Screw orientation and plate type (variable- vs. fixed-angle) effect strength of fixation for in vitro biomechanical testing of the Synthes CSLP Christian P. DiPaola, MD a, * , Justin A. Jacobson, MD a , Hani Awad, PhD a , Bryan P. Conrad, MSEng b , Glenn R. Rechtine, II, MD a a University of Rochester Medical Center, 601 Elmwood Avenue, Rochester, NY 14642, USA b University of Florida Medical Center, P.O. Box 100246, Gainesville, FL, 32610, USA Received 8 March 2007; accepted 27 June 2007 Abstract BACKGROUND CONTEXT: Two common justifications for orienting cervical screws in an an- gled direction are to increase pullout strength and to allow use of longer screws. This concept is widely taught and has guided implant design. Fixed- versus variable-angle systems may offer strength advantages. Despite these teachings, there is a paucity of supporting biomechanical evi- dence. The purpose of our study is to test the influence of screw orientation and plate design on the maximum screw pullout force. PURPOSE: This study evaluates the effect of screw orientation and plate type (fixed- vs. variable- angle) on screw pullout strength. STUDY DESIGN: Anterior cervical plates of both a fixed- and variable-angle CSLP, were tested for peak pullout strength in a direct plate pullout model using polyurethane foam bone, which models osteoporotic bone. METHODS: Self-tapping, locking screws (4.014 mm and 4.016 mm) were used. Screws were oriented in the fixed-angle plate in the standard fashion. In the variable plate, screws were instrumented in three different orientations. Biomechanical testing was performed on an Instron DynaMight 8841 servohydraulic testing machine, measuring maximum pullout force under a dis- placement-controlled pullout rate of 1 mm/min. Five samples were tested per group. RESULTS: When all screws were placed 90  to the plate, there was a significantly increased peak pullout strength (412.8622.2 N) compared with when all screws were placed 12  ‘‘up and in’’ (376.2624.3 N, p less than or equal to .03). When the 90  construct was reproduced using 14-mm screws and compared with 16-mm screws oriented 12  ‘‘all up and in,’’ there was still significantly higher pullout strength with the all 90  construct (434.2628.7 N vs. 376.2624.3 N, p less than or equal to .009). The fixed-angle plate had a significantly decreased peak pullout strength (288.2615.7 N) compared with the variable-angle plate (416.6612.6 N) (p less than .00001) when the screws were placed in the same orientation. Overall, the variable-angle plate, regardless of the orientation of screws, had a significantly greater pullout strength than the fixed-angle plate (p less than .001). CONCLUSIONS: In this system, a variable-angle plate has greater pullout strength than a fixed- angle plate, regardless of the orientation of screws. With the variable-angle plate, a construct of all screws 12  ‘‘up and in’’ is the weakest configuration. We found that with the 90  construct, both 16- and 14-mm screws performed significantly better than 16-mm convergent screws. These findings are remarkable because they contradict the current doctrine. This may be a function of plate- dependent factors and should not be applied universally to all plate systems. Further study of screw orientation in additional plating systems is warranted. Ó 2008 Elsevier Inc. All rights reserved. Keywords: Anterior cervical plating; Biomechanics; Instrumentation failure; Cervical screws; Screw pullout FDA device/drug status: approved for this indication (Medtronic Atlantis anterior cervical plate; Synthes cervical plating system; Instro Dynamight 8841 servohydraulic testing machine). The author acknowledges a financial relationship (support from Faculty member of Synthes, honorarium ad travel expense paid) that may indi- rectly relate to the subject of this research. * Corresponding author. University of Rochester Medical Center, Department of Orthopedic Surgery, 601 Elmwood Avenue, Rochester, NY 14642, USA. Tel.: (585) 262-4641; fax: (585) 756-4726. E-mail address: cpd62@yahoo.com (C.P. DiPaola) 1529-9430/08/$ – see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.spinee.2007.06.016 The Spine Journal 8 (2008) 717–722