Mechanical performance of the new posterior spinal implant: effect of materials, connecting plate, and pedicle screw design.
METHODS
A newly designed spinal implant was tested to evaluate multicycle stiffness and fatigue resistance.


OBJECTIVE
To investigate the effect of different materials, connecting plate, and pedicle screw design on the mechanical performance of the spinal implant.


BACKGROUND
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 Ti-6Al-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 for implants made 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.
Chen
Po-Quang
PQ
Department of Orthopedic Surgery, National Taiwan University, Taipei, Taiwan 10660, Republic of China.
Lin
Son-Jyh
SJ
Wu
Shing-Sheng
SS
So
Hon
H
D000497:Alloys; D001696:Biomechanical Phenomena; D001860:Bone Plates / Q000592:standards / Q000706:statistics & numerical data*; D001863:Bone Screws / Q000592:standards / Q000706:statistics & numerical data*; D004548:Elasticity; D004867:Equipment Design; D019544:Equipment Failure Analysis / Q000706:statistics & numerical data; D006801:Humans; D020007:Implants, Experimental / Q000592:standards / Q000706:statistics & numerical data*; D008422:Materials Testing / Q000295:instrumentation / Q000379:methods / Q000706:statistics & numerical data*; D013123:Spinal Fusion / Q000295:instrumentation*; D013314:Stress, Mechanical; D014025:Titanium / Q000502:physiology
D000497:Alloys; C031462:titanium alloy (TiAl6V4); D014025:Titanium