Introduction: Current treatments for osteoporosis do not completely eliminate the risk of fracture and bone loss may continue even at a low level. Enhanced bone formation and mineralization could minimize the risk of fracture in osteoporosis and prevent the pain and associated morbidity in these patients. Bone morphogenetic protein-type 2 has been successfully used to promote bone formation and to augment fracture repair in general and in the spine in particular [1]. The aim of this study was to increase local bone formation and mineralization in osteopenic vertebrae by administration of recombinant human morphogenetic proteins (rhBMP-2) in an ovine model.

Methods: Osteoporosis was induced in ten skeletally mature sheep with ovariectomy, low calcium diet and weekly steroid injection. Bone mineral density (BMD) of the lumbar spine was assessed monthly by DXA. When the BMD of the lumbar spine was reduced by at least 25% the induction treatment was stopped and pellets containing inert carrier alone (control) or rhBMP-2 in either slow or fast release formulation were implanted directly into three adjacent lumbar vertebrae of each animal in a random order. BMD was assessed at regular intervals and two and three months later five animals were euthanized and the lumbar spines were collected for histomorphometric analysis using the SkyScan 1076 Micro CT (SkyScan, Belgium). Significant differences between BMD and bone morphometric data (including trabecular bone volume, separation and number) were examined using ANOVA and Tukey’s test with significance set at P< 0.05.

Results: After five months of induction treatment BMD in the lumbar spines of all animals was reduced by at least 25% (p< 0.05). BMD increased insignificantly after cessation of the induction treatment but remained lower than the initial values. As there were no significant differences the histomorphometric data after two and three months were pooled. The trabecular bone volume in the vicinity of both the slow and fast release BMP implants increased by over 15% compared with the control (p< 0.05). Trabecular separation was reduced over 13% and trabecular number around both types of pellets increased by over 12% compared to the control (NS).

Discussion: This animal model provides an opportunity to evaluate systemic and local treatments for osteoporosis. The significant increase in bone formation adjacent to the implants as early as two months suggests that rhBMP-2 in either formulation improves bone quality at sites with high risk of fracture. The impact of the fast and slow release BMPs implants were not significantly different

Introduction Sheep are being used increasingly for spinal and other skeletal-related research. However, there is still limited information about the molecular pathways of bone remodelling in this species compared to rats or mice. It has been demonstrated in other animal models and in the human that the receptor activator of nuclear factor kappaB ligand (RANKL) and osteoprotegerin (OPG) play major regulatory roles in controlling osteoclast activity and their differentiation. We investigated the expression of RANKL and OPG in trabecular bone of an ovariectomised steroid-treated osteopaenic sheep model.

Methods Trabecular bone from the lumbar spine (LS) and proximal femur (PF) of ten osteopaenic ewes and four normal ewes were collected [1]. Total RNA was isolated and complementary DNA (cDNA) was synthesised. DNA encoding RANKL and OPG were sequenced and ovine specific primers were designed to amplify the cDNA by real time RT-PCR to generate products corresponding to mRNA encoding RANKL and OPG. The results were normalised to 18S RNA.

Results Total OPG expression (in trabecular bone) from the PF region was over two fold higher than the LS (P< 0.0001). The relative expression of OPG in the both LS and PF regions were significantly higher in the treated animals (steroid & oophorectomy) compared to controls (p< 0.05). The relative expression of RANK-L in the PF was significantly higher than in the LS (P< 0.0001). However, the relative RANK-L expression in the treated animals was not significantly different from the control animals in either region. The ratio of RANK-L:OPG in the PF and the LS was not significantly different but it was significantly reduced in the osteopaenic animals.

Discussion Based on this gene expression study and previous histomorphological data, it appears that trabecular bone loss is not due to increased osteoclastic activity but may rather due to lack of osteoblastic activity and function. Higher expression of OPG and RANK-L and greater bone loss compared to LS suggest that the rate of bone turnover is greater in the PF. Further investigation of the molecular pathways of bone loss in this animal model will increase its utility for osteoporosis research.