Abstract
BACKGROUND: The mismatch in the pedicle screw/rod interface angle is a common phenomenon after topping-off surgery, indicating that the implants were assembled under mechanical stress, which may negatively impact clinical outcomes. PURPOSE: This study aims to assess the incidence of postoperative screw/rod mismatch and its effect on clinical outcomes, including (1) Adjacent segment degeneration (ASD); (2) Screw loosening and rod breakage; (3) Visual analogue scale (VAS) scores; and (4) Oswestry disability index (ODI) scores. METHODS: This study is a monocentric retrospective observational study that included patients diagnosed with lumbosacral degenerative disease and underwent topping-off surgery. The outcome measures encompass: interfacing angle mismatch between pedicle screws and support rods, incidence of proximal ASD, screw loosening and rod breakage, VAS scores, and ODI scores. The impact of screw–rod interfacing angle mismatch on the development of postoperative ASD was further analyzed using univariate and multivariate logistic regression analysis. Postoperatively, lateral X-rays and CT scans were used for measurements. The interfacing angle mismatch between the pedicle screw and rod is measured as the angle between two parallel lines on either side of each pedicle screw head and a line laterally along the associated rod. Bonferroni correction was applied to counteract multiple comparisons, with the adjusted significance level set at *p < 0.05. RESULTS: Among the 237 patients who underwent topping-off surgery, 84 cases (35.44%) exhibited a mismatch in the interfacing angle between the pedicle screw and the support rod, involving 114 out of 1,208 screws (9.44%). After a mean follow-up period of 26.57 ± 2.88 months, the overall incidence of adjacent segment degeneration (ASD) was 9.28% (22/237), screw loosening occurred in 12.66% of patients (30/237), and rod breakage occurred in 2.53% (6/237).In patients with interfacing angle mismatch, the incidence of ASD was significantly higher, reaching 21.4% (18/84), compared to 2.6% (4/153) in patients without mismatch (P < 0.001). The increase in screw loosening was even more pronounced, with a rate of 28.6% (24/84) in the mismatch group versus only 3.9% (6/153) in the matched group (P < 0.001).There was no significant difference in rod breakage rates between the two groups (2.0% vs. 3.6%, P = 0.361). Additionally, patients with angle mismatch had significantly higher VAS scores than those without mismatch (3.63 ± 0.93 vs. 2.19 ± 0.78, P < 0.001). However, there was no statistically significant difference in ODI scores between the two groups (19.26 ± 2.51 vs. 19.70 ± 2.62, P = 0.214). CONCLUSION: A mismatch in the interfacing angle between the pedicle screw and the support rod may contribute to a higher incidence of ASD, increased screw loosening, and more severe postoperative pain following topping-off surgery. Refined preoperative planning of screw trajectories and precise intraoperative techniques are essential to optimize clinical outcomes and reduce the risk of ASD.