The central concept behind distraction is that introducing a mechanical force in bone will alter the steady state apposition-resorption equilibrium in favor of apposition and consequently reconstruction. Any part of the body that exists in this steady state has the potential to respond to distraction. A requirement for successful regeneration with distraction is an intact functional matrix. The muscles, nerves and soft tissue must be capable of accommodating the new bone tissue. The distraction device must not intrude on the functional matrix. The surgical procedure must be minimal to avoid disrupting the functional matrix.

An Intra-Oral Approach

Dr. Chin developed his interest in distraction when faced with the inability of traditional surgical techniques to improve mid-face deficiency. The classic LeFort III osteotomy to advance the mid-face in Crouzon Deformities yielded poor results as the child grew because the functional matrix had not been taken into account. His distraction technique in these cases has evolved over the past ten years.

He currently uses an almost entirely intra-oral approach as opposed to the large coronal incisions needed for the LeFort III operation. The maxilla is distracted 10mm at surgery with heavy forces from the distraction device that causes a controlled fracture of the orbital floor and pterygoid plates.

The child is then moved to the ICU and kept intubated and under light anesthesia for another 48 hours while the mid-face is distracted another 15mm. for a total of 25mm of advancement. This often produces a temporary Class II occlusion, accounting for future growth. The procedure can improve nocturnal breathing in these children, many of whom were dependent on tracheotomies.

As they grow into adulthood these children develop a normal facial appearance because the muscle, nerve and soft tissue comprising the functional matrix was transported along with the bone and was minimally disrupted by the surgery. Regenerating the soft tissue is more important than osseous morphology.

Mandibular distraction surgery is done with a traditional saggital split osteotomy and an intra-oral distraction device. The distraction rate is slower than in the maxilla due to the resistance of the functional matrix. The muscles restraining mandibular advancement are much more restrictive than in the maxilla. In Dr. Chin's opinion, the protocol followed by orthopedists where the distraction is delayed for five days while a callus forms has no validity in facial distraction. Occasionally if the distraction is not rapid enough the bone will heal across the surgery site during the distraction procedure.

Distraction in cleft lip and palate surgery

Cleft palate patients typically need a maxillary advancement and often a maxillary downgraft when growth is completed. Because of the previous surgeries to the maxilla, the soft tissue functional matrix is very restrictive to these movements and usually causes a near total relapse of the surgical correction. Conventional rigid fixation is not adequate to prevent relapse.

Distraction works well in these patients because the distractor can overcome the soft tissue pressure and remains in place until the bone heals. The distraction is always pushed a few millimeters beyond what is necessary in these patients to accommodate a small degree of relapse. Unrepaired alveolar clefts in patients over the age of 15 are not amenable to grafting due to a failure rate over 50%. A better plan is not to attempt a graft, but rather to close the cleft by further collapsing the maxilla and then to correct the Class III problem with maxillary advancement distraction. The gingival volume and subsequent health over the cleft site is dramatically improved with this technique.

Trauma Patients

A large number of Dr. Chin's orthognathic patients have been the victims of trauma to the anterior maxilla, usually motor vehicle accidents, where teeth have been lost. In most of these cases there is inadequate bone remaining for implant placement and bone grafting the anterior maxilla to the degree necessary is impossible. In these cases he performs a LeFort I osteotomy, often in three pieces to control the width and then distracts the maxilla anteriorly. Combined orthodontic treatment is necessary because the maxillary teeth are no longer in a Class I position with the mandible. The anterior maxilla is now positioned for proper placement of implants.

The basic theme is that the slower movement is more stable because it has less chance of damaging the matrix system that maintains morphology to begin with. Dr. Melvin Moss's concept of the Functional Matrix, while difficult to appreciate in his writings, is the overriding principle that governs the success of distraction surgery.

Alveolar Distraction

It is well known that grafting an alveolar site to increase vertical height will fail. Alveolar bone can be transported vertically with a distractor. Because the bone being transported is mature bone an implant can be placed much earlier than in grafted bone. Alveolar bone can also be distracted laterally to create space for implants, or, aveolar bone can be distracted laterally to create bone for orthodontic tooth movement into the site.

Bone Morphogenetic

Protein BMP - Human recombinant BMP as described by Urist will convert stem cells into osteoblasts. When this material is added into the distraction chamber the rate of new bone formation is dramatically enhanced. FDA approval for this technique has lagged behind the basic research.

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