Orthodontic Consideration of Class III Skeletal Deformity
Ellen Wen-Ching Ko, DDS, MS
Department of Craniofacial Orthodontics, Chang Gung Memorial Hospital
Graduate Institute of Craniofacial and Oral Science, Chang Gung University
Craniofacial Research Center, Chang Gung Memorial Hospital
Skeletal Class III malocclusion is the most common dentofacial deformities in Taiwan. The decision of the treatment to correct Class III malocclusion whether by orthodontic camouflage therapy or orthognathic surgery (OGS) depends on:1) facial esthetics; 2) 3-dimensional severity of jawbone skeletal imbalance; 3) combined with facial asymmetry; 4) the extent of molar relation from the Class I relation; and 5) compensated incisal inclination. The surgical-orthodontic plan should integrate the consideration of skeletal goal and ideal dental alignment and occlusion.
General skeletal morphologic features of skeletal Class III deformities are maxillary hypoplasia and mandibular prognathism, more than 60% with mandible deviation and asymmetry. Therefore bimaxillary surgical correction is preferred for a better total face correction. The clockwise rotation of maxilla-mandibular complex (MMC) by maxillary posterior impaction is a common strategy for skeletal Class III correction; it solve the problems to 1) forward augmentation of midface, 2) more backward rotation of mandible, 3) straighten the maxillary incisor inclination, 4) improve the smiling arc, 5) better control of lower facial height. Thus, the amount of mandible setback is mainly governed by the face esthetics rather than dental overjet correction.
The combination of segmental osteotomies provides efficiency of the surgical orthodontic treatment. The anterior maxillary segmental osteotomy could upright upper front teeth for a better dental decompensation, flatten the compensating curve in cases of skeletal anterior open bite. The 3-pieces maxillary osteotomy adjusts the maxillary transverse width by expansion or constriction. The anterior mandibular segmental osteotomy facilitate flattening of accentuated lower curve of Spee by lower front teeth intrusion and proclination; it ensure the post-surgical stability of mandible, reduce the amount of visible lower incisor show, and provide a better chin contour.
Computer aided surgical simulation foresee of the surgical outcome. In cases with surgery-first approach, the orthodontist‘s set-up treatable occlusion could be decided prior to skeletal simulation movement. The steps of MMC simulation follow the sequence of midline, occlusal cant, occlusal pitch angulation and yaw rotation.