Selection of Lowest Instrumented Vertebra Using Fulcrum Bending Radiographs Achieved Shorter Fusion Safely Compared With the Last "Substantially" Touching Vertebra in Lenke Type 1A and 2A Curves

Study Design. Retrospective study with prospective radiographic data collection. Objective. To compare fusion level determination criteria using the fulcrum bending radiograph (FBR) and the last substantially touched vertebra (SW) as the lowest instrumented vertebra (LIV) in the radiographic outcomes of correction surgery for Lenke 1A and 2A scoliosis patients with a minimum of 2-year follow-up. Summary of Background Data. The SW has been proposed as the LIV in Lenke 1A and 2A curves to avoid postoperative distal adding-on. However, the influence of the inherent flexibility of the curves on selecting the LIV in relation to the STV is not known. Methods. A total of 65 consecutive Lenke 1A and 2A patients who underwent posterior selective thoracic fusion were included in this study with a minimum of 2-year follow-up. LIV determination was compared with the FBR and SW methods. The curve correction, trunk shift, radiographic shoulder height, list, and the incidence of distal adding-on were documented. Results. Mean preoperative, postoperative, and final follow-up standing coronal Cobb angles of primary curves were 59.37 degrees, 15.58 degrees, and 16.62 degrees respectively. Using the FBR to determine the LIV, SW was selected in 16 patients (25%), SW-1 in 34 (52%), STV-2 in 11 (17%), and SW-3 in three (5%). Fusion level difference between using FBR and SW method was statistically significantly larger (P=0.019) in patients with more than 70% fulcrum flexibility (mean: 1.18 levels, range: 0-3 levels) than those with less than or equal to 70% flexibility (mean: 0.70 level, range: -1 to 3 levels). Mean fulcrum flexibility was 73.9% in patients who achieved a shorter fusion by FBR method and 66.3% in patients who did not achieve a shorter fusion. Addingon was observed in three patients (4.6%). Conclusion. By considering the curve flexibility, LIV determination using FBR method achieved a shorter fusion than STV method in over 70% of Lenke 1A and 2A patients, while being safe and effective at 2-year follow-up.