| Introduction & hypothesis. Distraction Osteogenesis (DO) is used clinically for bone lengthening, however its underlying mechanisms are not fully understood. The purpose of this study is to develop an animal model for DO and use it to test the hypothesis that linear stress will lead to temporal expression of PCNA, TGF-beta, alkaline phosphatase (ALP), and osteocalcin (OC) which should correlate with enhanced cell proliferation, cell differentiation and osteogenesis during the various stages of DO in comparison to fracture healing.;Materials & methods. 45 white New Zealand rabbits were divided into 3 groups: Group I comprised 20 rabbits to which mandibular distraction was performed. Group II comprised 20 rabbits, which represented the fracture-healing group. Group III comprised 5 rabbits, which served as normal control. GP I and II were subdivided into 4 subgroups, 5 rabbits each: A, B, C and D which represented different sacrifice time periods coinciding with mid-distraction, end of distraction, mid-consolidation and end of consolidation. Specimens were then subjected to histological, immunocytochemical, histomorphometric and densitometric techniques.;Results. Utilizing contact micro-radiography and bone histomorphometry, we have demonstrated the progressive increase in bone density and bone volume fractions from mid-distraction toward the end of consolidation. Bone density mean gray level at the end of consolidation attained 81.8% that of normal control (P > 0.01) and bone volume fractions at the end of consolidation attained 86.9% that of normal control (P < 0.01). PCNA and TGF-beta immunostaining showed increased number of PCNA (P < 0.001) and TGF-beta (P < 0.001) positive cells during the distraction phase followed by a rapid decline during early consolidation. ALP immunostaining showed maximal expression at the end of consolidation (P < 0.001), remained elevated during early consolidation and declined to normal levels by the end of consolidation. OC expression was downregulated during early distraction; peak at mid-consolidation (P < 0.01) and returned to normal control levels by the end of consolidation.;Conclusions. Our results confirmed the efficacy of the rabbit model in studying the cellular events during mandibular distraction. We demonstrated that the bone density and bone volume fractions attained near normal levels by the end of consolidation period. PCNA and TGF-beta expression together with quantification of marrow cells per unit area showed that the tension applied during distraction both increased and prolonged cellular proliferation within the regenerate. Also, mechanical strain may delay the differentiation of osteoblasts, demonstrated by the downregulation of osteocalcin during the distraction phase compared to fracture healing. An inverse relation was observed between the temporal expression of TGF-beta and OC, thus TGF-beta may also play a role in regulating OC expression in the distraction regenerate. |
