EFFECT OF BONE CYLINDER LENGTH ON DISTRACTION OSTEOGENESIS IN THE RABBIT TIBIA

The viability and osteogenic potential of bone cylinders for bone tran sport was investigated in one tibia of 18 mature male New Zealand rabb its. The length of the bone cylinder was equal to or twice that of the diameter of the tibia. The cylinder was cut subperiosteally with an e xternally cooled oscillating saw from a lateral approach, after a spec ific unilateral external fixator had been applied. To simulate bone tr ansport, one end of the cylinder was fixed to the distal bone stump by a cerclage wire and healing and revascularization was prevented by an interposed expanded polytetrafluoroethylene membrane. The periosteum was readapted and sutured, and distraction began 10 days postoperative ly at 0.25 mm/12 hours for 22 days. New bone formation in the distract ion gap was quantified by dual energy x-ray absorptiometry and by comp uter-assisted histomorphometry of polyfluorochrome-labeled undecalcifi ed bone sections and corresponding microradiographs. In half of the an imals with each size cylinder, osseous bridging occurred, so the findi ngs on distraction osteogenesis are reported only for the remaining ni ne animals. Generally, centripetal mineralization of the gap with two distinct zones of ripening bone structures and a central radiolucent f ibrocartilaginous zone could be distinguished. Neither absorptiometry nor histomorphometry showed significant differences in the overall amo unt of this new bone formation for the bone cylinders of two different lengths. However, osteogenesis was significantly greater at the proxi mal end than at the cylinder. New bone was formed predominantly from e ndosteal sites in the smaller cylinders and from periosteal sites in t he larger cylinders. Histologically, there was complete necrosis of bo th sizes of cylinders, followed by revitalization through newly formed vascular channels. This process started earlier and was more pronounc ed with the smaller bone cylinders. It is concluded from these experim ents that even careful subperiosteal osteotomy with an oscillating saw leads to necrosis of both small and large bone cylinders. However, di straction osteogenesis still can be achieved and is mediated mainly by the revitalized endosteum in the smaller cylinders and by the periost eum, which apparently can be better preserved, in the larger cylinders . These results demonstrate the feasibility of even extremely small bo ne cylinders for distraction osteogenesis; the minimum length is deter mined only by the possibility of stable fixation.