Vitamin D Deficiency and Secondary Hyperparathyroidism as Potential Causes of Idiopathic Knee Angular Deformity: A Prospective Cross-Sectional Comparative Study.

Background: Genu valgum and genu varum have various causes; however, the cause in some children remains unknown, leading to a diagnosis of idiopathic angular deformity. In this study, we investigated whether vitamin D deficiency could lead to idiopathic knee angular deformity in the absence of typical radiographic findings of rickets by examining serum markers in affected children and controls.

Methods: In this prospective cross-sectional comparative study, we evaluated 38 children aged 7 to 14 years with genu valgum or varum without medical conditions or radiographic findings affecting lower limb alignment and 29 controls. Laboratory parameters and the prevalence of vitamin D deficiency (serum 25-hydroxyvitamin D level <20 ng/mL) or hyperparathyroidism [serum parathyroid hormone (PTH) level >60 pg/mL] were compared between the 2 groups. The prevalence of angular deformities was compared among the groups based on the presence of vitamin D deficiency and hyperparathyroidism. Binary logistic regression analysis was used to calculate the odds ratios (ORs) for angular deformities based on vitamin D deficiency and hyperparathyroidism.

Results: The mean serum 25-hydroxyvitamin D levels did not significantly differ between the angular deformity (16.7±6.1 ng/mL) and control groups (19.9±7.1 ng/mL) (P=0.055). However, vitamin D deficiency was more prevalent in the angular deformity group than in the control group (79% vs. 48%, P=0.009). Angular deformities were more prevalent in children with both vitamin D deficiency and hyperparathyroidism than in those without these conditions [11/13 (85%) vs. 5/18 (28%), P=0.012]. The logistic regression analysis-adjusted for age, sex, and weight-showed a higher risk of angular deformity in children with both vitamin D deficiency and hyperparathyroidism (OR: 9.86, 95% CI: 1.36-71.47, P=0.024), but not in children with vitamin D deficiency alone.

Conclusions: Isolated vitamin D deficiency without other laboratory abnormalities or radiographic findings of rickets did not lead to knee angular deformity. However, in cases in which vitamin D deficiency elevates PTH levels, idiopathic genu valgum or genu varum might be observed.

Level of evidence: Level III-prognostic study.