Introduction

Ultrasound-based applications were recently developed to measure intervertebral disc (IVD) mechanical properties and its microstructure in vivo (Langlais et al., 2019; Vergari et al 2014). These are complementary evaluations to magnetic resonance imaging, which is the golden standard to assess IVD as it allows to quantify disc morphology and hydration, in addition to recent in vitro applications to determine IVD mechanical properties. However, ultrasound methods were limited to children because of poor ultrasound penetration in adult’s abdominal region. Nevertheless, such non-invasive and fast evaluation in adults could allow defining novel biomarkers for early disc degeneration. In this work, feasibility and reliability of IVD characterization by ultrasound in adults were explored, which were made possible by the continuous development of ultrasound technology.

Methods

Conventional ultrasound images and SWE (Figure 1) were acquired at the L3-L4, L4-L5 and L5-S1 IVD of 28 asymptomatic adults (13 women and 15 men, age range: 22-67 years). Images were acquired with an abdominal approach with a Mach30 (Supersonic imagine, Aix-en-Provence). Shear wave speed (SWS) and lamellar thickness were measured in the outer annulus with previously described protocols (Langlais et al., 2019; Vergari et al 2014). SWS was measured at all disc levels, while lamellar thickness only at L3-L4. Three operators, one with a 7-year experience and two with a one-day training, repeated the measurements on L3-L4 IVD of 8 subjects to determine measurement reproducibility. Results are reported as median [75% confidence interval], and uncertainty is reported as standard deviation. Non-parametric statistical tests were used (p < 0.05).

 Results

Inter-operator uncertainty of SWS was 8.7% (0.55 m/s), while uncertainty of lamellar thickness was 10% (24.7 µm). SWS was similar at the L3-L4 and L4-L5 levels (4.0 m/s [3.5-4.3]) and significantly lower at L5-S1 (3.4 m/s [3.2-3.6]. Median lamellar thickness at L3-L4 level was 253 µm ± [245-269]. Lamellar thickness was significantly correlated to SWS at the same disc level (p < 0.001, Spearman’s rho = 0.75).

Discussion

 A measurement protocol which was previously limited to application in children, was successfully applied to assess lumbar discs in adults. Results are promising, showing good reproducibility in the assessment of IVD mechanical and microstructural properties. Indeed, SWS is directly related to disc material stiffness, and there results suggest a correlation between disc microstructure and mechanical properties. This provides complementary information to other imaging modalities to characterize the IVD. Further research is underway to study the aging and degeneration of IVD by ultrasound.