The aim of this study was to investigate the relationship between masseter muscle size and craniofacial morphology, focusing on the maxilla. Twenty-four patients (11 males and 13 females; mean age 27.6 ± 5.6 years) underwent cephalometric analyses. Ultrasonography was used to measure the cross-sectional area (CSA) of the masseter muscle and bite force was measured using pressure sensitive film. The results showed that CSA-relaxed was positively correlated with upper anterior face height (UAFH)/total anterior face height (TAFH) and negatively with lower anterior face height (LAFH)/TAFH and LAFH (P < 0.05). CSA-clenched was correlated positively with SN-palatal, FH-palatal, UAFH/TAFH, and lower posterior face height (LPFH)/total posterior face height (TPFH) and negatively with LAFH/TAFH, LAFH, upper posterior face height (UPFH)/TPFH, and UPFH (P < 0.05). Bite force was positively correlated with LPFH/TPFH and negatively with UPFH/TPFH (P < 0.05). As the masseter became larger, the anterior maxillary region tended to shift downwards relative to the cranial base, whereas the posterior region tended to shift upwards. The decrease in LAFH/TAFH and increase in LPFH/TPFH as the size of the masseter muscle increases may be influenced not only by the inclination of the mandibular plane but also by the clockwise rotation of the maxilla.

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This study used an experimental dental arch model to examine the orthodontic forces generated by a quadhelix appliance in terms of parallel expansion, fan expansion, or a combination of the two. Strain gauges were attached to experimental brass rods that represented the teeth arranged in the shape of an average dental arch to detect forces in the buccal, lingual, mesial, and distal directions. Orthodontic forces generated by different types of activation were compared by Scheffe's multiple test. The largest orthodontic force generated during parallel expansion was observed at the first molar in the buccal direction. When fan expansion was applied, significant orthodontic force was observed at the canine in the mesial and labial directions, whereas force in the mesial and lingual directions was noted at the first molar. When a combination of 3 mm parallel and 5 mm fan expansion was used, the forces generated at the canine and first and second premolar, and first molar were nearly equivalent. Depending on the type of malocclusion, the most appropriate expansion technique may be parallel or fan expansion or a combination of the two. When expanding the entire dental arch simultaneously, a combination of 3 mm parallel and 5 mm fan expansion may be the most suitable.