The Changes Of Vertebral Bone Microcrack And Its Influence Factors In The Different Periods Of Ovariectomized Rats

OBJECTIVE:To explore the changes rule of bone microcrack in the lumbar trabecular and its influence factors and research the correlation between the bone microcrack and the bone mineral density, microstructure, biomechanics in the lumbar trabecular and biochemical markers in different periods of ovariectomized rats.METHODS:100 female 7-month-old Sprague Dawley(SD) rats were randomly divided into the following 3 groups:baseline control (unoperated group, n=20), ovariectomized group(OVX, n=40) and sham-operated group(SHAM, n=40). At beginning of the experiment,20 rats of the baseline control group were sacrificed, then 20 rats of the OVX group and the SHAM group were sacrificed at 3 and 15 weeks postoperative respectively. The lumbar vertebral bodies and artery blood samples were harvested and preserved for measuring the bone microstructure, mechanical, microdamage and biochemical markers. Tartrate-resistant acid phosphatase 5b(TRAP-5b),serum osteocalcin(OC), and bone alkaline phosphatase(ALP) were measured from serum specimens. The fifth lumbar(L5) vertebral bodies were performed axial compressive test for testing mechanical properties. The L6 vertebral bodies were performed micro-computed tomography (micro-CT) to estimate the bone mineral density (BMD) and microstructure parameters, then the fatigue damage test were performed. The bulk-stained in 1% basic fuchsin was finished to stain the microcrack. The microcrack and osteocyte density (Ot.N/T.Ar) parameters were measured.RESULTS:1. In the OVX rats, the bone mineral density, bone mineral content, trabecular number, trabecular cell density and trabecular connectivity density decreased gradually with estrogen decreased. The serum bone specific alkaline phosphatase and tartrate-resistant acid phosphatase 5b decreased. Trabecular separation and trabecular thickness increased, the vertebral microcrack density and microcrack surface density increased. The above changes in the OVX 15 week group were more evident than that of OVX 3 week group.2. There was significant positively correlation between vertebral microcrack density and microcrack surface density(r=0.807,p<0,001), but the two indexes were not significant correlation with the average length(p=0.623,p=0.232). Microcracks indicators were not significantly correlated(p>0.05) with TRACP-5b, ALP, OC and the mechanical indicators(ML, EM). The vertebral microcrack density significantly correlated with bone mineral content, bone mineral density, tissue bone mineral density, trabecular separation, trabecular connectivity density (p<0.05). The vertebral microcrack surface density significantly correlated with the trabecular thickness (p<0.05). There was significant negative correlation between the average length of vertebral trabecular microcracks and the average vertebral thickness (r=-0.522, p=0.004).CONCLUSION:In ovariectomized rats, the bone mass of the lumbar vertebrae losts rapidly while the microstructure of spongy bone deteriorated and microcrack increase gradually. At the different periods of ovariectomized rats, the microstructure indexes of the vertebral trabecular bone (bone mineral content, bone mineral density, tissue bone mineral density, trabecular separation, trabecular thickness, trabecular connectivity density) are correlated with microcrack development. The microcrack may play a role in the change of the bone structure and mechanical properties.