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.