Poster Presentation 50th International Society for the Study of the Lumbar Spine Annual Meeting 2024

Introduction

Disc degeneration involves evolving biomechanical changes¹, including increased annulus fibrosus stress that can lead to fissures and structural weakness². This weakness can disrupt disc alignment, progressing to instability, as demonstrated in vitro³. Deformation-field Magnetic Resonance Imaging (defMRI) enables non-invasive biomechanical assessment⁴, enhancing our understanding of degeneration's impact on disc stability in vivo.

This study aims to evaluate if and how annular fissuring, with CT discography as a reference, affects disc deformability in vivo using defMRI.

Methods

A total of 76 lumbar intervertebral discs in 28 patients (45±9 years (SD), 16 women (57%)) suffering from chronic low back pain were examined with T2-weighted conventional MRI. To reconstruct defMR images, the patients were also examined with MRI during axial loading in the supine position, mimicking the loading of the spine in an upright position. Low-pressure discography and computed tomography (CT) followed the MRI.

For each patient, the MR images with and without loading were registered to a common spatial volume using the Elastix software. Local disc compression or expansion due to axial loading was mapped in defMR images by calculating the Jacobian determinant of the registration deformation field in five midsagittal slices. The CT discography was used as a reference for annular fissuring. The defMR images were used to evaluate the absolute compression during loading for discs with 1) no fissures, 2) posterior fissures, 3) anterior and posterior fissures, and 4) severely disrupted annulus fibrosus, i.e., <50% continuously intact outer third annulus fibrosus lacking delimitable fissuring pathology.

Results

Fourteen discs displayed no fissures, 43 displayed fissures posteriorly, seven displayed fissures both anteriorly and posteriorly, and 12 discs displayed severely disrupted annulus fibrosus.

Discs without fissures or with only posterior fissures displayed similar loading patterns, with compression of the annulus fibrosus posteriorly and expansion anteriorly (Figure 1). Small disc compression differences were found between discs with only posterior fissures and those without fissures. In contrast, discs with fissures both anteriorly and posteriorly or with severely disrupted annulus fibrosus displayed an unstable compression pattern with significantly larger variance when compared to discs without fissures (Figure 1, Table 1).

Discussion

The findings show that discs with only posterior fissures seem to have similar biomechanical properties as normal discs without fissures and are compressed posteriorly and expanded anteriorly during axial loading. In contrast, discs with fissures both anteriorly and posteriorly and those with severely disrupted annulus fibrosus display large variations in internal deformation under stress and, as such, signs of micro-instability.

All compression of the disc results in mechanical stress on the annulus fibrosis, and fissures of different types seem to contribute to movements within the disc during load, as shown here. How disc deformation and fissures are linked to the experience of low back pain is unclear. However, the observation that discs with posterior fissures undergo compression similarly to those without fissures could suggest that mechanosensory of nerve fibers following the fissures may be stimulated here during axial loading.