Interscalene brachial plexus block: can the risk of entering the spinal canal be reduced? A study of needle angles in volunteers undergoing magnetic resonance imaging.

Background: Spinal cord damage during interscalene brachial plexus block has been attributed to needle entry into the spinal canal. The purpose of this study was to identify the angles and depths of needle insertion that increase the likelihood of such an event, using the traditional classic interscalene approach and two more proximal entry points.
Method: Magnetic resonance images of the neck from 10 healthy volunteers were used to obtain the three-dimensional spatial coordinates of three skin markers and the right-sided cervical nerves at the exiting neural foramina. The distance of the intervertebral foramina from the skin markers and the angles of the needle vector and the foramina were calculated.
Results: The distance from the skin to the intervertebral foramen may be as short as 2.5 cm with the classic approach. A caudal angulation greater than 50 degrees seemed to eliminate the risk of needle entry through the foramen.
Conclusion: With the classic approach to the interscalene block, there is a greater possibility of the needle passing through the intervertebral foramen if the needle is advanced too deeply. More proximal entry points and techniques that use a more steeply angled needle may reduce the risk of entry into the spinal space.