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We were interested to read the study by Laursenet al which highlights the important problem of decreased bone mass in the cystic fibrosis population.1The authors found that reduced bone size was primarily responsible for the decreased bone mass observed in patients ⩽19 years old. These results are at variance with the findings of Gibbens and colleagues, who, using quantitative computed tomography, showed that the volumetric (three dimensional) lumbar spine bone density of children with cystic fibrosis was 10% lower (p<0.001) than in local controls.2
Although these contrasting results may partially reflect true differences in bone mass between the study populations, they may also be due to regional differences in bone mass. In addition, quantitative computed tomography measurements in the spine are confined to cancellous bone whereas total body dual energyx ray absorptiometry measurements, as used by Laursen et al,1predominantly reflect cortical bone. Both measurements are therefore useful, but contribute differently to our knowledge of the skeleton. Another more recent study supports the fact that the reduced areal (two dimensional) bone mass density documented in young patients with cystic fibrosis is due to a combination of decreased volumetric bone mass density and reduced bone size, rather than reduced bone size alone.3 We would therefore suggest that it is important to monitor the skeletal health of patients with cystic fibrosis (starting in childhood), ideally by performing annual bone densitometry. As fractures involving trabecular sites such as the ribs and spine are more prevalent in adults with cystic fibrosis than in the general population,4 regional rather than total body bone mass density assessments should be performed to more accurately assess fracture risk.