Low-intensity, high-frequency vibration appears to prevent the decrease in strength of the femur and tibia associated with ovariectomy of adult rats

Abstract

The effect of low-intensity, high-frequency vibration on bone mass, bone strength, and skeletal muscle mass was studied in an adult ovariectomized (OVX) rat model. One-year-old female rats were allocated randomly to the following groups: start control, sham OVX, OVX without vibration, OVX with vibration at 17 Hz (0.5g), OVX with vibration at 30 Hz (1.5g), OVX with vibration at 45 Hz (3.0g). Vibrations were given 30 min/day for 90 days. During vibration each group of rats was placed in a box on top of the vibration motor. The amplitude of the vibration motor was 1.0 mm. The animals were labeled with calcein at day 63 and with tetracycline at day 84. The tibia middiaphysis was studied by mechanical testing and dynamic histomorphometry and the femur distal metaphysis by mechanical compression. OVX without vibration increased the periosteal bone formation rate and increased the medullary cross-sectional area, i.e., increased the endocortical resorption and outward anteromedial and lateral drifts of cortical bone at the tibia middiaphysis. OVX also resulted in a reduced maximum bending stress of the tibia diaphysis and a reduced compressive stress of the femur distal metaphysis. Vibration at the highest intensity, i.e., 45 Hz, of OVX rats induced a further increase in periosteal bone formation rate and inhibited the endocortical resorption seen in OVX rats. Furthermore, vibration at 45 Hz inhibited the decline in maximum bending stress and compressive stress induced by OVX. Neither OVX nor OVX with vibration influenced skeletal muscle mass. In conclusion, the results support the idea of a possible beneficial effect of passive physical loading on the preservation of bone in OVX animals.

Introduction

After the age of 30–40 years, there is a gradual decline in bone mass depending on genetic and hormonal factors, nutrition, physical activity, and life style; postmenopausal estrogen deficiency accelerates this process by increasing bone turnover, especially by increasing resorption [1]. It is the generally accepted view that exercising the musculoskeletal system should be advised in order to counteract or diminish this age-related bone loss. Exercise regimens, however, have been only marginally effective for improving bone mass in elderly individuals [2] and in experimental animals [3]. Several studies have shown that the osteogenic potential decreases markedly in relation to age [4], [5]. On the other hand, clinical conditions of immobilisation [6] and animal experiments of immobilisation [7], [8] result in severe osteopenia. Therefore, mechanical loading is necessary for the maintenance of bone structure and strength, but most individuals are not able to exercise sufficiently to prevent the age-related bone loss. The mechanisms and precise conditions behind the beneficial effect on bone induced by mechanical loading are not fully elucidated. One of the hypotheses is [5] that osteocytes, osteoblasts, and bone lining cells are influenced by strain-induced alterations in canalicular fluid flow [5], [9], [10], [11]. The strain-sensing cells then, by direct cellular contacts or release of messenger molecules (growth factors like IGF-I, prostaglandins, mediators that are secreted into the canalicular fluid), locally influence osteoblasts to increase their production of bone matrix and osteoprogenitor cells to proliferate and differentiate into bone-matrix-producing osteoblasts. The resulting increase in bone formation may be associated with a simultaneous decrease in bone resorbing activity [12], [13]. The number of osteoblasts, bone lining cells, and osteoprogenitor cells decrease with aging [14]. Likewise, the level of IGF-I and its binding proteins decrease with aging [15] in agreement with the age-related reduced osteogenic potential. Rubin and co-workers [16], [17], [18], [19] have shown that low-magnitude (0.3g), high-frequency (30 Hz) strain stimulates new bone formation in experimental animals and that the loading frequency also is important for the preservation of bone mass [20]. Flieger et al. [21] submitted 3-month-old ovariectomized (OVX) rats to low-intensity vibration (50 Hz, 2g, 30 min/day) for 3 months and found that this vibration regimen could prevent the early reduction in mineral density. Strains in this frequency range are found in the musculoskeletal system as a product of muscle dynamics associated with posture control. Also, the high-frequency muscle dynamics are reduced in relation to aging [16], [22].

In the present study, the adult OVX rat model was used to study the effect of whole-body, low-magnitude, high-frequency vibrations on the tibia middiaphyseal dynamic histomorphometry, mechanical properties of the tibia diaphysis, femur distal metaphysis, and the skeletal muscle mass.