Bone Abstracts

Hand X-rays from patients with hypophosphatemic rickets (HR) were assessed to evaluate if HR influences the dimensions of the metacarpal bones and the distribution between cortical and cancellous bone. In addition, we aimed to test the hypothesis that HR caused by mutation in DMP1 has a greater impact on bone dimensions than HR caused by mutations in PHEX.

Hand X-rays from 17 children with HR were evaluated. Three children had HR caused by a DMP1 mutation, and the remaining 14 had a verified PHEX mutation. BoneXpert^® (Visiana) was used to calculate the dimensions of the second, third and fourth metacarpal bones. HR patients were compared with age- and gender matched healthy children from Switzerland. For the whole HR group, data are reported as mean Z-scores (95% CI) and p value, followed by the mean Z-score in subgroups of patients with a PHEX mutation followed by patients with a DMP1 mutation.

Overall, patients with HR had significantly broader metacarpal bones, +2.5 (95% CI 1.7–3.2) S.D., P<0.001, (2.0; 4.6) S.D., with a wider medullary diameter of +2.4 (95% CI 1.8–2.1) S.D., P<0.001, (2.1; 4.2) S.D. and a reduced cortical thickness of -0.6 (95% CI –1.3 to –0.3) S.D., P<0.02, (−0.8; 0.3) SD. The metacarpal length of +0.7 (0.2–1.2) S.D., P=0.3, (0.4; 1.9) S.D., was not different compared with controls (Fig. 1).

Figure 1 Distribution of metacarpal dimensions in children with PHEX and DMP1 mutations respectively.

Children with HR have significantly broader metacarpal bones, a wider medullary diameter, and decreased cortical thickness. The metacarpal bones in children with a DMP1 mutation appear longer and broader with a wider medullary diameter compared to children with a PHEX mutation. Our data suggest that DMP1 mutations affect bone size more severely than PHEX mutations.