Ultrasonic wave velocity and bone mineral density in skull bones of ovariectomized and spontaneous-menopause mice

Osteoporosis is a public health concern strongly associated with menopause and aging. In this study, ovariectomized (OVX) and spontaneously menopausal (SM) C57BL/6J mice were used to experimentally investigate the effects of menopause and aging on the skull bone (unloaded bone). Ultrasonic longitudinal wave velocities and volumetric bone mineral density (v.BMD) were measured using micro-Brillouin scattering (μ-BS) and X-ray micro-computed tomography (μ-CT). The experimental findings revealed that v.BMD was significantly lower in OVX and SM mice than in Sham-operated (control) mice. The measured wave velocity in the anteroposterior direction of tibiae showed a significant reduction in SM mice, whereas no significant difference was found between Sham and OVX mice. A positive correlation was found between the square of wave velocity and v.BMD (r = 0.65). The skull exhibits complex elastic characteristics owing to the lack of mechanical loading. In the load-bearing bones (tibia), the direction of loading corresponded to the direction of the maximum wave velocity. Conversely, the directions of the maximum and minimum wave velocities in the skull varied depending on the sample. However, SM skull samples exhibited lower wave velocities.