Central and Juxta-Endplate Vertebral Body Screw Placement: A Biomechanical Analysis in a Human Cadaveric Model

T Lowe MD,
M O'Brien MD,
D Smith MSc,
D Fitzgerald BSME,
R Vraney MD,
J Eule MD,
J Wood, MD,
P Alongi, MD

PURPOSE:
To demonstrate the biomechanical characteristics of toggle and pull–out failure modes for juxta–endplate and centrally placed vertebral body screws.

MATERIALS & METHODS:
61 disarticulated human thoracic and lumbar vertebrae were potted in DynaCast Greenä epoxy and fixed in a custom grip system designed to allow axial and “toggle” (rostral/caudal) testing. Both juxta–endplate (JEP) and centrally placed vertebral body screws were tested with and without staples. Axial testing using “pull–out” was performed to failure on both juxta–endplate and centrally placed vertebral body screws. Toggle testing was performed on three different screw configurations with and without staples.

TOGGLE TESTING MODES AND YEILD STRENGTHS(NEWTONS):

	Staple JEP screw toggled towards endplate	JEP screws toggled away from endplate	Cent. Screws toward endplate
W/O	JT (sy = 219)	JA (sy = 301)	C (sy = 240)
W	JTS (sy = 267)	JAS (sy = 465)	CS (sy = 353)

RESULTS:
Axial Pullout: Unicortical fixation resulted in a 75% decrease in axial pull–out strength over bicortical fixation. Centrally placed screws failed at 765 N (SD 280) and juxta–endplate screws failed at 839 N (SD 383) (p=NS). Staples did not contribute to axial pullout strength. Toggle Testing: Toggle testing demonstrates that juxta–endplate screws with a staple, toggled away from the endplate (JAS), have the highest yield strength (465 N). When compared to other staple groups (JTS, CS) the differences were not significant. When JAS is compared to all non–staple groups (JT, JA, C), the differences are significant at p=0.001, p=0.045, p=0.003 respectively. Yield strength for centrally placed screws without staples (C) was 240 N and with a staple (CS) was 353 N (p=0.009). The yield strength of a juxta–endplate screw without a staple toggled away from the endplate (JA) is equivalent to a centrally placed screw with a staple (CS) (p=NS).

CONCLUSIONS:
Bicortical screw purchase is essential to maximize resistance to axial pull– out. Juxta–endplate and centrally placed screws with and without staples are equivalent in axial pull–out. If screws are placed centrally their resistance to toggle is maximized by using a staple. Juxta–endplate screws without a staple (JA) and centrally placed screws with a staple (CS) provide equivalent resistance to toggle. Therefore juxta–endplate screws without a staple (JA) should be considered when preservation of segmental vessels is desirable. Juxta–endplate screws with staples (JAS) provide the best resistance to toggle and should be considered for the superior end of long anterior constructs where screw pull–out is often a clinically relevant problem.