Transient Disturbance in Physeal Morphology Is Associated With Long‐Term Effects of Nitrogen‐Containing Bisphosphonates in Growing Rabbits

Bisphosphonates have clinical benefit in children with severe osteogenesis imperfecta or osteoporosis and potential benefit in children with Perthes disease or undergoing distraction osteogenesis. However, there is concern about the effects of bisphosphonates on the physis and bone length. In 44 growing rabbits, zoledronic acid caused a transient disruption of physeal morphology, retention of cartilaginous matrix in trabeculae and cortical bone of the metaphysis, and a minor decrement in tibial bone length at maturity. Introduction: Data from growing animal models suggest that bisphosphonates cause retention of longitudinal cartilaginous septa at the chondro‐osseous junction, extension of trabeculae to the metaphyseal‐diaphyseal junction, and varying dose‐dependent effects on longitudinal growth. However, there is a lack of data regarding effects of intermittent use of nitrogen‐containing bisphosphonates on the physis and on tibial length in models reaching maturity. Materials and Methods: Contralateral tibias of juvenile rabbits were examined after right tibial distraction osteogenesis from two previous studies. Animals were randomized to receive 0.1 mg/kg zoledronic acid (ZA) IV at 8 weeks of age (ZA1) or 8 and 10 weeks of age (ZA2) or saline. Body mass was analyzed from 5 to 44 weeks of age; tibial length and proximal physeal‐metaphyseal histology and histomorphometry were analyzed at 8–52 weeks of age. Results: Tibial length was 3% less at 14 weeks of age in the ZA2‐treated versus saline group (p < 0.05) in both studies, and this difference persisted at maturity in the long‐term study group (26 weeks of age, p < 0.05). Total body mass gain from 5 to 26 weeks of age was 14% less in ZA2‐treated than saline animals (p < 0.05). Rate of weight gain from 8 to 10 weeks of age was 76% less in ZA2 compared with saline animals (p < 0.05). Radiographs showed radiodense lines in the metaphyses of ZA‐treated bones, corresponding to the number of doses. Histologically, lines resulting from the first dose of ZA contained longitudinal cartilaginous matrix cores surrounded by bone, whereas those from the second dose contained spherical cores of matrix caused by transient disruption of physeal morphology after the first dose of ZA. Resorption of these lines at later times was radiographically and histologically evident, but remnants of cartilaginous matrix remained in the cortical bone of ZA‐treated animals. Conclusions: ZA treatment within the final 13.5% of the rabbit