Ultrasound-guided serratus posterior superior muscle block: an anatomical study investigating the extent of injected dye and the mechanism of action of a simulated injection in Thiel soft-embalmed cadavers.

PURPOSE: The extent of local anesthetic spread and mechanism of action of serratus posterior superior muscle (SPSM) block is still debatable. This cadaveric study aimed to evaluate the anatomical spread of 30 or 20 mL of dye following a simulated SPSM block in Thiel soft-embalmed cadavers.

METHODS

Simulated SPSM block injections were administered bilaterally in four cadavers (left side: 30 mL, right side: 20 mL). Anatomical dissection was performed to evaluate the extent of spread of the injected dye over the ribcage, and to document staining of the musculature of the back following a simulated SPSM block.

RESULTS

The extents of spread (mean ± SD) of 30 mL and 20 mL of dye over the ribcage were 8.5 ± 3.9 cm and 5.5 ± 1.6 cm, respectively. Dye spread to the peri-scapular fascia was observed in all four of the simulated SPSM blocks with 30 mL of dye, but not with 20 mL. Dye spread into the rhomboid major muscle was observed in two of the simulated SPSM blocks with 30 mL of dye.

CONCLUSION

Our findings suggest that the SPSM block using 30 mL of dye has a tendency to spread wider compared to the SPSM block using 20 mL of injectate, and that the block might serve as an interfascial block rather than a segmental nerve block. Further studies in a large sample of human participants are required to confirm the optimal volume of local anesthetic in the SPSM block.