Orthognathic surgery: a hierarchy of stability.

The stability and predictability of orthognathic surgical procedures varies by the direction of surgical movement, the type of fixation, and the surgical technique employed, largely in that order of importance. The most stable orthognathic procedure is superior repositioning of the maxilla, closely followed by mandibular advancement in patients in whom anterior facial height is maintained or increased. (If facial height is decreased by upward rotation of the chin, stability is compromised). The combination of moving the maxilla upward and the mandible forward is significantly more stable when rigid internal fixation is used in the mandible. Forward movement of the maxilla is reasonably stable, with or without rigid internal fixation, but mandibular setback often is not stable, and downward movement of the maxilla that creates downward rotation of the mandible is unstable. For mandibular setback, the inclination of the ramus at surgery appears to be an important influence on stability. It has been suggested that both interpositional synthetic hydroxyapatite grafting and simultaneous ramus osteotomy improve the stability of downward movement of the maxilla, but this has not been well documented. In two-jaw Class III surgery, the stability of each jaw appears to be quite similar to that of isolated maxillary advancement or mandibular setback. The least stable orthognathic procedure is transverse expansion of the maxilla. Although surgically assisted rapid palatal expansion has been suggested as a more stable alternative to segmental Le Fort I osteotomy, the patterns of movement resulting from the two procedures are different, and differences in stability have not been established.