Because of innovations in distraction devices for both facial bones and the long bones of legs and arms, distraction osteogenesis promises to gain wider acceptance. Once announced by huge, unsightly metal cages and bars, in vivo tissue engineering now occurs silently and invisibly, supported by ingenious internal mechanical scaffolds. It’s the second groundbreaking phase of Ilizarov’s remarkable procedure.

After a surgeon slices through bone that is to be distracted, there’s a one- to two-day wait before the bone can be pulled apart. During this latency period, fibroblasts, which play a crucial role in wound healing, and new blood vessels invade the fracture site. The fibroblasts start laying down a matrix of collagen fibers, and once this sticky bridge, called callus, has formed, the bone can be distracted. As it is pulled apart, bone-producing cells known as osteoblasts migrate from both sides of the fracture and begin depositing osteoid, a bone matrix that hasn’t yet calcified. “Think of stalactites coming out of both ends of the bone,” says plastic surgeon Michael Longaker, director of the Children’s Surgical Research Lab at the Stanford School of Medicine. Once distraction ceases, osteoblasts complete the bony bridge with mineral deposits that turn the soft bone into normal bone.

Compared with other craniofacial reconstructive techniques, distraction osteogenesis has several advantages. It doesn’t require surgery at a second site, as bone grafts do, and it can be performed on children as young as a recent two-year-old patient of Maria Troulis, director of the Skeletal Biology Research Center at the Massachusetts General Hospital and associate professor of oral and maxillofacial surgery at the Harvard School of Dental Medicine. Since birth, the girl had been forced to breathe through a hole in her neck because her lower jaw was too small to pull her tongue forward and keep the airway clear. To remedy that, the jawbone would have to be stretched six centimeters on either side. “She was so small, there was no way we could have taken 12 centimeters of bone from elsewhere in her body,” Troulis says. “In addition, the soft tissue would not accommodate such a large skeletal movement to pull the jaw back to its original size.” But using distraction, Troulis successfully lengthened the girl’s jaw.

Still, the large, external hardware needed for expanding facial bones is hardly ideal. Most distractors consist of an external metal bar with multiple joints to move the bone in different directions, and connected to the bone with metal pins inserted through the skin. A screw mechanism that the patient or a family member turns every day during the distraction phase moves the bone apart. For every millimeter the bone is distracted—at one millimeter per day—the external fixator stays in place twice as long. So if the distraction phase is 35 days, fixation is another 70. Add in the necessary latency period, and the patient must wear the device for about 16 weeks. The pins attached to the fixator track through the skin as the bone moves, causing permanent scars. Moreover, because the patient controls the distractor, there’s a chance of human error.

To improve on that, Leonard B. Kaban, chief of the department of oral and maxillofacial surgery at the MGH and chairman of oral and maxillofacial surgery at the Harvard School of Dental Medicine, resolved in 1995 to create a hidden, implantable, motorized device capable of three-dimensional movements that could be remotely activated. The bone could be distracted continuously in tiny increments, which may lead to faster bone growth, according to preliminary evidence.

Kaban figured his team could produce a prototype within six months, and he wanted Troulis to test it in mini pigs, which have a jaw shape similar to that of humans. Though he was off on the timetable—it’s been 13 years—the device exists, and Troulis and Kaban are inching toward a model for human use.

The first challenge was to devise a way for the fixator to move the jawbone in three directions in incremental steps. With an external fixator, surgeons can make adjustments to move the jaw, but with an implanted device, there’s no chance for mid-course changes of direction without operating. “Getting the bone movement right is a problem, because teeth have to interdigitate accurately,” Kaban says.

Ed Seldin, an oral and maxillofacial surgeon on Kaban’s research team, used Euclidean geometry to show that any three-dimensional surgical movement follows a curved path with specific parameters. In 1996, working in his home machine shop, he built a curvilinear distractor that uses a rack-and-pinion drive mechanism. Two years later, Troulis tested a more advanced version of the device in the jaws of mini pigs.