Effects of suppression of bone turnover on cortical and trabecular load sharing in the canine vertebral body

Interplay between low plasma RANKL and VDR-FokI polymorphism in lumbar disc herniation independently from age, body mass, and environmental factors: a case-control study in the Italian population

PURPOSE

Aim of this study was to investigate RANKL and osteoprotegerin plasma concentrations in patients affected by disc herniation, the most common epiphenomenon of disc degenerative diseases, and in a matched cohort of healthy subjects and whether the expression of these markers was associated to a polymorphism of the vitamin D receptor gene.

METHODS

For this case-control study, 110 consecutive cases affected by lumbar disc herniation (confirmed by MRI) and 110 healthy age- and sex-matched controls were enrolled. Subjects affected by any other pathology were excluded. RANKL and osteoprotegerin were measured in plasma by immunoassays. The difference in these markers between cases and controls was assessed by t test. The correlation between osteoimmunological markers concentrations, anthropometrical variables, and the expression of the pathology was statistically assessed (Pearson's test) along with the association (Fisher's exact test) with the vitamin D receptor gene genotype, determined elsewhere.

RESULTS

Despite comparable osteoprotegerin concentrations, cases, altogether or grouped for gender, express lower RANKL and, consequently, RANKL-to-osteoprotegerin ratio. While in cases RANKL and osteoprotegerin concentrations were independent from age and BMI, in controls they increased with age. Disc herniation was strongly associated with RANKL and the presence of the F allele of the VDR gene.

CONCLUSIONS

Whether vertebral bone changes precede or follow cartilage deterioration in intervertebral disc degeneration is not known. Our results suggest a reduced bone turnover rate, associated to a specific genetic background, in patients affected by lumbar disc herniation which could be one of the favoring factors for disc degeneration.

Heavy ion irradiation and unloading effects on mouse lumbar vertebral microarchitecture, mechanical properties and tissue stresses

Astronauts are exposed to both musculoskeletal disuse and heavy ion radiation in space. Disuse alters the magnitude and direction of forces placed upon the skeleton causing bone remodeling, while energy deposited by ionizing radiation causes free radical formation and can lead to DNA strand breaks and oxidative damage to tissues. Radiation and disuse each result in a net loss of mineralized tissue in the adult, although the combined effects, subsequent consequences for mechanical properties and potential for recovery may differ. First, we examined how a high dose (2 Gy) of heavy ion radiation ((56)Fe) causes loss of mineralized tissue in the lumbar vertebrae of skeletally mature (4 months old), male, C57BL/6 mice using microcomputed tomography and determined the influence of structural changes on mechanical properties using whole bone compression tests and finite element analyses. Next, we tested if a low dose (0.5 Gy) of heavy particle radiation prevents skeletal recovery from a 14-day period of hindlimb unloading. Irradiation with a high dose of (56)Fe (2 Gy) caused bone loss (-14%) in the cancellous-rich centrum of the fourth lumbar vertebra (L4) 1 month later, increased trabecular stresses (+27%), increased the propensity for trabecular buckling and shifted stresses to the cortex. As expected, hindlimb unloading (14 days) alone adversely affected microarchitectural and mechanical stiffness of lumbar vertebrae, although the reduction in yield force was not statistically significant (-17%). Irradiation with a low dose of (56)Fe (0.5 Gy) did not affect vertebrae in normally loaded mice, but significantly reduced compressive yield force in vertebrae of unloaded mice relative to sham-irradiated controls (-24%). Irradiation did not impair the recovery of trabecular bone volume fraction that occurs after hindlimb unloaded mice are released to ambulate normally, although microarchitectural differences persisted 28 days later (96% increase in ratio of rod- to plate-like trabeculae). In summary, (56)Fe irradiation (0.5 Gy) of unloaded mice contributed to a reduction in compressive strength and partially prevented recovery of cancellous microarchitecture from adaptive responses of lumbar vertebrae to skeletal unloading. Thus, irradiation with heavy ions may accelerate or worsen the loss of skeletal integrity triggered by musculoskeletal disuse.