• Vicente Roqués Escolar, MD
• Xavier Sala-Blanch, MD
Selective Block Techniques
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, femorocutaneous 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-l 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.
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 urologie 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
The obturator reflex is not abolished by spinal anesthesia. It can be suppressed only by a selective obturator nerve block.
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
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.
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.
The functions of the muscles innervated by the obturator nerve are adduction of the thigh and assistance with hip flexion.
The gracilis muscle assists knee flexion, and the obturator externus aids the lateral rotation of the thigh.
Asking the patient to adduct the thigh therefore tests the function of the nerve. The patient should be supine with knees extended. The leg is then adducted against resistance while the examiner supports the contralateral leg.
The paralysis (or block) of the nerve is characterized by a severe weakening of the adduction, although it is not completely lost as the adductor magnus (the most powerful adductor muscle) receives fibers from the sciatic nerve and eventually from the femoral nerve.
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.
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
• A 10-mL syringe
• Sterile gloves