Bone Health Among Adults with Cerebral Palsy: Higher bone mineral density may be deceiving

Summary

This study demonstrated these pitfalls of Bone Mineral Density (BMD) using a clinical cohort of 78 adults with CP with a hip DXA/DEXA scan. We found that higher BMD was associated with a greater risk of fracture, which is opposite of what we would expect. However, we picked apart the BMD ratio and found that bone size had the strongest association with fracture risk, such that smaller bones had a greater risk.

Why Does This Happen?

Our bones can be constructed in different ways. Like buildings, the architectural arrangement of the bone is a major determinant of how strong the bone is.

Children with cerebral palsy (CP) have low activity, problems in developing muscle, and face other challenges, like health issues, altered biology, and excess fat within muscle and bone. These factors can negatively impact the way their bones are constructed. Evidence suggests that children with CP are not simply developing a smaller version of a typically developing skeleton; instead, they may be developing a smaller bone with a different architectural arrangement, in a way that leads to bone weakness.

To assess bone strength, clinicians will often measure the bone's density as ‘bone mineral density’ (BMD) from a DXA (or DEXA) scan. A higher BMD will indicate a denser, stronger bone and lower BMD will indicate a less dense, weaker bone.

However, BMD is a ratio: bone mineral content/bone area. The problem is that because children with CP develop smaller bones, adults with CP will have small bones. The denominator (area) of the BMD ratio is decreased, which increases the BMD value in a way that is not reflective of the bone's actual strength. In addition, the size of the bone is a major determinant of how strong the bone is. When assessing the BMD value only, we are over-looking the bone size contribution to fracture risk.

This study demonstrated these pitfalls of BMD using a clinical cohort of 78 adults with CP with a hip DXA/DEXA scan. We found that higher BMD was associated with a greater risk of fracture, which is opposite of what we would expect. However, we picked apart the BMD ratio and found that bone size had the strongest association with fracture risk, such that smaller bones had a greater risk.

Whitney, DG, Caird, MS, ClineS, GA, Hurvitz, EA, Jepsen, KJ. Clinical bone health among adults with cerebral palsy: moving beyond assessing bone mineral density alone. Dev Med Child Neurol. 2022; 64: 469–475. https://doi.org/10.1111/dmcn.15093