Background The erector spinae plane block (ESP block) is frequently employed for thoracic, abdominal, and spinal surgeries, yet its precise mechanism of action remains a subject of debate. While initially postulated to influence both ventral and dorsal rami of the spinal nerve, recent studies indicate a predominant impact on the dorsal rami with limited involvement of the ventral rami. To elucidate this mechanism, we conducted an observational study to assess the distribution of ESP infiltration to the ventral rami.

Methods We performed 20 ESP infiltrations bilaterally in 10 unembalmed cadavers, targering the T9 transverse process level. A volume of 20 mL of ropivacaine 0.2% and methylene blue 0.01% was used. Dissection was carried out to assess dye distribution, with a focus on involvement of the ventral and dorsal rami, as well as lateral and longitudinal spread within the spinal muscular plane.

Results No evidence of dye staining was observed in the ventral rami in any of the cadavers; however, the dorsal rami consistently displayed staining. The dye exhibited extensive longitudinal diffusion across the paravertebral musculature, spanning a median of 10 vertebral spaces (T5–L2). The range extended from 3 to 18 spaces, with an IQR of 11 levels (T4–L2), predominantly affecting the longissimus and iliocostalis muscles.

Conclusions The infiltration of injectate into the ESP does not contact the ventral rami of segmental spinal nerves. The inference that an in vivo ESP block is a paravertebral “by-proxy” is, therefore, unlikely.