Biomechanical arrangement of threaded and unthreaded portions providing holding power of transpedicular screw fixation.
Failure of pedicle screw is a major concern in spinal surgery. The threaded and unthreaded portions of the pedicle screw provide the ability to anchor and squeeze the surrounding bone, respectively. This study aimed to investigate the anchoring and squeezing effects of different design of the threaded/unthreaded portions of a pedicle screw to vertebrae.



Four variations (one fully and three partially threaded, with a 1/3, 1/2, and 2/3 unthreaded designs at the proximal portion) of screws were used to measure pullout strength and withdrawn energy using synthetic and porcine specimens. The tests were conducted in static and dynamic fashions, in that the screws were axially extracted directly and after 150,000cycles of lateral bending. The load-displacement curves were recorded to gain insight into the peak load (pullout strength) and cumulative work (withdrawn energy).



The two testing results of the synthetic and porcine specimens consistently showed that the 1/3 unthreaded screw provides significantly higher pullout strength and withdrawn energy than the fully threaded screw. The withdrawn energy of the three unthreaded screws was significantly higher than that of the threaded counterpart.



The holding power of a pedicle screw was the integration of the anchoring (cancellous core) and squeezing (compact pedicle) effects within the threaded and unthreaded portions. The current study recommends the 1/3 unthreaded screw as an optimal alternative for use as a shank-sliding mechanism to preserve the holding power within the pedicle isthmus.
Division of Neurosurgery, Department of Surgery, National Taiwan University Hospital, Taiwan; Institute of Biomedical Engineering, National Taiwan University, Taiwan.
Fon-Yih
FY
Tsuang
Department of Orthopaedics, Shuang Ho Hospital, Taipei Medical University, Taiwan.
Chia-Hsien
CH
Chen
Department of Orthopaedics, Shuang Ho Hospital, Taipei Medical University, Taiwan.
Lien-Chen
LC
Wu
Department of Orthopaedics, Taipei Medical University Hospital, Taiwan; Department of Orthopaedics, School of Medicine, College of Medicine, Taipei Medical University, Taiwan.
Yi-Jie
YJ
Kuo
Graduate Institute of Biomedical Engineering National Taiwan University of Science and Technology, Taiwan.
Shang-Chih
SC
Lin
Department of Orthopaedics, Shuang Ho Hospital, Taipei Medical University, Taiwan; Department of Orthopaedics, School of Medicine, College of Medicine, Taipei Medical University, Taiwan. Electronic address: cjchiang@s.tmu.edu.tw.
Chang-Jung
CJ
Chiang
D000818:Animals; D001696:Biomechanical Phenomena; D004867:Equipment Design; D004868:Equipment Failure; D008422:Materials Testing; D065289:Pedicle Screws; D013131:Spine / Q000601:surgery*; D013552:Swine
Holding power; Pedicle screw; Pullout strength; Screw design; Screw loosening