Chapter 34. Obturator Nerve Block

Selective obturator nerve block was first described by Labat in 1922.1 More interest in obturator nerve block emerged a few years later when Pauchet, Sourdat, and Labat stated, “obturator nerve block combined with blocks of the sciatic, femoro-cutaneous nerves, anesthetized the entire lower limb.” However, a lack of clear anatomic landmarks, the block complexity, and inconsistent results were the reasons why this block had been used infrequently. Historically, Labat's classical technique remained forgotten until 1967, when it was modified by Parks.2 In 1993, the interadductor approach was described by Wassef,3 which was further modified by Pinnock in 1996.4 In 1973, Winnie introduced the concept of the “3-in-1 block,” an anterior approach to the lumbar plexus using a simple paravascular inguinal injection to anesthetize the femoral, lateral cutaneous nerve of the thigh and obturator nerves.5 Since its description however, many studies have refuted the ability of the 3-in-1 block to reliably block the obturator nerve with this technique. However, with the introduction of modern nerve stimulators, selective blockade of the obturator nerve has become more reliable and has seen a resurgence of interest in recent times.

Indications

Obturator nerve block is used to treat hip joint pain and is used in the relief of adductor muscle spasm associated with hemiplegia or paraplegia. Muscle spasticity is a relatively common problem among patients suffering from central neurologic problems, such as cerebrovascular pathology, medullar injuries, multiple sclerosis, and infantile cerebral palsy. Spasticity of the adductor muscle induced via the obturator nerve plays a major role in associated pain problems and makes patient grooming and mobilization very difficult. Obturator block, tenotomies, cryotherapy, botulin toxin infiltration, surgical neurolysis, and muscle interpositions all have been suggested to remedy this problem.6–9 A number of diagnostic or therapeutic procedures on the knee and thigh can be performed by combining obturator nerve block with block of the sciatic, lateral cutaneous nerve and femoral nerves. Common clinical practice is to combine a sciatic nerve with the femoral nerve block for surgical procedures distal to the proximal third of the thigh. When deemed necessary, addition of a selective obturator nerve block may reduce intraoperative discomfort, improve tourniquet tolerance, and improve the quality of postoperative analgesia in these cases.

Obturator nerve block is also occasionally used in urologic surgery to suppress the obturator reflex during transurethral resection of the lateral bladder wall. Direct stimulation of the obturator nerve by the resector as it passes in close proximity to the bladder wall results in a sudden, violent adductor muscle spasm. This is not only distracting to the surgeon, but also potentially dangerous, increasing the risk of serious complications such as bladder wall perforation, vessel laceration, incomplete tumor resection, and obturator hematomas.10,11 Prevention strategies include muscle relaxation, reduction in the intensity of the resector, the use of laser resectors, shifting to saline irrigation, peri-prostate infiltrations, and/or endoscopic transparietal blocks.12–16 However, a selective obturator nerve block remains the safest and most effective alternative to this problem.17–22

Neurolytic blockades with alcohol or phenol, performed with the help of a nerve stimulator and/or radioscopy, result in a cost-effective and effective reduction of muscle spasms.3,9,23–26 The main drawback to neurolytic blockade is its temporal duration and the need to repeat the blockade when the previous block wears off. Selective obturator nerve block has also been used in the diagnosis and treatment of chronic pain states secondary to knee arthrosis or pelvic tumors resistant to conventional analgesic approaches.27–31

Contraindications

Patient refusal, inguinal lymphadenopathy, perineal infection, or hematoma at the needle insertion site are typical contraindications to obturator nerve blockade.

Preexisting obturator neuropathy, clinically manifested by groin pain, pain of the posteromedial aspect at the thigh and occasionally paresis of the adductor group of muscles, are relative contraindications to this block. Obturator nerve blocks should be avoided in the presence of a coagulopathy.

Anatomy

The obturator nerve is a mixed nerve, which, in most cases, provides motor function to the adductor muscles and cutaneous sensation to a small area behind the knee. It is derived from the anterior primary rami of L2, L3 and L4 (Figure 34–1). On its initial course, it runs within the psoas major muscle. Taking a vertical course, it emerges from the inner border of the psoas, staying medial and posterior at the pelvis until it crosses at the level of the sacroiliac joint (L5) under the common iliac artery and vein and runs anterior/lateral to the ureter (Figure 34–2). At this level, it courses close to the wall of the bladder on its inferior/lateral portion and then it takes place anterior to the obturator vessels within the superior part of the obturator foramen, exiting the pelvis below the pubic superior branch. In its intrapelvic course, the obturator nerve is separated from the femoral nerve by the iliopsoas muscle and iliac fascia. It innervates the parietal peritoneum on the lateral pelvic wall and contributes collateral branches to the obturator externus muscle and the hip joint. It leaves the pelvis by passing through the obturator canal before entering the adductor region of the thigh (Figure 34–3). Here, 2.5–3.5 cm after leaving the obturator foramen, the obturator nerve divides into its two terminal branches, anterior and posterior, providing innervation to the hip adductor compartment (see Figure 34–3).32

The anterior branch descends behind the pectineus and adductor longus and in front of the obturator externus and adductor brevis. It gives muscular branches to the adductor longus, adductor brevis, gracilis, and occasionally the pectineus, and it terminates as a small nerve that innervates the femoral artery (Figure 34–4). In 20% of subjects, it contributes a branch, which anastamoses with branches of the femoral nerve and forms the subsartorial plexus, from which sensory branches emerge to supply sensation to posteromedial aspect of the inferior third of the thigh. The anterior branch contributes articular branches to anteromedial aspect of the hip joint capsule (Figure 34–5) but does not innervate the knee joint.

The posterior branch descends between the adductor brevis in front and the adductor magnus behind. It terminates by passing through the adductor hiatus to enter the popliteal fossa, supplying the posterior aspect of the knee joint and the popliteal artery. During its course, the posterior branch sends muscular branches to the obturator externus, the adductor magnus, and occasionally the adductor brevis muscles (see Figure 34–4).

Cutaneous innervation by the obturator nerve varies according to the authors and is illustrated in Figure 34–6.

Anatomic Variants

Numerous variations to the formation, course, and distribution of the obturator nerve can have clinical implications. For instance, in 75% of cases, the obturator nerve divides into its two terminal branches as it passes through the obturator canal. In 10% of cases, this division occurs before the nerve reaches the obturator canal; in the remaining 15% of cases, after entering the thigh.

Occasionally, the anterior and posterior branches descend through the thigh behind the adductor brevis. Note that the sensory cutaneous branch of the obturator nerve is often absent.

Up to 20% of subjects possess an accessory obturator nerve that can be formed from variable combinations of the anterior rami L2-L4 or emanate directly from the trunk of the obturator nerve.33 It accompanies the obturator nerve as it emerges from the medial border of the psoas but unlike the obturator, passes in front of the superior pubic ramus to supply a muscular branch, the pectineus. It contributes articular branches to the hip joint and terminates by anastomosing with the obturator nerve itself.

Equipment

To perform a block, the following equipment is required:

• Nerve stimulator
• Insulated stimulating needle (5–8 cm, depending on the approach chosen)
• Local anesthetic: 1% mepivacaine (onset of motor block 15 minutes, duration 3–4 hours) or 0.75% ropivacaine (onset of motor block 25 minutes, block duration 8–10 hours)
• Sterile fenestrated drape
• Marking pen
• Ruler
• A 10-mL syringe
• Disinfectant
• Sterile gloves

Landmarks

Anatomic landmarks vary depending on the chosen approach. The following landmarks are useful regardless of the approach chosen (Figure 34–7):

• Bony landmarks: Anterior and superior iliac spine and pubic tubercle, inguinal ligament
• Vascular landmarks: Femoral artery, femoral crease
• Muscular landmarks: Tendon of the long abductor muscle

Techniques

Several methods can accomplish block of the obturator nerve. These approaches can be grouped into plexus block techniques where the obturator nerve is blocked along with other components of the lumbosacral plexus and specific single-nerve block techniques for the obturator nerve.

Plexus Block Techniques

Several approaches have been described; however, the lumbar plexus block via the posterior approach (in the psoas compartment) is the only technique that ensures an acceptable success rate of obturator nerve blockade.

3‐in-1 Block Technique

Based on the theoretical existence of a suprainguinal compartment, in 1973 Winnie described the lumbar plexus block by an anterior approach or the 3 in 1.5 According to the 3-in-1 concept, a large volume of local anesthetics is injected over the femoral nerve to spread underneath the fascia iliaca. When combined with distal compression, the local anesthetic spreads proximally reaching the lumbar plexus. Unfortunately, several studies have repeatedly failed to demonstrate the reliability of this technique to obtain a block of the lumbar plexus or the obturator nerve.5,34–36 In addition, studies in human cadavers have documented the absence of a fluid-conducting compartment that would allow such an extensive proximal spread of the local anesthetic.37 For these reasons, the 3-in-1 technique results in anesthesia of the cutaneous branches of the obturator nerve. This is because the local anesthetic spread laterally or medially, rather than proximally, as once thought. Of note, increasing the volume of injectate does not increase the spread toward the lumbar plexus; no differences were found when local anesthetic injection volumes of 20 or 40 mL were compared.38 Theoretically, catheters inserted by an inguinal approach can ascend toward the psoas compartment, however, only a minor percentage (23%) of catheters can be reliably positioned.39,40

Iliofascial Block Technique

Dalens first described this approach in 1989 for use in pediatric patients.41 Following Winnie's reasoning for the 3-in-1 block, he took a more lateral approach and reported a 100% success rate for femoral and femorocutaneous nerve blockade and 88% success rate for the obturator nerve. However, follow-up studies in adults did not confirm these results.39,42 In adults, the iliofascial approach allows more successful blockade of the lateral femorocutaneous nerve when compared with the 3-in-1 technique. However, the obturator nerve remains spared.42,43

Psoas Compartment Block

Since Winnie's description of the posterior approach to the lumbar plexus in 1974 (psoas compartment block), numerous modifications of the technique have been described.43,44–47 The obvious advantage is the ability to obtain a complete lumbar plexus block with a single injection. Indeed, studies have demonstrated femoral nerve block close to 100% plexus block with this technique, whereas femorocutaneous and obturator nerve blocks are anesthetized 88–93% of the time.48,49

Parasacral Sciatic Block

Mansour initially described this technique in 1993 with the objective of achieving a more complete sciatic nerve block.50,51 Since this technique is a true plexus block, it provides more consistent anesthesia of all branches of the sciatic nerve. It successfully blocks the posterior cutaneous nerve of the thigh, the gluteal superior and inferior nerves, and the pudendal nerve. In addition, the splanchnic nerves, the inferior hypogastric plexus, the proximal portion of sympathetic trunks, and the obturator nerve are located close to the point of injection. Thus, a blockade of all these nervous structures would be theoretically achievable with a single injection. However, recent anatomic and clinical studies suggest that the parietal peritoneum and the pelvic fascia surrounding the sacral plexus are anatomically separated from the obturator nerve that runs along the medial border of the psoas. Consequently, although the parasacral approach to sciatic nerve block should result in a complete block of the sacral plexus, the obturator nerve may be spared.52

Selective Block Techniques

Labat's Classic Technique

Labat's classic approach was the most popular technique before the development of new approaches that are more easy to perform and less uncomfortable to patients. Originally described as a paresthesia method, the advent of nerve stimulation has increased the effectiveness and reduced patient discomfort, complications, and number of needle insertions. The procedure sequence consists of five phases, depicted in Figure 34–8.

Nerve stimulation is begun using a current intensity of 2–3 mA (2 Hz, 0.1–0.3 msec) and reduced to 0.3–0.5 mA before injection of local anesthetic. The patient lies supine, with the limb to be blocked at 30 degrees abduction. The pubic tubercle is identified by palpation, and a 1.5-cm long line is drawn laterally and caudally; the injection insertion site is labeled at the tip of the end of the caudal line (Figure 34–9). The classical approach consists of carrying out three consecutive movements of the needle until the tip of the needle is placed over the top of the obturator foramen, where the nerve runs before splitting into its two terminal branches. With a 22-gauge, 8-cm long needle, the skin is penetrated perpendicularly and the needle is advanced until it makes contact with the inferior border of the superior pubic branch at a depth of 2–4 cm. During the second phase, the needle is slightly withdrawn and then slipped along the anterior pubic wall (another 2–4 cm). After this, it is redirected anteriorly/posteriorly. Finally, the needle is withdrawn again and slightly redirected (cephalically and laterally) at an angle of 45 degrees for another 2–3 cm until contractions of the thigh adductor muscles are observed.

This technique can be simplified by eliminating the second movement of the needle. Hence, after making contact with the pubic branch, the needle can be redirected 45 degrees laterally to the obturator foramen (see Figure 34–9).

Paravascular Selective Inguinal Block

This technique consists of a selective block of the two branches of the obturator nerve (anterior and posterior), performed at the inguinal level and slightly more caudad than the previously described techniques.53 The femoral artery and the tendon of the long adductor muscle at the pubic tubercle are identified. For tendon identification, extreme leg abduction is required (Figure 34–10). A line is drawn over the inguinal fold from the pulse of the femoral artery to the tendon of the long adductor muscle. The needle is inserted at the mid-point of this line at an angle of 30 degrees anteriorly/posteriorly and cephalically (Figure 34–11). By following the needle a few centimeters in depth via the long adductor muscle, twitching responses from the long adductor and gracilis muscles are easily detectable on the posterior and medial aspect of the thigh. Subsequently, the needle is inserted deeper (0.5–1.5 cm) and slightly laterally over the short adductor muscle until a response from the major adductor muscle is obtained and can be visualized on the posterior-medial aspect of the thigh. After needle insertion, infiltration of 5–7 mL local anesthetic is recommended. Occasionally, a more caudal division of the obturator nerve is found; hence, the two branches are located within the same location at the inguinal fold, and two different motor responses may be observed with a single stimulation (injection).

Choice of Local Anesthetic

Ten to 15 mL of local anesthetic are adequate in patients. The type and concentration of the local anesthetic depend on the indication for the block. For diagnostic–therapeutic blockades, highly concentrated neurolytic solutions are used to achieve long-lasting blocks. In the literature, combinations of phenol, ethanol, bupivacaine, levobupivacaine, and/or steroids are well reported.3,9,23–26

For lower limb surgeries, the recommended anesthetic technique consists of the administration of medium- to long-lasting local anesthetics that are associated with adequate postoperative analgesia, such as bupivacaine 0.25–0.5%; ropivacaine 0.25–0.75%, and levobupivacaine 0.25–0.5%. To avoid adductor muscle spasms during transurethral surgery, the use of medium to long local anesthetics is not required because the surgery does not last more than 2 hours. Therefore, mepivacaine 1–2% or lidocaine 1–2% should be adequate for this purpose.17,54

The onset of motor blockade is seen approximately 15 minutes after administration of 1% mepivacaine and 25 minutes after injection of 0.75% ropivacaine. Evaluation of an obturator block by sensory testing is unreliable due to the variability in its sensory distribution (see Figure 34–10). In some cases, the obturator nerve may not contain any sensory branches that can be clinically tested for adequacy of the blockade. In addition, even when a sensory branch is present, there is considerable overlap of cutaneous innervation from the obturator, femoral, and sciatic nerves. It is often erroneously thought that the skin of the medial aspect of the thigh is innervated by the obturator nerve; in fact, sensory branches of the femoral nerve contribute sensory innervation to this region.

The area of skin most commonly regarded as having exclusive obturator nerve supply is a small region located on the posteromedial aspect of the knee. Also, the strength of the lower limb adductors relies 70% on the obturator nerve. Consequently, reduction in the strength of the adductors of the thigh is the most reliable sign of successful obturator nerve blockade. Adductor muscle strength can be objectively evaluated by comparing the maximal pressure exerted by the patient squeezing a sphygmomanometer that has been preinflated to 40 mm Hg and placed between the legs before and after block performance. Failure to demonstrate a reduction in adductor muscle strength from baseline is synonymous with block failure.

Patients must be warned that ambulation may be impaired becauseof the blockade of the thigh adductors.

There are no reports of complications associated with obturator nerve block. The lack of reported complications, however, is more likely due to the infrequent use of this block rather than to its inherent safety. Needle orientation for the classical pubic approach of Labat is toward the pelvic cavity. Therefore, if advanced too far in a cephalad direction, the needle can pass over the superior pubic ramus and penetrate the pelvic cavity, perforating the bladder, rectum, and spermatic cord. Accidental puncture of the obturator vessels could result in unintentional intravasacular injection and hematoma formation. A retropubic anastamosis between the external iliac and obturator arteries (corona Mortis) is present in up to 10% of patients: bleeding secondary to puncture of the corona Mortis can be difficult to control. Obturator neuropathy, secondary to needle trauma, intraneural injection, nerve ischemia, or local anaesthetic toxicity are also possible, as with other peripheral nerve block techniques.