CT-Navigated Spinal Instrumentations-Three-Dimensional Evaluation of Screw Placement Accuracy in Relation to a Screw Trajectory Plan

September 27, 2022

doi: 10.3390/medicina58091200.

Affiliations

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Arthur Gubian et al.

Medicina (Kaunas).

2022.

Free PMC article

doi: 10.3390/medicina58091200.

Affiliations

¹ Department of Neurosurgery, Heidelberg University Hospital, 69120 Heidelberg, Germany.
² Division of Medical Image Computing, German Cancer Research Center (DKFZ) Heidelberg, 69120 Heidelberg, Germany.
³ Pattern Analysis and Learning Group, Department of Radiation Oncology, Heidelberg University Hospital, 69120 Heidelberg, Germany.

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Abstract

Background and Objectives: In the literature, spinal navigation and robot-assisted surgery improved screw placement accuracy, but the majority of studies only qualitatively report on screw positioning within the vertebra. We sought to evaluate screw placement accuracy in relation to a preoperative trajectory plan by three-dimensional quantification to elucidate technical benefits of navigation for lumbar pedicle screws. Materials and Methods: In 27 CT-navigated instrumentations for degenerative disease, a dedicated intraoperative 3D-trajectory plan was created for all screws. Final screw positions were defined on postoperative CT. Trajectory plans and final screw positions were co-registered and quantitatively compared computing minimal absolute differences (MAD) of screw head and tip points (mm) and screw axis (degree) in 3D-space, respectively. Differences were evaluated with consideration of the navigation target registration error. Clinical acceptability of screws was evaluated using the Gertzbein-Robbins (GR) classification. Results: Data included 140 screws covering levels L1-S1. While screw placement was clinically acceptable in all cases (GR grade A and B in 112 (80%) and 28 (20%) cases, respectively), implanted screws showed considerable deviation compared to the trajectory plan: Mean axis deviation was 6.3° ± 3.6°, screw head and tip points showed mean MAD of 5.2 ± 2.4 mm and 5.5 ± 2.7 mm, respectively. Deviations significantly exceeded the mean navigation registration error of 0.87 ± 0.22 mm (p < 0.001). Conclusions: Screw placement was clinically acceptable in all screws after navigated placement but nevertheless, considerable deviation in implanted screws was noted compared to the initial trajectory plan. Our data provides a 3D-quantitative benchmark for screw accuracy achievable by CT-navigation in routine spine surgery and suggests a framework for objective comparison of screw outcome after navigated or robot-assisted procedures. Factors contributing to screw deviations should be considered to assure optimal surgical results when applying navigation for spinal instrumentation.

Keywords:

Gertzbein–Robbins classification; navigation-guided spine surgery; pedicle screw accuracy; spinal instrumentation; spinal navigation; three-dimensional accuracy.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

**Figure 1**

Flow diagram of case selection and workflow of data analysis.

**Figure 2**

Graphic illustration of the methodology for measuring screw accuracy: Euclidian distances were calculated in 3D-space from point coordinates to yield mean absolute distances (MAD) between screw heads (a), screw tips (b) and angular deviation (c) between planned screw trajectory (exemplified in green) and final screw position (exemplified in blue); Methodology illustrated for axial (A) and sagittal (B) planes and in 3D-space (C).

**Figure 3**

Illustration of superimposed screw positions for analysis of screw placement accuracy. Planned screw trajectories (colored screw illustrations) are superimposed on postoperative control-CT depicting final screw positions (white screw illustrations). Illustrations for transverse (a), sagittal (b), coronal (c) views and for 3D-reconstructions (d).

**Figure 4**

Deviation of final screw positions to preplanned screw trajectories illustrated in violin plots. Results are presented as angular deviation (in degrees) between screws for screw direction and mean absolute distances (in mm) between screw head and screw tip points. Differences for head and tip points were statistically significant in relation to navigation registration error recorded during surgery, respectively (ANOVA p < 0.001).

**Figure 5**

Evaluation of screw accuracy for screw direction (A), screw head (B) and screw tip (C) in relation to the spinal level treated. While deviation of final screw position and preplanned trajectory were comparable across spinal levels for screw head (B) (p = 0.1), significantly increased deviations were observed at the S1 level for screw direction (A) (p < 0.001) and screw tip (C) (p < 0.001), respectively. L1 was excluded from this statistical analysis due to paucity of data (n = 2 screws).

**Figure 6**

Qualitative evaluation of final screw position according to the Gertzbein–Robbins classification across spinal levels treated. There were no differences in qualitative screw results across spinal levels (p = 0.23).

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