Fatigue of human cortical bone under axial-torsional loading

Abstract

A b d r w c Since bone mass based measuremeuh have demonstrated limited ~uccess in predicting age-related increase in fracture incidence, recent empbasir bas been placed on quantiIkatiOQ of bone quality. Tbe current study reporb the multiaxial failure characteristie of bone (quality) by conduetiog fatigue tests on buman cortical bone specimens under physiologically relevant axial-torsional loading. Our resnlta show for the first time tbaf compared to uniaxial tesb, multiaxial fatigue testa sbow a four-fold reduction In the fatigue life of bumnu wrtical bone. Furthermore, the weak mode in physiologically relevant axial-torsional loading changes with age. Bones in the older group (>5Oyrs) were weaker in torsional than in adal loading and the bones in the younger donor group (em) were weaker in axial tban torsional loading (p4.05). IN-IRODUCXONOverlap in bone density values in patients with or without hip fmctures suggests that factors other than bone mass including bone quality may also contribute to fracture risk [SI. Traditionally, bone quality has been measured thrciugh material parameters determined from uniaxial loading [4,5].However. evidence is gradually accumulating that in vivo loading of bone is multiaxial [2,3,1 I] and that the hchlre of materials subjected to multiaxial loading cannot be predicted from uniaxial tests [9]. In contrast to uniaxial loading, multiaxial loading promotes mixed mode failure where damage forms in one mode [tension (mode 01 and propagates in another [in-plane (mode n) or out-of-plane (mode III) shear] [9].More significantly, aging of human bone is associated with changes in micmstn~ctural and ultrastructural features [ 1.7.81 that are likely to interact with the applied loading and alter the mode andor the magnituae of mixed mode failure. However, no studies have been done and the age-related changes in damage mechanisms under physiologically relevant multiaxial loading remain poorly understood. The overall goal of this study was to investigate multiaxial fatigue characteristics and damage mechanisms of human cortical bone under physiologically relevant loading involving simultaneous application of axial and torsional loading [1,2]. A secondary goal was to identify the loading mode that would accelerate fractwe under physiologically relevant axial-torsional loading. MATEFULS AND METHODSThirty-six cylindrical dumbbell specimens with reduced gage section diameter of 3 mm were wet machined from medial and lateral cortices of six male donor tibiae (Age: 21, 26, 36, 58, 65, 79) and separated into younger (40 yrs) and older 0 5 0 yrs) groups with three donors in each group.The fatigue protocols included subjecting specimens in each group to fully reversed (U-= 0 ) axial (n=6), fully reversed torsional (n=6) and fully reversed in-phase axialtorsional (n=12) cyclic tests until complete failure of the specimen. All specimens were tested on a MTS MiniBionix system (Model 858; MTS Systems, MN) under load control at a frequency of 2 Hz under continuous irrigation of 0.9% saline at 37°C...