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Unilateral and Bilateral Sacropelvic Fixation Result in Similar Construct Biomechanics

Tomlinson, Tucker MS*; Chen, Jack MD; Upasani, Vidyadhar MD; Mahar, Andrew MS*‡

doi: 10.1097/BRS.0b013e31817bd8d5

Study Design. In vitro biomechanical investigation of lumbosacropelvic spinal instrumentation.

Objective. Determine whether unilateral iliac fixation, with or without an L6 to S1 interbody graft, provides equivalent biomechanical stability compared with bilateral iliac fixation.

Summary of Background Data. Recent clinical evidence has shown improved clinical outcomes of unilateral iliac fixation compared with bilateral instrumentation that contradicts biomechanical data supporting bilateral instrumentation, although no specific investigation has compared unilateral versus bilateral instrumentation.

Methods. Sixteen porcine spines were instrumented with bilateral segmental pedicle screws from L1 to S1 and 5.5-mm titanium rods. Spines were randomized to either have an intact L6 to S1 disc space (n = 8/group) or a full discectomy and intervertebral cage at L6 to S1 (n = 8/group). Four reflective noncolinear markers were attached to both L6 and S1. Spines were tested with bilateral ilium, unilateral ilium, and sacrum-only fixation in flexion, extension, lateral bending, and axial torsion between ±7.0 Nm. L6 to S1 range of motion (°) and mechanical stiffness (Nmm/°) were compared between groups with a 2-way analysis of variance (P < 0.05).

Results. No significant differences were found in construct stiffness or L6 to S1 motion between unilateral or bilateral fixation for any test direction and both demonstrated significantly less L6 to S1 motion compared with sacrum-only fixation for all tests (all P values <0.005). Bilateral fixation was significantly stiffer than sacrum-only fixation in flexion and extension (P < 0.0001). The interbody cage significantly decreased construct stiffness in extension, lateral bending, and axial torsion (P < 0.002), and significantly increased L6 to S1 motion in torsion compared with an intact disc (P < 0.03).

Conclusion. There were no biomechanical differences between bilateral and unilateral iliac screw fixation. Intervertebral cage with full discectomy was significantly lessstiff than intact. This study provides biomechanical data to correlate with improved clinical outcomes using unilateral iliac screw fixation, and evidence contraindicating full discectomy with intervertebral cage placement.

An immature porcine spinal model was used to show that unilateral iliac screw instrumentation results in similar construct mechanical stiffness and L6 to S1 motion as bilateral iliac screws. An L6 to S1 interbody cage significantly decreased construct stiffness in extension, lateral bending and axial torsion, and significantly increased L6 to S1 motion during axial torsion.

From the *Orthopedic Biomechanics Research Center, Department of Orthopedics, Rady Children’s Hospital San Diego, San Diego, CA; †Orthopaedic Speciality Institute, Orange, CA; and ‡Department of Orthopaedic Surgery, University of California, San Diego, San Diego, CA.

Acknowledgment date: September 21, 2007. First revision date: February 20, 2008. Acceptance date: February 20, 2008.

The device(s)/drug(s) is/are FDA-approved or approved by corresponding national agency for this indication.

Corporate/Industry funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

This study was funded in part by a research grant from Alphatec Spine.

This study won the Thomas Whitecloud, MD Award for Best Basic Science Research Paper at the 2007 International Meeting for Advanced Spine Techniques.

Address correspondence and reprint requests to Andrew Mahar, Orthopedic Biomechanics Research Center, Department of Orthopedics, Rady Children’s Hospital, San Diego MC5054, 3020 Children’s Way, San Diego, CA 92123; E-mail:

© 2008 Lippincott Williams & Wilkins, Inc.