Distraction osteogenesis, or bone lengthening, is a surgical procedure which involves cutting and slowly separating bone, allowing the bone healing process to fill in the gap.
Distraction Osteogenesis (Bone Lengthening)
Distraction osteogenesis, bone lengthening, is an established surgical technique that allows for molding, shaping, and lengthening of bone. The procedure involves making controlled cuts, or osteotomies, in the bone, and using a device to gradually separate, or “distract”, the bone fragments. The fragments are then held apart by the distraction device over a period of 1-3 months, during which new bone forms, a process called “osteogenesis”. Distraction osteogenesis has been used in orthopedic and hand surgery to lengthen bones in the extremities, and the technique has more recently been applied to the craniofacial skeleton.
History of Distraction Osteogenesis
Distraction osteogenesis has been performed for over 100 years. It was first performed by Allessandro Codivilla in 1905 to lengthen femurs in a series of patients with malformed legs. In the 1950s, a Russian orthopedic surgeon named Gavril Ilizarov studied and improved the technique, applying it to the long bones of the lower extremities. His research helped elucidate how distraction osteogenesis works on a cellular and physiological basis, and the ideal tension, rate, and rhythm of distraction. Joseph McCarthy pioneered the use of distraction osteogenesis in the craniofacial skeleton in the early 1990s.
How does distraction osteogenesis work?
The process of distraction osteogenesis is divided into several stages: osteotomy, latency period, activation period, consolidation period. During surgery, the osteotomies (bone cuts) are made and distraction devices are applied to the bone segments. After surgery is the latency period, in which no distraction is performed. The duration of the latency period depends on the age of the patient and the part of the skeleton being distracted. Older children and adults generally have a latency period of 5-7 days. Infants undergoing mandible distraction have a latency period as short as one day. During the latency period, a hematoma is formed at the site of the osteotomy and inflammatory cells migrate into the bone gap. This process is similar to the beginning of normal fracture healing.
During the activation period, the distractors are gradually turned and the bone segments are gradually pulled apart. Careful design in surgery determines the vector of distraction, or the direction and angle the bone segments are pulled apart considering the desired final result. The timing and extent of distraction performed during each turn of the distraction devices is termed the rate and rhythm. The rate of distraction refers to the number of times each day the device is activated. The rhythm denotes the distance per turning episode that the bone distracted. For example, if the distraction device is turned twice a day for a distraction of 0.5 millimeters (mm) each time, the rate is two times a day, and the rhythm is 0.5 mm. This is a typical rate and rhythm for distraction osteogenesis, but may vary based on age and the part of the skeleton being distracted. Infants undergoing mandible distraction often have a rate of 2 times a day, with a rhythm of 0.9 mm each time. During the activation phase, new blood vessels develop in the area between the bone segments. Cells that create collagen, a structural protein crucial in wound healing and a key component of the structural support of the body, migrate into the bony gap and begin creating collagen.
After the desired length of bone is achieved by the activation phase, the consolidation phase begins. During this time, which lasts 1-3 months, the distraction devices are no longer turned, but act as rigid fixation devices holding the bone to length. The area between the bone segments forms new bone. Within the gap between the bone segments, the bundles of collagen become calcified, and osteoblasts appear. These are the cells that create new bone. The bone forms from the edges of the cut bone towards the center of the generate, which is the term for the newly formed bone. Finally, osteoclasts, the cells that remodel bone into mature bone, appear within the generate.
After the consolidation period, the distraction devices are removed. This is also an opportunity to revise scars if needed.
What are the types of distraction devices?
While there are many types of distractors, depending on the manufacturing company and area of the body being distracted, there are two broad categories: internal and external distractors. Internal distractors are largely buried beneath the skin, with only the turning arm protruding. External distractors are anchored into the bone using pins or screws, but the majority of the device is outside of the body. Each has advantages and disadvantages. Internal distraction devices (figure 1) are more subtle during the distraction process, but require an operation to remove. External distraction devices (figure 2) are overt during the distraction, but can be removed in a clinical setting. They potentially provide a more stable and predictable activation and consolidation phase for certain types of distraction. In reality, each type of device plays a role, and selection depends on a variety of variables, including the part of the skeleton being distracted and the extent of distraction.
For which patients is distraction osteogenesis appropriate?
When more bone is needed in the craniofacial skeleton, there are three options: bone grafting, free tissue transfer, and distraction osteogenesis. Each may be appropriate in reconstruction of the craniofacial skeleton depending on the situation. Bone grafting involves removal of bone from one part of the body and placing it elsewhere, where there is a paucity of bone. This works when there is healthy soft tissue, and when the amount of needed bone is small. Free tissue transfer involves taking bone with its blood supply (the artery and veins supplying the bone with blood) from one part of the body, transferring the bone, and connecting the blood vessels under a microscope to blood vessels near where the bone is needed (see the microsurgery page). This option is ideal when large amounts of tissue are needed, in areas with poor blood supply, and when extra soft tissue is needed in the defect, as soft tissue can be included in the flap. Distraction osteogenesis allows for creation of new bone without taking bone from elsewhere. It does require a healthy soft tissue envelope. An added benefit is that the process of distraction osteogenesis allows for expansion of the soft tissue, as well. This is called distraction histiogenesis.
In the craniofacial skeleton, distraction osteogenesis is used for a number of diagnoses. The most common application is in infants with small mandibles (micrognathia) leading to tongue-based airway obstruction, difficulty breathing, and sleep apnea (see Pierre Robin sequence). Distraction osteogenesis is used to lengthen the mandibles (figure 3) of patients with hemifacial microsomia, as well. In the midface (maxilla or upper jaw), the majority of anomalies can be treated with traditional jaw surgery. However, in patients with severe deficiencies (severe maxillary hypoplasia), traditional surgery leads to high rates of relapse, which is often secondary to an inability of the soft tissue to accommodate the bony movements. Examples of patients that benefit from distraction osteogenesis of the maxilla include patients with congenital facial anomalies such as Binder syndrome and some patients with cleft lip and palate. The appropriate level on the maxilla of the osteotomy and distraction depends on the level of maxillary deficiency. LeFort 1 osteotomies and distraction (figure 4) are used in patients whose deficiency involves the lower maxilla, and LeFort 2 and LeFort 3 osteotomies and distraction osteogenesis (figure 5) are utilized in patients with more extensive deformities. In certain patients with craniofacial syndromes that include craniosynostosis, such as Crouzon syndrome, distraction osteogenesis of the forehead, front of the skull, and midface are needed to correct the deformity. This is called frontofacial or monobloc distraction (figure 6). Distraction osteogenesis may also be a treatment option in cleft patients with severe defects on the alveolus, or tooth-bearing area of the maxilla.
More recently, distraction osteogenesis has been utilized in the management of certain patients with craniosynostosis (see craniosynostosis page). This is called cranial vault distraction osteogenesis (figure 7). Patients under 1-1 ½ years of age are able to spontaneously reform the bony gaps created in traditional cranial vault surgery. Older patients require either bone grafting or distraction osteogenesis. In addition, the scalp may be too tight to allow for traditional cranial vault remodeling in certain patients; distraction osteogenesis allows for a gradual expansion of the scalp. Certain patients with severe craniosynostosis and cephalocranial disproportion (brain pressure due to inadequate skull volume) may require urgent expansion, or multiple operations to address all of the areas where the skull needs expansion. Distraction osteogenesis allows for early significant skull advancement, and the ability to operate on other parts of the skull at the appropriate age.
If you would like more information about this craniofacial anomaly, please contact the Craniofacial Team of Texas by calling 512-377-1142 or toll free 877-612-7069 to schedule an appointment or complete an online appointment request.