An alternative method to evaluate lumbar interbody fusion status focusing on position change of screw-rod constructs

Objectives The evaluation of lumbar interbody fusion status is generally subjective and may differ among raters. The authors examined whether the assessment of position change of screw-rod constructs could be an alternative method for the evaluation of fusion status. Methods Sixty-three patients undergoing lumbar interbody single-level fusion were retrospectively reviewed. Three-dimensional images of screw-rod constructs were created from baseline CT examination on the day after surgery and follow-up CT examinations (3–5 months, 6–11 months, and ≥ 12 months) and superposed, with position change of screw-rod constructs being evaluated by the distance between the 3-dimensional images at baseline and follow-up. The evaluation was repeated twice to confirm the reproducibility. Fusion status on follow-up CT examinations was assessed by three raters, where inter-rater reliability was evaluated with Fleiss’ kappa. The results of the fusion status were classified into fusion and incomplete fusion groups in each timing of follow-up CT examinations, where the amount of position change was compared between the two groups. Results The evaluation of position change was completely reproducible. The Fleiss’ kappa (agreements) was 0.481 (69.4%). The medians of the amount of position change in fusion and incomplete fusion groups were 0.134 mm and 0.158 mm at 3–5 months ( p = 0.21), 0.160 mm and 0.190 mm at 6–11 months ( p = 0.02), and 0.156 mm and 0.314 mm at ≥ 12 months ( p = 0.004). Conclusions The assessment of position change of screw-rod constructs at 6 months or more after surgery can be an alternative method for evaluating lumbar interbody fusion status. Key Points • Lumbar interbody fusion status (satisfactory, incomplete, or failed) is associated with the quantification of position change of screw-rod in this study . • Reference values for the evaluation of position change in identifying interbody fusion status are provided . • Position change of screw-rod could be a supportive method for evaluating interbody fusion status .