Evaluating the safe zone for lumbar pedicle screws: are midline crossing screws indicative of pedicle breach?

Background Context: Pedicle screw breach (PSB) is not uncommon following lumbar instrumentation, and in some instances, it may lead to vascular and/or neurologic complications. Previous literature suggested that screws crossing the vertebral midline on an anterior-posterior (AP) radiograph (or midsagittal on CT) are concerning for medial pedicle breach.

Objective: Our primary aim was to map out the safe zones (SZ) of bilateral pedicle instrumentation and their relationship at each lumbar vertebral level. Our secondary aim was to evaluate the presence of SZs' intersection at each lumbar level, denoting safe midline pedicle screw crossing not otherwise associated with medial pedicle breach.

Study Design/setting: Retrospective Anatomical Study.

Patient Sample: Adult patients in the from "The Cancer Imaging Archive" (TCIA) database who have not had thoraco-lumbo-sacral fusion.

Outcome Measures: Physiologic measures obtained through 3D analysis of CT images and virtual pedicle screws.

Method: CT scans of 51 patients were randomly selected from "The Cancer Imaging Archive" (TCIA) online database for analysis. The Sectra 3D Spine software was used to create 3D renderings, place virtual screws, and make measurements. At each lumbar vertebra, the right and left pedicle corridors were mapped. At each pedicle, two screw positions were templated, the "medial limit screw" (MLS) and the "lateral limit screw" (LLS). Each limit screw was the most extreme position that the screw could exist in without causing a medial or lateral breach. The safe zone was defined as the zone between MLS and LLS. Measurements were taken for each level (between L1 and L5) and side (Left, Right).

Results: A total of 253 lumbar vertebrae from 51 patients (mean age 53.1, 56.9% male) were included. Two vertebrae from two patients were removed for poor image quality. Out of the 506 screw positions analyzed in our study, 97.4% had overlapping SZ and crossed the midplane without medial pedicle breach. The significant factors (p<.01) for safe midplane-crossing screws included: the screw length (L1-L5); the laterality of the screw entry point (L1-L4); and the pedicle diameter (L2 and L5).

Conclusions: A midline crossing pedicle screw on a lumbar AP radiograph is not necessarily indicative of a medial pedicle screw breach. Anatomical (ie, larger pedicle diameter) and technical (ie, longer screws, and lateral entry points) factors allow for safety zone intersections and indicate safe midline crossing by pedicle screws.