title: Longitudinal element size effect on load sharing, internal loads, and fatigue life of tri-level spinal implant constructs.
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)
authors.affiliation: University of Missouri-Columbia.
authors.forename: R C
authors.initials: RC
authors.lastname: Duffield
authors.forename: W L
authors.initials: WL
authors.lastname: Carson
authors.forename: L Y
authors.initials: LY
authors.lastname: Chen
authors.forename: B
authors.initials: B
authors.lastname: Voth
mesh_terms: D004867:Equipment Design; D004868:Equipment Failure; D006801:Humans; D008422:Materials Testing; D008962:Models, Theoretical; D009984:Orthopedic Fixation Devices; D013131:Spine; D013314:Stress, Mechanical