Selection of Optimal Lower Instrumented Vertebra for Adolescent Idiopathic Scoliosis Surgery

Abstract

Adolescent idiopathic scoliosis (AIS) affects approximately 2% of adolescents across all ethnicities. The objectives of surgery for AIS are to halt curve progression, correct the deformity in 3 dimensions, and preserve as many mobile spinal segments as possible, avoiding junctional complications. Despite ongoing development in algorithms and classification systems for the surgical treatment of AIS, there is still considerable debate about selecting the appropriate fusion level. In this study, we review the literature on fusion selection and present current concepts regarding the lower instrumented vertebra in the selection of the fusion level for AIS surgery.

Keywords: Adding-on phenomenon; Adolescent idiopathic scoliosis; Fusion level selection; Lower instrumented vertebra.

Fig. 1.

A representative case of using the LTV on supine radiographs. (A) Preoperative radiographs of a 15-year-old male patient diagnosed with a Lenke 6CN curve. (B) L4 was touched by the CSVL on an upright radiograph, but the supine LTV was L3 on a supine plain radiograph. (C) Based on the supine radiographs, L3 was chosen as the LIV, preserving one level. Clinical outcomes were excellent at the 18-month follow-up. LTV, last touched vertebra; CSVL, central sacral vertical line; LIV, lower instrumented vertebra.

Fig. 2.

A representative case of considering Lenke 1A-R (L4 vertebral tilt to the right) curves. (A) Preoperative radiographs of an 11-year-old female patient diagnosed with a Lenke 1A-R curve. (B) L3 was touched by the CSVL on an upright radiograph, but the supine LTV was L2 on a supine plain radiograph. (C) Since the LTV on supine radiographs was L2, L3 was chosen as the LIV considering Lenke 1A-R curves. At the 1-year follow-up, no AO phenomenon was observed. CSVL, central sacral vertical line; LTV, last touched vertebra; LIV, lower instrumented vertebra; AO, adding-on.

Fig. 3.

A representative case of considering LIV deviation from the CSVL. (A) Preoperative radiographs of a 14-year-old female patient diagnosed with a Lenke 1AN curve. (B) L3 was touched by the CSVL on an upright radiograph, but the supine LTV was L2 on a supine plain radiograph. (C) On an upright radiograph, the L3 deviation from CSVL was less than 1 cm and the L2 deviation was greater than 2 cm, but on a supine radiograph, the L2 deviation from CSVL was less than 2 cm. (D) L2 was chosen as the LIV because the LIV deviation from CSVL did not exceed 2 cm on supine radiographs. At the 1-year follow-up, no AO phenomenon or DJK was observed. LIV, lower instrumented vertebra; CSVL, central sacral vertical line; LTV, last touched vertebra; AO, adding-on; DJK, distal junctional kyphosis.

Fig. 4.

A representative case where sacral slanting was continued. (A) Preoperative radiographs of a 16-year-old female patient diagnosed with a Lenke 4CN curve and left-sided sacral slanting. (B) Supine LTV was L4 on a supine plain radiograph. (C) L3 was chosen as the LIV, considering left-sided sacral slanting. As a result, clinical outcomes were excellent at the 4-year follow-up. LTV, last touched vertebra; LIV, lower instrumented vertebra.