Shear Modulus Of The Human Nucleus Pulposus Measured Using Mr Elastography Correlates With Directly Measured Mechanical Properties

Introduction: Early diagnosis of disc degeneration is critical for the success of treatment strategies. Several MR parameters, including T2 and T1-rho maps, have been proposed as markers to quantitatively evaluate the progression of disc degeneration [1, 2]. Although MR parameters are correlated with degeneration grade and the decrease of glycosaminoglycan content, they represent the dynamics of water and/or proteoglycan molecules and may not be directly related to the function of the intervertebral disc. Since the function of the disc is to transmit forces through the spine, mechanical properties may be more sensitive markers for degeneration. Therefore, non-invasive measurements of changes mechanical properties of soft tissues are needed. Recently, our group demonstrated the feasibility of using the shear modulus of the nucleus pulposus (NP) as a marker for disc degeneration [3]. The shear modulus was selected because it is sensitive to degeneration and it can be non-invasively measured using Magnetic Resonance Elastography (MRE). Our MRE experiments on cadaveric human disc segments showed that the NP shear modulus decreased with degeneration and had good specificity and sensitivity for distinguishing between normal, mild, and severe degeneration [3]. Importantly, the shear modulus for degenerated discs was similar for MRE (47 kPa) and for torsion tests (60 kPa) [3]. However, the MRE measures had a decrease in shear modulus with degeneration, which was opposite to the increase of the NP shear modulus measured using torsion [4]. A possible explanation for these differences was that the intradiscal pressure may have a contribution to the MRE shear modulus (and to the in situ disc mechanics) that are not present in unconfined excised samples. Thus, while previous comparisons were promising, to date, quantitative verification of MRE shear modulus with direct mechanical property measurements have not been confirmed. The objectives of the current study were to validate MRE measurements by comparing the shear modulus to that of excised NP samples using confined compression, and to quantify the contribution of the intradiscal pressure and glycosaminoglycan (GAG) content to the MRE shear modulus. Methods: The NP shear modulus was measured non-invasively in 23 intact human bone-disc-bone segments from levels T12-L5 having degeneration grades from 1 to 4. An average shear modulus was calculated in a volume of 8 mm x 8 mm x 4 mm in the center of the NP. After MRE measurements, the discs were dissected and frozen at-20 C until the day of mechanical testing. Confined compression was …