6.5. Bone biomechanical properties

Reduction of mechanical strength is the ultimate reason for traumatical bone fractures (Melton et al. 1986, Alho et al. 1988, Lotz & Hayes 1990). Prevention of this reduction is the final determinant of the usefulness of antifracture therapy, and should be evaluated with respect to each compound before clinical use.

The 10 and 20 % dietary xylitol supplementation increased the tibial stress, the femoral shear stress, and the stress of femoral neck significantly as compared to the control rats fed the same diet without xylitol. Accordingly, 10% dietary xylitol protected significantly against the ovariectomy-caused decline of these properties. None of the measured biomechanical variables was weakened after dietary xylitol administration. These indicate a beneficial effect of xylitol on the mechanical properties of both the cortical and trabecular part of the bone, and negate the possibility that dietary xylitol in spite of the increased bone mineral content could cause such qualitative changes in bone that might compromise the mechanical properties of bone. In this respect the present results with xylitol differ from the results of recent rat model studies with fluoride (Einhorn et al. 1992, Jiang et al. 1996), where the increased bone mass and geometric properties did not unambiquously lead to an increased bone strength.

Our results, concerning increased bone density, increased ash weight and increased bone mineral content in the xylitol-fed healthy rats, and concerning a dietary xylitol-induced protective effect against the decrease of these values in ovariectomized rats, partly explain the advanced biomechanical properties. These results are derived mainly from the cortical part of the bone, and are thus associated especially with the results of the three-point bending test. The increased trabecular bone volume along with increasing dietary xylitol concentration, and the 10% dietary xylitol-induced preventive effect against the loss of bone trabeculae in ovariectomized rats must be partial explanations for the biomechanical results as well. Above all, they are very important reasons for the improved biomechanical properties of the trabecular bone, seen especially in the loading test of the femoral neck. Furthermore, it is of interest that the cancellous component of the femoral neck in humans is probably even more important for the strength properties, because the femoral neck of humans consists of much more cancellous bone than that of the rats (Bagi et al. 1997).

No significant differences in strain or Young"s modulus of the three-point bending test of the tibiae were detected between the groups. This indicates that the plastic-elastic properties of the cortical bone are not largely affected either by ovariectomy or by dietary xylitol, and neglects the possibility of decreased bone elasticity during dietary xylitol administration. Changes in the above parameters have previously been shown to depend on changes in bone collagenous structures (Burstein et al. 1975). Accordingly, no qualitative changes in bone collagen structures were detected in the present studies.

As seen from the results above, besides retarding bone resorption, and increasing bone trabeculation, the oral administration of xylitol also improves bone biomechanical properties, and protects against weakening of these properties during experimental osteoporosis. Thus, considering the results of these experimental studies, dietary xylitol seems to meet all critical requirements of a useful antifracture preparate for the use in the prevention of osteoporosis.