RESUMEN
BACKGROUND: The transversalis fascia plane and the quadratus lomborum blocks target the branches of T12-L1 nerves and provide analgesia in patients undergoing surgery involving the corresponding dermatomes. The transversalis fascia is believed to contribute to determine the spread of local anesthetic in such blocks. Nonetheless, the anatomy of this fascia and its possible role in these blocks still have to be precisely defined. METHODS: We conducted a series of 10 dissections and full-thickness specimens were obtained from one side for the microscopic analysis. RESULTS: Macroscopic study showed failed to identify a distinguishable fascial structure between the fascia of the transversus abdominis muscle and the peritoneum. Microscopic examination showed the presence of a further fascial layer (0.1-0.2 mm), ascribable to transversalis fascia. Transversalis fascia and transversus abdominis epimysium posteriorly diverge: transversus epimysium goes posteriorly to quadratus lomborum to joint the oblique internal aponeurosis, while transversalis fascia continues in front to quadratus lomborum. So, a little fascial triangle (2-3 mm) is formed on the lateral border of quadratus lomborum, defined by transversalis fascia and transversus abdominis. Inside this triangle, T12 and L1 nerves are present. CONCLUSIONS: Considering the thinness of the transversalis fascia and the small size of the triangular space that contains the target nerves, this is most likely a virtual, ideal rather than realistic injection site. Accordingly, it is probable that the local anesthetic is injected in the much wider retroperitoneal space and reaches the nerves by spreading backwards through the thin transversalis fascia.