OBJECTIVETo study the expression profiles of osteoprotegerin (OPG) and receptor activator of nuclear factor-KB ligand (RANKL) in the distraction region and to investigate the mechanism of bone remodelling during mandibular distraction osteogenesis.

METHODSOsteotomies were performed and external distractors were installed on the mandibles of 42 adult male SD rats. After a 5-day latency period, the distractors were activated at a rate of 0.4 mm/day for 6 days, followed by a 4-week consolidation period. Radiographs were taken, and specimens were harvested at the end of the latency period, when distraction was completed, and at of 1, 2, 3 and 4 weeks of the consolidation period. Tartrate-resistant acid phosphatase (TRAP) staining was used to detect the activated osteoclasts. Temporospatial expression of OPG and RANKL was investigated by using immunohistochemistry and reverse transcription polymerase chain reaction (RT-PCR). Semi-quantitative analysis was used to characterize OPG,RANKL and RANK/OPG ratio.

RESULTSIn all time points, OPG and RANKL were co-localized in bone marrow lining cells, osteoblasts and newly embedded osteocytes. OPG mRNA expression increased to a peak level when distraction was completed and maintained the level until the end of 2nd week of the consolidation period. RANKL mRNA expression increased steadily until the end of 1 st week of the consolidation period and maintained a peak level until the end of 3rd week, with a slight decrease at the end of 2nd week. The RANKL/OPG ratio increased continuously and reached its highest level at the end of 3rd and 4th week of the consolidation period. TRAP positive osteoclasts were mainly detected at 2, 3 and 4 weeks of the consolidation period in bone marrow cavities and bone surfaces.

CONCLUSIONSThe temporospatial expression patterns of osteoprotegerin and RANKL suggest that osteoblasts and the lineage cell network orchestrates bone remodelling during distraction. Osteogenesis and the most activated bone resorption takes place during 3rd and 4th week of the consolidation phase.