THE EFFECTS OF MECHANICAL FORCES ON BONES AND JOINTS - EXPERIMENTAL-STUDY ON THE RAT TAIL

We have used an experimental model employing the bent tail of rats to investigate the effects of mechanical forces on bones and joints. Mech anical strain could be applied to the bones and joints of tile tail wi thout direct surgical exposure or the application of pins and wires. T he intervertebral disc showed stretched annular lamellae on the convex side, while the annulus fibrosus on the concave side was pinched betw een the inner corners of the vertebral epiphysis. In young rats with a n active growth plate, a transverse fissure appeared at the level of t he hypertrophic cell layer or the primary metaphyseal trabecular zone. Metaphyseal and epiphyseal trabeculae on the compressed side were thi cker and more dense than those of the distracted part of the vertebra. In growing animals, morphometric analysis of hemiepiphyseal and hemim etaphyseal areas, and the corresponding trabecular bone density, showe d significant differences between the compressed and distracted sides. No differences were observed in adult rats. We found no significant d ifferences in osteoclast number between compressed and distracted side s in either age group. Our results provide quantitative evidence of th e working of 'Wolff's law'. The differences in trabecular density are examples of remodelling by osteoclasts and osteoblasts; our finding of no significant difference in osteoclast numbers between the hemiepiph yses in the experimental and control groups suggests that the response of living bone to altered strain is mediated by osteoblasts.