Abstract
Summary Statement
From a mechanical point of view, the clinical use of pedicle screws in the atlas is a promising alternative to lateral mass screws due to an increased biomechanical fixation.
Introduction
The most established surgical technique for posterior screw fixation in the atlas (C1) is realised by screw placement through the lateral mass [1]. This surgical placement may lead to extended bleeding from the paravertebral venous plexus as well as a violation of the axis (C2) nerve roots [1]. Using pedicle screws is an emerging technique which utilises the canal passing through the posterior arch enabling the use of longer screws with a greater contact area while avoiding the venous plexus and axis nerve roots. The aim of this ex vivo study was to investigate if pedicle screws in C1 bear the potential to replace the more common lateral mass screws. Therefore, the comparative biomechanical fixation strengths in terms of cycles to failure, stiffness, and removal torque were investigated.
Methods
Nine C1 cadaveric vertebrae from donors aged 58.0 ± 11.1 years were separated, CT scanned (Mx8000 IDT 16, Philips Healthcare, DA Best, The NL) with a phantom, and stored at −22°C. Each vertebra received one lateral mass screw and one pedicle screw of the same size (diameter: 3.5 mm, length: 26 mm, Synapse System, Synthes GmbH, Oberdorf, CH). The side on which each screw was placed into the vertebra was allocated based on BMD, age, gender, and testing order. Depending on the surgical technique the entry point varied; the pedicle screw entered through the posterior arch, and the lateral mass screw was inserted further inferior through the lateral mass. The screw tips converged to the same height and depth. Specimens were subjected to a sinusoidal, cyclic (0.5 Hz) fatigue loading at the screw head (858 Bionix®, MTS, Eden Prairie, MN). The peak compressive and tensile forces started from ±15 N and increased by 0.05 N every cycle. Testing was stopped at 5 mm displacement. Cycles to failure, displacement, initial and final cyclic stiffness were measured. After fatigue testing a surgeon evaluated each screw by hand for looseness. Final CT scans were taken and then the removal torque was measured.
Results
The specimens were of normal bone quality (BMD = 226 ± 69.0 mgHA/cm3). The pedicle screw technique consistently and significantly out-performed the lateral mass technique in cycles to failure (p=0.001, r2=0.48), initial stiffness (p=0.01, r2=0.29), end stiffness (p=0.005, r2=0.18), and removal torque (∗p=0.05, r2=0.18). After testing only 33% of pedicle screws were loose compared to 100% of lateral mass screws.
Discussion
Utilizing the C1 posterior arch, the pedicle screws were able to withstand a 32% higher toggle force than the lateral mass screws while maintaining a higher stiffness throughout and after testing. The advantages likely arise due to an increased depth into the bone and the smaller canal width. Due to the fixation benefits in the atlas, the clinical use of pedicle screws is a promising alternative to lateral mass screws. Funding from the State of Hamburg and the Marie Curie ITN project, SpineFX, is kindly acknowledged. The authors thank Synthes GmbH for providing the screws.
