Regular articleLow-intensity, high-frequency vibration appears to prevent the decrease in strength of the femur and tibia associated with ovariectomy of adult rats
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.
Section snippets
Materials and methods
Eighty-one female Wistar rats, 12 months old at the start of the experiment, were randomly divided into six groups. Group 1, the start control group, was killed at the beginning of the experiment. Group 2 was sham ovariectomized and not vibrated. The four remaining groups were ovariectomized at the start of the experiment. Group 3 was OVX without vibration; Group 4 was OVX with vibration, 17 Hz (0.5g); Group 5, OVX with vibration, 30 Hz (1.5g); Group 6, OVX with vibration, 45 Hz (3.0g).
Results
The periosteal MAR values are given in Table 1. In the start control and sham-operated groups, very little mineral apposition was found at the tibia middiaphysis. OVX without vibration resulted in a three-fold increase in MAR, primarily at the anteromedial aspect of the tibia compared with the sham-operated group. All three vibration regimens induced a further increase in periosteal MAR compared with the OVX group without vibration. At the lateral and medial aspects, the OVX rats vibrated at 45
Discussion
The skeleton has the ability to adapt to the functional demands and change bone mass, structure, and strength so that it can withstand the functional loading. The sensitivity of bone to physical stimuli is evident from exercise studies [2], [28], [29], bed rest [30], and local load as seen in the humerus of tennis players [31], [32]. The exact mechanical control of bone adaptation and cellular mechanisms are not fully understood. Strain magnitude, strain rate, strain energy density,
Acknowledgements
We thank Novo Nordisk A/S, The Danish Health Research Council, Grants 42875 and 9600822 (Aarhus University-Novo Nordisk Centre for Research in Growth and Regeneration), Aarhus University Research Foundation, and Novo Nordisk Foundation. The skilled technical assistance of C. Knæhus and linguistic revision of M. Fischer are gratefully acknowledged.
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