Background: Sagittal alignment after lumbar fusion relies on compensatory mechanics of the pelvis and lower extremities. Advanced knee osteoarthritis (OA) may restrict these mechanisms and undermine postoperative alignment. This study evaluated the effect of knee OA severity on radiographic and clinical outcomes after lumbar fusion.

Methods: 74 patients who underwent 1–2 level posterior lumbar interbody fusion with ≥2-year follow-up were reviewed. Patients were grouped by Kellgren–Lawrence grade (KLG): mild OA (KLG 1–2, n=39) and moderate–severe OA (KLG 3–4, n=35). Lumbar lordosis (LL), thoracolumbar kyphosis, pelvic tilt, C7SVA, and cranial axis parameters, along with clinical outcomes, were compared across preoperative, early postoperative, and final follow-up periods.

Results: Both groups showed early alignment improvement (p<0.05). However, the KLG 3–4 group demonstrated loss of correction: LL worsened (−42.3°→−35.9°, p=0.041) and C7SVA increased (28.3 mm→47.3 mm, p<0.001). At final follow-up, advanced OA patients had greater sagittal imbalance (C7SVA 43.7 vs 22.1 mm, p<0.001) and less functional recovery (p<0.05).

Conclusion: Moderate–severe knee OA limits the maintenance of sagittal alignment after lumbar fusion, likely due to restricted knee extension reducing compensatory ability. These patients exhibited radiographic deterioration and inferior clinical gains despite initial correction. Preoperative recognition and management of knee OA may improve postoperative alignment and spine-related outcomes.