A Short Distance Between Pedicle Screw Tip and Upper Endplate as a Risk Factor for Upper Instrumented Vertebra Fracture Following Adult Spinal Deformity Surgery

Study Design Retrospective cohort comparative study. Objective To investigate whether specific upper instrumented vertebra (UIV) pedicle screw characteristics—particularly screw slope (UIV SS), tip-to-endplate distance (TED), and tip-to-anterior cortex distance (TAD)—are associated with the development of upper instrumented vertebra fracture (UIVF) following adult spinal deformity (ASD) surgery. Methods This retrospective study included 132 patients (mean age: 68.4 years) who underwent corrective surgery from 2013 to 2022, with a minimum follow-up of 2 years. The radiographic parameters and UIV screw characteristics were analyzed. Patients were categorized into UIVF and non-UIVF groups. Logistic regression and Receiver Operating Characteristic (ROC) curve analyses were used to identify the risk factors and cutoff values. Results UIVF occurred in 28.8% (38/132) of the patients. Post-thoracic kyphosis (Post-TK) and TED of the upper UIV screw (Post-TED upper ) were independently associated with UIVF. Cutoff values predictive of UIVF were post-TK > 36.5° and Post-TED upper < 6.5 mm . Patients with a TED upper < 6.5 mm had a significantly higher incidence of UIVF (HR = 2.417, P = 0.010). Kaplan–Meier analysis showed that UIVF commonly occurred within 3 months postoperatively. Progressive reduction in TED was observed over time, particularly in the UIVF group. Conclusion Post- TK > 36.5° and Post-TED upper < 6.5 mm are significant predictors of UIVF. Based on the study findings, a postoperative TED of ≥ 6.5 mm may be associated with a reduced risk, and could be considered as a potential target in surgical planning, while acknowledging the limitations of the retrospective design and the model’s moderate predictive accuracy.