Search

A large proportion of overexertion injuries in sport and work related activities occur during high strain rate loading. The disc is known to have time dependent material properties (1). The joint load and stiffness may increase and the mode of failure may change from a location to another when higher loading rates are sustained (2). In order to quantify the response of the spine to dynamic loads, we need to account for the rate dependencies of the intervertebral disc in tension as well as other loading conditions. Bending tasks stretch the annulus fibrosus in the vertical direction and render the disc vulnerable to prolapse if the hydrostatic pressure in the nucleus pulposus is raised at the same time. Thus, the ability of the annulus to resist repeated vertical stretching may have a decisive influence on a disc’s susceptibility to prolapse. Tensile properties of the annulus fibrosus have been reported in the literature (3). However, to what extent loading rate can affect annulus tensile properties still needs to be clarified. The objective of this study was to investigate the loading rate sensitivity and regional variations in tensile mechanical properties of the annulus fibrosus, using disc annulus specimens from sheep.

This paper reviews the clinical and experimental findings on the effects of sodium fluoride (NaF) on human and animal bone. NaF has been shown to cause a significant increase in axial skeletal bone mass. However, there is concern that the new bone may not provide the desired increase in bone strength. Yet, NaF remains the most commonly used agent capable of stimulating bone formation in most patients (30% non-responders). But whether NaF reduces vertebral fracture rate (VFR) remains controversial. For a given treatment duration, the effect of F on bone quality appears to depend on dose: there is a marked detrimental effect on bone strength at high dose but there tends to be a beneficial effect at low dose. This biphasic NaF effect on bone strength has also been observed in fluoridated rat femurs. Unlike a study on young female rats which shows a linear dependence of cancellous bone volume (Cn-BV /TV) on NaF dose, a short-term study on young male rats, together with studies on chicks and dogs show biphasic NaF effects. Biphasic character is also observed in the effect of NaF on the packing of canine cortical bone mineral. When taken together, the animal models that show biphasic NaF effects seem to suggest that NaF at low dose improves Cn-BV/TV and bone strength and at high dose undermines them. These findings are in agreement with the clinical observations that high NaF dose does not help reduce VFR but low dose seems to help.