Key words: correction of protrusion and retrusion, combination with orthodontic treatment/transverse maxillary distraction/osteotomy, dental arch lengthening, large displacement, new bone formation, stretching of the mandibular segment nourishing mucosa
Anterior mandibular segmental distraction osteogenesis is indicated if the dental arch, which has been shortened due to missing teeth, eg after wrong indicated premolar extraction, is to be enlarged again or made congruent with the opposing arch. An alternative segment osteotomy (see Chapter 28) is not expedient because the necessary displacement distance is too great and the segment-adhering nourishing gingiva would tear. Only by gradually distracting the segment can the surrounding mucosa be stretched and stimulated to grow. New bone forms not only in the bony distraction crevices but also periosteally in the resulting ridge gap. Retrusion of the anterior mandible, eg after premolar extraction at a growing age with predominantly anterior gap closure, can also be an indication for anterior mandibular segmental distraction. It may also be indicated due to a narrow alveolar process if orthodontic measures would move the tooth roots out of the bone.
The most frequent indication after orthodontic dental arch shaping is the remaining/emerging protrusion of the anterior mandible and maxilla. This is corrected mainly in conjunction with orthodontic treatment or surgical procedures like maxillomandibular osteotomies. In the case of pronounced vertical overlap, fixed orthodontic treatment of the maxilla is performed first to form a regular dental arch with protrusion of the anterior teeth to create a sagittal anterior step, followed by anterior mandibular segmental distraction (see Fig 21-2). It can also be performed in combination with transverse maxillary distraction or as part of a mandibular or maxillomandibular osteotomy. In the present authors’ patient population, this last combination of therapies occurs most frequently. Anterior mandibular segmental distractions alone are indicated for decompensation of Angle Class III anomalies treated with premolar extraction or for pronounced Angle Class II anomalies when premolars have been extracted.
21.2 Surgical planning
Key words: anterior divergent alveolar ridge osteotomy, combination with mandibular osteotomy, LCR & panoramic radiograph with DVT if necessary, model surgery, orthodontic planning setup, safety distances between osteotomies and teeth
The prerequisite for planning the surgery is the agreement of the surgical measures, with the orthodontist treating before and after, and the orthodontic planning setup for the group of teeth that will be surgically adjusted.
In addition, a panoramic radiograph is necessary to identify the availability of sufficient interdental bone (> 3 mm) for interdental osteotomy. In cases of doubt, a DVT of the surgical region is more informative, as it allows the position of the tooth roots and the interradicular distances to be assessed in 3D.
An additional model surgery to saw out the anterior mandibular segment can be advantageous for determining the distraction direction, since the directions of the two alveolar ridge osteotomies must diverge anteriorly (Fig 21-1a).
As a rule, distraction is performed anteriorly, but a vertical effect can also be achieved by oblique horizontal displacement planes (Fig 21-1b). For this purpose, the direction of the basal horizontal osteotomy must be determined in the lateral cephalometric radiograph (LCR), using tracing. The treatment goal should be the guideline value of 90 to 95 degrees of the L1-MeGo angle.1 Note that there should be approximately 5 mm distance between the osteotomy and the anterior tooth root tips.
In addition, it is also possible to create an incisal or subapical segment tilt by using different distractors. An incisal segment tilt leads to protrusion, and a subapical segment tilt leads to retrusion of the anterior teeth (Figs 21-1c and 21-1d).
If the anterior mandibular segmental osteotomy is performed as part of a mandibular or maxillomandibular osteotomy, it is planned as part of the model surgery so that the information about the distraction movement is incorporated in the target splint, which is fixed in the maxilla.
Additional chin augmentation or chin wing correction can take place 1 year later to further enlarge the mandible and improve the facial profile if required.
21.3 Surgical methods and treatment strategies