Evidence of spinal stiffening following fusionless bipolar fixation for neuromuscular scoliosis: a shear wave elastography assessment of lumbar annulus fibrosus

Objectives There are no established criteria for stiffness after fusionless surgery for neuromuscular scoliosis (NMS). As a result, there is no consensus regarding the surgical strategy to propose at long-term follow-up. This study reports the first use of shear wave elastography for assessing the mechanical response of lumbar intervertebral discs (IVDs) after fusionless bipolar fixation (FBF) for NMS and compares them with healthy controls. The aim was to acquire evidence from the stiffness of the spine following FBF. Patients and methods Nineteen NMS operated on with FBF (18 +/- 2y at last follow-up, 6 +/- 1 y after surgery) were included prospectively. Preoperative Cobb was 89 +/- 20 degrees and 35 +/- 1 degrees at latest follow-up. All patients had reached skeletal maturity. Eighteen healthy patients (20 +/- 4 y) were also included. Shear wave speed (SWS) was measured in the annulus fibrosus of L3L4, L4L5 and L5S1 IVDs and compared between the two groups. A measurement reliability was performed. Results In healthy subjects, average SWS (all disc levels pooled) was 7.5 +/- 2.6 m/s. In NMS patients, SWS was significantly higher at 9.9 +/- 1.4 m/s (p<0.05). Differences were significant between L3L4 (9.3 +/- 1.8 m/s vs. 7.0 +/- 2.5 m/s, p=0.004) and L4L5 (10.3 +/- 2.3 m/s vs. 7.1 +/- 1.1 m/s, p=0.0006). No difference was observed for L5S1 (p=0.2). No correlation was found with age at surgery, Cobb angle correction and age at the SWE measurement. Conclusions This study shows a significant increase in disc stiffness at the end of growth for NMS patients treated by FBF. These findings are a useful adjunct to CT-scan in assessing stiffness of the spine allowing the avoidance of surgical final fusion at skeletal maturity.