Disclosure Distraction osteogenesis is a technique in which bone can be lengthened by de novo bone formation as part of the normal healing process that occurs between surgically osteotomized bone segments that undergo gradual, controlled distraction. Compared to conventional approaches, the ability of the soft tissue envelope (the skin, muscle, and neurovascular structures) to accommodate the gradual expansion of the underlying skeletal framework that contributes to the stability of the reconstruction is unique to distraction. The technique today is an important part of the reconstructive surgeon's armamentarium. History of the Procedure: The idea of gradual bone lengthening is not new. The concept originated with orthopedic colleagues solving the problem of leg length discrepancies. In 1905, Codivilla gradually lengthened a femur through serial application of casts that were cut and advanced using bed-frame traction. In 1927, Abbot replaced the casts with large pins placed through the fractured segments and used springs to lengthen the lower limb; in 1948, Allan incorporated a screw device that more accurately controlled the rate of distraction. However, these early attempts frequently were complicated by infection, ischemic necrosis of skin and muscle, malunion, and delayed ossification with fibrous union. The idea of distraction osteogenesis was largely abandoned by many until the 1950s, when Ilizarov demonstrated that performing a corticotomy with minimal disruption of the surrounding blood supply and using a system of tension ring fixators to control the distraction in multiple planes could minimize complications significantly. Through a series of experimental studies and clinical applications, Ilizarov established the foundation of distraction osteogenesis and its role in orthopedic management. However, application to the craniofacial region waited until 1973, when Synder et al first applied the approach to mandibular lengthening in a canine animal model. Almost another 20 years passed before McCarthy and colleagues published, in 1992, the first report of mandibular lengthening in 4 children with congenital mandibular deficiency, 3 with hemifacial microsomia, and 1 with Nager syndrome. Thereafter, its role rapidly expanded to the remaining upper craniofacial skeleton, and currently it is applied to nearly all classic approaches to craniofacial reconstruction. Pathophysiology: As an underlying principle, Ilizarov proposed the tension-stress model where "slow steady traction of tissues causes them to become metabolically activated, resulting in an increase in the proliferative and biosynthetic functions." The premise then is that the newly generated bone between distracted bony ends will result in a stable lengthening and behave as "new" bone, appropriately responding and adapting to the regional environmental loads placed on it. Whether in the long bones or in the craniofacial skeleton, distraction osteogenesis takes place primarily through intramembranous ossification. Histologic studies identified 4 stages that result in the eventual formation of mature bone. Stage I: The intervening gap initially is composed of fibrous tissue (longitudinally oriented collagen with spindle-shaped fibroblasts within a mesenchymal matrix of undifferentiated cells). Stage II: Slender trabeculae of bone are observed extending from the bony edges. Early bone formation advances along collagen fibers with osteoblasts on the surface of these early bony spicules laying down bone matrix. Histochemically, significantly increased levels of alkaline phosphatase, pyruvic acid, and lactic acid are noted. Stage III: Remodeling begins with advancing zones of bone apposition and resorption and an increase in the number of osteoclasts. Stage IV: Early compact cortical bone is formed adjacent to the mature bone of the sectioned bone ends, with increasingly less longitudinally oriented bony spicules; this resembles the normal architecture. As the bone undergoes lengthening, each of these stages are observed to overlap from the central zone of primarily fibrous tissue to the zone of increasingly mature bone adjacent to the bony edges. By 8 months, the intervening bone within the distraction zone achieves 90% of the normal bony architecture. It is believed that the architecture is maintained and that the bone responds to normally applied functional loads.
Indications for the use of distraction are broad, and its applicability depends on the particular clinical problem. As the technique is in its infancy, the indications are evolving, and it is applied to solving a wide range of craniofacial deformities. In hemifacial microsomia, distraction osteogenesis should be considered in children with Pruzansky Grade I and IIa type mandibular deformity. However, a child with a Pruzanky Grade IIb or III is unlikely to have sufficient bone to allow for a corticotomy and/or osteotomy and placement of pins for external or internal distraction devices. In such situations, conventional costochondral rib grafts or vascularized fibula grafts may be necessary. This grafting may be followed by distraction osteogenesis, if appropriate. Similarly, minimal facial skeletal asymmetry as result of mandibular hypoplasia (Pruzansky I) may be treated with conventional orthognathic surgery. In children with significant bilateral mandibular hypoplasia in whom the airway may be an issue or in those who are tracheostomy-dependent, early bone lengthening through distraction may be beneficial. Increasing experience with neonatal distraction has shown that in selected cases (eg, Pierre Robin sequence), the need for tracheostomy can be avoided. Moreover, distraction allows for correction of the hypoplastic mandible earlier in childhood rather than waiting until adolescence for maturity of the facial skeleton required with traditional approaches. Children with severe midfacial deformities also may benefit from distraction earlier in childhood rather than waiting until adolescence, as it can be applied to lessen the deformity. In adolescence, when the maxillary-mandibular discrepancy is significant and stability through conventional approaches is a concern, consider distraction. With the evolution of innovative devices, the technique is applied to an ever increasing range of reconstructive problems, from the deficient alveolar ridge to the frontofacial advancement. Nevertheless, as with any approach to solving a clinical problem, weigh the advantages and disadvantages of any technique carefully. Advantages cited in the literature include minimal likelihood of relapse, increased stability with large movements, simultaneous expansion of soft tissue, decreased operative time, and blood loss and morbidity associated with bone grafts.
Contraindications: Disadvantages include device failure, cutaneous scars with external pin-based devices, necessity of a secondary procedure for removal of internal devices, limited control of the distracting vector with internal devices, patient compliance and acceptance of the device, and the increased overall treatment time. Additional issues are related to the specifics of the osteotomies, such as neurovascular injury and dental injury. |
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Imaging Studies:
Surgical therapy: Regardless of which facial skeletal element is undergoing distraction, the treatment can be divided broadly into the following phases: (1) presurgical phase, (2) operative phase, (3) latency phase, (4) distraction phase, (5) consolidation phase, and (6) retention phase. Presurgical PhaseThis phase involves radiographic studies to determine the feasibility of placement of the distraction device, the vector (direction, amplitude) of the distraction, and whether an internal or external device is more appropriate. When possible, 3-D solid models help to visualize the placement of the device and simulate the distraction process. Involvement of the orthodontist is essential as presurgical orthodontic preparation typically is needed to guide the distraction at the occlusal level since the skeletal component is controlled by the device mechanism. Operative PhaseOsteotomies used with distraction are well described with the conventional reconstructive approaches and need only be modified to accommodate the specifics of the device being used. While the exact details may vary with the procedure, the following are guidelines: Mandibular distraction
Midfacial and frontofacial distraction
Latency PhaseThis is the initial postoperative phase when fracture healing is allowed to occur before distracting forces are applied. This period typically lasts 5-7 days. In younger patients (typically, younger than 4-5 years), the latency period may be significantly shortened or omitted altogether to prevent early consolidation. Distraction PhaseThe process of distraction is activated with the bone segments gradually pulled apart using either an internal or external device. Three variables must be set: the rate of distraction, the rhythm and/or frequency of distraction, and the total time of distraction. The rate of distraction is typically 1.0 mm/d. Some advocate up to 2.0 mm/d in younger children to avoid early consolidation and a slower rate of 0.5 mm/d or 0.25 mm qid in older patients to avoid fibrous unions. This can be accomplished either once a day or divided throughout the day, determining the rhythm or frequency of distraction. While the distraction rate is 1.0 mm/d, ideally maintain the tissues under constant tension by dividing the total daily rate of distraction into smaller increments throughout the day to favor histogenesis. The total time of the distraction phase depends on achieving the clinical goals; individualize it to each patient and to the severity of the deformity. Remember that the total length of bone desired does not necessarily equal the total time of the distraction phase. External devices that use pins to transmit the forces frequently bend, and the distance at the site of the distracting mechanism on the device rarely equals the distance of the gap at the osteotomy sites. Use clinical guidelines (eg, position of the chin point, distance from the lateral canthus to the commissure and the mandibular cant) to determine the end point in children with hemifacial microsomia. Consolidation PhaseOnce the desired correction is achieved with the distraction phase, allow mineralization of the immature bone to occur. Lock the distracting appliance into place to maintain stability until the newly formed bone has sufficient strength. The length of this phase varies depending on the circumstances. In general, 6-8 weeks is considered adequate. A guideline used by some centers is 2 days of consolidation to every day of distraction. Retention PhaseRemove the device and maintain stability, typically with the assistance of orthodontic appliances. In children with hemifacial microsomia, this may require occlusal splints to guide the maxilla into position when the leveling of the mandibular cant creates a posterior open bite. In children with midfacial deformity, retention may require a face mask with elastic traction for a period of time.
Complications specific to the distraction process include the following:
With increasing clinical experience, the long-term outcome and the specific role of distraction osteogenesis are today better defined. Clearly, distraction can generate bone with the capacity for remodeling and adapting to the loads placed on it. However, distraction osteogenesis is likely incapable of restoring the normal development of a once dysplastic pattern of growth of the facial skeleton. Distraction techniques allow the surgeon to intervene earlier in childhood to restore the facial form and function, but the extent to which it eliminates subsequent conventional procedures remains uncertain.
As with conventional orthognathic surgery, distraction osteogenesis of the craniofacial skeleton should be considered as one of the many tools in the armamentarium of a surgeon. Unlike axial lengthening and rotations of long bone, osteotomies required to mobilize facial elements are complex in shape; also, the geometry of the transport vector is complex and nearly always multidirectional. The extent to which it will replace conventional approaches depends largely on technical innovations that will allow for implantable, multidirectional devices that can be easily activated and controlled remotely with minimal incisions.
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