A Narrative Review of Radiological Signs of Dural Ossification in Thoracic Ossified Ligamentum Flavum: Implications for the Spine Surgeon

Ossified ligamentum flavum (OLF) is an important surgically treatable cause of thoracic myelopathy, most commonly seen in East Asian countries. Dural ossification (DO) is observed in 10%–60% of cases and is a significant cause of surgery-related complications. The aim of the present report is to provide a detailed narrative review of the incidence, pathogenesis, and various radiological signs described for the diagnosis of DO in OLF. We intend to present a simple practical approach for the pre-operative diagnosis of DO. Online databases were searched to retrieve studies that have described radiological signs for the diagnosis of DO in OLF, from the earliest date to the present, in the English language. Since 2009, ten radiological signs have been described in nine studies. They were mostly reported from computed tomography scans and all were conducted in Asian countries, namely China and India. These signs include the tram-track sign (TTS), comma sign, bridge sign, T2 ring sign, TOLF-DO grading system, cross-sectional area (CSA) occupying ratio, cerebrospinal fluid cross-sectional ratio, DO-OLF risk classification, Banner cloud sign, and three-dimensional occupying ratio. Each of these signs has good sensitivity and specificity, but none of them are 100% diagnostic. Because of comparable sensitivities and specificities, it may be opined that a combination of TTS, T2 ring sign, and a tuberous type of OLF (if present) will be sufficient for the diagnosis of DO in the majority of cases, as most of the others require complicated calculations that are difficult in routine clinical practice.