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TECHNOLOGY, COMPUTING, AND SIMULATION: Research Report

An Evaluation of the Cutaneous Distribution After Obturator Nerve Block

Bouaziz, Hervé MD, PhD*,; Vial, Florence MD*,; Jochum, Denis MD†,; Macalou, Dioukamaly MD*,; Heck, Michel MD*,; Meuret, Pascal MD*,; Braun, Marc MD, PhD‡,; Laxenaire, Marie-Claire MD*

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doi: 10.1213/00000539-200202000-00041
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Abstract

The obturator nerve has two divisions. Both carry motor fibers to the adductor muscles of the thigh, but only the anterior division provides any cutaneous innervation. The posterior division is important in that it contributes to innervation of the knee joint. The obturator nerve supplies the obturator externus and the adductor muscles of the thigh, has branches extending to the hip and knee joints, and, depending on the anatomical textbook consulted, has a variable cutaneous distribution to the medial aspect of the thigh and leg (Fig. 1) (1–3). When present, the cutaneous branch unites with branches of the saphenous and medial femoral cutaneous nerves to form the subsartorial plexus and contributes to the innervation of the skin over the distal two-thirds of the medial side of the thigh (1).

Figure 1
Figure 1:
The cutaneous distribution of the obturator nerve according to three classic anatomy textbooks.

Because of variability in the cutaneous distribution of the obturator nerve, several authors have stated that the only reliable indication of a successful obturator nerve block is paralysis of the adductor muscles of the thigh (4–6). According to this criterion, the three-in-one block described by Winnie et al. (7) was effective for lateral femoral cutaneous (LFC) and femoral (F) nerve blockade but often missed the obturator nerve.

This study was undertaken to evaluate the area of sensory loss produced by direct injection of local anesthetic around the obturator nerve.

Methods

After IRB approval, 30 patients presenting for total knee arthroplasty (TKA) and knee arthroscopy consented to be included in this study. Patients with conditions precluding obturator and three-in-one blocks (local infection, coagulopathy, or a history of central or peripheral nervous system disorders) were excluded. Patients were not premedicated and received no sedation. The technique used to block the obturator nerve was performed as follows: with the patient supine and legs slightly abducted, a 50-mm insulated needle (22-gauge Stimuplex™; B/Braun, Melsungen, Germany) was inserted almost perpendicularly to the skin, 2 cm caudal and 2 cm lateral to the pubic tubercle. The needle was advanced until it contacted the inferior border of the superior pubic ramus bone before it was redirected posteriorly and slightly laterally to walk off the inferior margin of the superior pubic ramus. The current was gradually decreased until the adductor muscle twitched at ≤0.5 mA of 0.1 ms at 2 Hz. At that time, 7 mL of 0.75% ropivacaine was injected.

Patients were assessed by the same investigator for sensory deficits and muscle weakness in the adductor muscles over 30 min. The area of blockade was assessed at 5-min intervals by loss of cold sensation with a swab soaked in ethyl (immediately applied to avoid evaporation) and light touch. The response was scored with the following scale: 0 = no perception, 1 = reduced sensation, and 2 = normal sensation. Results were immediately recorded on diagrams of the lower limb and compared with those obtained on the noninjected side. The assessment of adductor muscle strength was performed with a mercury sphygmomanometer as described by Lang et al. (6). Briefly, the patient was asked to flex his/her hips and knees and then to squeeze a blood pressure cuff previously inflated to 40 mm Hg between his/her knees. The maximum sustained pressure generated was then recorded as an index of adductor strength. This variable was measured both before and after the block. Because of the performance of a selective obturator nerve block, any degree of decrease in muscle strength was considered a positive sign of obturator paresis. Patients with evidence of motor deficit in adductors were considered to have a successful obturator nerve block and were subsequently included in this analysis. Patients were not able to watch the investigator performing the sensory or motor evaluation.

After this initial 30-min evaluation period, a three-in-one nerve block was performed as described by Winnie et al. (7). A 50-mm insulated needle was inserted just lateral to the fingertip palpating the lateral edge of the F artery. The needle was advanced cephalad in a sagittal plane at a 30° angle to the skin until an appropriate evoked motor response (the dancing patella sign) was elicited and still observed at ≤0.5 mA of 0.1 ms at 2 Hz. Then, 20 mL of 0.75% ropivacaine was injected over a 2-min period. During the injection, firm pressure was manually applied just distally to the puncture site to encourage the cephalad spread of the local anesthetic. The extent of the sensory and motor block was evaluated 10 min later on the anterior, lateral, and medial part of the thigh by using both cold and light-touch tests once the patient was on the operating table.

The second part of this study consisted of a magnetic resonance (MR) study performed on eight consenting volunteers and an anatomical study on five fresh cadavers. Three orthogonal planes and an oblique plane were imaged by MR to define the precise injection level in relation to the obturator nerve division. The obturator nerve course was imaged before, within, and after the obturator canal. The scanning variables were the following: 2-mm continuous slices covering the entire obturator nerve course, in SpinEcho, TE 13 ms, TR 500 ms, flip angle 90°, voxel size 0.5 × 0.6 × 2 mm, axial acquisition plane, and multiplanar reformation.

The obturator nerve blockade was simulated on five fresh cadavers by using the same technique as described previously. All experiments were performed on both legs, and 7 mL of blue dye solution was injected. Thereafter, the pelvis was dissected to examine the relation of the blue dye solution to the anterior and posterior branches of the obturator nerve.

Results

This study group comprised 30 patients. The mean age, weight, height and ASA physical status were 58 ± 20 yr, 69 ± 11 kg, 164 ± 10 cm, and II, respectively, with 16 female and 14 male patients scheduled to undergo TKA.

There were no systemic complications, nor were there any manifestations of local anesthetic toxicity. Motor weakness of the adductor muscles was observed in all patients. Adductor strength decreased by 77% ± 17% (mean ± sd) at 30 min. There was great variation in the area of sensory loss (Fig. 2). For 17 patients (57%), the cutaneous contribution of the obturator nerve was absent. Seven patients (23%) had a zone of hypoesthesia (cold sensation was blunt but still present, with no change in the perception of light touch) on the superior part of the popliteal fossa, and the remaining six patients (20%) had a sensory deficit located on the inferior part of the medial aspect of the thigh. The area of hypoesthesia was rectangular, with a surface area of approximately 40 cm2. The three-in-one nerve block was successful in all patients, as measured by the presence of cutaneous anesthesia to cold and light touch in the anteromedial aspect of the thigh and paralysis of the quadriceps muscle. The lateral aspect of the thigh was anesthetized (cold and light-touch tests) in 26 (87%) of 30 patients. MR imaging in eight volunteers demonstrated that at the level where the local anesthetic solution would be administered, the obturator nerve had already divided into its two branches in 100% of cases (Fig. 3). Injection of 7 mL of the dye into five cadavers resulted in spread of the injectate to both branches of the obturator in 9 of 10 blocks (Fig. 4). One side was injected outside the area of interest, with neither branch being impregnated with the dye.

Figure 2
Figure 2:
The cutaneous distribution of the obturator nerve according to findings of this study.
Figure 3
Figure 3:
Magnetic resonance image showing that at the level of the obturator canal, the two branches of the obturator nerve are already separated.
Figure 4
Figure 4:
Anatomy of the left obturator nerve where it exits the obturator foramen. The distribution of the dye clearly affects both the anterior and posterior branches of the obturator nerve. A, A’ = level of the magnetic resonance image.

Discussion

According to the definition chosen for a successful obturator nerve block (adductor muscle weakness), evidence of anesthesia of this nerve (77% ± 17% decreased strength) was demonstrated in all patients. The importance of the obturator nerve in the innervation of the adductor muscle is well known. Von Lanz and Wachsmuth (3) found that the obturator nerve was responsible for 56% of the loss of adduction strength, whereas the sacral plexus and the F nerve were responsible for 34% and 10%, respectively. We have confirmed the major role played by the obturator nerve in the adduction of the lower limbs.

In 1973, Winnie et al. (7) described a technique called three-in-one block that provided anesthesia of three nerves (F, LFC, and obturator nervous) in a single injection, providing that a volume of 20 mL or more of local anesthetics was injected. Unfortunately, the methodology used in this study to assess the extent of the block was not mentioned. In 1989, Parkinson et al. (4) stated that the only way to effectively evaluate the obturator nerve function was to assess the adductor’s strength. When blockade was assessed by testing motor function, the inguinal paravascular block nerve resulted in 5% to 10% of patients with paresis in hip adduction at 30 min, depending on the technique used (nerve stimulation or paresthesia). It was concluded that this block was effective for anesthetizing the F and LFC nerve, but not the obturator nerve. By using the same criteria, Lang et al. (6) demonstrated that the obturator nerve was successfully blocked in only 1 (4%) of 26 patients. Similar results were published in a study in which the effectiveness of obturator nerve motor block was assessed by recording the action potentials generated at the thigh muscles (5). Clinical data showing that the three-in-one block usually spares the obturator nerve are also supported by radiological studies. These studies demonstrated either unexpected lateral or medial distribution (8), a very rare spread of the solution to the lumbar plexus (9,10).

Unfortunately, in the previously mentioned clinical studies, either the cutaneous distribution of the obturator nerve was not assessed when obturator block was performed or the performance of the inguinal paravascular block made the assessment of cutaneous distribution difficult to interpret (4–6). Indeed, as mentioned by several authors, the cutaneous innervation of the obturator nerve to the medial aspect of the knee is highly variable and sometime missing (4,11,12).

The absence of a cutaneous contribution of the obturator nerve was found in 57% of our patients, and the remaining patients described vague sensory changes either in the medial part of the thigh or in the upper part of the popliteal fossa. This result is in agreement with others showing that patients who underwent a parasacral sciatic nerve block displayed evidence of anesthesia of the obturator nerve (defined as evidence of marked adductor motor weakness) without evidence of a clear cutaneous contribution (13). However, in the latter study, the obturator nerve block was difficult to ascertain because the sacral plexus also contributes to the innervation of the adductor muscles. The fact that the sensation was blunted but still present for 43% of patients is probably related to the presence of cutaneous territories innervated by several nerves. These territories were either innervated by the obturator and the F nerve forming a subsartorial plexus or by the obturator nerve and the posterior cutaneous of the thigh (a branch of the sacral plexus), as has already been shown for the deep innervation of the posterior knee capsule (14).

The medial cutaneous aspect of the thigh was supplied by the F nerve in 100% of our patients. This result is important because the inability to block the obturator nerve after a three-in-one block would explain clinical observations published as early as 1912, in which patients reported a pain sensation during knee surgery, despite a complete cutaneous sensory block of the lower limb (15). This observation and the demonstration of the role of the obturator nerve for patients undergoing TKA underline the importance of the articular branch of the obturator nerve, even in the absence of cutaneous distribution (16). Our results also explain why Madej et al. (17) observed a loss of sensation on the medial aspect of the thigh on all patients who received a three-on-one block, although there was no evidence of weakness of thigh adduction. The role of the F nerve in the innervation of the medial aspect of the knee has also been supported in cadaver studies. Indeed, in 1994, Horner and Dellon (14) studied the innervation of the human knee joint on 45 fresh cadavers and found that the most superficial constant branch innervating the medial aspect of the knee was the termination of the medial F cutaneous nerve. They also reported that the anterior branch to the obturator nerve contributed significantly to the perigenicular structures in 5 (11%) of 45 dissections, sending cutaneous filament to the inferomedial aspect of the thigh. Consequently, the results published by others have to be reconsidered if the only criterion used to assess obturator nerve block was based on the cutaneous blockade (18–20).

Marhofer et al. (9) traced the distribution of local anesthetics after a three-in-one block by means of MR imaging and stated that both anterior and posterior branches of the obturator nerve had sensory components for the medial part of the thigh and the popliteal fossa, respectively. They referred to a textbook dealing with techniques of neural blockade (21); however, we were not able to find an anatomical textbook in which the possibility of the presence of a cutaneous filament coming from the posterior branch of the obturator nerve was mentioned. Moreover, despite the administration of the local anesthetic solution at a level where both branches of the obturator nerve were already separated, our experiment on cadavers suggests that both the anterior and posterior branches of the obturator nerve had been impregnated by the local anesthetic solution. Therefore, our clinical results cannot be explained by the fact that we failed to anesthetized a branch of the obturator nerve.

The sensory evaluation of the popliteal fossa showed that 23% of our patients exhibited sensory analgesia in this area. Therefore, we deduce that the anterior branch inconstantly sends a sensory branch innervating a cutaneous area located on the popliteal fossa. Some authors have drawn a cutaneous area innervated by the obturator nerve located on the medial and very posterior part of the lower segment of the thigh (2,3). However, we must recognize that the sensory areas drawn by Von Lanz and Wachsmuth (3) were quite similar to those we found, in terms of both shape and location, when there was a cutaneous innervation. Nevertheless, in accordance with Horner and Dellon (14), we would stress the fact that the standard anatomy texts are often inaccurate in the description of the sensory innervation of tissues, because frequent anatomic variation is present in the practice of regional anesthesia.

In conclusion, after three-in-one block, an F nerve block may have been taken for an obturator nerve block in 100% of the cases when the cutaneous distribution of the obturator nerve was assessed on the medial aspect of the thigh. Therefore, the only way to effectively evaluate the obturator nerve function is to assess the adductor strength.

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