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Twenty-three Hoffmann ball-joint rods were subjected to sinusoidal loads, each at approximately constant amplitude, between ±400 and ±975 N at a frequency of 5 Hz. Ball-joint rod compliance was plotted against number of cycles for each specimen. The point of component failure was defined as an increase of compliance of 5% from its initial value. Fractographic methods were employed to substantiate a similarity of failure mode in specimens tested and a clear difference from non-tested specimens taken to failure. Experimental loads and number of cycles to failure were well within the clinical range. The fatigue limit of the ball-joint rods is less than 400 N (550 N/mm2). The likelihood of a given component failing is not predictable by inspection or manual manipulation.
fatigue (materials), ball-joint rods, external fixation, implant materials
Medical student, Burlington, VT
Kosair professor and chief, University of Louisville School of Medicine, Louisville, KY
Research assistant, University of Vermont, Hinesburg, VT