A bone defect that is not repaired with bone completely is designated a non-union defect or a critical-size defect. The biological mechanism that regulates the process of bone repair of the critical-size defect remains unknown. The present study was designed to investigate bone repair in a critical-size defect compared with that in a smaller or non-critical-size defect. Our original standardized rat calvarial bone defect model was used for the experiment. The rate of bone formation was examined with X-ray morphometry and the bone production of osteoblasts and osteocytes was assessed by molecular histology with in situ hybridization for type I collagen and osteocalcin. Formation of repaired bone ceased within 24 weeks in both critical- and non-critical-size defects i.e. regardless of completion of the defect repair. The results suggested that osteoblasts and osteocytes cease bone formation, and the differentiation of osteoblast progenitors declines in 24 weeks. Also, bone repair proceeds from the periosteum on both sides of the parietal bone but not from the surface of the bony edge around the original defect. The results could provide useful information for clinical research on bone repair.

S U M M A R Y Extracellular matrix (ECM)remodeling is achieved by both production and degradation of ECM molecules during bone development. ADAMTS (a disintegrin and metalloprotease with thrombospondin type 1 motifs) constitutes a family of extracellular proteases which are implicated in cleaving the protein versican. The present study was designed to investigate the expression of versican and ADAMTS1, 4, and 5 mRNA during bone development in rat mandibles and hind limbs by RT-PCR and in situ hybridization. Versican was localized by immunohistochemistry. The process of bone development from day 14 postcoitum through week 6 postnatum was divided into the beginning of osteogenesis, woven bone, and lamellar bone stages. Versican protein was abundant in the woven bone matrix, but decreased in the lamellar bone matrix. Versican mRNA was prominent in some osteoblasts with corresponding localization of the cognate protein. The temporal and spatial mRNA expression pattern of ADAMTS1, 4, and 5 was comparable to that of versican. These results suggest that woven bone rich in versican alters into lamellar bone containing little versican during bone development in both mandibles and hind limbs, where some osteoblasts may be involved in production as well as degradation of versican by secreting ADAMTS1, 4, and 5.

A disintegrin and metalloprotease with thrombospondin type 1 motifs (ADAMTS) is a family of extracellular proteases and implicated in cleaving proteoglycans, such as aggrecan, versican and brevican. No information is available about expression or localization of these ADAMTSs in teeth. Versican is a large chondroitin sulfate proteoglycan that is present in a variety of connective tissue including dental pulp, dentin, cementum and periodontal ligaments. The present study was designed to investigate expression of ADAMTSs and versican during rat tooth eruption. Rat maxillary first molars in weeks 1, 2, 3, 4 and 6 were examined. The mRNA expression of ADAMTS1, ADAMTS4, ADAMTS5 and versican was localized using in situ hybridization. ADAMTS1, ADAMTS4, ADAMTS5 and versican were expressed in dental pulp cells, odontoblasts, cementoblasts, cementocytes, periodontal ligament cells, osteoblasts and osteocytes. The temporal and spatial expression pattern in these cellular phenotypes was comparable among ADAMTSs and versican. The present study suggests that dental pulp cells, odontoblasts, cementoblasts, cementocytes, periodontal ligament cells, osteoblasts and osteocytes may be involved in both production and degradation of versican with secreting ADAMTS1, ADAMTS4 and ADAMTS5.

Piezosurgery is an innovative technique widely used for osteotomies in the field of oral and maxillofacial surgery. The surgical technique has been clinically supposed to cut mineralized bone selectively with reducing the risk of damage to adjacent soft tissues. However, none of the previous literature has reported any evidence of scientific experiments to examine performance of the piezoelectric device, i.e. the time required for cutting bone and the effect on soft tissues under the standardized conditions. This study was designed to test the hypothesis that cutting time of the piezoelectric device is longer than that of rotary instruments while the cut surface of bone is smoother and soft tissues are less damaged with piezosurgery under the standardized experimental system. We measured the time for cutting bone and soft tissues of rats with the piezoelectric device and rotary instruments. Damage to soft tissues was examined histologically, and the cut surface of bone was investigated using scanning electron microscopy. Our study demonstrated experimentally that piezosurgery provides a smooth cut bony surface with no damage to soft tissues and takes longer time to cut bone than conventional drillings. We propose that piezosurgery is beneficial for medical safety and usability.

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