A novel technique for in vivo deformation and strain measurements in human lumbar intervertebral discs using 3T clinical MRIs — The International Society for the Study of the Lumbar Spine
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A novel technique for in vivo deformation and strain measurements in human lumbar intervertebral discs using 3T clinical MRIs (#Z12)

Saman Tavana 1 , Julian Leong 2 , Brett Freedman 3 , Nicolas Newell 1
  1. Imperial College London, London, UK
  2. Royal National Orthopaedic Hospital, London, UK
  3. Mayo Clinic, Rochester, USA

Introduction: The intervertebral disc (IVD) transmits 80-90% of axial loads within the spinal column1. The mechanical response of the IVD to load is an important indicator of the IVD health and pathology2. In vivo evaluation of the IVD strains is crucial to better understand normal and pathological IVD mechanics, to investigate the effect of injuries on the mechanics of the IVD, and to evaluate the effectiveness of treatments. However, in vivo characterisation of IVD strains remained largely unexplored, likely due to the lack of a reliable and non-invasive strain measurement technique. The aim of this study was 1) to develop a novel in vivo technique based on 3T MRI and digital volume correlation (DVC) for non-invasive strain measurement within human IVDs and 2) to use this technique to resolve 3D strains within lumbar IVDs during extension.   

 

Methods: This study included 20 lumbar IVDs from four healthy subjects (M/F: 2/2, average age 30 years, range: 27-34 years). In order to find an optimal MR sequence to minimise DVC measurement errors, one subject was scanned two times using 3T MRI with four different sequences: CISS, T1 VIBE, T2 SPACE, and T2 TSE. DVC errors were quantified by conducting zero-strain tests3 (two repeated unloaded scans). Based on the DVC measured errors, the optimal MRI sequence was selected. To assess the repeatability of the strain measurements in spines with different anatomical variations and compositions all subjects (n=4) were scanned two times with the optimal sequence, and accuracy and precision of the strain measurements were quantified. In addition, to calculate 3D strains during lumbar extension, MR Images were acquired from subjects in both the neutral position (Figure 1(a)), and after full extension (Figure 1(b)).   

  

Results: DVC strain measurement errors were notably lower for the T2 TSE (0.33 %) in comparison with the CISS, T1 VIBE, and T2 SPACE (6.13, 6.98, and 2.05 %, respectively). The result of the repeatability study identified that DVC in combination with T2 TSE MRI is able to measure internal IVD strains with a precision of 0.33 ± 0.10 % and accuracy of 0.48 ± 0.11 % for a measurement spatial resolution of 4.64 mm. During lumbar extension, the site of peak maximum 3D principal (tensile) and shear strains were commonly observed at the anterior annulus (Figure 1(c)). Level L2-L3 experienced the largest peak tensile and shear strains (16.8 ± 0.4 % and 16.2 ± 2.4 %, respectively), while the L5-S1 level experienced the lowest peak tensile and shear strains (3.5 ± 0.1 % and 3.9 ± 0.2 %, respectively).

 

Discussion: The findings of this study establish clinical MRI based DVC (MRI-DVC) as a new tool for in vivo strain measurement within human IVDs. Medical imaging modalities that calculate tissue morphology alone cannot provide direct information regarding the biomechanics of the disc. MRI-DVC successfully provided internal strain distributions within IVDs and has great potential to be used for a wide range of clinical applications.

Acknowledgements: This work was supported by the EPSRC, New Investigator Award, EP/V029452/1.

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  1. 1. Adams et al, J Bone Joint Surg Br, 62 (3), 358-62, 1980.
  2. 2. Tavana et al, Front. Bioeng. Biotechnol, 8:610907, 2021.
  3. 3. Liu et al, Journal of Biomechanics 40, 3516–3520, 2007.
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