Evaluation of bone turnover in type I osteoporosis using biochemical markers specific for both bone formation and bone resorption

The aims of the study were to evaluate the use of bone-specific biochemical markers of turnover in type I osteoporosis, to test for evidence of heterogeneity of bone turnover in this condition, and to attempt to devise an uncoupling index by using the relationship between bone-specific biochemical markers of bone formation and bone resorption. In women with type I osteoporosis (mean age 64 years, SD 5;n=63) the mean level of serum osteocalcin, a specific biochemical marker of bone formation, was 9.9 ng/ml (SD 2.0), which was higher than the level in normal postmenopausal women (mean age 65 years, SD 6;n=8.9 ng/ml (SD 2.0;p<0.01). The variance of serum osteocalcin levels in the two groups was similar. Compared with this 11% increase in the biochemical marker for bone formation, the markers of bone resorption, total urinary deoxypyridinoline (bone-specific), pyridinoline and hydroxyproline were increased by 40% (p<0.0001), 61% (p<0.0001) and 25% (p<0.01), respectively. Furthermore, these biochemical markers of bone resorption had greater variance in women in type I osteoporosis than in the normal postmenopausal women (p<0.001). The urinary excretion of the free crosslinks deoxypyridinoline, pyridinoline and glycosylated pyridinoline were increased by 26% (p<0.001), 17% (p<0.01) and 13% (NS) respectively. An uncoupling index was calculated for the difference between urinary deoxypyridinoline and serum osteocalcin using the results from the normal women and expressed asz-scores. We conclude that the pyridinium crosslinks of collagen enable better discrimination between normal and osteoporotic women than does hydroxyproline. In osteoporosis there appears to be heterogeneity of bone resorption. Finally, an uncoupling index indicated that in osteoporosis bone resorption was increased to a greater extent than bone formation as compared with normal postmenopausal women.