Lowest Instrumented Vertebra (LIV) Index as a Supplemental Indicator to Lastly Touched Vertebra for Selection of the Fusion Level in Main Thoracic Adolescent Idiopathic Scoliosis

Study Design Retrospective Cohort Study. Objectives Selecting the last substantially touched vertebra (LSTV) or the lastly touched vertebra (LTV) as the lowest instrumented vertebra (LIV) has been shown to prevent distal adding-on (DA) in Lenke 1A and 2A curves or Lenke 1B and 1C curves, respectively. However, DA was still observed in our cohort even when fusing at or distal to STV or LTV when undergoing thoracic curve fusion (TCF). The purposes of this study are (1) to identify factors associated with the occurrence of DA when LIV for TCF is at or distal to LTV/LSTV, and (2) to demonstrate the effectiveness of LIV-index, defined as the summation of the proximity of LIV relative to neutral vertebra (NV) and stable vertebra (SV) as (LIV-NV) + (LIV-SV), in predicting postoperative DA. Methods A multicenter observational retrospective analysis was conducted on AIS patients who underwent TCF for major thoracic curves (MTCs, Lenke 1&2 curves). Subgroup analysis was performed between the DA and non-DA groups. Results 112 patients were included in the study with 10 patients presenting with DA during follow up. All patients in the DA group had an LIV-index < 0, indicating it a significant risk factor for DA (sensitivity = 100%, specificity = 92%, PPV = 55.5%, NPV = 100%, P < 0.001), especially for patients under 15-year-old (sensitivity = 100%, specificity = 94%, PPV = 77%, NPV = 100%, P < 0.001). Conclusion When fusing at or distal to LTV/LSTV in surgery for Lenke type 1 and 2 curves, the LIV-index can be used in clinical practice to prevent postoperative DA, particularly in younger patients.