Do microcracks decrease or increase fatigue resistance in cortical bone?

Abstract

Fatigue of cortical bone produces microcracks; it has been hypothesized that these cracks are analogous to those occurring in engineered composite materials and constitute a similar mechanism for fatigue resistance. However, the numbers of these linear microcracks increase substantially with age, suggesting that they contribute to increased fracture incidence among the elderly. To test these opposing hypotheses, we fatigued 20 beams of femoral cortical bone from elderly men and women in load-controlled four point bending having initial strain ranges of 3000 or 5000 microstrain. Loading was stopped at fracture or 10(6) cycles, whichever occurred first, and microcrack density and length were measured in the loaded region and in a control region that was not loaded. We studied the dependence of fatigue life and induced microdamage on initial microdamage, cortical region, subject gender and age, and several other variables. When the effect of modulus variability was controlled, longer fatigue life was associated with higher rather than lower initial crack density, particularly in the medial cortex. The increase in crack density following fatigue loading was greater in specimens from older individuals and those initially having longer microcracks. Crack density increased as much in specimens fatigued short of the failure point as in those that fractured, and microcracks were, on average, shorter in specimens with greater numbers of resorption spaces, a measure of remodeling rate.

Understanding Entheses: Bridging the Gap Between Clinical and Anthropological Perspectives

“…First, generation of new osteons will produce a smaller cutting cone, minimizing the effects of porosity, and thus the overall strength of the intracortical matrix. Second, dense concentrations of smaller osteons will absorb energy more efficiently than larger ones as the effects resulting from accumulated microdamage are reduced, with cement lines serving to limit crack propagation (Sobelman et al, 2004;O'Brien et al, 2005). Third, smaller osteons in regions of high strain may enhance the resistance towards osteon pullout or debonding of the osteon along the cement line surface (Skedros et al, 2007).…”

Bone as a composite material: The role of osteons as barriers to crack growth in compact bone

“…It has been proposed that a microstructural barrier concept may exist in bone (Martin and Burr [11], Taylor and Prendergast [12], Akkus and Rimnac [13]; Sobelman et al [14]) whereby the microstructure of osteonal bone provides barriers to crack growth in the form of cement lines which surround secondary osteons. The cement line interface between osteons and interstitial bone is relatively weak which means that it may reduce the shear strength of osteonal bone (Frasca [15]).…”

The influence of collagen fiber orientation and other histocompositional characteristics on the mechanical properties of equine cortical bone

“…The possibility that CFO and/or other aspects of collagen content/structure are mechanically adaptive in this context is supported by results of past investigations (Burstein et al, 1975;Shelton et al, 2000;Wang et al, 2001;Zioupos et al, 1999). For example, in human bone, Wang et al showed that the percentage of denatured collagen compared to the total collagen content is significantly related to failure energy and fracture toughness (work), supporting the idea that collagen in bone is a primary arrestor of microcracks (Burr, 2002;Sobelman et al, 2004).…”