The causes of false-positive medial branch (Facet joint) blocks in soldiers and retirees

Objective: The purpose of this study was to determine the factors associated with false-positive medial branch blocks (MBB), the nerve blocks used to diagnose facet arthropathy, in soldiers and retirees. Methods: The study subjects were 78 patients with chronic low back pain who underwent diagnostic MBB to determine whether or not the facet joints were pain generators. Radiofrequency denervation of these nerves was performed in all patients with positive responses. Patients who failed to obtain pain relief after the blocks (negative blocks) and those who obtained temporary pain relief after MBB but failed radiofrequency denervation (false-positive blocks) then proceeded to undergo discography. Based on patients' responses to diagnostic blocks, discography results, the presence of radicular pain, and previous back surgery, the data were analyzed to determine whether any of these variables correlated with false-positive MBB. Results: The presence of discogenic or radicular pain was not associated with a higher false-positive response rate to MBB. Conversely, the absence of discogenic pain was associated with a higher percentage of false-positive blocks. There was also a trend for patients with previous back surgery to have a higher false-positive rate than those who had not undergone previous surgery. Conclusions: Although a high incidence of epidural and foraminal spread occurs during the performance of MBB, this is unlikely to be a significant cause of false-positive blocks.