Thoracic spine ossified ligamentum flavum: single-surgeon experience of fifteen cases and a new MRI finding for preoperative diagnosis of dural ossification.

Ossification of ligamentum flavum (OLF) is an infrequent cause of thoracic spinal stenosis, mostly found in Japan and other East Asian countries, and uncommon in other populations. The incidence of dural ossification (DO) ranges from 11-62%. None of the previously described imaging findings can accurately diagnose DO preoperatively. Author reports outcomes of 15 cases of thoracic OLF from an Indian tertiary center, propose a new MRI finding to preoperatively diagnose DO and compare the various differences between subjects with and without DO. Three and half year retrospective analysis. Demographic, clinico-radiological and operative details, and outcome were analyzed. Correlation between new MRI finding and intraoperative DO was also noted. Fifteen patients (10 males, 5 females) with mean age of 51.3 years were analyzed. Ten (66.6%) cases had DO and the new MRI finding (T2 ring sign) had high correlation with intraoperative DO (sensitivity of 90%, specificity of 100%). The T2 hypointense rim corresponds to the calcified/ossified dura present around the thecal sac. Important clinico-radiological differences were observed in patients with and without DO. Canal diameter was uniformly <5 mm in Nurick grade V cases and <9 mm in those with DO. Mean follow-up duration was 19.5 months (range 4-45 months). All, except one, had significant symptomatic improvement, however, only two patients with DO attained Nurick grade I, perhaps because of the poor preoperative Nurick grade in those patients. Decompressive laminectomy (till the normal interlaminar space caudally and cranially) with complete removal of flavum including ossified dura, if possible, is the ideal treatment. Even after significant improvement, patients with DO may have mild residual myelopathy. Presence of DO, CSF leak and myelomalacia are neither related to wound problems nor clinical improvement. Higher preoperative Nurick grade was consistently associated with good clinical recovery. T2 ring sign is of benefit to surgeons in preoperative diagnosis of DO.