Proximity Between Screw and Endplate of Upper Instrumented Vertebra Using a Cortical Bone Trajectory Screw can Increase Radiographic Adjacent Segment Degeneration Risks in Patients With Lumbar Spinal Stenosis

Study Design. Retrospective observational study. Objective. To determine the proximity between the screw and endplate of the upper instrumented vertebra (UIV) using a cortical bone trajectory (CBT) screw as a predictive factor for radiographic adjacent segment degeneration (ASD) in patients surgically treated with transforaminal lumbar interbody fusion (TLIF) with CBT screws (CBT-TLIF) with lumbar spinal stenosis. Summary of Background Data. The risk factors for radiographic ASD after CBT-TLIF remain unknown. Methods. Among patients surgically treated with CBT-TLIF at a single institute, 239 consecutive patients (80 males and 159 females) were enrolled. ASD was defined by the presence of one or more of the following three radiologic criteria on the adjacent segment: >3 mm anteroposterior translation, >10° segmental kyphosis, or >50% loss of disc height comparing immediate postoperative and one-year follow-up radiographs. Clinical and radiological features associated with the development of ASD were retrospectively measured. Univariate and multivariate analyses were performed to identify risk factors associated with radiographic ASD. Results. Radiographic ASD was observed in 71 (29.7%) cases at one-year postoperative follow-up. The preoperative Pfirrmann grade of the adjacent segment (>grade 2), multi-level fusion (>2 levels), and proximity between the tip of CBT screws and endplate on the UIV were significantly associated with radiographic ASD (OR = 3.98, 95% CI [1.06–15.05], P =0.042 versus OR = 3.03, 95% CI [1.00–9.14], P =0.049 versus OR = 0.53, 95% CI [0.40–0.72], P <0.001). The cut-off value of the distance between the tip of the screw and endplate on UIV for radiographic ASD was ~2.5 mm (right-sided CBT screw; cut-off value 2.48 mm/ left-sided CBT screw; cut-off value 2.465 mm). Conclusion. Radiographic adjacent segment degeneration progression can occur when the cortical trajectory bone screw is close to the endplate of the upper instrumented vertebrae in patients with lumbar spinal stenosis undergoing fusion surgery.