|Mechanical Performance of the New Posterior Spinal Implant: Effect of Materials, Connecting Plate, and Pedicle Screw Design
|FATIGUE LIFE;INSTRUMENTATION SYSTEMS;BIOMECHANICAL ANALYSIS;TITANIUM IMPLANTS;FIXATION DEVICE;CROSS-LINKAGE
Study Design. A newly designed spinal implant was tested to evaluate multicycle stiffness and fatigue resistance. Objectives. To investigate the effect of different materials , connecting plate, and pedicle screw design on the mechanical performance of the spinal implant. Summary of the Background Data. The addition of cross-linkages did not significantly increase implant compression/flexion stiffness , but accelerated fatigue failure at the rod junctions. Both Ti-6Al-4V spinal implants and the 316L stainless-steel counterparts have been used extensively for clinical cases ; however, design factors establishing the proposed superiority of the Ti- 6Al-4V implant for fatigue resistance have not, as yet, been extensively studied. Methods. Twenty implants with connecting plates ( two materials by two screw designs by five implants) and five implants without connecting plates were assembled to UHMWPE blocks and cyclically loaded from 60 N to 600 N at a frequency of 5 Hz. Results. Failure sites for the tested prototypes were at the cephalic screw hubs or rod-plate junctions. All Ti6Al-4V implants demonstrated reduced stiffness compared to the structurally identical 316L analogs. The use of connecting plates raised the stiffness of the 316L prototypes without cross-links. However, elimination of the connecting plate avoided stress concentration at the rod/plate junctions and increased fatigue life. The Ti-6Al-4V new system with the minimal notch effect at the screw hubs achieved greater fatigue resistance than its 316L counterpart. By contrast, enlargement of the inner-hub diameter resulted in greater gains for fatigue resistance than for stiffness, especially for Ti-6Al-4V variants. Conclusions. Although Ti -6Al-4V was superior to 316L for endurance-limit properties, structural design of the Ti-6Al-4V implant dramatically affects fatigue resistance. This may explain the differences between existing studies and the current report, comparing fatigue life forimplantsmade from these two materials. Our results reveal that Ti-6Al-4V must be carefully treated because of sensitivity to notch, with special consideration given to screw-hub design .
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