Accuracy and safety of individualized 3D-printed guides for L4 pedicle screw placement: An in vitro comparison with the free-hand technique

Pedicle screw fixation is widely utilized for lumbar stabilization, but deviation from the planned pedicle corridor may result in cortical breach and neurovascular injury. Individualized 3D-printed guides may help transfer preoperative planning to screw insertion. However, their guiding performance and measurement reliability still require quantitative validation in controlled L4 models. This in vitro comparative study evaluated the accuracy, safety, and measurement reliability of guide-assisted L4 pedicle screw placement. Five L4 vertebrae were selected from a public spine CT dataset to design and print individualized guides and corresponding bone models. Bilateral screw placement was performed in both the guide-assisted and free-hand groups. Postoperative CT scans were registered to preoperative planning models to assess entry point deviation, 3D angular error, axial and sagittal plane angular errors, and Gertzbein-Robbins (GR) grading. Registration quality and the reliability of repeated measurements were also evaluated. The guide-assisted group showed lower positional and angular errors than the free-hand group across all quantitative metrics, and this pattern was consistent across the five paired specimens. A higher strict Grade A rate was observed in the guide-assisted group, although both groups achieved a 100% clinically acceptable rate (Grade A + B). These findings suggest that individualized 3D-printed guides may improve the reproduction of planned L4 pedicle screw trajectories in printed bone models. Because only five independent L4 specimens were included and PLA models cannot reproduce bone density or tactile feedback needed for free-hand placement, the between-group comparison should be interpreted as exploratory in vitro geometric evidence. The workflow provides a quantitative framework for the in vitro evaluation of guide-assisted screw placement.