Longitudinal Element Size Effect on Load Sharing, Internal Loads, and Fatigue Life of Tri-Level Spinal Implant Constructs

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Abstract
The effects of implant stiffness on load sharing and stress shielding, of vertebral column load sharing on implant fatigue life, and of instrumenting two versus one level adjacent to a comminuted segment on implant internal loads were studied. Finite element models of six screw constructs with 4.76 mm rod; 6.35 mm rod, and VSP plate tri-level instrumentation of two motion segments (healthy vertebra case and comminuted) and an adjacent healthy motion segment with dimensions representative of the human lumbar spine were used. Also a simplified model was developed to predict the percent of axial load passing through the column, which is a function of ki/kv the ratio of implant axial stiffness to instrumented vertebral column axial stiffness. For constructs with dimensions typical of the human lumbar spine, 77 to 80% of the axial load was predicted to pass through one or two healthy motion segments when instrumented with either 6.35 mm rod or VSP plates, compared to 90% when instrumented with 4.76 mm rods. When instrumenting smaller motion segments (in dogs) for comparison, 60% of the axial load was predicted to pass through the column for 4.76 mm rod and 33% for 6.35 mm rod constructs due to increased implant stiffness ki as a result of decreased AP and longitudinal construct dimensions, and lower canine motion segment stiffness kv.(ABSTRACT TRUNCATED AT 250 WORDS)

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