Hyper-Selective Posterior Fusion is Recommended When the Modified S-Line is Positive in Lenke 5C Adolescent Idiopathic Scoliosis

Abstract

Objective: Postoperative coronal decompensation and less fusion level are dilemmas and the proper selective posterior fusion (SPF) strategy should be investigated. We proposed a parameter, modified S-line, and aimed to investigate if the modified S-line could predict postoperative coronal decompensation in patients with Lenke 5C adolescent idiopathic scoliosis (AIS).

Methods: This is a retrospective radiographic study and Lenke 5C AIS patients undergoing SPF during the period from September 2017 to June 2021 were included. The modified S-line was defined as the line linking the centers of the concave-side pedicles of the upper end vertebra (UEV) and lower end vertebra (LEV) at baseline. A modified S-line tilt to the right is established as modified S-line+ (UEV being to the right of the LEV). The patients were further categorized into two groups: the Cobb to Cobb fusion group and the Cobb-1 to Cobb fusion group. Outcomes including thoracic Cobb angle, TL/L Cobb angle, coronal balance, upper instrumented vertebra (UIV) translation, lower instrumented vertebra (LIV) translation, UIV tilt, LIV tilt, LIV disc angle, thoracic apical vertebral translation, lumbar apical vertebral translation (L-AVT), L-T AVT ratio, L-T Cobb were measured at baseline, immediately after surgery, and the last follow-up. Radiographic parameters and the incidence of both proximal and distal decompensation between the two groups were compared by chi-square test.

Results: Among 92 patients, 48 were modified S-line+ and 44 were modified S-line-. Modified S-line+ status was identified as a risk factor for postoperative proximal decompensation (p = 0.005) during follow-up. In Cobb to Cobb group, a higher occurrence of proximal decompensation in individuals with modified S-line+ status (p = 0.001) was confirmed. Also, in the Cobb to Cobb group with baseline modified S-line+ status, patients presenting decompensation showed a significantly larger baseline of the UIV tilt and postoperative disc angle below the lower instrumented vertebra. However, In Cobb-1 group, the incidence of decompensation after surgery showed no association with baseline modified S-line tilt status (p = 0.815 and 0.540, respectively).

Conclusion: The modified S-line could serve as an important parameter in surgical decision-making for Lenke 5C AIS patients. Cobb to Cobb SPF is not recommended with a modified S-line+ status, and the Cobb-1 to Cobb fusion may serve as a potential alternative.

Keywords: Adolescent Idiopathic Scoliosis; Decompensation; Lower Instrumented Vertebra; Spinal Deformity; Upper End Vertebra.

FIGURE 1

The modified S‐line was defined as a line connecting the centers of the concave‐side pedicles of the upper end vertebra (UEV) and lower end vertebra (LEV). The modified S‐line tilt to the right of the CSVL was defined as positive modified S‐line status (modified S‐line+) and the tilt to left as modified S‐line− status.

FIGURE 2

An 18‐year‐old female Lenke 5C patient with positive modified S‐line. (A) Preoperative standing anteroposterior radiograph of this patient with a structural lumbar curve of 62° measured from T10 to L3. The preoperative thoracic curve was 42°. The modified S‐line tilt was positive at baseline, indicating a higher incidence of coronal decompensation. (B) Selecting upper instrumented vertebra at upper end vertebra and lower instrumented vertebra at lower end vertebra. Immediately postoperative standing anteroposterior radiograph with fusion from T10 to L3, with a thoracic curve of 24°. (C) Three‐year postoperative standing radiograph showing the thoracic curve as 32° with proximal and distal decompensation.

FIGURE 3

A 14‐year‐old female Lenke 5C patient with positive modified S‐line. (A) Preoperative standing anteroposterior radiograph of this patient with a structural lumbar curve of 53° measured from T10 to L3. (B) Selecting UIV at UEV‐1 and LIV at LEV. Immediately postoperative standing anteroposterior radiograph with fusion from T11 to L3, with a thoracic curve of 12°. (C) Two‐year postoperative standing radiograph showing the thoracic curve as 13° without proximal and distal decompensation.