Background Erector spinae plane (ESP) and retrolaminar (RL) blocks show unreliable spread. We hypothesize that the combination of ESP and RL blocks provides more extensive and reliable spread of dye than single ESP blocks. Our primary objective was to compare the spread of dye to the paravertebral spaces after the combination block and ESP block in Thiel embalmed cadavers. Spread, the primary end point, was defined as the number of paravertebral spaces colored with dye per injection.

Materials and methods A single anesthetist performed ultrasound-guided ESP (20 mL) and combination of ESP and RL (10 mL each) blocks at the third thoracic vertebra of eight soft embalmed Thiel cadavers. Tissue displacement was visualized on an adjacent strain elastography image. Cadavers were dissected 24 hours later and anatomical structures were inspected for the presence of dye.

Findings Dye was visualized in more paravertebral spaces with the combination block (median 3 (IQR 3–5 (range 0–8)) vs 1.5 (IQR 0.25–2.75 (range 0–3) and difference (1.5 (0–4), p=0.04). Six out of seven (86%) combined erector spinae and RL blocks spread to at least three paravertebral spaces compared with two out of eight (25%) ESP blocks (RR 3.4, 95% CI 1.0 to 11.8; p=0.04). Contralateral spread occurred in three combination blocks and in one ESP block (OR 9.0, 95% CI 4.0 to 21.1; p<0.001).

Conclusions In conclusion, the combination of ESP and RL blocks was more extensive and reliable than ESP block alone.