Isotope retention for assessment of bone turnover in involutional osteoporosis

A continuous bone remodelling takes place throughout life at different turnover speed according to age, physiological and pathological conditions. The evaluation of bone turnover may be of value for a prognostic and therapeutical assessment. Calcium bone exchange may be considered a suitable marker of bone turnover; for this reason47Ca or45Ca kinetics may be used; these methods have been employed in the past. Labelled diphosphonates, and in particular99Tecnetium-metilen-diphosphonate (99TcmCP) are simpler and safer, because these substances are strongly and almost completely stored in bone and not absorbed by the soft tissue; for this reason they are used at the present time. The evaluation of blood levels and 24 hrs urinary elimination of99TcmDP is used to measure whole bone diphosphonate retention (WBR). This parameter is positively correlated with other markers of bone turnover such as alkaline phosphatase (AP), osteocalcin (OC), urinary hydroxyproline (HOP). A bicompartimental analysis schedule of99TcmDP distribution has been proposed some years ago and therefore applied by our group, based on the mathematical evaluation of serum concentration at different times and urinary elimination of the label given intravenously. This method provides the possibility to calculate not only WBR but also total body retention (TBR) and a constant (Kbh) which reflects the influx speed of the tracer in the bone. Kbh probably represents a more sensitive index of bone turnover than WBR. It presents a better correlation with AP and OC values and also shows some (statistically less significant) correlations with some indices of bone remodelling obtained by hystomorphometry on bone biopsies. Kbh is fairly stable in rest conditions and is elevated in high bone turnover conditions (Paget, osteomalacia and, at a lower extent, high turnover osteoporosis).