Distal Adding-On After Selective Thoracolumbar/Lumbar Fusion for Lenke 5C AIS: The Role of Proximal Coronal Construct...

Selective thoracolumbar/lumbar fusion is a standard operative strategy for Lenke 5C adolescent idiopathic scoliosis (AIS); however, distal adding-on after this selective fusion remains incompletely characterized, and prior work has largely focused on distal foundation planning rather than proximal coronal construct position. We therefore aimed to compare radiographic and clinical outcomes between patients with and without distal adding-on and to identify radiographic factors associated with distal adding-on, with particular attention to proximal coronal construct position (including postoperative upper instrumented vertebra (UIV) translation) and distal junctional alignment around the lowest instrumented vertebra (LIV) and the first vertebra below the LIV (LIV + 1).

We retrospectively reviewed 74 patients with Lenke 5C AIS (age 11-18 years) who underwent single-stage posterior selective thoracolumbar/lumbar fusion using an all-pedicle-screw construct, with a minimum of 2 years of radiographic follow-up. Patients were classified according to whether distal adding-on developed during follow-up. Radiographic parameters were assessed preoperatively, immediately postoperatively, and at final follow-up; clinical outcomes were assessed using the Scoliosis Research Society-22 (SRS-22) questionnaire. Univariable screening and multivariable modeling were performed, and multivariable estimates were obtained using Firth penalized logistic regression.

Distal adding-on developed in 18 of 74 patients (24.3%). Age, Risser grade, number of fused segments, lowest instrumented vertebra distribution, and upper instrumented vertebra selection did not differ between groups, whereas sex distribution differed (p = 0.028). The main thoracolumbar/lumbar curve magnitude, sagittal and spinopelvic parameters, and SRS-22 scores were similar between groups at all time points (all p > 0.05). Several junctional coronal parameters differed, including preoperative LIV coronal disc angle (p = 0.029), postoperative LIV + 1 coronal disc angle (p = 0.008), final follow-up LIV + 1 coronal disc angle (p = 0.037), postoperative LIV tilt (p = 0.003), final follow-up LIV tilt (p = 0.048), and UIV translation preoperatively (p = 0.036) and postoperatively (p = 0.001). On univariable logistic regression, postoperative LIV + 1 coronal disc angle (p = 0.011), postoperative LIV + 1 tilt (p = 0.008), and postoperative UIV translation (p = 0.006) were associated with distal adding-on. In multivariable analysis, postoperative UIV translation remained associated with distal adding-on (OR 1.049, 95% CI 1.003-1.102; p = 0.036).

Distal adding-on occurred in approximately one-quarter of patients after selective thoracolumbar/lumbar fusion for Lenke 5C AIS. In addition to distal foundation planning at the LIV, surgeons should consider intraoperative coronal construct positioning, with particular attention to postoperative UIV translation and coronal alignment below LIV + 1.

Level III.