Abstract
Introduction
Distal adding-on is often accompanied by unsatisfactory clinical outcome and high risk of reoperation. However, very few studies have focused on distal adding-on and its attendant risk factors, and optimum treatment strategies remain controversial. In a retrospective study, we aimed to identify risk factors for the presence of distal adding-on in Lenke 1A scoliosis and to compare different treatment strategies.
Methods
Data for all surgically treated patients with adolescent idiopathic scoliosis (AIS) were retrieved from one institutional database. Inclusion criteria included: patients with Lenke 1A scoliosis treated with posterior pedicle screw-only constructs; and a minimum 1-year radiographic follow-up. Distal adding-on was defined as a progressive increase in the number of vertebrae included distally within the primary curve combined with either an increase of more than 5 mm in deviation of the first vertebra below instrumentation from the centre sacral vertical line (CSVL), or an increase of more than 5° in the angulation of the first disc below the instrumentation at 1 year follow-up. Wilcoxon rank-sum test, Fisher's exact test, and Spearman's correlation test were used to identify the risk factors for adding-on. A multiple logistic regression model was built to identify independent predictive factors. Risk factors included: age at surgery; preoperative Cobb angle; correction rate; the gap difference of stable vertebra (SV) and lowest instrumented vertebra (LIV), neutral vertebra (NC) and LIV, and end vertebra (EV) and LIV (gap difference means, for example, if SV is at L2 and LIV is at Th12, then the difference of SV-LIV is 2); and the preoperative deviation of LIV+1 (the first vertebra below the instrumentation) from the CSVL (the vertical line that bisects proximal sacrum). Five methods for determining LIV were compared in both the adding-on group and the no adding-on group.
Results
Of 278 patients reviewed, 45 met the inclusion criteria; 23 of these patients met the definition for distal adding-on and were included in the adding-on group. The remaining 22 patients were included in the no adding-on group. Mean follow-up was 3.6 years. Age, SV-LIV difference, EV-LIV difference, and LIV+1 deviation from CSVL differed significantly (p<0.05) between the two groups, and were significantly correlated with the presence of adding-on (p<0.05). Preoperative Cobb angle, correction rate, and NV-LIV difference were not associated with the presence of adding-on. Multiple logistic regression results indicated that preoperative LIV+1 deviation from CSVL was an independent predictive factor. Of the five methods, choosing EV as LIV nearly prevented distal adding-on; choosing EV+1 as LIV resulted in fusing many more segments than was necessary; and only choosing DV as LIV showed satisfactory outcome from both perspectives.
Conclusions
In Lenke 1A type scoliosis, the selection of LIV is highly correlated with the presence of adding-on; incidence increases substantially when the preoperative LIV+1 deviation from CSVL is more than 10 mm. Choosing DV (the first vertebra in cephalad direction from sacrum with deviation from CSVL of more than 10 mm) as LIV may provide the best outcome because it not only prevents adding-on but also conserves more lumbar motion.
