Anatomic basis of the PENG block: dissection, cryo-cross-sections, and histology challenge a pericapsular target

BACKGROUND: The pericapsular nerve group (PENG) block was developed to result in local anesthetic (LA) spread in the plane between the iliopubic ramus and the iliacus muscle. We aimed to investigate the anatomical and histological basis of injectate spread following a PENG block. METHODS: We performed a PENG block in 10 cryopreserved cadavers under ultrasound guidance. A 22-gage, 50 mm needle was inserted using an in-plane approach. After contacting the bone at the target site, 20 mL of the solution mixed with the marker (methylene blue or heparinized erythrocytes) was injected. The dispersion of the injectate was analyzed using anatomical dissection, cryo-cross-sections, and microscopy. RESULTS: Gross anatomical dissection demonstrated methylene blue staining in close proximity to the femoral nerve and its branches, without staining of the obturator nerve. Cryo-cross-sectional and histological analyses showed predominantly subepimysial and intramuscular localization of the injectate within the iliopsoas muscle compartment, without consistent involvement of the hip capsule. Histology identified small nerve branches within adipose-containing connective tissue planes. CONCLUSIONS: The injectate following a PENG block was predominantly confined to the iliopsoas muscle compartment. The presence of small intramuscular nerve branches suggests that intramuscular neural blockade may contribute to the analgesic effect. Potential femoral nerve involvement warrants further investigation.