Are uniform regional safety factors an objective of adaptive modeling/remodeling in cortical bone?

This study examined the claim that one of the key purposes of morphological variation in limb bone diaphyses is to achieve uniform safety factors across different cortical bones, using horse metacarpal bones as a model and finding that safety factors varied, contrarily to the original assumption.

Study Objectives and Methodology

• The research aimed to critically explore the theory that the process of morphological adaptation in the diaphyses of limb bones – the shaft or central parts of a long bone – aims for uniform safety factors in discrete cortical areas.
• This hypothesis was examined using a third metacarpal bone in horses (MC3) which had functionally generated strains measured by a sophisticated finite element model. This model is typically used in mathematics and engineering to understand how objects react under physical strain.
• To test different areas of the bone under habitual stress or compression, mechanical properties of numerous mid-diaphyseal samples were evaluated in both tension and compression. This approach is referred to as “strain-mode-specific” loading.
• The explored mechanical properties included the elastic modulus (measure of stiffness), yield strength (the stress force required to deform the bone), and ultimate strength and strain. These properties were correlated with strain data from a previously published finite element model.

Findings of the Study

• The investigation found slight variations in the bone’s elastic modulus, yield strength and ultimate strength in both tension and compression loading. However, there was a significant variety in physiological strains between the cortices, the outer layers of the bone that provide strength and flexibility.
• Contrary to the hypothesis of uniform safety factors, the horse metacarpal bone demonstrated a broad range of tension and compression safety factors. These safety factors are vital in determining the bone’s resistance to stress or deformation.
• The study concluded that the objective of morphological adaptation in limb-bone diaphyses achieving uniform safety factors across discrete cortical areas was not supported. The variation noted in both tension and compression safety factors reflects adaptive modeling/remodeling that is not directed towards a uniform regional safety factor.