This retrospective cohort study investigated how reciprocal changes (RCs) in unfused spinal segments influence global alignment after corrective surgery for adult spinal deformity (ASD). Ninety-eight patients who underwent staged lateral lumbar interbody fusion followed by posterior spinal fusion were analyzed and categorized at final follow-up into a balanced group (BG) or an imbalanced group (IG), with the latter further divided into proximal junctional kyphosis/failure (IG-PJK/PJF) and non PJK/PJF (IG-NPJK/PJF) subgroups. Radiographic and clinical parameters were assessed preoperatively, immediately postoperatively, and at 2-year follow-up.

Compared with the BG, patients in the IG demonstrated larger RCs in thoracic kyphosis (TK) and PI–LL mismatch from postoperative to follow-up (P = .030 and P = .008). Predictive thresholds associated with postoperative imbalance included age greater than 65.5 years (AUC .672) and postoperative pelvic tilt exceeding 17.5°  (AUC .852). A postoperative TK greater than 34.5° (AUC .755) was strongly associated with the development of PJK/PJF within the IG. Clinically, patients in the IG-PJK/PJF subgroup exhibited significantly worse Oswestry Disability Index scores than both the BG and IG-NPJK/PJF groups (P = .021 and P = .022). Total SRS-22 scores were also significantly poorer in the IG-PJK/PJF subgroup compared with IG NPJK/PJF (P = .021).

Greater RCs in TK were linked to mechanical complications and inferior patient-reported outcomes. Addressing excessive postoperative TK, particularly values exceeding 34.5°, and considering extension of the upper instrumented vertebra into the upper thoracic spine may help maintain sagittal balance and reduce junctional complications. These radiographic thresholds offer actionable guidance for optimizing surgical planning in ASD correction.