Adaptations in the mandible and appendicular skeleton of high and low bone density inbred mice

Oncologic doses of zoledronic acid induce site specific suppression of bone modelling in rice rats

OBJECTIVES

To examine the effect of zoledronic acid (ZOL) on cortical bone modelling and healing of extraction sockets in the jaw bones of a rodent model. We hypothesized ZOL suppresses both the bone formation in the modelling mode in the jaw bones and alters the extraction site healing.

MATERIAL & METHODS

Rice rats were administered saline solution and two dose regimens of ZOL: 0.1 mg/kg, twice a week, for 4 weeks (n=17, saline=8 & ZOL=9) and a higher dose of 0.4 mg/kg, weekly, for 9 weeks (n=30, saline=15 & ZOL=15). Two pairs of fluorochrome bone labels were administered. Extraction of maxillary teeth was performed in maxilla. Mineral apposition rate, mineralizing surface and bone formation rate (BFR) were quantified on periodontal (PDL), alveolar and basal bone surfaces, and in the trabecular bone of proximal tibia. Bone volume (BV) was evaluated at extraction sockets. Multivariate Gaussian models were used to account for repeated measurements, and analyzes were conducted in SAS V9.3.

RESULTS

ZOL suppressed bone modelling (BFR/BS) at the PDL surfaces in the mandible (P<.05), but its effect was not significant at the periosteal surfaces of both jaws. BV for the healing sockets of ZOL treated animals was not significantly different (P=.07) compared to the saline group. ZOL suppressive effect was higher in the tibia compared to the jaws.

CONCLUSION

ZOL severely suppresses coupled remodelling in the tibia, and the suppression of bone formation in the modelling mode in the jaws demonstrates the site specific effects of ZOL in rice rats.

Quasi-static and harmonic indentation of osteonal bone

The purpose of the study was to compare Quasi-Static (QS) and harmonic (CSM) methods of indentation testing. Bone sections were obtained from mid-femoral diaphyses of dogs which received a pair of calcein labels. Labeled (n = 35) and unlabeled (n = 112) osteons were identified. Indentation modulus (IM) and hardness (H) for the CSM method were collected during the entire loading cycle to peak depth, while IM and H for QS method were calculated at a peak depth of 500 nm. Results: The mean (SD) of the IM and H for labeled osteons were as follows: QS IM = 15.3 GPa (3.85) versus CSM IM = 14.7 GPa (3.58); P = .52 and QS H = .39 GPa (.171) versus CSM H = .42 GPa (.146); P = .32. The mean (SD) of the IM and H for unlabeled osteons were as follows: QS IM = 21.5 GPa (2.80) versus CSM IM = 20.6 GPa (2.53); P = .054 and QS H = .64 GPa (.117) versus CSM H = .70 GPa (.120); P = .017. There was no difference in IM and H for the two methods, except for H of the unlabeled osteons. In addition, for the CSM method, IM at 100 nm, 200 nm, 300 nm, 400 nm and 500 nm were not statistically significant different (P = .06). Bone is viscoelastic at an organ level. However, this component of its behavior was not detected at the length scale examined.

Zoledronic acid decreases bone formation without causing osteocyte death in mice

Bisphosphonates have been associated with osteonecrosis of the jaw. The purpose of this study was to examine the effect of a potent bisphosphonate, zoledronic acid (ZA) on osteocyte viability and bone formation. Ten experimental C57BL/6 mice were administered ZA (0.1 mg/kg-i.p.) weekly for 9 weeks while four control mice did not receive the drug. A pair of calcein (30 mg/kg) labels was administered 10 and 3 days prior to sacrifice of the 34-week-old mice. Fresh mandibular and femoral sections were obtained to evaluate osteocyte viability using a lactate dehydrogenase (LDH) assay. In addition, sections from the femur, mandible and maxilla were prepared for standard histomorphometry. The operator was blinded for data collection to eliminate bias. Data on necrotic area/total bone area from the LDH sections were collected. In addition, standard histomorphometric variables including bone formation rate were calculated. Mixed models were used to analyse data. The osteocytes were overwhelmingly viable and no necrotic areas were detected in the mandible and femur of both groups. ZA was not directly cytotoxic to the mouse osteocytes. There was suppression in indices of bone formation at all skeletal sites of the ZA group compared to the control group. While ZA administration in mice does not produce necrotic osteocytes, it severely suppresses bone formation. Such reductions can have a profound effect on bone healing.

Postpubertal architectural developmental patterns differ between the L3 vertebra and proximal tibia in three inbred strains of mice

An understanding of normal microarchitectural bone development patterns of common murine models is needed. Longitudinal, structural, and mineralization trends were evaluated by in vivo microCT over 12 time points from 6-48 wk of age at the vertebra and tibia of C3H/HeN, C57BL/6, and BALB/C mice. Longitudinal growth occurred rapidly until 8-10 wk, slowed as the growth plate bridged, and fused at 8-10 mo. Structural augmentation occurred through formation of trabeculae at the growth plate and thickening of existing ones. In the vertebrae, BV/TV increased rapidly until 12 wk in all strains. Between 12 and 32 wk, the architecture was stable with BV/TV deviating <1.1%, 1.6%, and 3.4% for the C57BL/6, BALB/C, and C3H/HeN mice. In contrast, the tibial architecture changed continuously but more moderately for BV/TV and TbTh compared with the vertebra and with comparable or larger changes for TbN and TbSp. Age-related trabecular deterioration (decreased BV/TV and TbN; increased TbSp and structure model index) was evident at both sites at 32 wk. In all strains, the cortex continued to develop after trabecular values peaked. The temporal plateau of BMD was variable across mouse strains and site, whereas tissue mineral density was attained at approximately 6 mo for all sites and strains. Geometric changes at the tibial diaphysis occurred rapidly until 8-10 wk, providing the C57BL/6 mice and C3H/HeN mice with the highest torsional and compressive rigidity, respectively. In summary, key skeletal development milestones were identified, and architectural topology at the vertebra was found to be more stable than at the tibia.