Biomechanical features of stress fractures

Abstract

The term stress expresses the force exerted by an external load on a solid body and the accompanying, opposed force (Newtons Law), expressed per unit of an imaginary area perpendicular to the loading direction. The internal loads generated this way deform (strain) proportionally the bodys structure. The resistance of the body to strain expresses its stiffness. Critical strain magnitudes may induce micro-fractures (microdamage), the confluence of which may fracture the body. The bodys resistance to separation into fragments determines its toughness. Hence, the bodys resistance to fracture is proportional to the stress the body can support (or give back) while it is not fractured by the load-induced strain (no stress, no strain -> no fracture). Therefore, the maximal stress the body can stand prior to fracture is determined by a combination of both, its stiffness and its toughness; and each of those properties is differently determined biologically. One or more deformations of the body may induce some microdamage but not a fracture. Microdamage accumulation determines the fatigue of the material constitutive of the body and reduces bodys toughness, leading to a fatigue-induced fragility. In case of bones, in general, both stress and fatigue have the referred, wide connotations, regarding any kind of fractures. In particular, bone fatigue predisposes to low-stress fractures, which are named (correctly) fatigue fractures and also misnamed stress fractures, to distinguish them from the current fractures that occur without any excess of microdamage, that are named (wrongly) fragility or insufficiency fractures. In fact, all fractures result from all stress and fragility or insufficiency as a whole; however, the gross distinction between fatigue or stress fractures, on one side, and fragility or insufficiency fractures, on the other, accepting the wide connotations of the corresponding terminology, is relevant to clinical practice. This article aims to explain the above biomechanical features and describe the different instances that predispose to fatigue or stress fractures in clinical practice, as a different entity from insufficiency or fragility fractures (maintaining this nomenclature), and describe their relevant features to their diagnosis and therapy.