DIFFERING LUMBAR VERTEBRAL MINERALIZATION RATES IN AMBULATORY PEDIATRIC-PATIENTS WITH OSTEOGENESIS IMPERFECTA

The purpose of this study was to evaluate serial changes in bone miner al density (BMD) of the lumbar spine in individual children and adoles cents with untreated osteogenesis imperfecta (OI) using dual X-ray abs orptiometry (DXA). Twenty-seven pediatric patients with OI who had no historical or radiographic evidence of lumbar fracture, required no as sistive device for mobility, and were taking no medications known to a ffect skeletal mineralization during the study period comprised the in vestigational group. Absolute BMD and age-and gender-matched BMD (Z-sc ores) were assessed relative to standard parameters of growth (height, weight, age, height adjusted for age and gender and body surface area ) and severity of disease (lifetime fracture rate). The spinal mineral ization rate (SMR) between examinations for 15 patients with more than one measurement (n = 20 intervals) was expressed as the magnitude of the change in BMD Z-score per year. Both BMD and BMD Z-score were clos ely correlated with height, height Z-score, weight and body surface ar ea and were inversely related to fracture rate (P < 0.001 for all comp arisons). BMD was also highly correlated with patient age (P < 0.001). Stepwise regression analysis showed that together height Z-score and lifetime fracture rate improved the prediction of BMD Z-score (r = 0.7 1; P < 0.001). SMRs ranged from -0.5 to 3.5, The average change in SMR between sequential measurements was 168% for the five children who ha d more than two DXA examinations. Linear regression showed a significa nt negative correlation between SMR and height Z-score (r = -0.79, P < 0.001). We conclude that vertebral body size is a critical determinan t of BMD and BMD Z-score in OI because DXA results are expressed per u nit area, not per unit volume. Pediatric patients with OI mineralize t heir lumbar vertebrae at rates similar to healthy children but tend to lag behind in overall mineralization. The rate of mineralization at a ny age appears to be related to the patient's height (adjusted for age -and gender-matched controls) and inversely related to the patient's l ifetime rate of fractures. Our data suggest that vertebral mineralizat ion in children with OI is related primarily to rapid increases in ver tebral volume and only secondarily to increases in vertebral mineral d ensity.