Immobilization-dependent bone collagen breakdown appears to increase with time: Evidence for a lack of a new bone equilibrium in response to reduced load during prolonged bed rest

The purpose of this study was to evaluate the effect of prolonged immobiliz ation on bone, in order to investigate how skeletal turnover adapts to bed rest. We examined indices of bone formation and bone resorption in the seru m and urine of fifty-four patients (26 males and 28 females) immobilized af ter an episode of paralytic stroke. The length of immobilization ranged fro m 30 to 180 days. A significant, time-dependent increase in markers of reso rption - urinary pyridinoline (Pyr) and deoxypyridinoline (D-Pyr), serum Ty pe I collagen cross-linked C-telopeptide (ICTP)- was observed in immobilize d patients, as compared to free-living healthy subjects. The positive corre lation between resorption markers increase and the length of immobilization suggests that the rate of bone resorption did not decrease with time. On t he other hand, the levels of markers of bone formation - bone-specific alka line phosphatase (B-ALP), and the carboxyl-terminal propeptide of Type I pr ocollagen (PICP) - remained within the normal range in all patients, regard less the length of immobilization. Our results would indicate an uncoupling between bone formation and bone resorption during bed rest, and suggest th at the bone collagen breakdown was not a self-limiting process in immobiliz ed patients, and that a new equilibrium or "steady state" in response to th e reduced load was not reached in the skeleton.