Distal adding-on phenomenon in Lenke IA and Lenke IIA: Risk Analysis and Selection of the LIV.

January 1, 2020

OBJECTIVE:

To analyze the risks of the distal adding-on phenomenon and identify the ideal lowest instrumented vertebra (LIV) for Lenke IA and IIA.

METHODS:

A total of 84 patients with Lenke IA or Lenke IIA treated with posterior all-pedicle-screw instrumentation were enrolled in this cohort study. Radiographs that were obtained before, immediately after, and 2 years after the operation were measured. Patients were grouped based on the occurrence of the adding-on phenomenon. Independent risk factors were evaluated between these two groups via univariate analysis and logistic regression analysis.

RESULTS:

All patients obtained optimal correction of the main thoracic curve and lumbar curve after selective thoracic fusion. Eighteen patients among a total of 84 patients suffered from the distal adding-on phenomenon during the 2-year follow-up. Multivariable analysis revealed that the primary factors were preoperative thoracolumbar or lumbar curve (TL/L) size in supine side-bending films (OR = 0.75, P = 0.008), preoperative thoracic kyphosis (T5-T12) (OR = 0.743, P = 0.022) and the difference between the LIV and the LSTV (lowest substantial touched vertebra). All seven (100%) patients whose LIVs were proximal to the LSTV suffered from the distal adding-on phenomenon, while seven of forty (17.5%) suffered from the distal adding-on phenomenon when the LIV was distal to the LSTV. Patients whose LIV was distal to the LSTV had the lowest incidence of the distal adding-on phenomenon (10.8%).

CONCLUSIONS:

An LIV located proximal to the LSTV should be avoided during selective thoracic fusion for Lenke IA and IIA in order to prevent the distal adding-on phenomenon. For patients who have a small TL/L size in bending films or a small T5-T12 angle before surgery, the next vertebra distal to the LSTV may be an optimal choice.