Specificity of lumbar medial branch and L5 dorsal ramus blocks. A computed tomography study.

STUDY DESIGN: A cross-sectional study.

OBJECTIVES: To determine the face validity of lumbar medial branch blocks.

SUMMARY OF BACKGROUND DATA: Lumbar medial branch blocks have been used increasingly to diagnose zygapophysial joint pain. The course and relations of the medial branches of the dorsal rami have been demonstrated in previous anatomic studies. What is not known is whether blocks of the medial branches anesthetize these nerves exclusively or whether they anesthetize other structures that are potential sources of pain.

METHODS: In a cadaveric study, the branches of the dorsal rami were exposed. Spinal needles were placed over the nerves, and plain radiographs were taken to demonstrate the precise radiographic locations of the nerves. In the second phase of the study, healthy volunteers underwent injections of radiographic contrast over the nerves, and plain radiographs and computed tomographic images were taken. Injections were performed using different rates of injection and in two positions for each nerve.

RESULTS: Radiographic contrast incorporated the medial branches of the dorsal rami in every injection. When injections were performed using the upper position, aberrant flow of contrast medium was demonstrated with extension into the epidural space or intervertebral foramina. When a position lower on the transverse process was selected, aberrant flow was very uncommon. Eight percent of injections were found to be intravenous.

CONCLUSIONS: When the appropriate technique is used, medial branch blocks are target specific. To guard against false-negative responses due to intravenous up-take, contrast medium must be used before the injection of local anaesthetic.