Validation of the of Mid-Thigh Bone, Muscle and Fat Mass by Densitometry as a Sensitive and Practical Region of Interest for the Diagnosis of Tissue Loss Syndromes

Abstract

\n Purpose: The prevalence of musculoskeletal tissue loss syndromes (osteoporosis and sarcopenia), as well as obesity and their combinations, is on the rise. Previous reports indicate mid-thigh dual-energy x-ray absorptiometry (DXA) is well-suited for the simultaneous assessment of bone, muscle, and fat mass in a single scan.Methods: Using DXA images of 1322 community-dwelling adults (57% women, age 58.2 ± 15.6 SD), bone, lean and fat mass were quantified in five regions of interest (ROIs): a) a 2.6 cm; and b) a 13 cm thick slice of mid-thigh; c) whole thigh; d) whole calf; and e) forearm. Conventional indices of tissue mass i.e. appendicular lean mass, hip, and spine bone mineral density (BMD) and total, gynoid and android fat mass, were also calculated. Their associations with muscle strength (handgrip strength) and performance (timed-up-and-go [TUG] and gait speed), as well as falls and fractures, were investigated.Results: Lean mass in all ROIs was well-correlated with outcomes, and according to regression analyses, lean mass explained two-thirds, a third and a quarter of the variability in the handgrip strength (r2= 0.63 to 0.64, p<0.001), TUG (r2= 0.34 to 0.37, p<0.001) and gait speed (r2= 0.25 to 0.27, p<0.001), respectively. Lean masses were negatively associated with falls only when corrected for BMI. In the ≥60yo cohort, only mid-thigh lean mass corrected for BMI was associated with decreased odds of falls (24%, p=0.029). In all assessed ROIs, BMD was associated with the number of incident fractures within five years (OR= 0.72 to 0.93, p=0.034 to <0.001). Increased fat mass was associated with increased TUG time and decreased odds of fractures. Muscle indices corrected for BMI performed better, compared to those uncorrected or corrected for height2.Conclusion: Compared to the conventional indices and four other ROIs, mid-thigh tissue masses were associated with markers of muscle performance. Mid-thigh BMD and lean mass were superior and showed more consistent and more robust associations with adverse outcomes (falls and fractures), particularly in the older population.