Multi-Domain Load Hypothesis

Evidence of different load-complexity categories/domains within the same bone lead to the development of Teambone’s multi-domain load hypothesis. As illustrated in figure 1 (at top), we have applied the concept of distinct load-complexity ‘domains’ to help make sense of differences in structural and material (histomorphological) characteristics, and their potential “interactions”, between different bones in the same animal (Skedros et al., 2009). Figure 1 (at bottom) and figure 2 show how to apply these categories to the hypothesized presence of different load domains within human and chimpanzee femora. In these examples, we argue that the greater and lesser trochanter region (the “intertrochanteric region”) in each bone is a transition zone between the proximal and distal “load domains” (Skedros and Baucom, 2007).

Fig. 1 At top are lateral-to-medial views of the right forelimb and hindlimb skeletons of an adult horse showing a spectrum from simple loading to complex loading, respectively: calcaneus (A), radius (B), and third metacarpal (MC3) (C). The drawings below are simplified renditions of each bone type, showing: A) the calcaneus as a cantilevered beam, B) the radius as a curved beam with longitudinal loading; the curvature accentuates bending. Torsion (dotted line) is also present but is less than the torsion in the MC3 (solid circular line in C), and C) the MC3 with off-axis longitudinal loading producing bending and torsion, the latter being greater than in the other two bones. Several studies reporting in vivo strain data were used to create these drawings (Biewener et al. 1983a; Biewener et al. 1983b; Gross et al. 1992; Lanyon 1974; Rubin and Lanyon 1982; Schneider et al. 1982; Turner et al. 1975).
At bottom are the hypothesized multi-domains for a human femur (left) (also see Fig. 2) and chimpanzee femur (right); the letters within the femora correspond to the basic load conditions in the horse bones. The intertrochanteric regions of these bones (indicated by ellipses with oblique lines) are transition zones between the neck and proximal diaphyseal domains. In accordance with predominant CFO data (Beckstrom et al. 2010; Skedros et al. 1999b), these drawings show the speculation that the chimpanzee femoral neck (also more robust and elliptical in cross-sectional shape) receives more prevalent/predominant bending than the human femoral neck.

Fig. 2 Habitual loading of a modern human femur showing the multi-domain load hypothesis. Torsion is depicted by the curved lines. B.W. = body weight. In the lower portion of figure 1 are drawings that contrast the hypothesized habitual loadings of the femoral neck domains between chimpanzee and human femora.

In the human femur one of the “proximal domains” — the femoral neck — is likely habitually loaded in net compression by the gluteus medius/minimus and in torsion by the actions of other muscles, body weight, and variable joint reaction forces (Lovejoy, 1988; Skedros and Baucom, 2007) (the femoral head could also be considered a separate “domain” but this possibility is not addressed further here). By contrast, it has been argued that the upper portion of the “distal domain” — the proximal metaphyseal-diaphyseal region — is customarily loaded in bending and torsion (but with relatively less torsion than seen in the mid-neck and mid-diaphysis — discussed below), which is restrained to some extent by the iliotibial band (Skedros and Baucom, 2007). Although the iliotibial band might reduce lateral pitching of the trunk in humans, in vivo strain data obtained during walking at peak loading of stance phase reveal that net tension is still present on the lateral aspect of the proximal metaphysis-diaphysis in the human femur (Aamodt et al., 1997). This study by Aamodt and co-workers is the most important experimental support for considering the human proximal femoral diaphysis as a habitual lateral-medial “tension-compression” environment (Skedros and Baucom, 2007). Quantitative data showing clear patterns of CFO in the sub-trochanteric region and proximal diaphysis of adult human femora are consistent with this interpretation (Skedros et al., 1999; Skedros et al., 2012).

The mid-diaphyseal “load domain” of the human femur.

The mid-diaphyseal load domain extends from the proximal (sub-trochanteric) domain to the distal third of the diaphysis where the strain distribution again changes (Duda et al., 1998). Several studies of CFO patterns in the mid-diaphysis of the human femur have attempted to detect the “expected” history of habitual medial-to-lateral bending (Portigliatti Barbos et al., 1984; Portigliatti Barbos et al., 1987; Goldman et al., 2003). This hypothesized load history is based on the idea that the habitual medial-lateral (compression) to anterior-lateral (tension) bending seen in the proximal diaphysis would be transmitted distally and would be of sufficient intensity/duration to evoke similar regional strain-mode-specific CFO patterns at the mid-diaphysis. However, these studies failed to detect evidence of this load history. Why are these mid-diaphyseal data so different from the proximal diaphyseal data?

Although the strain environment at the mid-diaphyseal femur has, to Teambone’s knowledge, never been measured experimentally in vivo, in vitro strain measurements on femora loaded in simulated single-legged stance contradict the idea of a habitual medial-lateral bending moment at the mid-diaphysis. Results of in vitro strain gauge studies of femora loaded in simulated single-legged stance show both a reduction in the magnitude of the medial-to-lateral bending moment at mid-diaphysis (the bending moment is substantially greater in the sub-trochanteric area) and increased inter-specimen variability of the strain distribution at mid-diaphysis (Oh and Harris, 1978; Cristofolini et al., 1996).

Consequently, the relatively complex load environment of the femoral mid-diaphysis (where torsion > bending) might explain why “expected” tension/compression (lateral/medial) CFO differences are typically absent there, but are present in the proximal diaphysis and subtrochanteric regions (where bending > torsion) (Skedros et al., 1999; Beckstrom et al., 2010; Skedros et al., 2011a; Skedros et al., 2011b). Hence, the “load complexity changes along the femoral diaphysis — from less complex/variable at the proximal diaphysis to more complex/variable at the mid-diaphysis. An explanation for the relatively uniform CFO in adult human mid-diaphyseal femora is similar to that stated in the sections on “Load-Complexity Categories” and the “Shear Resistance-Priority Hypothesis” — bone regions that receive varying amounts of torsion and bending would not be expected to produce clear regional patterns of predominant CFO, and possibly also osteon-related characteristics.

The hypothesized differences in load history from the proximal femoral diaphysis (bending > torsion) to the mid-diaphysis (torsion > bending) in humans are also consistent with the CFO data reported in chimpanzee femora (Beckstrom et al., 2010; Skedros et al., 2011a). This explanation, based on changes in habitual load-complexity, for the dissimilar histological findings between mid- and proximal-diaphyseal chimpanzee femora is also consistent with that offered in a previous study of equine third metacarpals from our laboratory (Skedros et al., 2006) — non-stereotypical and/or complex load histories that produce significant amounts of torsion would not be expected to produce clear regional patterns of CFO-based histological organization (Skedros and Hunt, 2004). Additional support for proximal-distal changes in habitual load complexity within a bone’s diaphysis, and how this could be mediated by strain-related/site-specific differences in thresholds for modeling/remodeling activities, can be found in Hsieh et al. (2001), Skerry (2006), and Espinoza Orías et al. (2009).

It has also been suggested that the relatively circular cross-sectional shape of the human femoral mid-diaphysis reflects cross-sectional morphology expected in a loading environment characterized by prevalent torsion (and the increased variability of the neutral axis location produced by such loading) (Ruff, 1981; Carter and Spengler, 1982; Wainwright et al., 1982). But a quasi circular shape is also typical in the proximal diaphysis where the bending moment is the greatest in the entire femur. Again, this reinforces the idea that cross-sectional shape can be misleading when interpreting load history. These data again support the assertion that patterns of predominant CFO and/or osteon morphotypes are strong predictors of load history because they help distinguish bending from torsional load histories.

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