomical three-dimensional (3D) characteristics of masseter muscle in growing subjects with different vertical patterns by using an ultrasound (US) method. The sample comprised 60 prepuberal subjects (33 males, 27 females) with a mean age of 11.5 +/- 1.6 years with late mixed or permanent dentition and Class I molar and skeletal relationship. For each subject, a lateral cephalogram was required, and according to the mandibular plane angle (Frankfort horizontal plane/mandibular plane angle [FMA]), the subjects were divided into three groups of different underlying vertical facial patterns: brachyfacial: FMA < 22 degrees, mesofacial: 22 degrees <= FMA <= 28 degrees, and dolichofacial: FMA > 28 degrees. For each subject, an US scan was carried out to analyze the width, the thickness, the cross-sectional area, and the volume of the masseter muscle. Mean differences in measurements between vertical facial subgroups were contrasted by means of analysis of variance (ANOVA) with Tukey's post hoc tests (p < 0.05). Measurements of the whole masseter in dolichofacial patients were significantly smaller when compared with brachyfacial and mesofacial individuals during relaxation and contraction. The volume of the masseter decreased significantly by 10% going from the brachyfacial group to the mesofacial group and from the mesofacial group to the dolichofacial group with no difference between the left and the right sides. A significant negative correlation was found between the US measurements and the divergency (FMA degrees). Ultrasound is a technique indicated in children for evaluating muscles of mastication in vivo. Growing patients with a dolichofacial vertical pattern present with a reduced dimension of the masseter when compared with brachyfacial and mesofacial subjects.