Indications for Additional Pedicle Subtraction Osteotomy in Iatrogenic Flatback After Short-Segment Fusion Surgery

Background and Objectives: This study aimed to identify radiographic predictors and optimal cut-off values for determining the need for additional pedicle subtraction osteotomy (PSO) in patients with iatrogenic flatback syndrome following short-segment (≤3 levels) fusion surgery. Materials and Methods: From 2011 to 2022, a total of 49 patients who underwent deformity correction for iatrogenic flatback following short-segment fusion at a single institution were included. We divided all patients into group A (n = 33, only anterior column realignment, ACR) and group B (n = 16, ACR combined with PSO). Among group A patients, we further divided them into two subgroups: The Excessive group, who developed excessive anterior disc height distraction (EADH) during surgery, and the Non-excessive group, who did not. The Receiver Operating Characteristic (ROC) curve was used to determine the cut-off values for spinopelvic parameters associated with the decision to perform additional PSO. Results: Group A had a significantly lower number of previously fused segments compared to Group B (p < 0.001). Preoperative C7 sagittal vertical axis (C7SVA, p = 0.026) and its correction (p = 0.003) in group B were greater than those in group A. Group B showed a significantly more kyphotic preoperative fused segment angle (FSA) compared to Group A (p = 0.001). Postoperatively, EADH occurred in 7 patients (21.2%) in Group A, while no cases were observed in Group B. Subgroup analysis revealed that the dynamic segment angle (DA) was significantly lower in the Excessive group compared to the Non-excessive group (p < 0.001). The optimal cut-off values of preoperative radiographic parameters for selecting PSO were: C7-SVA > 242.8 mm, FSA > -3.2°, and DA < 4.3°. Conclusions: ACR alone and ACR combined with PSO showed satisfactory outcomes in patients with iatrogenic flat back. For selected patients with preoperative C7SVA > 242.8 mm, FSA > -3.2°, or DA < 4.3°, additional PSO may be reasonable to help optimize sagittal alignment.