Comparative biomechanical analysis of an improved novel pedicle screw with sheath and bone cement.
METHODS
A human cadaveric biomechanical study of fixation strength of an improved novel pedicle screw (NPS) with cement and a conventional screw.


OBJECTIVE
To clarify whether the NPS has adequate fixation strength without leakage in vertebrae with low bone quality.


BACKGROUND
The fixation strength of pedicle screws decreases in frail spines of elderly osteoporotic patients. Augmentation of screw fixation with bone cement must be balanced against increased difficulty of screw removal and risk of cement leakage. We developed the NPS consisting of an internal screw and an outer sheath to mitigate the disadvantages of cement augmentation.


METHODS
The T12 and L1 vertebrae obtained from 18 formalin preserved cadavers (11 males and 7 females; mean age, 82.7 y) were used. The mean bone mineral density was 0.39 +/- 0.14 g/cm2. The NPS was inserted into one pedicle of each vertebra and the control screw, a Compact CD2 screw, was inserted into the contralateral pedicle. Both screws were 6mm in diameter and 40 mm in length. Pull-out tests were performed at a crosshead speed of 10 mm/min. Cyclic loading tests were performed with a maximum 250 N load at 2 Hz until 30,000 cycles.


RESULTS
Cement leakage did not occur in any of the specimens tested. The mean maximum force at pull-out was 760 +/- 344 N for the NPS and 346 +/- 172N for the control screw (P < 0.01). Loosening of 50% of the screws was observed after 17,000 cycles of the NPS and after 30 cycles of the control screw. The hazard ratio of loosening was 19.6 (95% confidence interval 19.3-19.9) (P < 0.001).


CONCLUSIONS
The NPS showed a significantly higher mechanical strength than the control screw in both pull-out tests and cyclic loading tests. The NPS showed more than adequate strength without cement leakage.
Takigawa
Tomoyuki
T
Department of Orthopaedic Surgery, Okayama University Graduate School of Medicine, Dentistry and Pharmaceutical Sciences, Japan. takigawa2004@yahoo.co.jp
Tanaka
Masato
M
Konishi
Hitoshi
H
Ikuma
Hisanori
H
Misawa
Haruo
H
Sugimoto
Yoshihisa
Y
Nakanishi
Kazuo
K
Kuramoto
Koichi
K
Nishida
Keiichiro
K
Ozaki
Toshifumi
T
D000368:Aged; D000369:Aged, 80 and over; D001843:Bone Cements / Q000627:therapeutic use*; D001863:Bone Screws; D002102:Cadaver; D004867:Equipment Design; D019544:Equipment Failure Analysis; D005260:Female; D006801:Humans; D016268:Internal Fixators; D008159:Lumbar Vertebrae / Q000503:physiopathology / Q000601:surgery*; D008297:Male; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods*; D013904:Thoracic Vertebrae / Q000503:physiopathology / Q000601:surgery*
D001843:Bone Cements