Biomechanics and orthodontic treatment protocol in maxillofacial distraction osteogenesis.

As in the traditional combined surgical and orthodontic procedures, an Orthodontist can plays vital role in treatment planning and the orthodontic treatment of patient undergoing distraction osteogenesis. This role includes predistraction assessment of the craniofacial skeleton and occlusal function, pre-distraction, and post-distraction orthodontic care. Based on clinical evaluation, dental study models, photographic analysis, cephalometric evaluation, and three-dimensional computed tomographic analysis, the Orthodontist, in collaboration with the Surgeon, plans distraction device placement and the predicted vectors of distraction. Finally, as in other forms of orthognathic surgery, the practice of distraction osteogenesis depends on the cooperation and planning between orthodontist and surgeon as a team. Purpose of this paper is to review biomechanics and orthodontic treatment protocol of distraction osteogenesis in the maxillofacial region.

INTRODUCTION

Distraction osteogenesis (DO) is a surgical-orthopedic method for lengthening bone by separating or distracting a fracture callus. It represents an important technique for surgeons and orthodontists to reconstruct and recontour the facial skeleton. Hard tissue engineering by distraction is critical for correcting craniofacial deformities resulting from birth defects especially in situations where conventional orthognathic surgery and bone grafting techniques are not possible. As distraction osteogenesis becomes more refined, this technique may also serve as an alternative to some of the standard osteotomies currently in use for lengthening mandibles and maxilla. In the craniofacial team setting, the orthodontist assists in planning distraction surgery, developing or adapting distraction devices, and managing postsurgical results. The application of distraction to dentofacial problems will broaden and influence the way in which surgery and orthodontics interact to resolve skeletal discrepancies. Interest in distraction osteogenesis will likely overlap into other disciplines of dentistry as distraction osteogenesis promises to provide new approaches for solving old problems such as alveolar ridge augmentation for dental implants[1] and cleft defects.

BIOLOGICAL BASIS OF DISTRACTION OSTEOGENESIS

In its simplest form, distraction osteogenesis describes the generation of new bone in the stretched fracture callus.[2] When a fracture occurs, the bony ends are usually set and stabilized so that the bony ends align and form one continuous length of bone. If the two bony ends are slowly pulled apart by a screw-driven appliance after the callus forms, then new bone can fill in the stretched callus tissue. This process is called distraction osteogenesis [Figure 1]; the success of new bone formation is influenced by the rate and rhythm of the separating forces. The sequential period in distraction osteogenesis is as follows:

Figure 1

Showing basis principle of distraction osteogenesis

Osteotomy

Each osteotomies bone segment must contain sufficient number of viable osteocytes to initiate and perpetuate the distraction osteogenesis.[3] Complete osteotomy instead of corticotomy is now preferred because periosteum provides adequate osteoblastic activity in well-vascularized craniofacial region.

Latency

Histological sequence in latency period in distraction osteogenesis is similar to that of fracture bone healing and in this phase soft callus formation takes place. Initial latency period is recommended is between 5 to 10 days.

Distraction

In this phase normal process of fracture healing interrupted by the application of gradual traction to the soft callus. As a result of the tension created by this traction force a dynamic microenvironment created which encourages new tissue formation in the direction parallel to the vector of traction. During distraction, four zones[4] appear: a fibrous, less vascular center with collagen fibers parallel to the distraction vector, a transition zone of early bone formation, a bone remodeling zone, and mature bone at the ends. Distraction process is generally carried out at the rate of 0.5 to 1 mm per day.

Consolidation and remodeling

Bone maturation occurs and continues over a period of a year or more before the structure of the newly formed bony tissue is comparable to that of preexisting bone and in which soft tissue adaptation also occurs. After distraction ceases, this soft callus ossifies and a distinct zone of woven bone completely bridge the gap mainly by intramembranous ossification.[5]

HISTORICAL REVIEW

This procedure was used as early as 1905 by Codivilla,[6] who published first report on lengthening of limb. The physician who is credited for developing current methods of lengthening limbs through the gradual distraction of a fracture callus and whose name is synonymous with distraction osteogenesis is G. A. Ilizarov.[7] His pioneering research[589] helped to define the parameters that would lead to successful bone lengthening by distraction. In 1992, distraction osteogenesis was first applied to the human mandible by McCarthy et al.[10] Since then it has been applied to all the bones of the craniofacial skeleton, including the midface and maxilla. In 2001, Liou et al.[11] first applied this concept to orthodontic tooth movement and performed rapid canine retraction through distraction. Various distractor[12] that are used in maxillofacial distraction osteogenesis can be classified [Figures 2a and b].

Figure 2

Various types of distractors used in maxillofacial region (a-d)

VARIOUS TYPES OF DISTRACTOR USED IN THE MAXILLOFACIAL REGION

Based on site

Mandibular

Midface or maxillary

Alveolar

Transport[13] (Reconstruction of neo-mandible/ neo-condyle)

Rigid external distractor (RED)

Based on use

External distractor

Unidirectional (activated in one plane of space)

Bidirectional (activated in two plane of space)

Multiplanner (activated in three plane of space)

Rigid external distraction (RED) system

Internal distractor

Mandibular intraoral distractor [Figures 2c and d]

Modular internal distractor (MID)

Tooth borne distractor e.g. rapid canine distractor, alveolar distractor

Advantages

Large maxillo-mandibular advancement possible[14]

Can be done at any age. There have been reports of rapid, early mandibular distraction to prevent tracheotomy in a newborn with micrognathia that was causing severe airway obstruction.[1415]

Minimized need for orthognathic surgery, hence reduce complication

Minimized relapse because of histogenesis during distraction

Shorter hospital stay

No bone graft required

New bone formed in distraction osteogenesis is more native and permit orthodontic tooth movement

Disadvantages

Lack of precision[14]

Poor three-dimensional control, but current distractor are constantly modified for desired results

In some cases multiple daily outpatient visit required

Pain due to manipulation of healing corticotomy daily

Difficult access for orthodontist during distraction osteogenesis

Difficult plaque control

Damage to TMJ due to incorrect vector orientation

Indication

Distraction osteogenesis is indicated[16-18] in all cases of mandibular shortening or maxillary hypoplasia where orthognathic surgery is not first choice. Various acquired and congenital condition in which distraction osteogenesis is indicated are as follows:

Congenital deformities

Pierre- Robin Syndrome

Severe retrognathic syndrome e. g. Treachers Collins and Goldenhar syndrome

Non-syndromic congenital micrognathia

Severely constricted mandible/ maxilla

Craniofacial microsomia unilateral/bilateral

Midfacial hypoplasia

Obstructive sleep apnea (OSA)

Facial asymmetry

Acquired conditions

Post-traumatic growth disturbances of mandible, e.g. temporomandibular joint ankylosis

Non union fractures

Atrophy of edentulous segments

Oncologic mandibular osseous defects

Others

Rapid canine distraction for rapid distalization of canine to reduce orthodontic treatment time[11]

Distraction for ankylosed teeth to create optimum height of alveolar bone[19]

TREATMENT PLANNING AND BIOMECHANICS OF DISTRACTION OSTEOGENESIS FROM AN ORTHODONTIC PERSPECTIVE

Management[20] of a case with craniofacial deformities with distraction osteogenesis involves a team approach among the surgeons and orthodontists. Orthodontist plays a crucial role in diagnosis and treatment planning. This would involve a thorough clinical examination of the face and structures. A diagnostic data base is created with the aid of study models, photographs, lateral head and PA cephalograms, orthopantomograms, computed tomography scans. In addition to treatment planning, the data base is also useful for evaluation of treatment results.

VECTORS OF DISTRACTION OSTEOGENESIS: BIOLOGICAL AND MECHANICAL

Various factors combine to determine the ultimate selection and placement of the distraction device on the mandible/ maxilla. The biological and mechanical forces[21] that shape the regenerate (the newly formed bone during the active period of distraction osteogenesis) are key elements in determining appliance position. The desired mandibular change in shape and function can be achieved by selecting and controlling the force vectors that operate during active distraction. The biological forces influencing the morphology of the bone regenerate (newly formed bone) arise from the surrounding neuromuscular envelope [Figure 3]. The mechanical forces under the clinician's control originate from activation of the distraction devices, their specific orientation to skeletal anatomy, the application of intermaxillary elastics during the active phase of distraction, and the intercuspation of the dentition. During the process of planning mandibular distraction, it is imperative to consider the powerful impact of both biological and mechanical force systems to anticipate their resultant effects. A review of clinical records has shown that device orientation to the mandible has a direct influence on the change in skeletal morphology.[21] Device placement can be described as vertical, horizontal, or oblique. It is important to note that the position of the device is best described in relation to the long axis of the mandibular body [Figure 4]. This is in distinction to orientation of the devices to the posterior border of the ramus or the inferior border of the body because of the variability in contour of these borders. When there are significant irregularities in the occlusal plane, the long axis of the mandibular body is the preferred reference line.

Figure 3

Biological vectors of distraction osteogenesis in maxillofacial region (a-b)

Figure 4

Vectors of distraction. Straight lines indicate the long axis of the device vertical (a), horizontal (b), and oblique (c)

VERTICAL DEVICE PLACEMENT

Vertical device placement results in an increase in the vertical dimension of the mandibular ramus [Figure 5a and b]. During activation, a change occurs in appliance orientation that appears to be caused by the nonlinear molding effect of the neuromusculature on the regenerate as it is formed. The mandible autorotates in a counterclockwise direction and the lower incisors take a more advanced position. A posterior open bite may occur on the side that has undergone vertical distraction in the ramus [Figure 5c]. Bilateral vertical lengthening of the ramus results in counterclockwise up righting of the mandibular symphysis. When combined with the sagittal advancement of the mandibular body, the increased prominence of the lower third of the face is evident [Figure 5d]. The unilateral vertical ramal lengthening is usually associated with transverse correction of the chin position and the cant correction of the mandibular occlusal plane [Figure 6].

Figure 5

Vertical mandibular ramus distraction osteogenesis and its effect (a-d)

Figure 6

Transverse correction of chin position and cant correction of the mandibular occlusal plane by unilateral vertical ramal lengthening

HORIZONTAL DEVICE PLACEMENT

The most efficient approach for achieving sagittal projection of the mandibular body and symphysis is by placement of the distraction device in a horizontal position in relation to the mandibular body [Figure 7a and b]. There is a tendency in horizontal distraction of the mandibular body to rotate in a clockwise direction, resulting in an open bite[22] [Figure 7c]. The suprahyoid musculature, in balance with the muscles of mastication and the distraction device itself, has a role in this occurrence. It is reported that here is an improvement in the patency of the oropharyngeal airway and tongue position subsequent to mandibular sagittal advancement.[23] Neonatal mandibular distraction has been performed when life-threatening airway problems exist.

Figure 7

Horizontal mandibular distraction and its effect (a-c)

OBLIQUE DEVICE PLACEMENT

Oblique device placement results in an increase in both the vertical and horizontal dimensions of the ramus and body [Figure 4c]. The effect of oblique device placement is a combination of the vertical and horizontal changes previously described. Overjet and both ramal and body size deficiency may be addressed by oblique device placement.

ORTHODONTIC TREATMENT PROTOCOL

Predistraction orthodontics

Orthodontics during distraction and consolidation phase

Post distraction orthodontics

Retention

Predistraction orthodontics

Maxillary and mandibular dental arches are prepared for distraction osteogenesis by leveling and alignment, decompensation, and correction of curve of spee. The teeth should be moved to the ideal positions relative to the basal bone so that an ideal maxillo-mandibular relationship is not compromised by existing dental compensations. Standard pre distraction orthodontic protocol includes appropriate transverse arch width coordination of both maxillary and mandibular arches followed by passive rectangular arch-wires with the hooks for engaging interarch elastics during and after distraction. Predistraction preparation should consider root divergence of the tooth with a fixed orthodontic appliance at the osteotomy site. This is to facilitate the osteotomy and to ensure adequate alveolar bone on the both sides for periodontal health.[20]

Orthodontics during distraction and consolidation phase

During distraction and consolidation phase, orthodontics’ management[24] aimed to direct the tooth bearing segment to their post distraction positions. In mandibular distraction, interarch elastics during this phase influence the vector and are useful in remodeling of regenerated bone and close the open bite. Unilateral mandibular distraction (hemifacial microsomia) can lead to posterior open bite on distraction side and crossbite on normal side. Expansion of maxillary arch along with use of interarch elastics can be used to correct developing transverse and vertical discrepancies [Figure 8]. Occlusal bite-block can be used during the consolidation and post-consolidation phase to supra-erupt posterior dentition to correct the cant and open bite [Figure 9].

Figure 8

Maxillary arch expansion and intermaxillary elastics to correct developing transverse and vertical discrepancies in case mandibular unilateral distraction

Figure 9

Occlusal bite-block to correct the cant and open bite in case of mandibular unilateral distraction (a-d)

For the maxillary and mid-facial distraction, force should be directed through the center of resistance (CR) of maxilla. If force is directed below the center of resistance, an open bite is the result. Light vertical elastics from the maxillary arch to the lower arch would help in controlling clockwise rotation of the mandible and open bite.

Post distraction orthodontics and retention

Post distraction orthodontics should be initiated after the consolidation phase which is aimed at finishing and settling the occlusion. Essix type retainers may not be sufficiently rigid to maintain the increased transverse dimension. If an Essix retainer is required for patient esthetics and compliance, it should only be used during the day and a Hawley retainer used for evening and night time wear. Fixed lower canine-to-canine wire will adequately maintain the canine width and anterior alignment, but cannot be expected to aid in maintaining any posterior expansion. Consequently, a Hawley retainer with integral lingual support wire is a good form of mandibular retention.

Periodontal ligament distraction

Reducing the orthodontic treatment time and controlling the anchorage loss are common aims of research in modern orthodontic practice. Rapid canine distalization is a cornerstone for these goals and was first introduced by Liou and Huang in 1998. Since then, however, studies of rapid canine distalization through distraction of the periodontal ligament are limited because individual canine retractors are not available in the market and the long-term effects are unknown. The major advantages of rapid distalization were shortening the treatment time, eliminating the need for additional anchorage, and rapid retraction of incisors using the new bone tissue distal to the lateral incisors. The average orthodontic treatment time can be reduced three to four months by this technique. A case report[25] of the patient treated by us showing rapid retraction of canine through the periodontal ligament distraction [Figure 10]. At the time of first premolar extraction, the interseptal bone distal to the canine was undermined with a bone bur, grooving vertically inside the extraction socket along the buccal and lingual sides and extending obliquely towards the socket base. Then, a tooth-borne, custom-made, intraoral distraction device was placed to distract the canine distally into the extraction space. It was activated 0.5 mm/day immediately after the extraction. Canine was distracted 6.5 mm into the extraction space within 2 weeks.

Figure 10

Complete canine distraction procedure with retention device on left side in mandibular arch (a-f)

FUTURE PROSPECTS

Several new developments are on the horizon in the field of craniofacial distraction osteogenesis. A successful combination of endoscopic techniques to create osteotomies and insert distraction devices will move distraction into the field of minimally invasive surgery. New work using bio-resorbable materials may lead to the implementation of devices that do not require a second surgical procedure to remove them and following resorption leave no trace that they had ever been inserted. In addition, use of microprocessors and miniature motorized distraction devices may provide the ability to insert submerged appliances capable of auto-distraction according to preprogrammed data.

CONCLUSION

Orthodontists are well suited to manage and use this new clinical procedure because of their knowledge of biomechanics and long-term patient management. As distraction osteogenesis gains more clinical acceptance, it will be important for orthodontists to understand the next generation of internal distraction devices and learn how to integrate distraction osteogenesis into their treatment plan. Finally, as in other forms of orthognathic surgery, the practice of distraction osteogenesis depends on the cooperation and planning between orthodontist and surgeon. Distraction osteogenesis is an important clinical procedure for lengthening mandibles and other craniofacial bones; however, it requires interdisciplinary and coordinated care in order to ensure a successful clinical outcome for the patient.