Abstract
Introduction
The causes of low back pain (LBP) are poorly defined and indistinct; the most often implicated tissue as the origin for pain is the intervertebral disk (IVD)1,2. Traditionally, lumbar spine range of motion measurements and IVD gross morphology (from MR images) are used to assess spinal health, but largely fail to correlate well with pain or provide clinically useful patient stratification3,4,5. Due to the inadequacies of current imaging techniques in differentiating symptomatic versus nonsymptomatic changes, a new series of quantitative protocols is emerging6. Quantitative T2* relaxation time measurements probe biochemical properties of the tissues. T2* mapping techniques are employed for cartilage assessment, probing spatial collagen architecture in conjunction with water molecule mobility7. As a first step, the goal of this work is to correlate quantitative MR mapping of the IVD with 3D kinematic data to probe the relationship between disk biochemical properties and functional kinematic parameters.
Materials and Methods
Total 18 osteoligamentous cadaveric lumbar spines (L3-S1), acquired from the UofM Bequest Program (aveage: 53.2 ± 15.5 years; range: 21 to 71 years) were examined using conventional and quantitative MR imaging protocols and biomechanically exercised in the cardinal planes. MR imaging was performed on a Siemens 3T scanner (Magnetom Trio; Siemens Health care). Conventional T2-weighted sagittal anatomic images were acquired for Pfirrmann disk grading (Fig. 1A)8. T2* relaxation maps were obtained using [TR(ms): 500; TE(ms): 4.18, 11.32, 18.46, 25.60, 32.74, 39.88; Voxel Size(mm): 0.5 × 0.5 × 3.0, Slices: 33]. Quantitative T2* maps were calculated (MapIt, Siemens Health care) and mean T2* values were recorded using Osirix Imaging Software across five regions of interest (ROI) (Fig. 1B). Utilizing MATLAB (Mathworks Inc. Natick, MA), the central ROI mean relaxation value was evaluated in the coronal plane as the intensity changed from lateral to medial to lateral (Fig. 1C). Using a least squares method, the slope of the transition zone (ms/mm) between the annulus fibrosus and nucleus pulposus on each side was computed (Fig. 1D). The area underneath the curve was calculated and normalized to the width of the IVD (Fig. 1D). After imaging, the specimens were embedded in polymethylmethacrylate and tested in a six-axis Spine Kinetic Simulator (8821 Biopuls, Instron, Norwood, MA, USA). Pure moments of up to 7 Nm were applied sinusoidally (0.015 Hz). Three cycles were performed and the final cycle was analyzed. Load and moment data was collected at 100 Hz. Segmental displacements were recorded using a 3D visual motion analysis 5-camera system (Vicon MX-F40NIR, Vicon Motion Systems, Centennial, CO) capturing a four-ball infrared reflecting marker set attached to each vertebral body. Kinematic outcomes (ROM, Neutral Zone, NZ/ROM, Stiffness) were correlated with imaging parameters using Pearson correlation test with alpha acceptance of 0.05.
Results
The T2* relaxation measurements in the nucleus pulposus were significantly correlated with flexion stiffness (p = 0.024) and lateral bending stiffness (p = 0.021) wherein increased stiffness was associated with lower signal and higher Pfirrmann grade. The slope of the transition zone for the 54 disk samples was found to be correlated with the flexion stiffness (p = 0.026), flexion range of motion (p = 0.002), lateral bending stiffness (p = 0.029), and lateral bending range of motion (p = 0.0028). The area under the T2* intensity curves in the coronal plane was likewise found to be significantly correlated with following biomechanical measures: flexion stiffness (p = 0.043), flexion range of motion (p = 0.014), lateral bending stiffness (p = 0.021), and lateral bending range of motion (p = 0.023).
Conclusion
MR imaging of IVD using T2* relaxation measurements provide quantitative assessment of the disk's biochemical properties which is related to its biomechanical competence. This new methodology and analysis technique may provide enhanced quantitative data on the degenerative cascade which alters disk function and in some cases leads to painful outcomes. Limitations of this research include the biomechanical testing of cadaveric samples without musculature and their postmortem imaging at room temperature. Next, we aim to evaluate this relationship in human subjects and determine its clinical efficacy. This research linked kinematic data with quantitative T2* mapping of IVD from across the degenerative spectrum. By coupling quantitative MR imaging with functional kinematic outcomes, these data may improve patient stratification and targeted therapeutic intervention for patients suffering from LBP.
Yes
None declared
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