Distraction osteogenesis (DO) is a surgical technique that takes advantage of natural wound healing mechanisms to augment bone and soft tissues. DO is extremely versatile and can be applied to nearly any bone. In the craniofacial skeleton, the cranial vault, midface, maxilla and mandible are the most common sites for DO. This technique allows larger skeletal movements than could be achieved with conventional techniques, decreases operative time and blood loss, eliminates the need for bone grafts and associated donor site morbidity, and may improve postoperative stability. DO can be used in preparation for, in lieu of, or in combination with orthognathic surgery to correct dentofacial deformities. DO requires meticulous planning to achieve optimal results.
Distraction osteogenesis (DO) has been used for lengthening of long bones since the early 20th century. DO was adapted to the craniofacial skeleton in dogs by Snyder in 1973. McCarthy is credited with popularizing the technique in humans with his 1992 publication of mandibular DO in patients with hemifacial microsomia and Nager syndrome, though DO had been used commonly by orthodontists for maxillary expansion much earlier.
Advantages of DO compared to conventional techniques include the ability to achieve larger bony movements, concomitant expansion of surrounding soft tissues and neurovascular structures, and potential for greater stability. Distraction eliminates the need for bone grafts and associated donor site morbidity. DO has been shown to decrease operating time, blood loss, and hospital length-of-stay compared to standard techniques.
Conversely, compared to conventional operations, DO requires a higher degree of patient and family compliance. Total treatment time and time away from school, work and activities may be lengthened with DO. DO introduces the potential for device failure, and a second operation is required for device removal at the conclusion of the process. Additionally, the vector of movement may be more difficult to control with DO compared to standard orthognathic surgery.
Biology and stages of distraction
There is an important difference in the healing process for DO compared to traumatic fracture repair. Because of the controlled microtrauma and slow expansion in DO, bone formation in the distraction gap occurs by membranous rather than endochondral ossification, and is mediated by a series of growth factors, cytokines and extracellular matrix proteins. This biology has informed the development of the 3 phases involved in the distraction process: latency, distraction and consolidation.
Latency is the time period from osteotomy and device application to device activation. The purpose of this delay is to allow formation of a primary bone callus, thereby stimulating the influx of biochemicals to support bone growth. Ilizarov proposed a latency period of 5–7 days, though other studies have called the need for this delay in to question.
Distraction is the phase during which the device is gradually activated, and neoformation of tissue occurs parallel to the vector of distraction. The rate of activation may influence ossification in the gap and expansion of surrounding tissues; distracting too quickly may lead to nonunion and/or increased neuropraxia, and activating too slowly may lead to premature consolidation. The original Ilizarov protocol for long bones dictated total activation of 1 mm/day, separated in to 4 equal 0.25 mm activations. In the maxillofacial skeleton, in which a rich blood supply supports predictable healing, distraction protocols of up to 3 mm/day have been used successfully. Though no universally accepted protocol for distraction rate exists, many clinicians use a distraction rate of 1 mm/day in adults and 2 mm/day in infants and children, taking advantage of the faster healing of youth. As devices and technology improve, continuous rather than incremental distraction may prove to support the most robust ossification.
Consolidation begins after distraction is completed. The distraction device remains in place and acts as a stabilizer to prevent micromotion of the separated segments as ossification occurs. Commonly reported consolidation periods vary from 4–12 weeks ; 8 weeks has been found to be sufficient. Inadequate consolidation, however, may lead to nonunion. Once the consolidation period has concluded, the distraction device is removed.
Types of DO and distraction devices
The simplest form of DO is the creation of a “distraction gap” by slow separation of two cut bone surfaces. This is termed monofocal distraction and serves to augment a region of existing bone. Bifocal distraction is used to fill a continuity defect, after tumor resection, or construct a missing portion of bone, as in a congenital ramus deformity (e.g. hemifacial microsomia). In this approach, a “transport segment” of bone is distracted from one side of the defect to the other. Trifocal DO is a modification of the bifocal technique, in which two transport segments are created- one on each side of a larger defect- and they are distracted toward one another until they meet. This approach may be useful in treating large bony defects.
Distraction devices are necessary to facilitate controlled, gradual separation of the osteotomized segments during the distraction phase, and to stabilize those segments to promote ossification during the consolidation phase. Many devices are available from several manufacturers. The choice of device is based on: (1) type and location of movement planned, (2) surgical access for osteotomy and device placement, (3) rigidity of device needed, (4) characteristics of the bone to which the device will be fixed, (5) desired vector of movement, and (6) surgeon’s preference and experience.
Devices are available for each area and type of craniofacial distraction, and in both external forms, in which the device is anchored to the bone via transcutaneous pins, and internal designs, in which the entire mechanism and fixation are subperiosteal and the activation mechanism exists the wound percutaneously. Advantages of external devices include easier application and removal, and improved access to the device mechanism to manage malfunction compared to internal devices. However, external devices may leave scars as the pins travel through the skin, have a higher likelihood of pin dislodgement due to the long lever arm created by the percutaneous pins, are more easily damaged by head movement and are unsightly. In contrast, internal devices are more difficult to place and always require a second operation for removal, but are more stable, do not leave pin scars, and are more socially acceptable. Different applications of craniofacial DO have associated considerations for device type; for example, difficulty with control of the central portion of the midface using internal distractors has been noted by some surgeons who prefer external devices for midfacial advancement after Le Fort III subcranial osteotomy. The vector of distraction may also dictate device choice; distractors are available in single vector, multi-vector and curvilinear forms.
DO in preparation for orthognathic surgery
Distraction osteogenesis techniques are useful to the orthognathic surgeon both in coordination with and in preparation for other orthognathic procedures.
Surgically assisted maxillary expansion (SAME)
The most common use of DO techniques prior to orthognathic surgery is for management of maxillomandibular transverse discrepancies. In fact, “rapid palatal expansion” is performed routinely by orthodontists using tooth-borne devices without an operation in children prior to fusion of the midpalatal suture. Conventional teaching has been that, following sutural fusion in early adolescence, maxillary expansion can only be achieved by surgically-assisted maxillary expansion (SAME), in which at least the maxillary buttresses and midpalatal suture are osteotomized prior to initiating distraction. Superior expansion with pterygomaxillary disjunction, particularly in older patients. With the advent of bone-borne palatal distraction devices, however, reports of successful non-surgical maxillary expansion in patients over 20 years-of-age are beginning to emerge.
When presented with a patient with an absolute maxillomandibular transverse discrepancy (maxilla narrower than mandible), the orthodontist and surgeon have several available options:
Orthodontic resolution of discrepancy via dental tipping. This approach should be used cautiously, as orthodontic relapse may occur if the teeth are maneuvered to or through the borders of the alveolar bone.
Non-surgical orthopedic resolution of discrepancy via maxillary distraction using a tooth-borne or bone-borne device.
Surgically assisted maxillary expansion prior to fixed appliance treatment and/or orthognathic surgery.
Delay correction of transverse discrepancy until orthognathic surgery and perform segmental Le Fort I osteotomy.
Advantages of a SAME procedure compared to a segmental osteotomy include: (1) equal widening of the entire maxilla from anterior to posterior (two-segment Le Fort I provides separation in a V-shape, with more posterior than anterior widening), (2) increase in arch length that can be used by the orthodontist to resolve crowding and manage impacted teeth prior to definitive orthognathic surgery, (3) ability to create larger movements than can be achieved with Le Fort I osteotomy, (4) potential to slightly over-correct thereby compensating for relapse, (5) no need for bone grafting. Conversely, advantages of a segmental Le Fort I osteotomy are (1) single rather than staged operations, (2) decreased total treatment time, (3) lack of creation of a temporary diastema between the central incisor teeth, which occurs with SAME, (4) slightly decreased transverse relapse rate. With segmental Le Fort I osteotomy, however, the magnitude of achievable transverse widening is less than what can be obtained with SAME, there is a risk of devascularization if the palatal tissues are violated, and bone grafts are sometimes needed to maintain larger movements.
Distraction in congenital and acquired craniofacial deformities
Many patients presenting for orthognathic surgery will have had prior DO for management of congenital or acquired craniofacial deformities in childhood. For example, patients with Robin sequence, the triad of micrognathia, glossoptosis and airway obstruction, may have undergone mandibular distraction during infancy. In this population, the orthognathic surgeon must pay close attention to: (1) the underlying diagnosis as Robin sequence is a heterogenous group that including syndromic and non-syndromic etiologies, (2) potential for additional mandibular growth, (3) function of the temporomandibular joints, as either the underlying diagnosis and/or the prior DO procedure may increase the risk for hypomobility and temporomandibular joint ankylosis, (4) potential for recurrent obstructive sleep apnea and potential to adjust the orthognathic surgical plan to address this, and (5) history of pharyngeal surgery, as ∼90% of patients with Robin sequence will also have a cleft palate and may require speech surgery; presence of a pharyngeal flap should be communicated with the anesthesia team to allow special care during intubation.
Patients with other craniofacial anomalies may have had DO prior to definitive orthognathic surgery. These include patients with:
Hemifacial microsomia. Early treatment of hemifacial microsomia using transport distraction of the mandibular ramus during the mixed dentition in patients with Pruzansky-Kaban Type I and IIA mandibular deformities was popularized in the 1990s. Long-term outcomes have demonstrated a high rate of recurrent asymmetry and need for subsequent orthognathic surgery and therefore this technique has fallen out of favor for routine use. Bimaxillary DO to reduce skeletal asymmetry and expand soft tissue in patients in the permanent dentition in preparation for future conventional orthognathic surgery has been proposed in its stead.
Le Fort III osteotomy and midfacial distraction in patients with syndromic craniosynostosis, particularly Apert, Crouzon and Pfeiffer syndromes. In some patients, midfacial advancement is delayed until skeletal maturity and combined with routine orthognathic surgery. In others, including those with associated obstructive sleep apnea, midfacial advancement may occur during childhood. As the end-point of midfacial distraction is typically dictated by orbital-globe relationships rather than occlusion, most patients who have had midfacial advancement will also require routine orthognathic surgery ( Fig. 1 ).
Combined midfacial and mandibular distraction for counter-clockwise rotation in Treacher Collins syndrome.
Unilateral or bilateral mandibular ramus distraction in patients with progressive mandibular resorption, such as in juvenile idiopathic arthritis.
DO in lieu of conventional orthognathic surgery
There are some scenarios in which the orthognathic surgeon may choose to use DO instead of conventional osteotomies for correction of a dentofacial deformity. There are three types of situations that frequently dictate this: (1) need for large and/or complex movements, (2) excessive scar tissue or other soft tissue limitation to planned movement, and/or (3) expected need for bone grafting with acute movement. No precise measurement exists for magnitude of advancement after which DO should be used in lieu of acute movement. While Bell and Guerrero proposed use of DO for movements exceeding 6 mm in their 2007 textbook on the topic, most surgeons routinely advance well beyond this threshold with conventional techniques. As advancement surpasses 15 mm in a previously unoperated patient, DO likely provides the highest likelihood of achieving the desired movement; DO may be useful for much smaller movements in patients with substantial scar tissue and a tight soft tissue envelope.
Maxillary DO in patients with repaired cleft lip and palate
Patients with repaired cleft lip and palate may have significant maxillary hypoplasia and dense scar tissue of the lip and palate ( Fig. 2 ). The large maxillary advancement needed to correct the skeletal malocclusion may not be achievable with conventional techniques and bone grafting may be necessary for these large advancements. There is some evidence that stability is inferior with conventional techniques and improved with DO. A meta-analysis, however, did not find a significant difference in stability between the two techniques.