The principle of lengthening the mandibular ramus with distraction osteogenesis is that the positioning of the distractor and the vector of distraction should be that the force is vertical and passes through the condylar process. In effect this creates a lateral open bite, lengthens the ramus, creates a new position for the mandibular angle, and corrects chin asymmetry.
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. The virtual osteotomy is completed and the distractor virtually activated. 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 osteotomy. Intraoperative navigation can achieve a similar result.
3. Surgical approach
The surgical approach depends to some extent on the age of the patient and the severity of the deformity. The type of distractor used is also a factor.
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 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).
The wounds are then closed with a dressing applied to the external port.
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 distraction device is removed.
6. Aftercare following distraction osteogenesis
Apply ice packs (may be effective in a short term to minimize edema).
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.
Patients with intraoral wounds must be instructed in appropriate oral hygiene procedures. A small soft toothbrush with toothpaste should be used. Antiseptic rinses can be used in the early postoperative period. 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.
Early post-operative x-rays are obtained to verify correct device placement. Additional postoperative imaging is performed as needed.
Remove sutures from skin after approximately 5 days if nonresorbable sutures have been used.
Regular follow up examinations to monitor healing and the postoperative occlusion are required.
Avoid sun exposure and tanning to skin incisions for several months.