title: Biomechanical stability according to different configurations of screws and rods.
abstract: METHODS
  Comparison of biomechanical strength according to 2 different configurations of screws and rods.
  
  
  OBJECTIVE
  To compare the biomechanical strength of different configurations of screws and rods composed of the same material and of the same size.
  
  
  BACKGROUND
  Many complications related to instrumentation have been reported. The incidence of metallic failure would differ according to the materials and configurations of the assembly of the screws and rods used. However, to our knowledge, the biomechanical effects of implant assembly rods and screws with different configurations and different contours have not been reported.
  
  
  METHODS
  Biomechanical testing was conducted to compare top tightening (TT) screw-rod configuration with side tightening (ST) screw-rod configuration. All tests were conducted using a hydraulic all-purpose testing machine. All data were acquired at a rate of 10 Hz. Both screw systems used spinal rods of 6 mm diameter and were made of TiAl4V ELI material. Among 5 types of tests, 3 were conducted on the basis of American Society for Testing and Materials (ASTM) F 1798 to 97 and F1717-10. The other 2 tests were conducted for comparing the characteristics between TT and ST pedicle screws according to modified methods from ASTM F 1717-10 and ASTM F 1798-97. All results including axial gripping capacity and yield forces were obtained using the same methods on the basis of the mentioned ASTM standards.
  
  
  RESULTS
  In the axial gripping capacity test, the mean axial gripping capacity of the TT screw-rod configuration was 3332 ± 118 N and that of ST was 2222 ± 147 N in straight rods (P = 0.019). In 15-degree contoured rods, TT was 2988 ± 199 N and ST was 2116 ± 423 N (P = 0.014). In 30-degree contoured rods, TT was 2227 ± 408 N and ST was 1814 ± 285 N (P = 0.009). In the pulling-out test, the pulling-out force of ST was 8695 ± 1616 N and that of TT was 6106 ± 195 N (P = 0.014). In the rod-pushing test, the failure force of ST was 4131 ± 205 N and that of TT was 5639 ± 105 N. In the compressive fatigue test, the maximum load was 145 N in ST and 119 N in TT. In the cycle fatigue test, the fatigue strength of ST was higher than that of TT. In the rod-pushing test, the failure force of ST was 4131 ± 205 N and that of TT was 5639 ± 105 N (P=0.046).
  
  
  CONCLUSIONS
  Two different configurations of rod-screw systems found statistically significant differences with axial gripping, pulling out, and fatigue failures. ST constructs improved fixation stability over TT constructs. It was concluded that ST configuration may reduce complications related to implantation.
authors: Department of Orthopedic Surgery, Seoul St Mary's Hospital, College of Medicine, The Catholic University of Korea, Seoul, Korea. kyh@catholic.ac.kr
authors: Kee-Yong
authors: KY
authors: Ha
authors: Sung-Chul
authors: SC
authors: Hwang
authors: Tae-Hyuk
authors: TH
authors: Whang
mesh_terms: D001696:Biomechanical Phenomena; D001863:Bone Screws; D006801:Humans; D008159:Lumbar Vertebrae / Q000601:surgery; D008422:Materials Testing; D013123:Spinal Fusion / Q000295:instrumentation*; D013314:Stress, Mechanical; D013904:Thoracic Vertebrae / Q000601:surgery