INTRODUCTION
Spinal pathologies and intervertebral disc degeneration (IVD) are common problems that can be associated with pain and disability. Though not fully understood, function of the muscles around the lumbar spine appears to be associated with low back pain and lumbar degeneration. Computed tomography (CT) and magnetic resonance imaging (MRI) provide a non-invasive means of evaluating structural changes related to muscle quality. The purpose of this work was to quantify muscle quality through imaging modalities of CT and MRI in canines being treated for IVD extrusions. It was hypothesized that muscles at the level of IVD extrusion would be of lower quality when compared to adjacent spinal levels. Additionally, it was hypothesized that the erector spinae (ES) and multifidus (MF) would also have lower muscle quality, at the surgical level, when compared to the psoas (PS).
METHODS
Preoperative imaging of the thoracolumbar spine was performed using CT (n=13) or MRI (n=11) in canines surgically treated via hemilaminectomies for IVD extrusions. DICOM images were taken at the level of IVD extrusion and one and two levels cranially and caudally. ImageJ was used to assess spinal muscles with regions of interest (ROIs) drawn to include the bulk of each muscle. In CT scans Hounsfield units (HU) of each muscle were normalized to the ascending aorta to analyze muscle quality. In T1-weighted MRIs pixel intensity of each muscle was normalized to a region of adjacent subcutaneous fat to create a muscle-fat index to analyze muscle quality.
RESULTS
No significant differences were found between any of the muscles on either CT (p=0.76) or MRI (p=0.20) at the surgical level and surgical side. No significant differences were found amongst any of the five levels on the surgical side using either CT or MRI for ES (p=0.93, p=0.95), MF (p=0.71, p=0.75), or PS (p=0.34, p=0.41). Similarly, no significant differences were found amongst any of the five levels on the surgical side using either CT or MRI for ES (p=0.62, p=0.35), MF (p=0.99, p=0.92), or PS (p=0.13, p=0.95).
DISCUSSION
The muscles at the level of IVD extrusion were expected to be of lower quality than the adjacent levels. However, no differences were observed amongst any of the five levels nor between any of the muscles. This could be due to the highly acute nature of intervertebral disc extrusion in chondrodystrophic canines, where emergency surgery is required. Chondrodystrophic canines have a very high prevalence of IVD degeneration with onset often by three months of age [1]; as such, any degenerative muscle changes may be relatively uniform along the length of the spine. Next steps are to accumulate more data and additionally grade intervertebral disc degeneration at the surgical and adjacent levels to determine if any relationship exists between disc degeneration, extrusion and muscle quality in this population of canines.