What causes osteoporosis in children with spina bifida?

• A. Reduced bone mineral density secondary to hypotonic/flaccid musculature with decreased loading of the long bones
• B. Low intake of calcium in diet
• C. Excess physical activity leading to microfractures
• D. Vitamin D oversynthesis

Answer: (A)

Disuse of the bones due to paralysis or marked muscle weakness in spina bifida leads to reduced mechanical loading on the long bones. Bones adapt to the stresses they experience, so when leg muscles are hypotonic or flaccid and weight-bearing activity is limited, the stimulus for bone formation diminishes and resorption can outpace formation. This results in lower bone mineral density, i.e., osteoporosis, particularly in the long bones that normally bear most of the load. While adequate calcium and vitamin D support bone health, the primary mechanism in this scenario is the loss of mechanical loading from decreased mobility. The other options don’t fit this pattern: calcium intake deficiency or vitamin D excess aren’t the main drivers here, and excessive physical activity would not typically occur in children with spina bifida.