This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. FULL TITLE: HIGH RESOLUTION MR IMAGING OF PARASPINAL MUSCLE INJURY FOLLOWING MINIMALLY INVASIVE AND CONVENTIONAL OPEN POSTEROLATERAL LUMBAR FUSION Introduction: Degenerative disk disease with nerve root compression or neuroforamen stenoses and spondylosysthesis occurs with increasing incidence in our population. The current standard of care is cord and nerve root decompression which is often combined with a postero-lateral fixation of adjacent vertebral bodies to immobilize the affected segments and thus to reduce the likelihood of recurrent compression. Conventionally, the fixation approach is performed with open surgery. However, despite a large success rate for the fixation procedure, only a small number of patients present favorable clinical outcome post surgically; a much larger fraction of patients have either unchanged or worsened symptoms and therefore has major socio-economic implications. A large body of evidence exist that muscle degeneration and lipid infiltration after open surgery is a major determinant for the poor outcome. Here, the retraction duration and the high pressure applied to the paraspinal muscles are indicative that muscle ischemia is a significant contributor to muscle degeneration. Recently, minimally-invasive tools have been introduced to reduce the amount of muscle compression and total surgery time. Preliminary studies demonstrates that a much larger fraction of patients have a favorable outcome, however, currently no established methods exist to study whether or not these minimally invasive methods are less damaging to the paraspinal muscles. The purpose of this prospective and randomized study was to use high resolution MR imaging to evaluate the paraspinal muscles, and to determine whether minimally invasive lumbar spinal fusion results in less paraspinal muscle damage than conventional open spinal fusion. Methods: Patients with a recent minimally invasive or open single level posterolateral lumbar spinal fusion were recruited at a single institution by 2 spinal surgeons. MRI scans were performed approximately 6 months post operatively (range 4-10 months). All subjects were imaged in a supine position using a 1.5 T Signa LX (General Electric, Milwaukee, WI) with a phased-array spine coil. Axial multi-slice T1-weighted spin echo (SE) and axial flow-compensated T2-weighted fast spin echo (FSE) sequences were used for high-resolution structural imaging. In order to differentiate between edema and fatty atrophy, a combination of axial STIR T2-weighted FSE and fat-saturated T2-weighted FSE sequences were used. An oblique coronal T2 FSE or inversion recovery sequence was performed for better assessment of the overall extent of muscle damage. Edema and atrophy within the multifidus (MF) muscle was scored on a visual scale of 0 to 3 with 0 = none, 1= mild, 2= moderate, and 3 =severe. For quantitative T2-mapping, an axial dual spin echo sequence was performed, with multiple axial slices at the levels of the lumbar intervertebral discs. ROIs with a minimum area of 10 mm2 were placed on the individual bundles of the multifidus muscles bilaterally, and a T2 relaxation time was calculated using a monoexponential fit on a per-pixel basis in the region of maximal signal intensity. In order to compare the 2 surgical groups, the average T2 relaxation time was also calculated in the multifidus muscles at the level of the fusion. Results: Twelve patients (9 females, 3 males) with a mean age of 52.7 years (range 38 71yrs) were enrolled in the study. The average duration of symptoms prior to fusion was 7.4 years (range 1-20 yrs). Six had a minimally Endius Atavi fusion, and 6 had conventional open spinal fusion. The average operative procedure times for the Atavi and Open groups were 251 minutes (range 198-285 mins) and 221 minutes (range 140-262 mins) respectively. There was a striking difference in muscle edema between the minimally invasive and open spinal groups. T2 values correlated well with the visual estimate of edema at each site (r = 0.76, p < 0.0001). The mean T2 relaxation time at the level of the fusion was 85.8 ms ( 19.5) in the open group, compared with 51.3 ms ( 13.4) in the minimally invasive group (p=0.0051). In most patients, edema was seen in a chevron configuration on coronal images, following the anatomy of MF fibers. Therefore edema and T2 measurements were also analyzed in this distribution, using measurements from the medial bundles at the level above the fusion, the medial and central bundles at the level of the fusion and from all three muscle bundles at the level below the fusion. The mean T2 was 98.6 ms ( 18.1) in the open group and 53.2 ms ( 8.9) in the Atavi group (p=0.0005). Muscle atrophy was present in both groups, but there was no significant difference between the atrophy scores in the open and minimally invasive groups. References: 1Chan Y, et al . Pediatr Radiol 1999; 29: 360-363. 2Gejo R, et al. Spine 1999; 24 : 1023-1028. Acknowledgements. This work was supported in part by the NIH (1R01EB002771), the Center of Advanced MR Technology at Stanford (P41RR09784), Lucas Foundation, and Oak Foundat
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