Introduction

The detailed response of the intervertebral disc (IVD) to mechanical loading, in vivo, remains largely undocumented. Ex vivo experiments are not adequate surrogates for in vivo studies as they cannot capture the IVD’s biomechanics during loading conditions. Methods for evaluation of the biomechanical properties of the IVD in vivo may yield greater insight into the pathophysiology of the IVD and assist in treatment planning Magnetic resonance imaging (MRI), a non-ionizing and non-invasive tool, may reveal important intradiscal deformation patterns when performed during spinal loading.

The purpose was to investigate how the lumbar IVD structure deforms in vivo during spinal loading, quantified with a novel non-invasive method utilizing MRI and image registration.

Methods

T2-weighted lumbar spine images (L1-S1) of 24 low back pain patients and 12 matched controls, acquired during axial loading, were compared with the corresponding unloaded T2-weighted MR images. To display the intradiscal deformation of the IVDs pixel by pixel, the Jacobian determinant matrix was retrieved from the deformation field of the image registration. The mean deformation over the whole IVD and in different sub-regions of the IVD was automatically determined using in-house softwares. With these softwares, also the center of mass of the deformation for each IVD was determined. The IVDs were grouped according to degeneration (Pfirrmann grade). Mann-Whitney’s U and Kruskal-Wallis H tests were used to compare global as well as regional deformation measures between groups.

Results

The degeneration distribution of the IVD cohort was; Pfirrmann 1:10%, 2:50%, 3:21%, 4:16%, 5:2%. Differences in deformation for IVDs at different spine levels were found, where IVDs at the lower lumbar levels displayed more compressive deformation with axial loading (p<0.001; Fig.1), even when correcting for degeneration. With the novel imaging method, the regional variation in the deformation over the IVD was displayed, with generally more compression at the nucleus pulposus and a slightly left asymmetric pattern (Fig.2). IVDs with advanced degeneration displayed more compressive deformation at the posterior region (p=0.035-0.045), also when correcting for spine level. No difference between patients and controls was found.

Discussion

The present method reflected the loss in stiffness with more compression for IVDs with pronounced degeneration [1]. Also, the dependence of the intradiscal deformation on the IVD degeneration demonstrates the potential with new diagnostic tools for evaluation of the biomechanical properties of the IVD in vivo. Another interesting finding was the presence of asymmetries in the deformation pattern over the IVDs, most probably caused by varying disc height over the IVDs [2]. The proposed method offers the possibility to depict and track biomechanical changes non-invasively while characterizing disc structures in detail.

Figure 1. Deformation heat map where Jacobian determinant <1 = expansion and >1 = compression. Where more compression can be seen in the lower lumbar spine in the (A) sagittal projection and in the (B) projection from head to feet.

Figure 2. Mean deformation (<1 = expansion, >1 = compression) of the different spine levels for (A) the nine mid slices (B) the five anterior to posterior regions for IVDs.