Orthopaedic diagnostic pitfalls in fibroblast growth factor 23-mediated hypophosphatemic rickets/osteomalacia in fibrous dysplasia/McCune-Albright syndrome: Two burosumab-treated cases.

Fibrous dysplasia/McCune-Albright syndrome (FD/MAS) can obscure fibroblast growth factor 23 (FGF23)-mediated hypophosphatemic rickets/osteomalacia. We report two FD/MAS patients with recurrent fractures, hypophosphatemia, elevated FGF23, and serial imaging showing overlooked rachitic changes years before diagnosis. Burosumab improved serum phosphate levels after dose adjustment and was associated with mobility gains in Case 1 and increased growth velocity with physeal normalization in Case 2. Age-appropriate phosphate assessment and FGF23 testing are warranted when orthopaedic findings suggest impaired mineralization.