The LL-TK index as a regional marker of sagittal balance and disability in adults with chronic low back pain.

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 it remains unclear which indices are most closely associated with sagittal imbalance and functional disability. This study therefore evaluated the associations between regional and global spinopelvic parameters and clinical status in an adult outpatient cohort with chronic low back pain, and compared the ability of the LL-TK and PI-LL indices to discriminate concurrent sagittal imbalance.

METHODS: Cross-sectional study including 142 patients (mean age 56.6 ± 13.2 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 4.5 ± 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. For the identification of concurrent sagittal imbalance, ROC analysis yielded 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 AUC of LL-TK was numerically higher, but the two correlated curves were not compared statistically. 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 (positive predictive value 78%; negative predictive value of the favorable combination 98%). The remaining 72 patients (50.7%) had discordant index values and were not classified by this rule.

CONCLUSIONS: In this cross-sectional cohort of adults with chronic low back pain, the regional LL-TK index was moderately associated with SVA and with patient-reported disability, and its AUC for discriminating sagittal malalignment was numerically higher than that of PI-LL; the two AUCs were not formally compared, so no superiority is claimed. The combination of regional and pelvic parameters yielded a high predictive values in the concordant subgroups, suggesting potential value as an adjunct screening aid rather than a basis for outcome prediction or surgical planning. Multicenter studies with standardized radiography, bone quality assessment and prospective validation of the proposed thresholds are needed.