Long Bone Fractures in Fibrous Dysplasia/McCune-Albright Syndrome: Prevalence, Natural History, and Risk Factors.

Fibrous dysplasia/McCune-Albright syndrome (FD/MAS) is a rare bone and endocrine disorder arising along a broad spectrum. Long-bone fractures are a common, painful, and potentially disabling complication. However, fracture prevalence and risk factors have not been well-established, making it difficult to predict which patients are at risk for a severe course. Clinical and imaging data were reviewed from two large, well-phenotyped cohorts (National Institutes of Health [NIH] in the United States and the Leiden University Medical Center [LUMC] in the Netherlands) to identify long-bone fractures at FD sites. Skeletal burden score was quantified using bone scintigraphy. Multiple linear regressions were performed to identify clinical associations with fractures. A total of 419 patients were included (186 NIH, 233 LUMC); 194 (46%) had MAS endocrinopathies. Median age at last follow-up was 30.2 years (range 3.2-84.6, interquartile range [IQR] 25.5), and median skeletal burden score was 16.6 (range 0-75, IQR 33). A total of 48 (59%) patients suffered one or more lifetime fracture (median 1, range 0-70, IQR 4). Median age at first fracture was 8 years (range 1-76, IQR 10). Fracture rates peaked between 6 and 10 years of age and decreased thereafter. Lifetime fracture rate was associated with skeletal burden score (β = 0.40, p < 0.01) and MAS hyperthyroidism (β = 0.22, p = 0.01). Younger age at first fracture was associated with skeletal burden score (β = -0.26, p = 0.01) and male sex (β = -0.23, p = 0.01). Both skeletal burden score >25 and age at first fracture ≤7 years were associated with a higher total number of lifetime fractures (median 4, range 1-70, IQR 5 versus median 1, range 1-13, IQR 1) (p < 0.01). In conclusion, higher skeletal burden score and MAS hyperthyroidism are associated with long-bone fractures in FD/MAS. Both skeletal burden score ≥25 and age at first fracture ≤7 years are associated with a higher lifetime long-bone fracture risk and may predict a more severe clinical course. These results may allow clinicians to identify FD/MAS patients at risk for severe disease who may be candidates for early therapeutic interventions.