Anatomical variations of the external obturator muscle and their impact on injectate spread to the obturator canal: a cadaveric study related to ultrasound-guided proximal obturator nerve block.

We anatomically investigated whether the superior fasciculus (SF) of the external obturator muscle (EOM) influences the spread of dye into the obturator canal, in a cadaveric model simulating a proximal ultrasound-guided obturator nerve block. Thirteen sides from seven Thiel-embalmed cadavers (five male cadavers, nine sides; two female cadavers, four sides) were studied. Blue dye was injected either between the pectineus muscle and the EOM, the pectineus muscle and the SF, or the SF and the main belly of the EOM. After injection, gross dissection was performed to evaluate dye spread along the obturator nerve and into the obturator canal. In the SF-absent sides (four of thirteen), the dye injected between the pectineus muscle and the EOM broadly stained both the anterior and posterior branches and spread into the obturator canal, with staining of the obturator nerve trunk in all cases. In the SF-present sides (nine of thirteen), dye injection between the pectineus muscle and the SF (five sides) failed to produce canal spread, and neither the posterior branch nor the trunk was stained. In contrast, dye injection between the SF and the main belly of the EOM (four sides) stained both branches in all cases and achieved canal spread with obturator nerve trunk staining on three sides. Because the SF may restrict obturator-canal spread during the proximal approach, the injection site may need to be adjusted according to the presence of the SF.