Where to stop distally in Lenke modifier C AIS with lumbar curve more than 60°: L3 or L4?

Objective: Selecting lowest instrumented vertebra (LIV) in adolescent idiopathic scoliosis (AIS) with large lumbar curve can be difficult. Stopping the distal fusion at L3 could save more mobile lumbar segments but may increase the risk of decompensation. This study was designed to evaluate preoperative radiographic factors that were associated with the selection of either L3 or L4 as LIV in posteriorly treated AIS patients with large lumbar curve.

Patients and methods: A total of 84 AIS patients with lumbar curve >60° were analyzed with a minimum of 2-year follow-up after posterior instrumentation with lumbar curves included in fusion. Patients were grouped according to the selection of LIV, either L3 or L4 group. All radiograph parameters were measured pre- and post-operatively including Cobb angle, lumbar flexibility, L3 translation and rotation on posteroanterior (PA) and side-bending (SB) film, etc. The SRS-22 score was used to assess clinical outcomes. Radiographic and clinical parameters were compared between the two groups.

Results: There were 24 patients in L3 group and 60 patients in L4 group. At last follow-up, no difference was found in the clinical and radiographic parameters between the two groups. Preoperatively, the L3 group had lower L3 translation on PA view, L3 translation on concave SB film, L3 rotation on convex SB film, more L3/4 disc opening on convex SB film and larger lumbar flexibility. Multivariate regression found L3 translation on concave SB film was the single most important predictor of LIV selection. Specifically, L3 translation on concave SB film <10 mm was a potential threshold for selecting L3 as LIV.

Conclusions: For AIS patients with large lumbar curve, instrumentation can be reliably stopped at L3 if L3 translation on preoperative concave SB film was less than 10 mm, with the same radiographic and clinical outcomes as fusing to L4.