Principles of orthodontic mechanics in orthognathic surgery cases.

In orthognathic surgery cases the treatment objectives, extraction patterns, and types of mechanics used are frequently the reverse of those used in conventional orthodontics. Thus, starting cases orthodontically and then, if unsuccessful, referring them for surgery often produces compromised results. Presurgical intra-arch objectives include positioning of the incisors in "ideal" positions, establishment of correct torque, and elimination of tooth-size discrepancies so as to permit the establishment of Class I canine and molar relationships after surgery. Presurgical objectives in the sagittal plane focus on removal of dental compensations. This may require the use of Class III elastics in Class II cases (and vice versa), thus allowing for maximal surgical correction of the underlying skeletal deformity. In the transverse plane, differentiation of skeletal from dental problems as well as identification of relative and absolute discrepancies should be carried out presurgically. Lateral corticotomies or segmental maxillary procedures should be used, depending on individual circumstances. Presurgical objectives in the vertical plane include maximizing the amount of presurgical orthodontic treatment carried out in open bites and minimizing the presurgical mechanics in deep bites. Encouragement of opening mandibular rotation at surgery while avoiding an increase in posterior face height contributes to stability. Orthodontic mechanics should not always include presurgical leveling of the curve of Spee and should actively avoid movements that may cause relapse tendencies. Careful attention to the use of surgical arch wires and splints during surgery and fixation, along with controlled elastic therapy and exercise programs after fixation, can greatly facilitate treatment.