LONGITUDINAL ELEMENT SIZE EFFECT ON LOAD SHARING, INTERNAL LOADS, AND FATIGUE LIFE OF TRI-LEVEL SPINAL IMPLANT CONSTRUCTS

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. Single level instrumentation adjacent to a comminuted segment resisted the entire axial load which produced a max bending moment of 11.4 Nm per 445 N axial lumbar load at each corresponding screw-longitudinal element junction as compared to less than 2 Nm when load sharing by a healthy motion segment existed. Thus finite implant fatigue life was predicted when instrumenting comminuted lumbar segments, with low probability of fatigue when load sharing equivalent to a healthy motion segment existed. Instrumenting two levels adjacent to a comminuted human lumbar segment was predicted to reduce the flexion moment at screw-longitudinal element interconnections by 16% when using 4.76 mm rods, 32% for 6.35 mm rods, 36% for VSP plates, and 50% when ki = kv. These results illustrate the clinical need to create load sharing when possible, to select an implant creating the desired ki/kv ratio depending upon the trade off between potential iatrogenic effects of stress shielding relative to the fatigue strength/life of the selected implant, and to control the patient's type of activity and number of cycles until fusion has satisfactorily been achieved.