I. Introduction
Guided bone regeneration(GBR) is a well-established technique, both in the reconstructing of atrophic alveolar bone prior to implant placement and around exposed implant surfaces at the time of implant installation. [1] [2] [3] [4] For this promising technique, two basic components are needed -membrane and graft material. 1) The membranes itself placed and closely Although the use of autogenous bone is suggested as a 'gold standard' for effecting bone regeneration, the comparable results can be obtained using nonautogenous grafting materials.
7)
Among available graft materials, natural bone mineral of bovine origin has shown good osteoconductive properties in various types of defects. [8] [9] [10] [11] [12] In this study, histologic and histomorphometric observations of bone sections obtained from defect sites at surgically created mandibular defects of beagle dogs that had been augmented with new bovine hydroxyapatite/collagen membrane are reported.
II. Materials and Methods

Surgical procedure in beagle dog mandible
Four beagle dogs were used in this study.
All animals enrolled in this study were cared and processed in accordance with the Seoul National University Guidelines for the care and use of laboratory animals. 
Statistical analysis
The overall data was statistically analyzed with repeated measures of ANOVA with posthoc(Tukey)(p<0.01).
III. Results

<Histologic findings>

Non-graft(NG) group
The new bone primarily consisted of intensely stained woven bone. However, the regenerated bone close to the pre-existing bone exhibited a more lamellar structure.
The defect showed a continued bone formation at 6 weeks.
Newly formed peripheral bone, mainly at the lower half of the defect, resulted in a significant bony defect, which primarily healed by connective tissue repair (Figure 1-A) .
Graft(G) group
Newly formed bone was well evident in the defects filled with bovine hydroxyapatite.
The grafted materials are in direct contact with the newly formed bone as well as with connective tissue (Figure 2-A) . The osseous tissue around the grafted particles presented numerous osteocytes, typical to a woven bone pattern. At 6 week, the bridging of new bone tissue around graft particle was observed (Figure 3-A) . However, at periphery of the defect, the new bone tissue was sparse compared to the center of the defect.
The defect was not collapsed as severely as 'NG' group and maintained its shape, but some specimens showed the slight ridge deformities (Figure 1-B) .
Graft+Membrane(GM) group
Bone regeneration was similar to 'G' group( Figure 2-B, 3-B) . However, the newly However, no significant difference was found with regard to the use of membrane.
Regenerated bone area(Bone area fraction) ( showed smaller regenerated bone area than 'G' group and 'GM' group, and the difference between 'NG' group and 'G' group or 'GM' group was statistically significant.
IV. Discussion
Bovine hydroxyapatite used in this study was capable of completely restoring experimental defects in dogs. The border between the old and newly formed bone was clearly distinguishable. Newly formed bone was observed in the grafted sites, as well as Araújo et al. 13) reported the successful lateral ridge augmentation using bovine-derived bone substitute in canine model.
Compared to the autogenous block bone graft which underwent marked resorption, the biomaterial could retain its dimension, and allowed amounts of new bone to be formed within the graft material.
In Hockers's animal study 14) , the deprote- Zitzmann et al. 15) used Bio-oss® for alveolar ridge augmentation with collagen membrane, Bio-Gide® in humans. After 6 to 7 months following grafting, the histologic analysis revealed an intimate contact between woven bone and graft particle along 37% of the particle surfaces.
Anorganic bovine bone used in sinus augmentation procedures was also showed a good osteoconductivity. And the resorption rate itself is controversial, because some studies reported a predictable resorption of particle after 12 to 13 months postoperative in augmented sinus and extraction socket 21, 22) .
In this study, new bovine hydroxyapatite was proved to be an excellent osteoconductive agent in grafted sites, which biologically incorporated with newly formed osseous tissue. Further studies about the resorption rate of bovine hydroxyapatite and its safety in humans are needed.
V. References 

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