Background: Unforeseen dural ossification (DO) increases the risk of complications in the surgical management of thoracic ossification of the ligamentum flavum (OLF). Several methods have been proposed to identify DO; however, these approaches either have low diagnostic accuracy or poor feasibility.Therefore, we aimed to determine the relationship between DO and the severity and range of thoracic OLF compression using a 3-dimensional (3D) imaging analysis and to evaluate its superiority in diagnosing DO over conventional measurement methods.Methods: A total of 114 consecutive patients who underwent decompressive laminectomy for thoracic OLF in 4 institutions were retrospectively enrolled and divided into DO and non-DO groups. Univariate analysis was performed to determine the relationship between OLF compression and DO. We measured the 3D occupying ratio (OR; 3D OR = OLF volume/normal canal volume × 100%), calculated its cutoff values, and compared its diagnostic value in DO with that of conventional 1D and 2D radiological parameters in the whole thoracic spine.Results: The 3D OR in the DO group (50.9%±7.9%) was significantly higher than that in the non-DO group (30.8%±7.5%; P<0.01). The overall reliability and reproducibility for measurements of the 3D OR (intra-and interobserver correlation coefficients 0.94 and 0.90, respectively) were excellent. Thus, the 3D OR could be used as an indicator to distinguish between DO and non-DO, with high diagnostic accuracy (91.2%). Moreover, a 3D OR of >43%, known as the "ossification zone", was indicative of DO in OLF, whereas a value of <37% was considered the "safe zone". Additionally, the 3D OR [area under the curve (AUC) =0.98, 95% confidence interval (CI): 0.93-0.99] showed a statistically higher diagnostic value for DO in the upper, middle, lower, and whole thoracic spine than did both 1D (AUC =0.81; 95% CI: 0.73-0.88) and 2D (AUC =0.87; 95% CI: 0.79-0.92) parameters (P<0.01).Conclusions: DO was significantly associated with the severity and range of OLF compression. The 3D OR could be used as a critical diagnostic indicator for identifying DO in the whole thoracic spine, owing to its superiority over conventional radiological parameters. Classification of the 3D OR could maximize the clinical feasibility and thus help surgeons to decrease the incidence of DO-related surgical complications.