Kristof Racz MD1,2, María Luz Padilla del Rey MD, FIPP, CIPS, EDPM3, and Agnes R. Stogicza MD, FIPP, CIPS, ASRA-PMUC1 1 Saint Magdolna Private Hospital, Budapest, Hungary The obturator nerve (ON) arises from the lumbar plexus and provides both sensory and motor innervation to the thigh. This nerve is clinically relevant in certain pathologic conditions and procedures involving the hip and knee. Moreover, its course in the pelvis and upper thigh makes the ON a relevant anatomic structure in several clinical settings [1] and puts it at risk for entrapment, compression, and damage at many different sites. The ON entrapment, also known as obturator tunnel syndrome, is not commonly diagnosed or considered and can present as groin, pelvic, and/or lower extremity pain [2], weakness with leg adduction and sensory loss in the medial thigh. ON injections have utility in diagnosing and treating a wide variety of pelvic, hip, and groin pains. This chapter discusses ON procedures, possible complications and ways to avoid them. The ON is a mixed nerve related to the motor and sensitive innervation of the medial compartment of the thigh. It arises from the anterior divisions of L2, L3, and L4 ventral rami [1]. The ON has the most medial course of any branch in the lumbar plexus. It enters the lesser pelvis area from the anteromedial face of the iliopsoas muscle; rarely, the entire ON can be positioned posterior to this muscle. The nerve then travels over the pelvic brim at the level of the sacroiliac joint, behind the common iliac vessels, and follows the lateral pelvic wall, close to the wall of the bladder, to enter the obturator foramen through a fibro-osseus obturator tunnel (the obturator canal). This tunnel is formed superiorly by the obturator sulcus of the pubic bone and inferiorly by the internal and external obturator muscles [2]. Once it leaves the pelvis through the obturator canal, it enters the adduction region of the thigh. The ON divides into anterior and posterior branches. The bifurcation of these two branches can occur intrapelvic (23.22%), within the obturator canal (51.78%) or in the medial thigh (25%) [3]. The anterior branch travels anterior to the external obturator muscle and posterior to the pectineal muscle and then distally between the adductor longus and adductor brevis muscles, terminating as a cutaneous branch. The larger posterior branch pierces the external obturator muscle and the adductor magnus and then continues between the adductor brevis and the adductor magnus [2, 4] passing through the adductor hiatus to enter the popliteal fossa where it terminates. According to most anatomic studies, the accessory obturator nerve (AON) is present in 8–12.6% of the cases, while other sources mention higher incidence (29–54%) [5, 6]. AON arises from the L2 and L5 lumbar roots (most commonly L3 and L4), and it may communicate with the femoral nerve [2]. It runs along the medial margin of the psoas and passes over the iliopubic eminence, behind the pectineal muscle, to terminate on and supply the medial-inferomedial aspect of the hip capsule [5]. Its presence may have a negative effect on the success of the hip capsular block [7]. At its origin, the ON passes posterior to the 2nd, 3rd, and 4th lumbar arteries. After entering the pelvis, it travels alongside the external iliac artery, then turning toward the obturator canal, it leaves the bladder from lateral and meets the obturator artery. The obturator nerve crosses the obturator foramen in the superior part, anterior to the obturator vessels. A retropubic arterial or venous anastomosis between the obturator and the external iliac or inferior epigastric vessels (corona mortis or “crown of death”) is present in up to 70% of patients [8]. The ON (together with the AON) provides sensory articular branches, derived from the common ON or its divisions, to the anteromedial section of the hip joint capsule, and also the knee joint [3, 9]. The hip joint capsule receives innervations from the ON, femoral nerve, sciatic nerve, and superior gluteal nerve [10]. The ON anterior division sends an arterial branch to the femoral artery and then it communicates with cutaneous branches of the femoral nerve in the adductor canal (subsartorial plexus) [11] innervating a small area on the medial thigh and just above the medial knee; however, research has shown that, in more than 50% of cases, ONs provide no cutaneous innervation [4]. The ON posterior division, combined with the tibial nerve, gives rise to the popliteal plexus in the popliteal fossa supplying the posterior aspect of the knee and the popliteal vessels. The ON is the only nerve from the lumbar plexus that does not innervate any of the intrapelvic structures [2] except for the parietal peritoneum on the lateral pelvic wall [12]. Broadly, the ON contributes motor innervation to the leg adductor muscles which, in addition to adducting the lower extremity at the hip joint, serves an important role in pelvic stability and gait balance. The anterior branch of the ON innervates the adductor brevis, adductor longus, gracilis and, in rare circumstances, the pectineus muscles. The posterior branch of the ON provides innervation to the adductor magnus, adductor brevis and, occasionally, the adductor longus and obturator externus muscles [4]. The adductor brevis is the only muscle solely innervated by the obturator nerve [2]. The AON gives muscular branches to supply the pectineus muscle. Diagnostic injection may be performed at any level as described below, but proximal to the pathology of the nerve. Needles: 22–25G 2–3.5 inch needle. Medication: 1–3 ml of local anesthetic ± steroid. RFA cannula: 18–22G, 5–10 mm active tip. Cryoablation cannula: 1.3 mm diameter cryo probe (creates a lesion 4–5 × 3 mm) or 2 mm probe (that creates a 6–7 × 4 mm lesion). Once the electrode is confirmed to be located at the target site, a stimulation test must be performed. Motor stimulation (50 Hz) (thigh adduction) is appropriate to localize the nerve when treating spasticity. For pain, on the other hand, sensory testing (50 Hz) is more effective, avoiding strong motor stimulation if repositioning is possible [2]. After appropriate stimulation 80°C, RFA lesioning or cryoablation at -70°C follows for 60–90 seconds. As the ON contains mostly motor fibers, one must consider the functional loss and carefully weigh the risk and benefit of the procedure on an individual basis. For preservation of motor function, PRF treatment or, in case of hip pain, articular nerve lesioning are the treatment of choice. Performed similarly to the ablative procedures. Stimulation once the cannula is in place may increase procedure success. Patient position: Supine with slightly abducted and externally rotated limb. The proximal standard landmark-technique requires the identification of the pubic tubercle. The needle is inserted perpendicularly to the skin 1.5 cm lateral and 1.5 cm caudal from the tubercle. When the needle makes bony contact with the inferior edge of the superior pubic ramus, the needle is withdrawn, and redirected 45° lateral to enter the obturator foramen. For the distal landmark technique, the patient should flex the hip in order to draw a line marking the inguinal crease. The needle is inserted at the midpoint of the adductor longus tendon, the most superficial palpable tendon in the medial thigh and the femoral artery and advanced at a 30° cephalad direction with a peripheral nerve stimulator. Gracilis and adductor contractions indicate the anterior branch is identified; whereas, adductor magnus contractions show the posterior branch has been reached [2].
47
Complications of Obturator Nerve Block
2 Semmelweis University, Budapest, Hungary
3 University Hospital Complex of Cartagena, Cartagena, Region of Murcia, Spain
Introduction
Anatomy
Origin
Path
Neurovascular Relations
Innervation
Sensory
Motor
Indications for Obturator Nerve Block (ONB)
Indications for Obturator Nerve Ablation and Pulsed Radiofrequency Treatment
Contraindications
Technique
Diagnostic Injection
RFA and Cryoablation
PRF Treatment
Landmark-Guided Technique
US-Guided Technique
Related posts:
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
