Study design:

A retrospective study.

Objective:

To determine in which case one level proximal to last substantially touching vertebra (LSTV-1) could be a valid lowest instrumented vertebra (LIV), in which case distal fusion should extend to last substantially touching vertebra (LSTV), and to identify risk factors for distal adding-on.

Summary of background data:

Posterior thoracic fusion to save more lumbar mobile segments has become the mainstay of operative treatment for adolescent idiopathic scoliosis (AIS) with Lenke 1A/2A curves. Although previous studies have recommended selecting the LSTV as LIV, good outcomes could still be achieved in some cases when LSTV-1 was selected as LIV.

Methods:

Ninety-four patients were included in the study with a minimum of 2-year follow-up after posterior thoracic instrumentation, in which LSTV-1 was selected as LIV. Patients were identified with distal adding-on between first erect radiographs and 2-year follow-up based on previously defined parameters. Factors associated with the incidence of adding-on were analyzed.

Results:

The mean follow-up duration was 37.7 ± 15.8 months. Forty patients (42.6%) with LSTV-1 selected as LIV achieved good outcomes at the last follow-up. Several preoperative risk factors significantly associated with distal adding-on were identified, including lower Risser (P = 0.001), longer thoracic curve length (P = 0.005), larger rotation and deviation of LSTV-1 (P < 0.001), and preoperative coronal imbalance (P = 0.013).

Conclusion:

Skeletally immature patients with long thoracic curve, preoperative coronal imbalance, large rotation, and deviation of LSTV-1 are at increased risk of distal adding-on when selecting LSTV-1 as LIV. Under this condition, distal fusion level should extend to LSTV; while in other case, LSTV-1 could be a valid LIV.Level of Evidence: 4.