Comparison Of Craniofacial Structures And Biomechanic Analysis In Adult High-angle And Low-angle Facial Skeletal Types

Orthodofltists have been interested in the differences of the diagnosis, treatment,and treatttient response between high-angle and low-angle facial skeletaI types fOr along time.A computer search of the literature reveals no published edicles on comparingthe craniofacial differences of adult high-angle and lowangle facial skeletal types,specifically on the the mongolian race.The purpose of this study was to compare the differences of the craniofacialcharacteristics and the suPerficial masseter muscle morphology between the adulthigh-angle and low-angle facia1 skeletal types, and fringe analysis the differencesofbiomechanic between two grouPs in sagittal p1ane.The subjects consisted of 37 yellow race adults (l7 high-angle, 20 low-angle).The investigation included three parts.The first pwt of this study was the cephalometric analysis of the adulthigh-angle and low-angle facial skeletal tyPes. Comparing the considerabledifferences in skeletal fOrm between tWo groups, the results indicated that;i. The craniofacia1 morphology of the adult high-angle and low-angle facialskeletal types showed significant differences, especially in lower facia1 form.ii. The thickness of the mandibular symphysis and the alveolar process inhigh-angle grouP were thicker than that in low-angle group.iii. The adaPtability of the dentoalveolar processes and compensatory toothmovements can compensate for the hyperdivergency of the jaw-baserelationship.The second part of this study was to use ultrasound scanning fOr measuringmasseter muscle perimeter, area, width, average thickness, maximal thicAness, andmasseter muscle length. They were measured under relaxed, maximal clenching andmaximal protrusive mandibular position. The result as follows:i. UltrasonograPhy produced a well-defined depiction of the masseter muscle- 5 -with distinct tendinous structures. During the different mandibular positions, the masseter muscle size of adult with high-angle facial skeletal type were always smaller than these of the high-angle group.ii. Except for the changes of width from the relaxed to maximal clenching position and the relaxed to maximal protrusive position, comparing with the changes of masseter muscle sizes between the different mandibular positions, there were no significant differences between two groups.iii. It was also showed that stature and weight influenced the ultrasound image of the masseter muscle.At present, little effort was made to measure the insertion site of the masseter muscle by the ultrasonography in the world. In the third part of this study, the anterior border of the insertion site of the superficial masseter muscle was located by the 9MHz ultrasound scanning probe, and compared the muscle orientation and biomechanics differences between two groups in sagittal plane. It was concluded that:i. The high ultrasonic frequency of the probe can detect the anterior border of the superficial masseter muscle precisely.ii. The angles of the sella-nasion plane, Frankfort plane, and occlusal plane along to the masseter muscle border showed there were no significant differences between two groups. As compared with the angle of the mandibular plane to masseter border, according to the statistic analysis, there was a significant difference between two groups.iii. In low-angle group, there was more masseter muscle force distributed on the axis of the upper and lower first molars.iv. The biomechanics analysis showed that there was little relationship between the occlusal force and the masseter orientation, the muscle strength and the length of the moment arm may be the primary reasons that the adults with high-angle and low-angle facial skeletal types have different occlusal force.v. As the different masseter orientation, the mandible of individual with low-angle facial skeletal type has more counter-clock rotational tendency.