HR-pQCT skeletal phenotype clustering associated with muscle function in older men and women: The Study of Muscle, Mobility and Aging (SOMMA)

Age-related changes to BMD, morphometry, and microarchitecture do not occur uniformly across the population and the common skeletal phenotypes beyond BMD are not well defined. Additionally, the associations between bone and muscle are critical to understanding fall and fracture risk. We hypothesized that unsupervised clustering of High Resolution-peripheral Quantitative Computed Tomography (HR-pQCT) measures at the distal tibia (DT) and radius (DR), separately, would reveal unique skeletal phenotypes; and certain phenotypes would be associated with worse muscle function. In the Study of Muscle, Mobility and Aging (SOMMA; first annual follow-up visit), a cohort of community-dwelling older women and men (61% women; 87% White), HR-pQCT parameters acquired at the DT (N = 321; 76.3 ± 4.6 yr) and DR (N = 295; 76.1 ± 4.5 yr) were standardized within-sex then combined to form clusters. This resulted in 3 phenotypic clusters, (C1) high total BMD (Tt.BMD) and cortical area (Ct.Ar); (C2) medium Tt.BMD, Ct.Ar and low trabecular BMD (Tb.BMD); and (C3) low Tt.BMD, and Ct.Ar. DT C2 and C3 exhibited lower micro finite element analysis failure loads, with the cortical load fraction higher in C2 and lower in C3. C2 and C3 both had a similar proportion of osteoporotic and osteopenic/low bone density individuals, highlighting the novel granularity of HR-pQCT clusters vs. aBMD clinical cutoffs. In linear regression models for women, DT C3 was associated with lower leg power (p < .05). For men, DT C3 was associated with lower stair climb and leg power (p < .05). No significant difference was found in grip strength between DT clusters. For DR, no significant difference or association was found between muscle function and clusters for women and men. These findings suggest the concept of bone phenotypic-specific associations with lower but not upper extremity muscle function and have possible implications for the interaction between skeletal phenotypes and muscle function as potential contributory factors to fracture risk.