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The same screw-rod interconnection was tested at two labs. One used the fixed-fixed end straight rod assembly in ASTM F 1798-97. The other used a fixed-free end equivalent assembly unilateral construct. The fixed-fixed assembly consistently produced higher fatigue strength, and a different failure mode with only one exception. Freebody diagram analysis of the fixed-fixed end rod-connector assembly revealed that the superior and inferior ends of the rod shared in resisting the applied load, which resulted in lower internal forces and moments than in the inferior rod end of the fixed-free assembly. Lordotically contoured rods had lower flexion fatigue life than straight rods due to the residual tensile stress from contouring. Changing ASTM F 1798-97 to fixed-free end assembly testing is recommended since the interconnection loading conditions created are “worst case” and are more clinically relevant (particularly at ends of constructs), and tests can be performed with bent or straight rods.
ASTM 1798-97, interconnection, flexion, fatigue, contoured
Professor Emeritus, University of Missouri-Columbia, Columbia, MO