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A Stronger Bicortical Sacral Pedicle Screw Fixation Through The S1 Endplate: An In Vitro Cyclic Loading and Pull-Out Force Evaluation

Luk, Keith D. K. FRCSEd, MCh Orth, FRACS, FHKAM(Orth)*; Chen, Liang MD†; Lu, William W. PhD*

doi: 10.1097/01.brs.0000154649.55589.bf
Biomechanics

Study Design. The insertion torque and pull-out force after cyclic loading of the bicortical sacral pedicle screw through the S1 endplate were tested using human cadaveric specimens.

Objectives. The purpose of this study was to (1) evaluate the effect of cyclic loading on the pull-out force of two different techniques of bicortical sacral pedicle screw fixation and (2) to correlate the pull-out force after cyclic loading with the screw insertion torque.

Summary of Background Data. Biomechanical studies using conventional sacral pedicle screw fixation techniques have demonstrated reduction in stiffness and strength after cyclic loading. Technical difficulties with anterior sacral cortex penetration and frequent screw loosening have been reported in clinical studies. In the authors’ center, a new method of sacral pedicle screw fixation bicortically through the S1 endplate has been used successfully in the clinical setting. However, the mechanical stability of this new technique after undergoing cyclic loading has not been documented in the literature.

Methods. Seven-millimeter compact Cotrel-Dubousset sacral screws were randomly assigned by side (left vs. right) and inserted bicortically either anteromedially through the anterior sacral cortex or superiorly through the S1 endplate of 17 fresh frozen human sacrum. The tk;4maximum insertion torque for each screw was measured. Cyclic loading from 40 N to 400 N was applied to each screw at a frequency of 2 Hz for 20,000 cycles. Pull-out tests were conducted after completion of the cyclic tests.

Results. The mean maximum insertion torque and mean pull-out force following cyclic loading were significantly higher for bicortical fixation through the S1 endplate (mean 3.17 N·m and 1457 N) than bicortical fixation through the anterior sacral cortex (mean 1.98 N·m and 1122 N). Both S1 endplate and anterior cortical fixation techniques demonstrated significant correlations between insertion torque and pull-out force following cyclic loading.

Conclusions. In sacral pedicle screw fixation, screw trajectory through the S1 endplate was significantly stronger than screws penetrating the anterior sacral cortex. Insertion torque was a good intraoperative indicator of screw pull-out force after cyclic loading.

This study examined the effect of cyclic loading on screw pull-out force and the correlation between the pull-out force and the screw insertion torque of bicortical sacral pedicle screw fixations penetrating the anterior sacral cortex or the S1 superior endplate. The mean maximum insertion torque and mean pull-out force following cyclic loading were significantly higher for bicortical fixation to the S1 endplate than bicortical fixation to the anterior cortex. The pull-out force and insertion torque showed high correlation in both groups.

From the *Department of Orthopaedics and Traumatology, The University of Hong Kong, Pokfulam, Hong Kong; and †Department of Orthopaedic Surgery, The First Affiliated Hospital of Suzhou University, Suzhou, China.

Acknowledgment date: July 31, 2003. First revision date: November 11, 2003. Acceptance date: March 29, 2004.

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

No 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.

Supported by The Hong Kong Research Grants Council (Grant HKU472/92 M).

Address correspondence to Prof. Keith D. K. Luk, Department of Orthopaedics and Traumatology, University of Hong Kong, Queen Mary Hospital, Pokfulam, Hong Kong; E-mail: hrmoldk@hkucc.hku.hk

© 2005 Lippincott Williams & Wilkins, Inc.