Age and BMI variations in bone, muscle, and fat on AP mid-thigh radiographs

摘要

Background: Body mass index (BMI) uses height and weight to determine relative fat content. BMI is used in numerous epidemiologic and mechanical studies of hip and knee osteoarthritis (OA). There are more precise, but cumbersome and expensive, measurements of the contribution of body fat to BMI. Our hypotheses were that there would be a large variation in relative mid-thigh bone, muscle, and fat width within specific BMI range and that relative widths change with age and sex. Methods: The hospital IRB approved a retrospective review of 340 patients who had a recent knee radiograph for knee OA. Using 5.08-cm (2-inch) increments, six groups were created. At height-adjusted reference points on the lower mid-thigh total thigh diameter (TD), muscle diameter (MD), bone diameter (BD), and nonfat diameter (NFD) were measured. Relative widths within BMI groups (<25, 25-30, 30-40, >40) were compared to age and sex. Results: MD, BD, and NFD ratios showed a wide variation in all BMI groups. As would be expected MD, BD, and NFD ratios decreased with increasing BMI. These ratios did not significantly change with age. However, the MD ratio remains similar for each BMI in men, in women it decreases with BMI. Conclusion: Adding relative thigh measurements to BMI data could enhance mechanical and epidemiologic studies.