Temporal Shape Changes of Pedicle screw-rod Constructs After Lumbar Interbody Fusion

Study Design. Retrospective multicenter study. Objective. To examine the shape change of screw-rod constructs over time following short-segment lumbar interbody fusion and to clarify its relationship to clinical characteristics. Summary of Background Data. No study has focused on the shape change of screw-rod constructs after short-segment fusion and its clinical implications. Methods. One hundred and eight patients who had single-level lumbar interbody fusion with pedicle screws and cages were enrolled. Three-dimensional (3D) images of screw-rod constructs were generated from baseline CT on the day after surgery and follow-up CT, and were superposed on the right and left side, respectively, using the iterative closest point algorithm. The shape change was quantitatively assessed by computing the median distance between the 3D images, which was defined as the shape change value. Among the five time-course categories of follow-up CT (≤1 month, 2–3 months, 4–6 months, 7–12 months, ≥13 months), the shape change values were compared. The relationships between the shape change values and clinical characteristics, such as age, CT-derived vertebral bone mineral density, screw and rod materials, and postoperative interbody fusion status, cage subsidence, and screw loosening, were evaluated. Results. A total of 237 follow-up CTs were included (≤1 month [34 scans], 2–3 months [33 scans], 4–6 months [80 scans], 7–12 months [48 scans], ≥13 months [42 scans]) because many patients underwent multiple follow-up CTs. There were significant differences in shape change values among the time-course categories ( P <0.001 in Kruskal-Wallis test). Most shape changes occurred within 6 months postoperatively, with no significant changes observed at 7 months or more. There were no significant relationships between the shape change values and each clinical characteristic. Conclusion. The temporal shape changes of screw-rod constructs following short-segment lumbar interbody fusion progressed up to 6 months after surgery but not significantly thereafter.