The Effects of Hormonal and Mechanical Changes on Mandibular Bone

Objectives: To compare the relative effects of ovariectomy and hypofunction on jaw bone mineralisation, volume density and cellularity. Methods: Female rats were ovariectomised (OVX) at 50 days, or had left maxillary molars removed at 64 days. Control animals remained untreated. At 7 months, all animals were sacrificed, ulnae and mandibles removed, fixed in formaldehyde and either embedded for quantitative backscattered electron analysis in a scanning electron microscope (qBSE-SEM) and microtomography, or decalcified in 10% EDTA, and cryosectioned. Nuclei in 10micron thick sections were counted using an Olympus BHS microscope linked to a Kontron image analysis system. Known field areas comprising matrix only were captured and osteocyte numbers counted. The mandibular molar region was analysed for: mineralisation density (using qBSE-SEM); bone volume fraction (using the muCAT x-ray microtomography system at QMUL www.microtomography.org and Imaris software to calculate number of voxels per unit jaw length) and osteocyte density (OD). Results: Ovariectomised animals had a significantly greater mandibular bone volume fraction (mean voxels/unit length: ovx 10,537; control 8,160 p=0.04). By contrast, hypofunction animals had a reduced bone volume fraction cf. controls (7,340, ns). However, when corrected for body weight, both ovariectomised and hypofunction animals had a significantly reduced bone volume fraction than controls (p=0.02 and 0.007, respectively). In the mandible, there was significantly greater OD in alveolar (control 1,592 ± 238/mm2) compared with basal (control 807 ± 175/mm2) mandibular bone. In the ovariectomised group, OD was reduced in alveolar bone (1,228 ± 160/mm2, p=0.01) and ulna (660 ± 73/mm2, p=0.03), however, in hypofunction OD increased (2,007 ± 131/mm2, p<0.001) in alveolar bone cf. controls. No differences in OD were observed in basal mandibular bone between the groups. Conclusions: Hypofunction has a different effect upon alveolar bone in terms of osteocyte density compared with ovariectomised animals. Supported by a DoH Clinician Scientist Fellowship Award.