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Table 1 Rydell classification

From: Salvianolic acid B reduced the formation of epidural fibrosis in an experimental rat model

Grade 0 Epidural scar tissue was not adherent to the dura mater.
Grade 1 Epidural scar tissue was adherent to the dura mater, but easily dissected.
Grade 2 Epidural scar tissue was adherent to the dura mater and difficultly dissected without disrupting the dura matter.
Grade 3 Epidural scar tissue was firmly adherent to the dura mater and could not be dissected.