Relationship between the LL-TK index as a regional predictor of sagital balance and quality of life in adults

Purpose Population aging has increased both the prevalence and clinical impact of adult spinal deformity (ASD). Sagittal and regional parameters (e.g., SVA, PI–LL, LL–TK) guide assessment and treatment planning, but there remains debate over which indices best predict sagittal imbalance and functional disability. Therefore, this study seeks to evaluate associations between regional and global spinopelvic parameters and clinical outcomes in an adult outpatient cohort, and to compare the diagnostic performance of the LL–TK and PI–LL indices for detecting sagittal imbalance. Methods Cross‑sectional study including 142 patients (mean age 56.6 ± 8.24 years) with chronic low back pain, evaluated between 2018 and 2019. Standing full‑length AP and lateral radiographs were analyzed using Surgimap to measure SVA, LL, TK, PI, PT and SS. Regional indices (LL–TK and PI–LL) were calculated. Patients were classified as balanced (SVA < 4 cm) or unbalanced (SVA ≥ 4 cm). Pearson correlations, independent‑samples t tests and ROC analysis were performed; significance threshold p < 0.05. Results Mean values were: ODI 23.6 ± 14.2; VAS 45 ± 2.3; LL 55.3° ± 13.0; TK 44.1° ± 13.8; PI 52.8° ± 10.9; PT 16° ± 9.0; SS 36.9° ± 9.0; PI–LL − 2.5° ± 12.8. SVA correlated significantly with age (r = 0.50), ODI (r = 0.40), VAS (r = 0.42), LL (r = − 0.28), LL–TK (r = − 0.43) and PI–LL (r = 0.42) (all p < 0.001). LL–TK also demonstrated significant correlations with clinical and radiographic measures. Twenty‑nine patients (20.4%) were sagittally unbalanced. ROC analysis identified optimal cutoffs of 12° for LL–TK (AUC = 0.755; sensitivity 96.6%; specificity 55.8%) and 9° for PI–LL (AUC = 0.699; sensitivity 44.8%; specificity 86.7%). The combined criterion LL–TK > 12° and PI–LL ≤ 9° was met by 56 patients, of whom 55 (98%) had satisfactory sagittal alignment (SVA < 4 cm). Conversely, LL–TK ≤ 12° combined with PI–LL > 9° identified 14 patients, 11 (78%) of whom were sagittally unbalanced; in this combination the positive predictive value for imbalance was 78% and the negative predictive value 98%. Conclusions The regional LL–TK index correlates strongly with SVA and patient‑reported disability and demonstrated greater discriminative power than PI–LL for detecting sagittal malalignment in this cohort. Combining regional and pelvic parameters produced high predictive values and may aid screening and surgical planning in ASD. Multicenter studies with standardized radiography and inclusion of bone quality assessment are needed to validate and generalize these findings.