Selective blockade of the obturator nerve is not as common as other major peripheral nerve blocks (e.g., sciatic or femoral nerves).

When femoral nerve block combined with sciatic nerve block is used for lower limb surgery, combined sedation or general anesthesia are frequently reported in 13–37% of patients scheduled for knee arthroscopy [1-4], and in 42% of patients for open knee surgery [5].

Femoral nerve block is effective for femoral nerve blockade, sometimes effective for lateral femoral cutaneous nerve blockade and not effective at all for obturator nerve blockade [6-9]. The concept of three-in-one block is now defunct and it is necessary to separately block individual nerves to consistently provide complete anesthesia for procedures on the knee joint or involving the medial aspect of the thigh [10-13].

In half of the cases, the cutaneous contribution of the obturator nerve is missing or overlapped by the femoral, posterior cutaneous, or sciatic nerve; in half of the cases, hypoesthesia is either in the medial part of the knee or in the inner part of the popliteal fossa. For the vast majority of patients, the femoral nerve supplies the medial cutaneous aspect of the thigh. The cutaneous distribution of the obturator nerve must not been assessed on the medial aspect of the thigh and the only way to effectively evaluate obturator nerve function is to assess the adductor strength [10-13].

Some practitioners are reluctant to perform an ONB using the classical pubic approach described by Labat [14] witch is reputed to be difficult. Moore said that the block could be missed, even in the most expert hands [15]. The success rate is now between 89 and 100 % with the aid of the nerve stimulator [11,16]. With the pubic approach, 90% of patients reported moderate to severe discomfort and patient satisfaction is low [17]. This could be due, in part, to the puncture site at the mons pubis close to the genitals and because of painful bone contact for a great number of patients.

An inguinal approach for ONB offers various advantages. In clinical experience, this technique is easy, successful, less painful and more comfortable than the pubic approach [18].The inguinal approach is more acceptable to the patient because palpation and penetration of the mons pubis is avoided. Delineating the groove between the vascular bundle and adductor muscles, drawing only in the inner part of the thigh and introducing the needle at the level of the inguinal crease probably improve patient acceptance.

Anatomy

The medial (adductor) compartment of the thigh consists of the adductor longus, brevis, magnus, and the gracilis muscles, mainly innervated by the obturator nerve.

The obturator nerve, which originates from the ventral rami of L2, L3, L4, is the most anterior and medial branch of the lumbar plexus. It emerges from posterior and medial border of the psoas muscle, piercing the iliaca fascia between L5 and S1. It runs on the lateral side of the pelvis over the obturator muscle. It crosses the obturator foramen with the obturator vessels and gives two divisions, anterior and posterior, for the medial side of the thigh.

The posterior branch runs over adductor magnus and behind the adductor brevis, and contributes to the articular capsule of the knee.

The anterior branch runs in front of the adductor brevis muscle and behind the adductor longus, and contributes to the cutaneous posterior and / or medial aspect of the knee.

Indications

Obturator nerve block at the pubic level combined with spinal or general anesthesia is indicated in transurethral resection (TUR) of tumors located on the lateral and inferolateral bladder. Direct electrical stimulation of the obturator nerve by the surgical resectoscope by way of contractions of the thigh adductor musculature and sudden movements of the leg pose a potential for bladder wall perforation, bleeding, incomplete resection, and/or dissemination of the tumor.

The block is also used for the diagnosis and treatment of adductor muscle spasticity and in chronic pain conditions (hip pain, pelvic tumor). For these indications, the inguinal obturator block has not been evaluated.

Obturator nerve block improves the quality of anesthesia during knee surgery under peripheral nerve block [19,20] and postoperative analgesia after total knee arthroplasty [21,22].

Inguinal obturator block

Anterior and posterior branches of the obturator nerve are selectively blocked at the inguinal level.

Sedation

All patients do not require sedation if only an inguinal ONB is performed. Patients who are anxious or who demonstrate needle phobia, trauma patients in whom multi-nerve stimulation may be particularly painful should clearly benefit from sedation.

Positioning of the patient

The patient is placed supine and legs slightly abducted as for a femoral block.

Equipment

• a nerve stimulator;
• a 50 mm short bevel needle at the inguinal fold, 70 mm at the pubis or in overweight patients;
• a 10 ml syringe of local anesthetic.

Cutaneous landmarks and puncture site

The patient is first asked to flex his/her hip and a line marked the inguinal crease.

The adductor longus tendon is identified as the most superficial palpable tendon in the medial part of the thigh.

A mark on the skin is made in the inguinal crease at the midpoint of the line drawn between the inner border of the adductor longus tendon and the femoral arterial pulse. This point corresponds to the center of an easily palpable groove between the vascular bundle and the adductor longus muscle.

The stimulation is begun using a current of 2 mA for 0.1 ms at 1 Hz. The needle is inserted in a 30° cephalad direction until contractions of the gracilis and/or adductor longus muscle is elicited.

The adductor longus twitch (anterior branch) is observed at the anterior part of the inner thigh. Weak contraction of the gracilis, which frequently accompanies the former, forms a narrow muscular band down to the medial part of the knee. The current is gradually decreased until the muscle twitch stopped between 0.2 and 0.7 mA. At that time, 5 mL (0.07 ml.kg-1) of LA is injected (anterior branch of the obturator nerve).

The needle is inserted deeper and in a 5° lateral direction until contractions of the adductor magnus muscle are elicited.

The strongest adductor magnus (posterior branch) twitch appears at the posterior part of the inner thigh and produces a noticeable hip adduction. In the same manner, 5 mL (0.07 ml.kg-1) of LA is injected (posterior branch of the obturator nerve).

Tips

The needle direction from its point of entry is mainly perpendicular to the plane of the anterior and posterior branches which spread out and diverge downward on leaving the obturator canal. Thus, needle access to the branches is highly likely at the first attempt.

When the nerve is not located after some attempts, the current charge may be increased, then the insertion point is moved 1 cm more laterally, then 1 cm more medially if necessary.

A 50-mm needle seems adequate for the majority of subjects except for those who are overweight.

One may easily differentiate one branch from the other, even when they are still on the same plane.

A bone contact in the inguinal approach indicates that the needle is too deep and touches the ischiopubic ramus.

The main limitation is failure to reach obturator branches contributing to hip joint innervation which arises frequently prior to entry of the nerve into the thigh.

The total dose must be taken into consideration in the case of combined blocks.

The patient is placed supine and legs slightly abducted and externally rotated.

The pubic tubercle (pubic spine) and the inguinal ligament are palpated and a line is drawn between the tubercle and the anterior-superior iliac spine.

Stimulation is begun using a current of 2 mA for 0.1 ms at 1 Hz.

The needle is inserted posteriorly and 20° laterally, 2 cm caudal and 2 cm lateral to the pubic tubercle.

Intentional bone contact and sliding off the inferior border of the superior pubic ramus is not necessary with the aid of the nerve stimulator.

The current is gradually decreased until the muscle twitch stopped between 0.2 and 0.7 mA. At that time, 10 mL of LA is injected for an adult patient. A volume of 0.3 ml.kg-1 is suggested for children.

One may be unable to demonstrate cutaneous loss of sensation following ONB.

The only way to effectively evaluate obturator nerve function is to assess the adductor strength asking the patient to squeeze his (her) knees.

With a complete combined femoral, sciatic and obturator nerve block, the patient only adduct the contralateral leg toward the midline.

Complications and contraindications

No complications directly related to an obturator block have been reported in the literature. Some specific complications are likely with the traditional approach [23-26]. The needle may pass above the pubic ramus penetrating the pelvic cavity, particularly when identifying the pubic spine is difficult. Care must be taken not to advance the needle too far and to damage surrounding structures (e.g., bladder, rectum, spermatic cord). This approach is performed in a highly vascularized region (obturator vessels, circumflex arteries and veins). Intravascular injection and hematoma may occur. The vascular connections between the obturator and external iliac systems behind the pubic ramus (i.e., corona mortis) can be life-threatening in case of injury [27].

The puncture at a distance from the pelvis and large vessels in the inguinal approach minimize the risk of complications and allows compression in the event of a hematoma.

These blocks should be avoided in cases of entrapment neuropathy or compression [28].

Conclusion

Inguinal obturator nerve block is easy, efficacious and safe [29]. An ONB is mandatory for adequate anesthesia when surgery involves the medial thigh or the knee.