Pull-out strength of patient-specific template-guided vs. free-hand fluoroscopically controlled thoracolumbar pedicle screws: a biomechanical analysis of a randomized cadaveric study.

Abstract

To assess the pull-out strength of thoracolumbar pedicle screws implanted via either a patient-specific template-guided or conventional free-hand fluoroscopically controlled technique in a randomized cadaveric study, and to evaluate the influence of local vertebral bone density, quantified by Hounsfield units (HU), on pedicle screw pull-out strength. Thoracolumbar pedicles of three spine cadavers were instrumented using either a free-hand fluoroscopically controlled or a patient-specific template-guided technique. Preoperative bone density was quantified by HU measured on CT. Pedicle perforation was evaluated on postoperative CT scans by an independent and blinded radiologist. After dissected vertebrae were embedded in aluminum fixation devices, pull-out testing was initiated with a preload of 50N and a constant displacement rate of 0.5mm/s. Subgroup analyses were performed excluding pedicle screws with a pedicle breach (n=47). Pull-out strength was significantly different with 549±278 and 441±289N in the template-guided (n=50) versus fluoroscopically controlled (n=48) subgroups (p=0.031), respectively. Subgroup analysis limited to screws with an intrapedicular trajectory revealed a tendency toward a higher pull-out strength in the template-guided (n=30) versus fluoroscopically controlled screws (n=21) with 587±309and 454±269N (p=0.118), respectively. There was a trend toward a higher pull-out strength (709±418 versus 420±149N) in vertebrae with a bone density of (>171 HU) versus (<133 HU), respectively (p=0.061). There was a significantly higher pull-out strength of thoracolumbar pedicle screws when inserted via a patient-specific template-guided versus conventional free-hand fluoroscopically controlled technique, potentially associated with screw trajectory.