Parathyroid hormone induces formation of new cancellous bone with substantial mechanical strength at a site where it had disappeared in old rats

The present study addresses the question--can PTH induce formation of trabeculae in areas where cancellous bone has disappeared? Two-year-old male rats were chosen, because in this aged animal model the distal femurs have almost no cancellous bone, and the marrow cavity has reached a substantial dimension. The rats were injected for 56 days with either PTH(1-34), 15 nmol/kg/day (62.5 microg/kg/day), or vehicle. Transverse specimens, 2-mm high, were cut from the distal femoral metaphysis. Marrow cavity diameters and cancellous bone trabeculae were analysed by a micro-computerized tomography scanner. The cancellous bone within the cortical and endocortical rim of each specimen was submitted to a biomechanical compression test. Furthermore, the cancellous bone was studied by dynamic tetracycline labelling and histomorphometry. In the vehicle-injected group the trabecular bone volume was 0% (0-1.4), median (range). All PTH-injected rats had trabeculae in the distal metaphysis and the trabecular bone volume (6.7% (2.3-12.0)) was markedly increased (P