Distraction Osteogenesis 

Distraction osteogenesis is a technique that relies on the normal healing process that occurs between surgically osteotomized bone segments. De novo bone lengthening occurs by gradual, controlled distraction. In contrast to traditional approaches, the soft tissue envelope (the skin, muscle, and neurovascular structures) is simultaneously expanded, which stabilizes the skeletal reconstruction. The technique today is an important part of the reconstructive surgeon's armamentarium.

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 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 fractured femur segments and used springs to lengthen 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. Ilizarov minimized complications by 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. Through a series of experimental studies and clinical applications, Ilizarov established the foundation of distraction osteogenesis and its role in orthopedic management.^[1] Applications in craniofacial surgery were first seen in 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.^[2] Thereafter, its role rapidly expanded to the mid face and nearly all classic approaches to craniofacial reconstruction.

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."^[1] The premise 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.

Osteogenesis may be through a cartilaginous or fibrous intermediate. The former, known as endochondral bone formation, is found in the axial skeleton; the latter, intramembranous ossification, is primarily in the craniofacial skeleton. Distraction osteogenesis mimics intramembranous ossification where recruitment and differentiation of primitive cells create a new bony framework. Histologic studies identified 4 stages that result in the eventual formation of mature bone.

• Stage I, Fibrous tissue precursor: The intervening gap initially is composed of fibrous tissue comprised of longitudinally oriented collagen with spindle-shaped fibroblasts within a mesenchymal matrix of undifferentiated cells.
• Stage II, Creation of bony scaffold: 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, Bony remodeling: Remodeling begins with advancing zones of bone apposition and resorption and an increase in the number of osteoclasts.
• Stage IV: Structural modification: 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 is 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. Its architecture is believed to be able to withstand normal functional loads.

Indications for the use of distraction are broad, and it is applied to solve a wide range of craniofacial deformities. While its use continues to evolve, its appropriateness depends on the particular clinical problem.

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 dependent on tracheostomy, early bone lengthening through distraction may be beneficial.^[3] Increasing experience with neonatal distraction has shown that in selected cases (eg, Pierre Robin sequence), the need for tracheostomy can be avoided.^{[4, 5, 6]} Moreover, distraction allows for correction of the hypoplastic mandible earlier in childhood. Traditional approaches waited until adolescence when the facial skeleton had matured. See the image below.

Children with severe midfacial deformities may also benefit from early distraction.^{[5, 7]} Lessening the maxillary deformity in childhood can minimize dentofacial abnormalities in adolescence and provide a more stable outcome.

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 orbitofrontal advancement. Nevertheless, as with any promising innovation, the benefits and risks of distraction must be weighed 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 decreased blood loss and morbidity associated with bone grafts.

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.