A diagnostic study of thoracic myelopathy due to ossification of ligamentum flavum

We set out to establish a magnetic resonance imaging (MRI) and computed tomography (CT)-based diagnostic method for determining the responsible segments in thoracic myelopathy due to ossification of the ligamentum flavum (OLF). Forty-four patients who underwent surgery for treatment of myelopathy due to OLF between June 2005 and May 2013 were enrolled in this study as the myelopathy group. Forty-four patients who were identified through CT and MRI scans to have OLF but had no definite neurologic deficits prior to the examination were included as the control group. MRI and CT examination were reviewed, and the degree of spinal canal compromise was graded on axial T2-weighted MRI. Anteroposterior spinal canal diameter was measured at the maximally stenosed level on axial and sagittal CT. The canal grade and the cross-section area-occupying ratio were measured and calculated on the CT scans. The diagnostic coincidence rates for the indices were then compared. Cases of Grade IV were all in the myelopathy group while cases of Grade II were all in the control group. The canal grade (paramedian) was the most relevant continuous variable with the largest JOA score (r = 0.685, P < 0.005). A canal grade (paramedian) of < 60 % can be used as a critical value for determining OLF-induced myelopathy (sensitivity and specificity, 95.5 %). Spinal canal compromise is relevant to spinal cord deficits in patients with OLF, and a canal grade (paramedian) can be used to quantify spinal cord deficits. Additionally, a canal grade (paramedian) of < 60 % on axial CT scan can serve as a critical value for diagnosing OLF-induced myelopathy, especially for Grade III compression on T2-weighted MRI.