O2084: A Multisite Study Evaluating Frequency and Characteristics of Non-ossifying Fibromas in Children with Congenital Forms of Rickets

Background: Non-ossifying fibromas (NOF) are benign, fibrous lesions of the skeleton estimated to occur in about 30% of children and adolescents. In the general pediatric population, they are twice as common in males, usually asymptomatic and self-limiting. Etiology is largely unknown, though multiple NOF are described with genetic syndromes such as neurofibromatosis 1. We have observed NOF in children with various forms of rickets. However, there is limited information on the frequency and characteristics of NOF in rickets. The purpose of this study was to evaluate the frequency of NOF in various congenital forms of rickets under care in tertiary care centers and describe their characteristics.

Methods: This was a retrospective chart review of patients with congenital forms of rickets and at least one bilateral lower extremity radiograph taken between 4 and 18 years of age at two academic children’s hospitals in the United States and Canada between 2004 to 2020. Rickets diagnosis was confirmed by clinical, biochemical, radiographic, and genetic testing where available. Data are described as proportions and percentages for categorical variables and means with standard deviations for continuous variables.

Results: Sixty-four patients, mean age 13.3 ± 5.1 years old (66%F, 46% Hispanic) were included with the following diagnoses: X-linked Hypophosphatemia (XLH, n=55, 86%), renal hypophosphatemia [n=4, 6% (3 Fanconi syndrome, 1 Dent disease)], hereditary vitamin D resistant rickets (HVDRR, n=3, 5%), and hereditary vitamin D dependent rickets type 1 (VDDR1, n=2, 3%). Fifteen patients (23%, 67%F) had at least one NOF, of whom 13 (87%) had XLH, one (7%) had HVDRR, and one (7%) had VDDR1. All NOF were localized to the lower extremities (60% femur, 27% tibia, and 13% fibula); there were no NOF in the upper extremities or at other sites among those with relevant skeletal imaging (n=32 with upper extremity and/or full skeletal survey data). The mean NOF height was 24.1 ± 13.8 mm and width 10.1 ± 5.3 mm. Nine of 15 patients with NOF (60%) compared with 21/49 without NOF (43%) required angular deformity surgery. Three patients (20%) experienced fractures within the lesions, two (13%) had two lesions each, and one (7%) had a progressive lesion requiring surgery with grafting.

Conclusion: We describe the frequency of NOF in children with congenital forms of rickets followed in academic centers. NOF were identified in almost a quarter of patients, and patients with NOF required angular surgery more often. Fractures were present within the lesions in one fifth of those with NOF, suggesting clinically important osteomalacia at that site. Interestingly, in contrast to the general pediatric population, we found no male preponderance. Long-term follow up and future studies are important to further characterize NOF in various forms of rickets, their pathogenesis, progression, and association with the disease severity.