Do regional modifications in tissue mineral content and microscopic mineralization heterogeneity adapt trabecular bone tracts for habitual bending? Analysis in the context of trabecular architecture of deer calcanei

Abstract

Calcanei of mature mule deer have the largest mineral content (percent ash) difference between their dorsal 'compression' and plantar 'tension' cortices of any bone that has been studied. The opposing trabecular tracts, which are contiguous with the cortices, might also show important mineral content differences and microscopic mineralization heterogeneity (reflecting increased hemi-osteonal renewal) that optimize mechanical behaviors in tension vs. compression. Support for these hypotheses could reveal a largely unrecognized capacity for phenotypic plasticity - the adaptability of trabecular bone material as a means for differentially enhancing mechanical properties for local strain environments produced by habitual bending. Fifteen skeletally mature and 15 immature deer calcanei were cut transversely into two segments (40% and 50% shaft length), and cores were removed to determine mineral (ash) content from 'tension' and 'compression' trabecular tracts and their adjacent cortices. Seven bones/group were analyzed for differences between tracts in: first, microscopic trabecular bone packets and mineralization heterogeneity (backscattered electron imaging, BSE); and second, trabecular architecture (micro-computed tomography). Among the eight architectural characteristics evaluated [including bone volume fraction (BVF) and structural model index (SMI)]: first, only the 'tension' tract of immature bones showed significantly greater BVF and more negative SMI (i.e. increased honeycomb morphology) than the 'compression' tract of immature bones; and second, the 'compression' tracts of both groups showed significantly greater structural order/alignment than the corresponding 'tension' tracts. Although mineralization heterogeneity differed between the tracts in only the immature group, in both groups the mineral content derived from BSE images was significantly greater (P < 0.01), and bulk mineral (ash) content tended to be greater in the 'compression' tracts (immature 3.6%, P = 0.03; mature 3.1%, P = 0.09). These differences are much less than the approximately 8% greater mineral content of their 'compression' cortices (P < 0.001). Published data, suggesting that these small mineralization differences are not mechanically important in the context of conventional tests, support the probability that architectural modifications primarily adapt the tracts for local demands. However, greater hemi-osteonal packets in the tension trabecular tract of only the mature bones (P = 0.006) might have an important role, and possible synergism with mineralization and/or microarchitecture, in differential toughening at the trabeculum level for tension vs. compression strains.

Fig. 1

(a) Lateral-to-medial view of the ankle region of a skeletally mature mule deer showing the calcaneus shaft ‘length’ and other associated bones. The trabecular patterns are stylized, and are based on the lateral-to-medial roentgenogram (at bottom). The distance from 0 to 100% was typically about 6.3 cm in the bones used in this study (range: 6.1 cm immature to 6.5 cm mature). The dotted line at the tip of the 100% arrow indicates the deeper location of the contour formed by the astragalus-calcaneus articular surfaces. The large dorsally directed arrow at the distal end of the bone indicates the direction of force imparted by the Achilles tendon (i.e. common calcaneal tendon) during mid-stance, placing the dorsal cortex in compression (converging arrows). The section at right is from 50% to 60% of the defined bone ‘length’, and shows the relatively thicker compression cortex. An approximate location of a theoretical neutral axis (NA) is shown. (b) The trabecular patterns of the artiodactyl calcaneus resemble stress trajectories of an idealized, homogeneous, isotropic cantilever subject to bending from a force (arrow) applied at the free (distal) end (redrawn from Currey, 1984, p. 140). The principal stress trajectories are diagrammatically represented; they were not drawn using mathematically derived coordinates (J.D. Currey, personal communication). The more crowded the trajectories, the greater the stress. The trajectories at the base of the cantilever have been omitted for clarity. The beam and a transverse cross-section of the beam show the location of the neutral axis (NA).

Fig. 2

Diagrammatic depictions of the cut surfaces of the 40% and 50% segments showing P-values of selected paired comparisons of mineral content data (average values and standard deviations are shown). The dark circles indicate locations where the cylindrical bone cores were removed for mineral content analysis. In the trabecular bone, the dark circles also indicate the locations where micro-CT measurements were made. There are no significant differences (P < 0.05) between the age groups in terms of the tract comparisons (i.e. immature dorsal tract vs. mature dorsal tract). No attempt is made to show the age-related differences in size of the cross-sections.

Fig. 3

On the left are BSE images showing differences in gray levels that represent differences in mineral content (Bloebaum et al. 1997). On the right are the gray level profiles for each of the images shown [M = mature (red curve); I = immature (blue curve)]. Mineralization heterogeneity is quantified as the FWHM of the main gray level peak in each of these profiles. The FWHM data reflect bone mineral density distribution (BMDD). Packets, which are the basic structural units typically formed by hemi-osteons, can be distinguished by their differences in mineralization, directions and/or intensity of lamellations (banding patterns), and/or cement lines. Gray level profiles that are shifted more to the right represent increased mineralization. The gray level contrasts in these images are directly comparable because they were obtained in the same imaging session, with the magnification and all other electron beam conditions being identical. D.C., dorsal cortex; D.T., dorsal tract; P.T., planar tract; P.C., planar cortex.

Fig. 4

BSE images of immature and mature deer calcanei at 200 × . The gray level contrasts in these images are directly comparable because they were obtained in the same imaging session, with the magnification and all other electron beam conditions being identical.

Fig. 5

Three-dimensional reconstructed images obtained from micro-CT scans of the 50% segment of a mature mule deer calcaneus. The views are longitudinal (‘on axis’) and approximately 10° off axis in the dorsal ‘compression’ trabecular tract (a) and plantar ‘tension’ trabecular tract (b). Cylinder diameter = 3 mm.