Bimaxillary distraction osteogenesis

Bimaxillary distraction osteogenesis is only carried out in Hemifacial microsomia from 12 years of age onwards. It allows correction of mandibular asymmetry with simultaneous correction of the position of the maxilla. If lengthening of the mandibular ramus by distraction is carried out in early adolescence, a lateral open bite is created and sometimes the maxillary teeth may not erupt into the space. Bimaxillary distraction osteogenesis effectively solves this problem.

This procedure is therefore indicated in Pruzansky II cases in which the dental occlusion is manageable orthodontically. Preoperatively the orthodontist needs to place fixed orthodontic appliances with surgical hooks in place to facilitate intermaxillary fixation.

Distractors can be placed free-hand without any guides, but may produce unpredictable results. This section will therefore describe a more predictable approach using computer-assisted planning.

Planning is carried out in virtual reality on a 3D-CT scan using appropriate software. An estimate can then be made of the amount of distraction required and the bone stock available. This is helpful in deciding which distractor to use.

The greater the deformity, the smaller will be the mandibular ramus and the greater will be the distraction distance required. This is problematic because there is less space available for an internal distractor. This can be solve by the use of either an external distractor or an internal distractor that employs an external activation rod. External pin distraction has the disadvantage of leaving unsightly facial scars.

The virtual distractor is selected and placed on the mandibular ramus. A horizontal osteotomy is marked above the lingula.

A virtual Le Fort I osteotomy is made and MMF is simulated.

The virtual osteotomies are completed and the distractor virtually activated. The mandibular ramus will lengthen and bring the maxilla down with it. If the movement is not satisfactory, the virtual distractor position can be adjusted until the desired vector is achieved.

A guide can be constructed which allows the accurate positioning of the distractor and the osteotomies. Intraoperative navigation can achieve a similar result.

A trans oral approach can be used but with a few modifications:

• The ramus is exposed subperiosteally up to the condylar neck and the sigmoid notch, and down to the mandibular angle and the posterior border.
• Stab incision in the submandibular region to allow for the exit of the activation rod.

Additionally a transbuccal approach is used for screw insertion.

In very young children, or when the deformity is very severe, a purely external approach should be considered. The whole lateral aspect of the mandibular ramus including the condylar stump is exposed.

The standard approach to the maxilla is through a vestibular incision. This author prefers the approach described for cleft le Fort I osteotomies. An alternative approach is described in the orthognathic module.

The surgical guide is positioned and stabilized with one screw. The holes for the distractor screws are drilled and the osteotomy line marked on the bone.

The guide is removed and the osteotomy completed with a saw and osteotome.

The distractor is placed and if a transoral approach has been used, a small submandibular incision is required to exteriorize the activation rod. The distractor is stabilized in position using screws in the previously made screw holes.

The distractor is activated to ensure that it is working properly and is then deactivated (returned to starting position).

A periosteal elevator is inserted between the nasal mucosa and the lateral wall of the nose on one side. A curved retractor is inserted behind the maxillary tuberosity. A further instrument is used to retract upwards the lip and mucoperiosteal flap, exposing the antero-lateral maxilla.

The cut starts at the piriform rim just below the inferior turbinate. It then traverses the anterior maxilla approximately 5 mm below the infraorbital foramen and crosses the maxillary buttress at which point the direction of the cut should be inferior.

Pitfall: The direction of the cut posteriorly is important as it must not travel upwards. That could result in a posterior osteotomy fracture line that may either result in excessive bleeding or travel upwards into the orbit.

The osteotomy is completed anteriorly and laterally with fine osteotomes. A planned wedge of bone is removed from the contralateral side of the maxilla (that is the non Hemifacial microsomia affected side) to allow for rotation upwards.

Posteriorly a fine gently curved osteotome is used with the curvature pointing downwards to complete the cut up to the pterygomaxillary junction.

Pitfall: The direction of the cut posteriorly is important as it must not travel upwards. That could result in a posterior osteotomy fracture line that may either result in excessive bleeding or travel upwards into the orbit.

The lateral walls of the nose are then divided with nasal osteotomes while protecting the nasal mucosa. Care is taken to preserve the connection of the maxilla to the bony nasal septum.

A curved pterygoid chisel is placed with the curvature pointing medially and inferiorly between the tuberosity and the pterygoid plates.

A mallet is used to drive the osteotome medially to complete the pterygomaxillary dysjunction. The position of the tip of the osteotome can be checked with a palpating finger.

When the above osteotomies have been made, Rowe's disimpaction forceps are inserted and a limited mobilization of the maxilla is carried out. The main purpose is to ensure that the maxilla twists in the way that it will be moved by distraction.

Maxillo-mandibular fixation is established. The distractor is once again activated to ensure that the correct movements occur, and then wound back to its starting position.

After a suitable latency period, the ramus is distracted at a rate of 1.0 mm per day.

Weekly review of the patient is valuable until such time as the required distraction has been achieved.

After reaching the desired distraction the device is left in place to retain the distraction and to allow for bone consolidation.

It is valuable to check that distraction is progressing well periodically with radiographs. It is also useful to slightly overcorrect the deformity -- the younger the patient the more this should be done.

Consolidation will take approximately 8 weeks and the regeneration of bone into the distraction gap can also be verified with radiographs. When sufficient bone is present in the gap, the distractor is removed.

Before bimaxillary distraction osteogenesis.

After bimaxillary distraction osteogenesis.

MMF is maintained during the distraction phase to allow for the correction of the canted occlusion.

To reduce swelling, the application of ice-packs or cooling devices during the early post-operative phase is advice. Intravenous steroids should also be continued for a short period postoperatively for the same purpose.

The sterile dressing placed over the skin incisions is maintained for a minimum of 48 hours. Thereafter routine wound care should be instituted around the activation rod.

Antibiotic prophylaxis is continued for 1-5 days depending on the nature, complexity, and duration of the surgical procedure.

Airway control is of major importance. An individual decision has to be taken if the patient can be extubated or should remain intubated until it is clear that safe airway can be established.

Early post-operative x-rays are obtained to verify correct segment and distractor position. Additional postoperative imaging is performed as needed.

Remove sutures from skin after approximately 5 days if nonresorbable sutures have been used. Avoid sun exposure and tanning to skin incisions for several months.

Regular follow up examinations to monitor healing are required.

At each appointment, the surgeon must evaluate the patient's ability to perform adequate oral hygiene and wound care and should provide additional instructions if necessary.

Postoperatively, patients will have to follow three basic instructions:

1. Diet
A liquid diet should be used during the distraction phase while the patient is in MMF.

2. Oral hygiene
Patients with intraoral wounds must be instructed in appropriate oral hygiene procedures. The presence of orthodontic appliances, the splint, and elastics makes this a more difficult task. A small soft toothbrush with toothpaste should be used. Any elastics are usually removed for oral hygiene procedures. Additionally, antiseptic rinses can be used in the early postoperative period. An oral irrigator (eg, Waterpik) is a very useful tool. If a Waterpik is used, care should be taken not to direct the jet stream directly over intraoral incisions to prevent wound dehiscence in the early postoperative phase.

3. Functional training/physiotherapy
After the release of MMF, the patient is instructed in how to perform functional training (opening and excursive exercises) as soon as possible. The progress should be monitored by the surgeon. If available and needed, a physiotherapist can support the functional rehabilitation. An undisturbed mouth opening of minimum 35 mm interincisal jaw opening should be attained by 4 weeks after removal of MMF.

In case of undisturbed healing, the postoperative orthodontic treatment can usually start 2 to 6 weeks after consolidation depending on the case.