NEEDLE TIP POSITION AND INJECTION SITE IN LUMBAR SELECTIVE NERVE ROOT BLOCKS: A PROSPECTIVE AUDIT OF LOCATION ACCURACY

Abstract

Background: Although needle tip position has been correlated with outcome with respect to pain relief, and to complication rate, to our knowledge, no previous study assesses the location of the needle tip with respect to its ability to isolate injectate around the exiting nerve root without blocking the traversing nerve root to the next caudal level.

Aim: To study the location of injectate when diagnostic selective nerve root blockade is performed.

Method: 87 consecutive selective nerve root blocks performed by a single surgeon were assessed. A consistent surgical technique was utilised. Antero-Posterior fluoroscopy films were analysed to determine the location of injectate in relation to the foraminal and pedicle anatomy. A pro-forma operation note ensured all necessary data was collected prospectively.

Results: Of needle tips positioned lateral to the middle third of the superior pedicle on the AP view, 45 of 51 flowed into the nerve sheath alone, and 6 flowed into both the nerve sheath, and spinal canal. Of these 6, 2 were due to a larger volume of contrast injected, and 2 were due to abnormal anatomy from marked lumbar degenerative scoliosis. Of tips located below the middle third of the pedicle, 2 of 29 flowed into the nerve sheath alone, 2 flowed into the canal alone, and 27 flowed into both. Of those placed medial to zone below the middle third of the pedicle, all seven flowed into the canal only. Analysis using Fisher’s Exact test yielded an extremely statistically significant result, with p < 0.001 comparing needle tip positions in the lateral position with the mid-zone and medial tip positions, and their relationship with injectate reaching the traversing nerve root.

Discussion: For a nerve root block to be truly selective, no injectate must flow past the exiting root to the traversing root. Low volumes of injectate must be placed predictably and accurately. This paper demonstrates the importance of needle tip location in preventing flow beyond the foramen. It is also recommended that contrast be used when significant deformity is present. In cases where there is a therapeutic rather than diagnostic intention, such accuracy is unnecessary. This study does not address the efficacy of the selective nerve root block as a non-operative intervention, nor does it assess the ability of the block to predict operative benefit. It does, however, provide a benchmark for accuracy achievable in patients without significant spinal deformity.

Conclusion: Care must be taken to ensure that the needle tip is positioned lateral to the zone below the middle third of the pedicle if a selective nerve root block is to be used for diagnostic purposes, particularly if there is significant deformity or no contrast is used. In the absence of deformity, however, selective nerve root block may be performed reliably, with a location accuracy of 96%.