The Optimal Screw Length of Lumbar Pedicle Screws during Minimally Invasive Surgery Fixation: A Computed Tomography-Guided Evaluation of 771 Screws

Chua, Min Jia; Siddiqui, Shiblee; Yu, Chun Sing; Nolan, Colum Patrick; Oh, Jacob Yoong Leong

Abstract

STUDY DESIGN: A retrospective study of radiographic parameters of patients who underwent lumbar spinal pedicle screw insertion. PURPOSE: The optimal length of pedicle screws is often determined by the lateral radiograph during minimally invasive surgery (MIS). Compared with open techniques, measuring the precise length of screws or assessing the cortical breach is challenging. This study aims to ascertain the optimal pedicle screw lengths on intraoperative lateral radiographs for L1-L5. OVERVIEW OF LITERATURE: Research has revealed that optimal pedicle screw length is essential to optimize fixation, especially in osteoporotic patients; however, it must be balanced against unintentional breach of the anterior cortex, risking injury to adjacent neurovascular structures as demonstrated by case reports.
METHODS
We reviewed intra- and postoperative computed tomography scans of 225 patients who underwent lumbar pedicle screw insertion to ascertain which of the inserted screws were "˜optimal screws.' The corresponding lengths of these screws were analyzed on postoperative lateral radiographs to ascertain the ideal position that a screw should attain (expressed as a percentage of the entire vertebral body length).
RESULTS
We reviewed 880 screws of which 771 were optimal screws. We noted a decreasing trend in average optimal percentages of insertion into the vertebral body for pedicle screws going from L1 (average=87.60%) to L5 (average=78.87%). The subgroup analysis revealed that there was an increasing percentage of screws directed in a straight trajectory from L1 to L5, compared to a medially directed trajectory.
CONCLUSIONS
During MIS pedicle screw fixation, this study recommends that pedicle screws should not exceed 85% of the vertebral body length on the lateral view for L1, 80% for L2-L4, and 75% for L5; this will minimize the risk of anterior cortical breach yet maximize pedicle screw purchase for fixation stability.