Abstract
Ultrasound shear wave elastography is a radiation-free and low-cost technique for evaluating the mechanical properties of different tissues. This study systematically reviewed all relevant literature on shear wave elastography of the intervertebral disc. The purpose was twofold: first, to determine the validity of the elastography method, that is, the correlation between elastographically measured shear wave speed and disc mechanical properties, and inter-/intra-operator reliability; and second, to explore if disc elastography is potentially useful in identifying children at risk for idiopathic scoliosis. This systematic review was performed according to the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-analyses) guidelines. A comprehensive search was performed in PubMed and Embase, and study quality was assessed using the AXIS (Appraisal Tool for Cross-sectional Studies) critical appraisal instrument. Seven articles were included. Three animal ex vivo studies reported moderate-to-good correlations between shear wave speed and disc mechanical properties (r=0.45-0.81). Three studies reported high intra-operator repeatability (intra-class correlation coefficient [ICC] 0.94-0.99) and inter-operator reproducibility (ICC 0.97-0.98). Four clinical studies measured shear wave speed in asymptomatic children. Two studies reported significantly higher shear wave speeds in scoliosis patients compared with healthy controls, measured in discs both inside and outside the scoliotic curve. In conclusion, shear wave elastography appears reliable in assessing intervertebral disc mechanical characteristics. Despite its promising capabilities to distinguish patients with asymptomatic from those with pathological discs, the exact correlation between disc mechanical properties and shear wave speed remains unclear.
Highlights
Adolescent idiopathic scoliosis (AIS) is the most prevalent form of scoliosis: a 3-D structural deformity of the spine, which includes lateral curvature, anteroposterior deviation and axial rotation, without any obvious underlying condition (Kuklo et al 2005; Illes et al 2011)
Three animal ex vivo studies reported on the correlation between shear wave speed (SWS) measurements of the annulus fibrosus and apparent intervertebral disc (IVD) stiffness and/or elastic modulus
All seven included studies were of high quality according to the AXIS critical appraisal tool (Appendix B)
Summary
Adolescent idiopathic scoliosis (AIS) is the most prevalent form of scoliosis: a 3-D structural deformity of the spine, which includes lateral curvature, anteroposterior deviation and axial rotation, without any obvious underlying condition (Kuklo et al 2005; Illes et al 2011). The main problem with elucidating the role of the IVD in the etiology of AIS is the invasive nature of harvesting IVD tissue to study its mechanical properties, especially because it concerns a population of young age. Most studies on AIS etiology include patients with already established scoliotic curves; distinguishing between cause and effect is practically impossible. We hypothesize that in AIS patients, pathological mechanical IVD properties may already be present before the onset and/or progression of the scoliotic curve. If this were to be true, and these IVD properties could and safely be measured in children, this could potentially be used as a proxy for the risk of AIS development and progression of the scoliotic curve. There currently is no established diagnostic method for non-invasive characterization of the IVD to determine the tissue’s mechanical properties
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