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 opti-mal 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 un-derwent 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 ACR) and group B (n= 16, ACR combined with PSO). Among group A patients, we further di-vided 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 val-ues for spinopelvic parameters associated with the decision to perform additional PSO. Results: Group A had a significantly lower number of previously fused segments com-pared 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  242.8mm, FSA > -3.2°, and DA < 4.3°. Conclusions: ACR alone and ACR combined with PSO showed satisfactory outcomes in patients with iatro-genic flat back. In particular, we highly recommend ACR combined with the PSO in pa-tients with preoperative C7SVA > 242.8 mm, FSA > -3.2°, or DA < 4.3°.