Lumbosacral transitional vertebra and S1 radiculopathy: the value of coronal MR imaging

The association of a lumbosacral transitional vertebra with accelerated degeneration of the disc above has been described. Lumbosacral transitional vertebrae have also been reported as a cause of extraforaminal entrapment of the L5 nerve root between the transverse segment of the transitional vertebra and the sacral ala optimally demonstrated by coronal MRI. The association of the lumbosacral transitional vertebra pseudoarthroses and S1 nerve root entrapment due to degenerative stenosis of the nerve root canal has never been described. We present 12 patients with lumbosacral transitional vertebrae that were referred for symptoms and signs of S1 nerve root radiculopathy in which the sagittal and axial MRI sequences failed to identify a plausible cause for the patients' S1 nerve root symptoms. A coronal T1-weighted imaging (T1WI) MRI sequence was consequently added to the investigation. The coronal T1WI MRI sequence demonstrated hypertrophic degenerative stenosis of the S1 nerve root canal at the level of the lumbosacral transitional vertebra pseudoarthrosis, with entrapment of the respective S1 nerve root in all patients. We emphasize the value of coronal T1WI MRI of the lumbosacral junction and sacrum if the cause for S1 radicular symptoms was not identified on conventional sagittal and axial MRI sequences in patients with lumbosacral transitional vertebrae.