Fatigue damage development in cortical bone was investigated in vitro under different mechanical components of physiological loading including tension, compression, and torsion. During each test, stress and strain data were collected continuously to monitor and statistically determine the occurrence of the primary, secondary, and tertiary stages associated with fatigue and/or creep failure of bone. The resultant microdamage and failure modes were identified by histological and fractographic analysis, respectively. The tensile group demonstrated Mode I cracking and the three classic stages of fatigue and creep suggesting a low crack initiation threshold, steady crack propagation and final failure by coalescence of microcracks. In contrast, the compressive group displayed Mode I1 cracking and a two-stage fatigue behavior with limited creep suggesting a high crack initiation threshold followed by a sudden fracture. The torsion group also displayed a two-stage fatigue profile but demonstrated extensive damage from mixed mode (Modes I1 and 111) microcracking and predominant time-dependant damage. Thus, fatigue behavior of bone was found to be uniquely related to the individual mechanical components of physiological loading and the latter determined the specific damage mechanisms associated with fatigue fracture.

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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...

In contrast to the traditional classroom environment that promotes passive learning, the multimedia-based studio approach is considered to be a more effective tool for delivering course content as it increases active in-class involvement, teamwork experience and cooperative learning (Wilson 1994). More significantly, the studio environment provides a plethora of opportunities to include case studies that promote hands-on experience and problem-solving, illustrate real-life problems and increase student interest in the course content (Starrett and Morcos 2001). In general, engineering courses benefit from the availability of simulation and analysis software in a multimedia studio environment. Biomedical engineering courses including biomechanics, however, are not always amenable to simulation and often require the setting of complicated and expensive tests involving human subjects and hazardous materials. Furthermore, unlike traditional medical courses, biomedical engineering departments do not have extensive teaching laboratories and students have little or no clinical exposure.