Cortical and Trabecular Bone Density in X-Linked Hypophosphatemic Rickets

Context: X-linked hypophosphatemic rickets is caused by mutations in PHEX. Even though the disease is characterized by disordered skeletal mineralization, detailed bone densitometric studies are lacking. Objective: The aim of the study was to assess volumetric bone mineral density (vBMD) in X-linked hypophosphatemic rickets using forearm peripheral quantitative computed tomography. Setting: The study was conducted in the metabolic bone clinic of a pediatric orthopedic hospital. Patients: Thirty-four patients (age, 6 to 60 years; 24 female) with PHEX mutations were studied, of whom 7 children (age, 6 to 11 years) were actively being treated with calcitriol and phosphate supplementation. Twenty-one patients (age, 16 to 40 years) had received the same therapy before but had discontinued the treatment; 6 patients (age, 12 to 60 years) had never received this treatment. Main Outcome Measures: Trabecular and cortical vBMD of the radius. Results: Trabecular vBMD was elevated (mean age-specific and sex-specific z-score: +1.0) when all patients were analyzed together, due to very high results in currently treated patients (mean z-score: +2.4) and slightly above-average mean values in the other patients. Cortical vBMD was low when the entire cohort was analyzed together (mean z-score: -3.3), but was higher in currently treated patients (mean z-score: -1.3) than in patients who had discontinued therapy (mean z-score: -3.8) or who had never been treated (mean z-score: -4.1). Conclusions: Patients with PHEX mutations have elevated trabecular vBMD at the distal radius while receiving calcitriol and phosphate supplementation, but low cortical vBMD at the radius diaphysis. Low cortical vBMD presumably reflects the underlying mineralization defect that is not entirely corrected by current treatment approaches.