Mandibular bone stiffening and increased bone calcium mass in rats permanently stunted by hypophysectomy

Effects of aging, gender, and hypogonadism on mandibular bone density

AIM

The aim of the present study was to evaluate how aging, sex, and hypogonadism influence mandibular bone density with and without the benefits of hormone treatment.

METHODS

Three-month old Wistar rats were randomly assigned to three experimental groups, with eights animals per group: controls, castrated (orchiectomized [ORX], ovariectomized [OVX]) and castrated with hormonal treatment (ORX + testosterone, OVX + estradiol benzoate). Females were previously evaluated by vaginal cytology. The corporal mass was verified weekly, and after three experimental periods (90, 120, and 150 days), the animals were killed. Blood was collected, and bones underwent densitometric and biomechanical analyses.

RESULTS

After castration, the male rats demonstrated low gain in body weight compared to females (P < .05). Male and female castrated animals presented serum concentrations of sex steroid hormones lower than the control group (P < .05). Bone mineral density and biomechanical properties of the L4 vertebrae and femur were reduced earlier in females than in males (P < .05). However, mandibles were affected only in the male rats at the most chronic experimental period.

CONCLUSION

Hypogonadism promotes alterations in the mandible over chronic periods, especially in males, and these alterations could be minimized by hormone treatment.

Biomechanical properties of the mid-shaft femur in middle-aged hypophysectomized rats as assessed by bending test

Both stiffness and strength of bones are thought to be controlled by the "bone mechanostat". Its natural stimuli would be the strains of bone tissue (sensed by osteocytes) that are induced by both gravitational forces (body weight) and contraction of regional muscles. Body weight and muscle mass increase with age. Biomechanical performance of load-bearing bones must adapt to these growth-induced changes. Hypophysectomy in the rat slows the rate of body growth. With time, a great difference in body size is established between a hypophysectomized rat and its age-matched control, which makes it difficult to establish the real effect of pituitary ablation on bone biomechanics. The purpose of the present investigation was to compare mid-shaft femoral mechanical properties between hypophysectomized and weight-matched normal rats, which will show similar sizes and thus will be exposed to similar habitual loads. Two groups of 10 female rats each (H and C) were established. H rats were 12-month-old that had been hypophysectomized 11 months before. C rats were 2.5-month-old normals. Right femur mechanical properties were tested in 3-point bending. Structural (load-bearing capacity and stiffness), geometric (cross-sectional area, cortical sectional area, and moment of inertia), and material (modulus of elasticity and maximum elastic stress) properties were evaluated. The left femur was ashed for calcium content. Comparisons between parameters were performed by the Student's t test. Average body weight, body length, femur weight, femur length, and gastrocnemius weight were not significantly different between H and C rats. Calcium content in ashes was significantly higher in H than in C rats. Cross-sectional area, medullary area, and cross-sectional moment of inertia were higher in C rats, whereas cortical area did not differ between groups. Structural properties (diaphyseal stiffness, elastic limit, and load at fracture) were about four times higher in hypophysectomized rats, as were the bone material stiffness or Young's modulus and the maximal elastic stress (about 7×). The femur obtained from a middle-aged H rat was stronger and stiffer than the femur obtained from a young-adult C rat, both specimens showing similar size and bone mass and almost equal geometric properties. The higher than normal structural properties shown by the hypophysectomized femur were entirely due to changes in the intrinsic properties of the bone; it was thus stronger at the tissue level. The change of the femoral bone tissue was associated with a high mineral content and an unusual high modulus of elasticity and was probably due to a diminished bone and collagen turnover.