Introduction

This assignment seeks to establish from the literature, the principles, clinical technique, applications, and limitations of this technique of bone augmentation.

Distraction Osteogenesis has been described as ‘the biological process of bone formation occurring between vital bone segments that are gradually separated by incremental traction’. Traditionally bone healing has required immobilisation of the fracture, whereas the technique described by McCarthy et al. in 1992, showed that providing bone separated by an osteotomy was stabilised by a distraction device, the segment can be slowly moved away from the base, and gap-healing occurs becoming bone contiguous with both the base and distracted segment.

Clinical Technique and Timescale

As with all implant therapy, proper planning is necessary to obtain the desired results. Pre-treatment models, wax-ups and stents are necessary to obtain the correct plane of distraction for increased bone volumes in the correct position for prosthetic restoration. Such planning should also reveal whether a distraction device can be placed in the correct plane free of occlusal interference, and damage to adjacent structures.

Once planning is complete, the surgical technique as described by various studies is as follows:
A horizontal incision is made on the labial surface of the bone where the segment is to be released. Where possible, vertical relieving incisions are to be avoided. If the incision is in attached mucosa, then attached mucosa will be regenerated; similarly with free gingivae. The buccal mucosa is lifted without elevating the crestal mucosa, to reveal the underlying bone cortex. The distraction device is offered up, checked for fit, and any adaptation made to the fixing wings. (Care is needed to avoid separating the wings from the distractor – this is an expensive item!). The osteotomy is then made using bone-saws, fissure burs, or piezo-cutters. The horizontal incision is made, and then the vertical incisions as close to the adjacent teeth without damaging the periodontal membranes of these teeth. The vertical incisions need to be slightly convergent apically to allow free movement of the separated segment in the desired plane. All cuts are made through the full thickness of bone whilst maintaining the integrity of the lingual or palatal mucosa to ensure the blood supply to the mobilised segment.

Some operators like to attach the distraction device before completing the vertical bone cuts, as this make attachment of the mobilised segment easier. Where large segments are mobilised two distraction devices can be attached parallel to each other in every plane. The device(s) is then activated a few turns to ensure unimpeded movement of the mobilised segment, and then returned to its starting position. The wound is closed around the distraction site by suturing. If possible the gap is covered by periosteum to encourage bone healing. However secondary healing is inevitable where the distractor passes through the incision.

There are four phases of distraction

1. Latency period.
   Chiapasco et al.¹ waited seven days post-surgery for soft-tissue healing and an osteogenic response in their initial paper based on eight patients in 1991. Their results were successful in that they were able to place all the implants that they had planned, and that recovery from surgery was uneventful for all eight patients. They also reported less morbidity and shorter treatment times in comparison to GBR and bone-grafting procedures used to obtain vertical augmentation. Rachmiel et al^.2 in their study of 14 patients a decade later, started distraction four days post-surgery, and suffered the loss of one implant as a result of improper distracted segment stability. Neither study is big enough to reveal whether the latent period was responsible for relative success or failure, but most subsequent studies have used a seven day latency period.
2. Distraction.
   A distraction rate of between 0.8 and 1mm per day has been used in most studies, with distraction movements being made between 2 and 4 times a day.
3. Consolidation.
   A period of 6-12 weeks is normally described in the studies following active distraction when the distractor is left in situ (Often the protruding screw is sectioned to aid patient comfort), and the newly-formed bone matures. After three months the new bone can be demonstrated radiographically. Stabilisation of the distracted segment should be demonstrable before implant placement.
4. Distractor removal and Implant placement.
   In more recent studies these are concomitant unless bone defects are observed, and additional grafting is necessary.

Possible Clinical Applications of ADO

ADO appears to be most successful when used to repair vertical deficiencies in partially edentulous ridges where the edentulous span is greater than one tooth. (Jensen et al.3 in their study of 30 maxillary distractions concluded that single tooth osteotomies had the highest complication rates because of devascularisation. Again the study can be criticised for low numbers, but 30 consecutive distractions were carefully followed for five years to obtain the results.)

Risks and benefits of ADO

In their study comparing autogenous onlay bone-grafts with ADO in 17 patients, Chiapasco et al.4 concluded that there was no difference in survival and success rates of implants placed with either technique. ABGs demonstrated higher morbidity due to two surgical sites, whereas ADO was more limited in applications.

Various studies have described complication rates of between 50 and 75% with AOD. In their small but thorough study of 20 (all successful) ADOs, Wolvius et al.5 describe an implant success rate of 98%, but a complication rate of 55% mainly occurring during the distraction phase, but also as a result of relapse of the bone formed by distraction. They concluded that ‘Distraction seems to be a suitable treatment for vertically deficient alveolar bone, but a relatively high although manageable complication rate must be confronted, including considerable relapse’.

Other benefits include shorter treatment times, less soft tissue management, and potentially greater height gain of bone achievable. Risks include failure to mobilise or distract the segment in the correct plane, and failure of the blood supply to the segment leaving an even bigger defect. It is difficult to temporise these patients during the distraction phase, and successful distraction depends on patient compliance.

References:

1. Chiapasco M, Romeo E, Vogel G. Vertical Distraction Osteogenesis of Edentulous Ridges for Improvement of Oral Implant Positioning: A Clinical Report of Preliminary Results. Int J Oral Maxillofac Implants 2001;16:43-51.
2. Rachmiel A, Srouji S, Peled M. Alveolar ridge augmentation by distraction osteogenesis. Int J Oral Maxillofac Surg. 2001;30:510-517.
3. Jensen O, Cockrell R, Kuhlke L, Reed C. Anterior Maxillary Alveolar Distraction Osteogenesis: A Prospective 5-Year Clinical Study. Int J Oral Maxillofac Implants 2002;17:52-68.
4. Chiapasco M, Zaniboni M, Rimondini L. Autogenous onlay bone grafts vs. alveolar distraction osteogenesis for the correction of vertically deficient edentulous ridges: a 2-4 year prospective study on humans. Clin Oral Impl Res., 18, 2007: 432-440.
5. Wolvius E, Scholtermeijer M, Weijland M, Hop W, van der Wal K. Complications and relapse in alveolar distraction osteogenesis in partially dentulous patients. Int J Oral Maxillofac Surg, 2007; 36:700-705.