The Effect of Spinal Fusion on the Water Content and Mobility of the Adjacent Intervertebral Disks Measured with Quantitative MR and Dynamic CT

The PI of this project was:

This project was funded by: Radiological Society of North America Seed Grant

The term of this project was: December 2006 to July 2008

The number of subjects scanned during this project was: 21

Specific Aims

The goal of this study is to obtain pilot data to evaluate the effect of spinal fusion on junctional disks (adjacent to a spinal fusion) and non-junctional disks in a small cohort of patients and to obtain additional validation of methodology. The junctional intervertebral disk, that is, the disk adjacent to a surgically-fused spinal segment, presumably has an increased risk of undergoing degeneration. However, prospective studies to measure that risk have been lacking. Because of the recent development of techniques for fusion of the spine with “non-rigid” devices, the assessment of junctional disks has even greater clinical importance. Two new imaging techniques, quantitative MR (assessment of the water content in a disk) and dynamic CT, may provide tools to study intervertebral disk chemistry and biomechanics. This study will provide objective measurements of the effect of spinal fusion on the biomechanical and biochemical integrity of junctional intervertebral disks in a small cohort of patients who undergo a spinal fusion. These techniques will enable the detection of changes in disk hydration and in disk function that hypothetically predict the development of degenerative changes detectable in MR images. The study will provide pilot data to design a longitudinal study of the effect of spinal fusion on junctional disks. Ultimately the functional MR and CT techniques may provide tools to evaluate disk degeneration in clinical and experimental studies more accurately than is now possible.

Study Design

This is a prospective pilot study of twenty adult UWHC patients who are scheduled for lumbar spinal fusion for degenerative disk disease. MRI, quantitative MRI, and dynamic CT studies will be performed prior to the spinal fusion and at 12 months post-fusion. MRI scans are performed routinely as part of the work-up prior to spinal fusion. If an MRI has already been done at another institution, a limited MRI will be performed for the study. The MRI exam at 12 months post-fusion, as well as the two CT exams, will be done for research purposes only. Study participation also includes completion of a questionnaire that includes a modified Oswestry disability index and a visual analogue scale for pain (VAS), which will be completed at both time points.

Standard MRI and quantitative MRI scans will be performed before and 12 months after the fusion procedure. Each MRI scanning session (including both standard MRI and quantitative MRI scans) will take about 45 minutes, and will be performed at the MRI Division of the UWHC Radiology Department. Subjects will be screened for MRI compatibility using the UWHC MRI screening questionnaire. The quantitative MR technique will be used to measure the T2 relaxation voxel by voxel in the intervertebral disk. T2 relaxation is an intrinsic property of tissue, unlike the signal intensity in MR images which depends on extrinsic factors such as the echo time, relaxation time, number of averages, type of receiver coil, and other acquisition parameters used to create the MR image. With T2 measurements, the spatial distribution of water in the disk can be monitored over time. Measurements of T2 in intervertebral disks have been reported. T2 correlates directly with the unbound water content in the intervertebral disk, which in turn varies directly with maturation and aging and degeneration in the disk. Degenerated intervertebral disks have shorter T2 relaxation times than normal disks.

Prior to surgery, the MR appearance of each intervertebral disk will be noted and classified into one of five stages of degeneration using the Thompson staging criteria. T2 will be mapped preoperatively in each lumbar disk by quantitative MR. The magnitude of rotation in each lumbar motion segment will be measured with the dynamic CT. Twelve months after fusion, CT, MR imaging, and quantitative MR will be repeated. The change in the MR appearance of each intervertebral disk will be noted. Changes in T2 and in axial rotations at the fused level and the adjacent (junctional levels) will be calculated. The data will be used to perform powers calculations for a future prospective study of junctional disks. To measure the precision of the CT and T2 calculations, mean differences and standard deviations for disks distant from a fusion will be computed.

With data stratified by disk degeneration grade, we will calculate the average change in T2 over 12 months for non-junctional and in junctional disks. From published results, we expect an average change of less than 1 percent in stage II non-junctional disks. We anticipate that the change in T2 for junctional disks is greater than 1 percent on average. An objective of this study is to obtain sufficient data to perform a powers calculation for a study of longer duration or more patients. We will correlate Oswestry scores, T2, and rotational mobility of motion segments in postoperative patients. Differences in age, weight, and other variables between groups will be analyzed for evidence of confounding factors.

Dynamic CT exams will be performed before and 12 months after the fusion procedures. For the CT study, the subject will be placed on a specially built table (University of Wisconsin Biomedical Engineering) that provides separate supports for the torso and for the hips and legs. The support under the legs and hips rotates 8 degree either clockwise or counterclockwise to provide 16 degrees of isocentric rotation about the spinal axis.

We will calculate the difference between the first and the second measurement of rotation and level by level determine the average and standard deviation of the change. We anticipate that on average at non-junctional and non-fused levels, rotations will not change (average difference = 0). We will use the standard deviation of this change as a measure of the precision of the measurement. In the disks that are fused, we anticipate that rotation will diminish to zero on average, due to the fusion. We anticipate that the rotation in the adjacent motion segments (junctional disks) will increase marginally after fusion. For these disks, we will correlate the change in rotation with the change in T2, on the hypothesis that the disks with a greater change in T2 have a greater change in rotation. We will compare the change in rotation with the change in the VAS and Oswestry scores. One goal of the study is to collect sufficient data to determine the power of the dynamic CT to detect changes in biomechanical function in motion segments over time.

Subjects and Recruitment

The study population consists of twenty adult UWHC patients who are scheduled to have a lumbar spinal fusion for degenerative disk disease. Specific inclusion criteria are: pain duration greater than 6 months, pain localized to the low back, single level degenerative disease on imaging, and an Oswestry disability index greater than 50 percent. Exclusion criteria are significant systemic disease, contraindication to MRI, claustrophobia, history of neoplasm or osteoporosis, weight greater than 250 pounds, inability to ambulate, age more than 80 years or less than 20 years, or history of systemic arthritis or previous spinal surgery.

Potential subjects will be identified by Drs. Paul Anderson, Daniel Resnick, Thomas Zdeblick, and Clifford Tribus (Departments of Orthopedic and Neurological Surgery, Neurosurgery) from their clinical practice of patients being referred for spinal fusion. No clinical procedures will be delayed because of this study.