The effects of hibernation and forced disuse (neurectomy) on bone properties in arctic ground squirrels.

Bone loss is a well-known medical consequence of disuse such as in long-term space flight. Immobilization in many animals mimics the effects of space flight on bone mineral density. Decreases in metabolism are also thought to contribute to a loss of skeletal mass. Hibernating mammals provide a natural model of disuse and metabolic suppression. Hibernating ground squirrels have been shown to maintain bone strength despite long periods of disuse and decreased metabolism during torpor. This study examined if the lack of bone loss during torpor was a result of the decrease in metabolic rate during torpor or an evolutionary change in these animals affording protection against disuse. We delineated changes in bone density during natural disuse (torpor) and forced disuse (sciatic neurectomy) in the hind limbs of the arctic ground squirrel (AGS) over an entire year. We hypothesized that the animals would be resistant to bone loss due to immobilization and disuse during the winter hibernation season when metabolism is depressed but not the summer active season. This hypothesis was not supported. The animals maintained bone density (dual-energy X-ray absorptiometry) and most bone structural and mechanical properties in both seasons. This was observed in both natural and forced disuse, regardless of the known metabolic rate increase during the summer. However, trabecular bone volume fraction (microcomputed tomography) in the distal femur was lower in neurectomized AGS at the study endpoint. These results demonstrate a need to better understand the relationship between skeletal load (use) and bone density that may lead to therapeutics or strategies to maintain bone density in disuse conditions.

Repetitive coronal polishing yields minimal enamel loss.

PURPOSE: The American Dental Hygienists' Association recommends selective polishing because of risk of enamel removal and lack of documented therapeutic value. The initial study documenting enamel loss from polishing used methods not acceptable for clinical use, while results from other studies are inconsistent. This study examines the effect of simulated life-time polishing on enamel thickness. Enamel loss from polishing is compared to the enamel thickness just coronal to the cementoenamel junction (CEJ) to relate results to clinical application. METHODS: Eight premolars and 18 molars were polished 150 times with coarse prophy paste, then pre- and post-polishing micrometer measurements were compared. Eight unpolished premolars and 18 unpolished molars were used as control groups. Average enamel thickness from 10 premolars and 10 molars just coronal to the CEJ was chosen to represent minimal enamel thickness, and was calculated using digital radiography. T-tests were used to compare group means. RESULTS: The mean measurement difference was significantly higher for the premolar treatment group than the control group, but no difference was noted between molar treatment and control groups. Neither treatment group demonstrated significant abrasion when compared to average minimal enamel thickness. Root abrasion was noted on 5 molars. CONCLUSION: The results of our study indicate that polishing may remove enamel, but the quantity removed is unlikely to be clinically relevant. Root surface abrasion seen on molars is disturbing, considering stain often occurs on exposed mandibular anterior root surfaces and may cause repeated and prolonged polishing. Further investigation into alternative stain removal methods is recommended.

Dual-energy X-ray absorptiometry (DXA) can accurately and nondestructively measure the body composition of small, free-living rodents.

Dual-energy x-ray absorptiometry (DXA) is a nondestructive technique that can potentially measure specific components of whole-body composition in free-living and lab-raised animals. Our aim was to test the ability of DXA to measure the composition of a common arvicoline rodent, the northern red-backed vole (Clethrionomys rutilus). We used a DXA apparatus to obtain measurements of fat mass (FM), lean mass (LM),bone mineral content, bone mineral density, and fat-free mass(FFM) in carcasses of free-living and lab-raised voles. We then used chemical carcass analysis to derive predictive algorithms for actual values of FM, total body water, total protein, total mineral, LM, and FFM. Unexplained error in the equations for all voles grouped collectively ranged from R(2) = 0.82 to R(2) = 0.98. The DXA FM measurement had the highest coefficient of variation, and it was higher for free-living voles than for lab-raised voles. However, FM can be determined by difference with excellent precision by using the FFM equation (R(2) = 0.98). We also derived corrective terms for passive integrated transponder-tagged animals. Thus, DXA is a nonlethal, nondestructive tool capable of precisely and accurately measuring many specific parameters of whole-body composition in small free-living and lab-raised rodents.