1 INTRODUCTION

Orthognathic surgery has been used to treat obstructive sleep apnea (OSA) since the mid-1980s.¹ In the classic phase II surgery for OSA described by Riley and Powell, 10 mm advancement of the maxilla and mandible resulted in an impressive 97% cure rate in patients who had failed phase Isurgery and 91% in patients treated solely by phase IIsurgery.² Other studies on bimaxillary advancement techniques have also shown success rates ranging from 75% to 100%, which are superior to other surgical treatments for OSAS.^3–5 Maxillomandibular advancement has now been accepted as an effective modality in the treatment of OSAS.

Many studies that have evaluated the perceived facial changes associated with such advancement surgeries reported favorable perceived aesthetic result from the patients’ viewpoint^6–9; however, all of these studies were done on Caucasian cohorts. There is currently no literature on the aesthetic outcome of maxillomandibular advancement of such a large extent for the treatment of OSA in Asians. The aesthetic outcome after maxillomandibular advancement is different between Caucasians and Far East Asians as there is a greater incidence of bimaxillary protrusion in the Far East Asian face. These and other ethnic differences in the cephalometric parameters between the Far East Asian and the Caucasian have been previously shown.^12,13 Though the soft tissue measurements such as the distance from the mandibular plane to the hyoid bone (MP-H), posterior airway space (PAS), and the distance from the posterior nasal spine to the tip of the soft palate (PNS-P) did not vary significantly, the angle measurement from the sella to nasion to point A, subspinale (SNA), and the angle from sella to nasion to point B, supramentale (SNB), were significantly different between the racial groups.

From a purely aesthetic point of view, profiles that are flat or slightly bimaxillary protrusive are considered more attractive in the Western sense of aesthetics than those which have extreme bimaxillary protrusion. ¹⁶ On the contrary, in a comparative study of the perception of Chinese facial profile aesthetics by native Chinese dental professionals, students and lay people, facial profiles that were normal or had bimaxillary retrusion were perceived to be more attractive than profiles that had bimaxillary protrusion, protrusive mandible, retrusive mandible, retrusive maxilla or protrusive maxilla.¹⁷ Therefore considering the relatively greater maxillary and mandibular protrusion in the Asian population and the biased perception by the native Chinese favoring bimaxillary retrusion, the aesthetic effects of increased bimaxillary protrusion after maxillomandibular advancement would be undesirable in the Far East Asian face.

Patients with bimaxillary protrusion may have lip seal problems due to the protrusion as evidenced by upward strain of the mentalis and increased activity of perioral muscles; this may be unfavorably increased after maxillomandibular advancement. The increased tension on advanced maxillary and mandibular segments in a patient with bimaxillary protrusion may also affect the stability of the advancement, which may increase regression and failure rates postop-eratively. The long-term stability of the advanced segments due to decreased tension after an anterior segmental setback was also a factor in the consideration of the development of the modified technique for the Far East Asian face.

2 PATIENT SELECTION

Anterior segmental surgeries have been practiced in oral and maxillofacial surgery since the 1960s. It is generally indicated for cases of bimaxillary protrusion. This involves the posterior setback of the anterior maxilla (premaxilla) and the anterior alveolar segment of the mandible, usually after the extraction of a bicuspid from each quadrant of the dental arch. Together with the standard Le Fort I and bilateral sagittal split osteotomies, the surgery, when applied to OSAS patients, achieves the aim of increasing the posterior airway space without significant aesthetic facial alterations.

The indications for modified MMA to treat obstructive sleep apnea are the same as for standard maxillomandibular advancement. The modification is offered in patients with bimaxillary protrusion who do not want their facial profile and appearance altered postoperatively. These patients can be identified on physical examination by the presence of lip closure problems, excessive proclination, wide nasal alar base, excessive show of teeth or gums when smiling and protrusive lips on facial profile examination and cephalometric measurements.

Preoperative model surgery is undertaken to assess suitability. Several dental considerations exist with the maxillary segmental setback surgery. Presurgical orthodontic preparation of the teeth involves arch co-ordination and elimination of dental crowding. When combined with the Le Fort I maxillary advancement procedure, the setback is likely to result in some retroclination of the anterior segment with tipping and reduction of the maxillary to sella–nasion angle. Orthodontic tipping of the canine and premolar teeth away from the osteotomy site will allow for maximum bone removal during surgery and reduce this tendency.

Another dental consideration with the maxillary setback surgery is the superior/upward movement of the upper canine as the segment is moved back. This potentially creates an open bite at the canine/premolar positions and post-surgical orthodontics may be required to track the canine tip down. Care should be taken to reduce this tendency at the planning and surgical phase without compromising the vitality to the canine root tip. Reducing tipping and a conscious attempt to effect a bodily movement of the segment in the posterior direction will allow for maximum corresponding maxillary advancement without compromising facial aesthetics. Occasionally, a careful division of the anterior segment in the midline is useful to help swing the canines into position. It should be stressed that maintenance of the palatal pedicle is paramount, and a careful handling is essential to prevent aseptic necrosis of the small sub-segment.

Smoothing of the arch form at this surgery is also possible, especially if the patient has a pre-existing skeletal/transverse discrepancy. A midline split of the palate is sometimes helpful to co-ordinate the arch to reduce the amount of postoperative orthodontic movement necessary. This, coupled with the division of the anterior segmental component, creates a four-piece Le Fort I osteotomy. We find that transverse expansion beyond 4 mm to 5 mm is difficult to achieve without risking an oro-nasal perforation and should be avoided in treatment planning. Excessive surgical expansion of the arch usually results in a buccal tipping of the teeth, as opposed to the entire dentoalveolar complex moving laterally. A preformed arch bar should be fabricated, in addition to the occlusal wafer, to control the segments as much as possible. Fixation is otherwise standard with four bone plates for the entire maxilla.

Several considerations exist for the mandibular segmental surgery for the treatment of OSA. The primary concern is that the genial tubercles are attached to the lingual surface of the mandible and the segment should not be too large or gains created by the advancement surgery will be nullified by the anterior segmental setback. In patients who require a concurrent advancement or lengthening genioplasty for facial aesthetics, we find it useful to preserve a bony bridge between the segmental and genioplasty cuts for muscle attachment or plication. Preservation of an intact bony bridge is important (more so in female patients), as this prevents a potential broadening of the mandible with the tendency for opening up of the bone fragments laterally and the creation of a widening of the mandible. This is, however, a welcome side effect, especially in males with an already dolichofacial structure, as it creates a more masculine face form. Advancement genioplasty should be very carefully considered in patients with an already long facial height, as elongation is likely to be magnified with the setback, especially in females where an undesirable ‘witch-like’ profile may result.

The tooth positions are less critical in the lower segment as the lower teeth are fairly vertical at the canine/premolar position. Again care should be taken with the root positions in the presurgical orthodontic preparation, as this will allow for a maximum set-back of the segment, without jeopardizing tooth vitality. The lower canine root is the longest tooth in that part of the jaw, and care should be taken not to transect the root with the horizontal bone cut.

The lower dental arch form may also be smoothed out with a midline split of the mandible and a reduction in the transverse width of the arch can be carried out to smooth the arch form and eliminate the transverse step created by the extraction of the lower premolar teeth. On occasion, a Lindorf genioplasty may be carried out as it will simultaneously advance the genial tubercles, bringing the tongue forward in the process. The application of bone plates is challenging in situations where many bone fragments are present and a prefabricated arch bar wired to the teeth is imperative in helping to stabilize these segments.