Disparity between early radiographic decompression and one-year patient satisfaction after single-level LLIF: proposal of the structural improvement score (SIS) and QOL improvement score (QIS)

Study design Retrospective cohort study. Purpose To develop and internally assess two exploratory composite measures—the Structural Improvement Score (SIS) and the QOL Improvement Score (QIS)—that integrate early postoperative radiographic changes and multidomain patient-reported outcomes after single-level lateral lumbar interbody fusion (LLIF). Methods A total of 101 patients who underwent single-level LLIF were retrospectively analyzed. Morphologic changes in the spinal canal were quantified using the change in midsagittal canal diameter (ΔCD) and central canal area (ΔCCA) on MRI, which were standardized to the Z-score and summed to define the SIS. Functional improvement was evaluated using the Japanese Orthopaedic Association Back Pain Evaluation Questionnaire (JOABPEQ), and Z-standardized changes across five domains were aggregated to compute the QIS. Associations between SIS, QIS, postoperative satisfaction, and low back pain improvement (ΔNRS LBP ) were examined using Spearman’s correlation and the Mann–Whitney U test. ROC analysis assessed the discriminative ability of QIS, while multivariate logistic regression identified independent predictors of satisfaction. Results SIS demonstrated strong correlations with ΔCD ( r = 0.864, p < 0.001) and ΔCCA ( r = 0.849, p < 0.001), confirming its structural measurement validity, but showed no significant association with satisfaction ( p = 0.197) or ΔNRS LBP ( r = − 0.046, p = 0.645). QIS was moderately correlated with ΔNRS LBP ( r = − 0.432, p < 0.001) and was significantly higher in satisfied patients ( p = 0.004). ROC analysis showed that QIS predicted 1-year satisfaction with an AUC of 0.745 (95% CI, 0.608–0.861), sensitivity of 68.9%, and specificity of 81.8%. In multivariable logistic regression, only ΔNRS LBP ( p = 0.041) and postoperative NRS LBP ( p = 0.024) remained independent predictors of satisfaction. Conclusions SIS and QIS provide complementary information on structural and functional recovery after LLIF. QIS demonstrated a moderate association with satisfaction, whereas residual pain remained the dominant determinant. These indices should be considered exploratory and require external validation, psychometric testing, and prospective multicenter studies before routine clinical use.