The Muscle-Bone Relationship in X-Linked Hypophosphatemic Rickets

Context: We recently found that patients with X-linked hypophosphatemic rickets (XLH) have a muscle function deficit in the lower extremities. As muscle force and bone mass are usually closely related, we hypothesized that patients with XLH could also have a bone mass deficit in the lower extremities. Objective: The study objective was to assess the muscle-bone relationship in the lower extremities of patients with XLH. Setting: The study was carried out in the outpatients department of a pediatric orthopedic hospital. Patients and Other Participants: Thirty individuals with XLH (6 to 60 y; 9 male patients) and 30 age- and gender-matched controls participated. Main Outcome Measures: Calf muscle size and density as well as tibia bone mass and geometry were assessed by peripheral quantitative computed tomography. Muscle function was evaluated as peak force in the multiple 2-legged hopping test. Results: Muscle force was significantly lower in XLH patients than in controls but muscle cross-sectional area did not differ (after adjustment for tibia length). External bone size, expressed as total bone cross-sectional area, was higher in the XLH group than in controls. The XLH cohort also had statistically significantly higher bone mineral content. Conclusions: Patients with XLH have increased bone mass and size at the distal tibia despite muscle function deficits.