The aim of adaptation is to “accommodate” not “resist” habitual/beneficial strains, at least up to a point.
While the best way to avoid fatigue failure may be to decrease microcrack incidence, Martin et al. (1998) argued that bone that is highly resistant to microdamage initiation will be inefficient at controlling microcrack propagation. The trade-off for highly stressed, fatigue-prone bone, therefore, may be efficient repair and a decrease in propagation, rather than prevention of microcrack initiation (Reilly et al., 1997). Studies have suggested that secondary osteonal bone is rather poor at minimizing microcrack formation, but rather good at attenuating the distance and rate of microcrack propagation (Burr et al., 1988; Reilly et al., 1997; Taylor, 1997; Martin et al., 1998; Reilly and Currey, 1999). From this perspective it is important to reconcile the contrary teaching of conventional wisdom that the overall goal of adaptation is to “resist” excessive bone stresses/strain. This conventional view tends to suggest that microdamage formation should be avoided altogether. An overbuilt bone then results, and the consequence could be increased microdamage anyway (please refer to section on the “volume effect”). As noted, recent empirical data do not support the conventional view. Therefore, it is best to consider adaptation as “accommodating” the safe range of strains engendered by habitual loading (Bertram and Biewener, 1988) — this is especially important when considering the benefits described below for regional variations in osteon morphotypes and predominant CFO.
The capacity of bone to accommodate/resist microdamage formation/propagation through microstructural/nanostructural modifications is called ‘toughening’. One of the most important, but non-specific, toughening mechanisms includes the introduction of microstructural interfaces such as cement lines of secondary osteons (Skedros et al., 2005; Ural and Vashishth, 2005; Gibson et al., 2006). Another important and specific, toughening mechanism has been revealed in studies that have examined thin sections of bone using polarized light. In these studies, specific osteon ‘morphotypes’ have been identified, and their increased regional prevalence can differentially/specifically toughen bone for its mechanical disparities in tension, compression, and shear (Fig. 1) (Hiller et al., 2003; Bigley et al., 2006; Skedros et al., 2009; Skedros et al., 2011a; Skedros et al., 2011b). Examples of how osteon morphotypes influence bone failure from the material level to the structural level of the whole bone are described by Ebacher et al. (2007) in their in vitro study of human tibia fractures.
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