Which Lenke 1A curves are at the greatest risk for adding-on... and why?

Multicenter review of prospectively collected data.

The purpose of this study was to evaluate the incidence of distal adding-on and associated risk factors in each of the 2 Lenke 1A curve patterns.

Previous work has demonstrated 2 distinct Lenke 1A curve patterns on the basis of the tilt of L4 (1A-L and 1A-R) that are different in form and treatment.

A query of a prospective multicenter adolescent idiopathic scoliosis database identified 195 patients with Lenke 1A curves. Patients were grouped on the basis of the direction of the L4 vertebral tilt: 1A-L (left) and 1A-R (right). The incidences as well as clinical and radiographical risk factors for adding-on were identified for each group. Adding-on was defined as an increase in the Cobb angle of at least 5° and distalization of the end vertebra of the thoracic curve or a change in disc angulation of 5° or greater below the lowest instrumented vertebra from the first erect to 2-year postoperative radiographs.

Forty (21%) patients met the criteria for adding-on. The average increase in Cobb angle was 11.9° for those categorized as having adding-on compared with 3.8° in the non-adding-on group. Lenke 1A-R curves were 2.2 times more likely to experience adding-on than 1A-L curves. In the 1A-R curves, patients who added-on were fused at an average of 1.6 levels proximal to the neutral vertebra versus an average of 0.9 levels proximal to the neutral vertebra for the patients who did not add-on (P = 0.023). Patients who added-on were fused at an average of 2.5 levels above stable versus 2.1 levels above stable in those who did not (P = 0.06). Age and skeletal maturity were not identified as risk factors in the 1A-R curves. In 1A-L curves, younger (12.7 vs. 14.7 yr, P = 0.002) and less skeletally mature patients based on Risser grading (70% vs. 14% Risser 0, P = 0.004) were more likely to experience adding-on.

Understanding the difference between Lenke 1A-L and 1A-R curve types may be helpful in preventing the adding-on phenomena postoperatively. To prevent adding-on in 1A-R curves, we recommend fusing distally to 1 level above the neutral vertebra or 1 to 2 levels above the stable vertebra. In 1A-L curves, adding-on may simply be a need to balance some lumbar curve progression in a young, skeletally immature patient.