3 MODES OF OSSIFICATION DURING DISTRACTION OSTEOGENESIS IN THE RAT

We developed a rat model of limb lengthening to study the basic mechan ism of distraction osteogenesis, using a small monolateral external fi xator. In 11-week-old male rats we performed a subperiosteal osteotomy in the midshaft of the femur with distraction at 0.25 mm every 12 hou rs from seven days after operation. Radiological and histological exam inations showed a growth zone of constant thickness in the middle of t he lengthened segment, with formation of new bone at its proximal and distal ends. Osteogenic cells were arranged longitudinally along the t ension vector showing the origin and the fate of individual cells in a single section. Typical endochondral bone formation was prominent in the early stage of distraction, but intramembraneous bone formation be came the predominant mechanism of ossification at later stages. We als o showed a third mechanism of ossification, 'transchondroid bone forma tion'. Chondroid bone, a tissue intermediate between bone and cartilag e, was formed directly by chondrocyte-like cells, with transition from fibrous tissue to bone occurring gradually and consecutively without capillary invasion. In situ hybridisation using digoxigenin-11-UTP-lab elled complementary RNAs showed that the chondroid bone cells temporar ily expressed type-II collagen mRNA. They did not show the classical m orphological characteristics of chondrocytes, but were assumed to be y oung chondrocytes undergoing further differentiation into bine-forming cells. We found at least three different modes of ossification during bone lengthening by distraction osteogenesis. We believe that this is ease first report of such a rat model, and have shown the validity of in situ hybridisation techniques for the study of the cellular and mo lecular mechanisms involved in distraction osteogenesis.