Does the degree of laminarity correlate with site-specific differences in collagen fibre orientation in primary bone? An evaluation in the turkey ulna diaphysis

de Margerie hypothesized that preferred orientations of primary vascular canals in avian primary cortical bone mediate important mechanical adaptations. Specifically, bones that receive habitual compression, tension or bending stresses typically have cortices with a low laminarity index (LI) (i.e. relatively lower cross-sectional areas of circularly (C) orientated primary vascular canals, and relatively higher areas of canals with radial (R), oblique (O) or longitudinal (L) orientations. By contrast, bones subject to habitual torsion have a high LI (i.e. relatively higher C-orientated canal area) [LI, based on percentage vascular canal area, = C/(C + R + O + L)]. Regional variations in predominant collagen fibre orientation (CFO) may be the adaptive characteristic mediated by LI. Using turkey ulnae, we tested the hypothesis that site-specific variations in predominant CFO and LI are strongly correlated. Mid-diaphyseal cross-sections (100 +/- 5 mum) from subadult and adult bones were evaluated for CFO and LI using circularly polarized light images of cortical octants. Results showing significant differences between mean LI of subadult (40.0% +/- 10.7%) and adult (50.9% +/- 10.4%) (P < 0.01) bones suggest that adult bones experience more prevalent/predominant torsion. Alternatively, this relationship may reflect differences in growth rates. High positive correlations between LI and predominant CFO (subadults: r = 0.735; adults: r = 0.866; P < 0.001) suggest that primary bone can exhibit potentially adaptive material variations that are independent of secondary osteon formation.