The parasacral sciatic nerve block does not induce anesthesia of the obturator nerve

The ability of the parasacral sciatic nerve block (PSNB) to induce anesthesia of the obturator nerve remains controversial. Our objective was to evaluate the anesthesia of the obturator nerve after a PSNB.

Forty patients scheduled to undergo knee surgery (anterior cruciate ligament reconstruction) were included in this prospective, randomized, controlled study. Patients were randomized to receive PSNB alone (control group, n = 20) or PSNB in combination with an obturator nerve block (obturator group, n = 20). After evaluation for 30 min, the two groups received a femoral nerve block, and patients were taken to surgery. The obturator nerve blockade was assessed by measurement of adductor strength at baseline (T0) and every 10 min during the 30-min evaluation (T10, T20, and T30). Pain scores after tourniquet inflation and during surgery were compared between the two groups. The requirement for additional intravenous analgesia and/or sedation was also recorded.

The two groups had comparable demographic and surgical characteristics. Four patients were excluded from the study because of PSNB or femoral nerve block failure. The adductor strength values were similar between groups at T0 but were significantly lower in the obturator group at T10, T20, and T30 (p < 0.0001). Patients in the obturator group reported less pain than those in the control group (p < 0.05). They also required less additional intravenous sedation and/or analgesia (p < 0.05).

This clinical study demonstrated that the PSNB is an unreliable means of inducing anesthesia of the obturator nerve and emphasizes the need to block this nerve separately to induce adequate analgesia during knee surgery.