The postoperative deterioration of the curve below spinal fusion instrumentation resulting in a distal adding-on (AO) phenomenon in idiopathic adolescent scoliosis (IAS) frequently requires surgical revision with disappointing secondary clinical results.

Analysis of AP (coronal) range of motion (cROM) and lateral (sagittal) range of motion (sROM) on dynamic (side-bending, flexion, extension) X-rays to determine the choice of the lowest instrumented vertebra (LIV) can help reduce distal adding-on. The goal of this study was to study the postoperative progression of the lumbar curve in Lenke 1 scoliosis operated on with a LIV based on dynamic X-ray results.

Right-sided Lenke 1 IAS that was treated surgically by posterior arthrodesis alone with a follow-up of at least 2years was included in the study. The following radiographic parameters were evaluated: the Cobb angles of the curves, reducibility of the curves, the apex of the scoliosis, the central sacral vertical line, the stable vertebra (SV), the neutral vertebra (NV), the distances between the CSVL and the centroids of the LIV and of the first vertebra below instrumentation, as well as the tilt of the superior endplates. sROM and cROM were determined on dynamic X-rays.

Fifty IAS were evaluated/185 files. Only three cases fulfilled the criteria for AO including two that were secondary to peri- or postoperative complications. The lumbar curve presented with a loss of correction of 0.9° at one year and 1.14° at the final follow-up. None of the parameters studied were correlated to the deterioration of the lumbar curve.

The choice of the LIV has been shown to influence the deterioration of the lumbar curve and the development of AO. The choice of the LIV based on an analysis of AP (coronal) and lateral (sagittal) range of motion seems to prevent the development of AO.

4, retrospective study.