Bone mechanical properties and changes with osteoporosis

Abstract

This review will define the role of collagen and within-bone heterogeneity and elaborate the importance of trabecular and cortical architecture with regard to their effect on the mechanical strength of bone. For each of these factors, the changes seen with osteoporosis and ageing will be described and how they can compromise strength and eventually lead to bone fragility.

Keywords: Biomechanics; Bone fragility; Bone loss; Bone resorption; Collagen.

Fig. 1. Cortical bone trabecularization

Trabecularization of cortical bone at the endocortical aspect of the cortex. Light microscopy of a quadrant of a female (age 91 years) femoral cortex at midshaft level.

Fig. 2. Association between bone loss and fracture incidence

(a) Cortical and trabecular bone loss in different age groups as shown by Zebaze et al. [20]. Early bone loss occurs in the trabecular bone, but with increasing age the bone loss becomes mainly cortical. (b) Incidence of osteoporotic hip and vertebral compression fractures in different age groups in Switzerland as shown by Svedbom et al. [21]. Vertebral compression fractures are more common in individuals aged less than 65 years. With increasing cortical bone loss after the age of 65 years, hip fractures become the most frequent entity.

Fig. 3. Radiographic frontal view of the proximal femur. Courtesy of Dennis Carter

Fig. 4. Bone heterogeneity and vertebral endplate collapse

Regions of endplate collapse (outlined in blue and red) and distribution of structure model index (SMI) in the trabecular bone directly underlying the endplate (grayscale): The lightest blue outline corresponds to the loading increment at which endplate collapse clearly initiated. The boundaries at subsequent loading increments are represented with progressively darker shades of blue. The red outline corresponds to the region of endplate collapse that remained after loading was complete and all load was removed. Modified from Jackman et al. [52].

Fig. 5. Trabecular mineralization in estrogen deficiency

Spatial distribution of calcium (wt% Ca) between superficial, intermediate, and deep lamellae in the greater trochanter (GT), head (H) and lesser trochanter (LT) regions of the proximal femur from 12 month ovariectomized sheep (OVX) and aged matched controls (CON). * indicates statistical significance between trabecular regions indicated by brackets ( p ≤ 0.02). Figure adapted and data from [64].

Fig. 6. Trabecular mineralization in prolonged estrogen deficiency

Spatial distribution of calcium (wt% Ca) between superficial, intermediate, and deep lamellae in the greater trochanter (GT), head (H) and lesser trochanter (LT) regions of the proximal femur from 31 month ovariectomized sheep (OVX) and aged matched controls (CON). * indicates significantly different to deep lamellae within the same femoral region of the indicated group. + indicates significant difference to the same ROI of the CON group. Data from [65].

Fig. 7. Cortical bone porosity and mechanical strength

Relationships among bone mineral density, and pore size in cortical bone and mechanical strength assessed by yield stress. Data from [4,6]

Fig. 8. Collagen cross-links

A schematic illustration of enzymatic crosslinks (e.g. pyridinoline [PYD], deoxypyridinoline [DPD]) and non-enzymatic crosslinks (e.g. pentosidine [PEN]) at the molecular level.