Context: Intravenous pamidronate treatment is beneficial to children and adolescents with osteogenesis imperfecta (OI) but the effects of prolonged therapy are not well characterized.

Objective: To assess the effect of long-term pamidronate treatment on the bone tissue of children and adolescents with OI.

Design: Observational study on OI patients receiving intravenous pamidronate for >4 yr.

Setting: A pediatric metabolic bone research unit.

Patients: 25 OI moderately to severely affected OI patients (7 girls) aged 1.4 to 15.3 yr at baseline.

Intervention: Cyclical intravenous pamidronate at a dose of 9 mg/kg per year.

Main Outcome Measures: Iliac bone biopsy and lumbar spine bone mineral density measures were obtained at treatment start, after 2.7 ± 0.5 yr (mean ± SD), and after 5.5 ± 0.7 yr of therapy.

Results: Average areal bone mineral density increased by 72% in the first half of the observation period, but by only 24% in the second half. Mean cortical width and cancellous bone volume increased by 87% and 38%, respectively, between baseline and the first time point during treatment (P < 0.001 for all changes). Thereafter, cortical width did not change significantly, but there was a trend (P = 0.06) toward higher cancellous bone volume. Average bone formation rate on trabecular surfaces decreased by 70% after pamidronate treatment was initiated, and showed a trend (P = 0.08) toward a further decline in the second part of the study interval.

Conclusions: The gains that can be achieved with pamidronate treatment appear to be largely realized in the first two to four years.