Relationships between quantitative ultrasound (QUS), density (bone volume d
ensity [BV/TV]), and trabecular architecture were investigated in 69 calcan
eal cancellous bone cubes. Ultrasound signal velocity, phase velocity, atte
nuation, and broadband ultrasonic attenuation (BUA) measurements were made
along the mediolateral axis. Density and architectural parameters were meas
ured using microcomputed tomography (mu CT). Density yielded the best corre
lations with QUS (r(2) = 73-77%). Of the individual architectural parameter
s, correlations with QUS were highest for the Structure Model Index (SMI),
a parameter quantifying the relative proportion of rods and plates (r(2) =
57-63%). After adjustment for density, significant associations with QUS re
mained for SMI, trabecular spacing (Tb.Sp), and trabecular number (Tb.N), a
lthough the variance in QUS attributable uniquely to individual architectur
al parameters was at best 4%. In multivariate regression models, combinatio
ns of density and architectural parameters explained 76-82% of the variance
in QUS, representing an r(2) increase of, at most, 8% compared with using
density alone. However, multivariate models using combinations of architect
ural parameters alone (i.e., density excluded) also had a good predictive a
bility for QUS (r(2) = 73-81%). Thus, although these data show modest but s
ignificant density-independent relationships between QUS and trabecular arc
hitecture in the human calcaneus for the first time, the causal relationshi
ps behind the variation in acoustic properties remain obscure. Given the re
lative weakness and complexity of the emerging associations between QUS and
architecture, it is prudent to regard QUS measurements in calcaneal bone p
rimarily as an indicator of bone density.