Microarchitectural and mechanical characterization of the sickle bone

Abstract

Individuals with sickle cell disease often experience acute and chronic bone pain due to occlusive events within the tissue vasculature that result in ischemia, necrosis, and organ degeneration. Macroscopically, sickle bone is identified in clinical radiographs by its reduced mineral density, widening of the marrow cavity, and thinning of the cortical bone due to the elevated erythroid hyperplasia accompanying the disease. However, the microstructural architecture of sickle bone and its role in mechanical functionality is largely unknown. This study utilized micro-CT and biomechanical testing to determine the relationship between the bone morphology, tissue mineral density, and trabecular and cortical microarchitecture of 10- and 21-week-old femurs from transgenic sickle male mice and littermates with sickle trait, as well as a wild-type control. While bone tissue mineral density did not vary among the genotypes at either age, variation in bone microstructure were observed. At 10 weeks, healthy and trait mice exhibited similar morphology within the cortical and trabecular bone, while sickle mice exhibited highly connected trabeculae. Within older femurs, sickle and trait specimens displayed significantly fewer trabeculae, and the remaining trabeculae had a more deteriorated geometry based on the structure model index. Thinning of the cortical region in sickle femurs contributed to the displayed flexibility with a significantly lower elastic modulus than the controls at both 10- and 21-weeks old. Wild-type and trait femurs generally demonstrated similar mechanical properties; however, trait femurs had a significantly higher modulus than sickle and wild-type control at 21-weeks. Overall, these data indicate that the progressive damage to the microvasculature caused by sickle cell disease, results in deleterious structural changes in the bone tissue׳s microarchitecture and mechanics.

Keywords: Biomechanics; Bone; Micro-CT; Microarchitecture; Sickle cell disease.

Fig. 1

Schematic setup of interest point bending and femur placement. The regions of four that were scanned and analyzed by micro- and analyzed by micro CT for morphological assessment are a black highlighted. Location of growth plate between trabecular regions is represented as line. The distance between the two upper points is 2.2 mm (not shown). [Reprinted from Bone, 39/4]. One Column

Fig. 2

Micro-CT measured area and volume occupied by mineralized tissue within cortical (mid-diaphyseal) and trabecular (metaphyseal and epiphyseal) bone respectively at the ages of 10 and 21 weeks. Data shown represent mean ± SEM. * indicates significance (P < 0.05). One-column

Fig. 3

Micro-CT generated representative 3D heat maps of AA, AS, and SS femoral mid diaphyseal cortical thickness of at 21 weeks of age. Thickness is depicted in pseudocolor scale, red (0.2 mm) to blue (0 mm). Scale bar equals 200 μm. Full-page

Fig. 4

Micro-CT generated representative 3D map of epiphyseal (top) and metaphyseal (bottom) trabecular morphology with respect to genotype and age. Measurements are depicted in color scale, red (0.09 mm) to blue (0 mm). Scale bar equals 300 μm.

Fig. 5

Epiphyseal and metaphyseal trabecular connectivity densities determined by micro- CT scans in AA, AS, and SS from 10- and 21-week old mice femurs. Data shown represent mean ± SEM. * indicates significance (P < 0.05). One column

Fig. 6

Force-displacement curves representative of the mechanical behavior of AA, AS, and SS mice femurs at 10- and 21-weeks old as determined by four-point bending. One column