Obturator Nerve Palsy due to Fixation of an Acetabular Reinforcement Ring with Transacetabular Screws. A Case Report*

FRICKER, RENATO M. M.D.†; TROEGER, HANS M.D.†; PFEIFFER, KARL M. M.D.†, BASEL, SWITZERLAND

Journal of Bone & Joint Surgery - American Volume:
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†Division of Hand and Peripheral Nerve Surgery, Department of Surgery, University Clinic of Basel, Kantonsspital, CH-4031 Basel, Switzerland.

Article Outline

Clinically evident disturbance of the obturator nerve following total hip replacement is rare, although one prospective study demonstrated subclinical electromyographic abnormalities in thirteen of thirty extremities9. Anatomical studies, however, have revealed that there is a risk of serious injury of the obturator nerve if fixation screws in the anterior or central region of the acetabulum perforate the medial wall of the acetabulum4,8. We report the case of a patient in whom a symptomatic lesion of the obturator nerve developed after fixation of an acetabular reinforcement ring with screws during a total hip arthroplasty.

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Case Report

A sixty-three-year-old woman who had osteoarthrosis of the left hip joint was managed with total hip replacement with use of an acetabular reinforcement ring that was made of titanium. Postoperatively, mobilization of the patient was hampered by pain in the medial aspect of the thigh and weakness of the adductors of the hip; these symptoms had not been present preoperatively, and transient disturbance of the obturator nerve was suspected. One year after the operation, a neurological examination revealed atrophy and paresis of the adductor muscles (strength, grade 2 of 5), absence of the adductor reflex, and hypoesthesia in the medial aspect of the distal part of the left thigh. Electromyography demonstrated normal findings in the rectus femoris and iliopsoas muscles but partial denervation of the adductor magnus muscle; these results were consistent with a lesion of the obturator nerve. The patient was first seen in our clinic fourteen months after the operation, at which time the neurological symptoms were identical to those that had been noted at one year; the patient still used crutches for walking outdoors. Radiographs made at that time showed that two of the four screws that had been used for fixation of the acetabular reinforcement ring had perforated the medial pelvic wall; a lesion of the obturator nerve was suspected at this site (Fig. 1).

Operative exploration was performed with use of an extraperitoneal approach, which involved a curvilinear skin incision in the left lower quadrant of the abdomen, division of the aponeurosis of the external oblique muscle superior to the inguinal ligament, and splitting of the internal oblique and transverse abdominis muscles. After blunt dissection, the peritoneum was retracted upward and the urinary bladder was retracted medially. This approach permitted the surgeon to explore the intrapelvic course of the obturator nerve without exposing the patient to the risks associated with a transperitoneal approach. The nerve was dissected from the surrounding scar tissue and was found to contain a neuroma-in-continuity that was located approximately three millimeters away from the tip of the inferior perforating screw, on an extrapolated line along the axis of that screw. Intraoperative electrical stimulation both central and peripheral to the lesion failed to produce any contraction of the adductor muscles. Examination of an unstained section of the neuroma with use of loupe magnification showed what appeared to be fibrous tissue. Additional resection of both nerve ends was necessary to demonstrate a normal-appearing fascicular structure; to obtain this finding on the distal side, it was necessary for the nerve to be dissected distal to the obturator foramen after lengthening of the incision to the anterior part of the thigh. The defect was bridged with two twelve-centimeter-long sural-nerve grafts with use of the operating microscope and 9-0 epi-perineural sutures. The grafts did not follow the anatomical course of the obturator nerve through the obturator foramen, but they descended underneath the inguinal ligament. This course was chosen because it facilitated the distal sutures and allowed the grafts to be directed away from the tips of the screws, which were not removed.

Four months later, a positive Tinel sign was present distal to the inguinal ligament; this finding subsequently progressed distally. Two years after the reconstruction of the obturator nerve, the pain in the medial aspect of the thigh had resolved, the hypoesthesia in the distal third of the medial aspect of the thigh was less severe than it had been before grafting, the patient was able to walk without any assistive devices for as long as one hour, the circumference of the thigh had increased by 1.5 centimeters, and the strength of the adductor muscles had improved from grade 2 to grade 4.

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Discussion

The obturator nerve arises from the anterior divisions of the second, third, and fourth lumbar ventral rami, descends in the psoas major muscle, and pierces that muscle at its medial border. It continues downward over the pelvic brim into the lesser pelvis, passes along the pelvic wall, and descends through the obturator foramen. The nerve enters the thigh posterior to the pectineus and adductor longus muscles and divides into anterior and posterior branches, which provide motor function to the adductor muscles as well as sensory fibers to the hip and knee joints and to the skin on the medial aspect of the distal part of the thigh3.

The possibility that the obturator nerve has been damaged should be considered when a patient has pain as well as anesthesia or hypoesthesia in the medial aspect of the thigh, weakness of the adductors, and a gait disturbance. Electromyography may help to confirm the diagnosis. Although clinically evident lesions of the obturator nerve rarely have been reported after total hip arthroplasty, such lesions have occurred as a result of excessive retraction during the operation9 or intrapelvic extrusion of cement through the anchoring holes of the acetabular component6. Anatomical studies have demonstrated that, in addition to the risk of serious hemorrhage due to lesions of the external iliac vein, tributaries of the internal iliac vein, and the obturator vessels, there also is the risk of an injury of the obturator nerve when screws in the anterior and central regions of the acetabulum4 or the anterior-inferior quadrant of the acetabulum8 perforate the inner pelvic cortex. Nevertheless, we are aware of only one report2 in which a lesion of the obturator nerve was thought to have occurred during drilling through the medial wall of the acetabulum; in that study, however, the lesion and its etiology were not verified by operative exploration and no muscle weakness or gait disturbance was noted.

To our knowledge, we are the first to report an intraoperatively confirmed lesion of the obturator nerve that occurred in association with the use of screws for fixation of an acetabular reinforcement ring. Although there is only slight soft-tissue interposition between the obturator nerve and the pelvic brim8, it is unlikely that the injury in our patient was caused by the tip of the screw itself. As the neuroma was situated on an extrapolated line along the axis of the inferior protruding screw, the nerve lesion most likely occurred when the powered drill—after perforating the medial wall of the acetabulum—was advanced farther into the pelvis.

Traumatic transections of the lumbosacral plexus and the peripelvic nerve segments are rare, and reconstruction has been described infrequently5. Intrapelvic lesions of the obturator nerve, including complete operative division, are known to be a complication of pelvic lymphadenectomy during the resection of gynecological and urological tumors. We are aware of the cases of two patients who were managed with primary repair after an inadvertent transection of the obturator nerve was recognized intraoperatively; one patient had no neurological deficit at the time of the most recent examination7, and the outcome for the other patient was not described1. We are not aware of any report on secondary grafting of a lesion of the obturator nerve, but we think that such grafting should be considered if disabling symptoms are not ameliorated by non-operative treatment. Microsurgical reconstruction of the obturator nerve was performed for our patient in an attempt to decrease the persistent pain and to correct the major gait disturbance that was due to paresis of the adductor muscles. Despite the unfavorable conditions of delayed repair, the need for grafting, and the relatively older age of the patient—all of which impair nerve regeneration—the over-all result was satisfactory.

Whether the resolution of symptoms in our patient can be attributed to successful nerve-grafting remains speculative: the improvement in the strength of the adductors may have been due to co-innervation of the adductor magnus by the sciatic nerve (which also may explain the incomplete denervation of that muscle as demonstrated by preoperative electromyography); the improvement in sensibility may have been due to better adaptation resulting from the overlap of the sensory areas of the obturator, femoral, and posterior cutaneous nerves in the distal third of the medial aspect of the thigh; and the resolution of the disabling pain may have been related to the resection of the neuroma at the time of the reconstruction of the obturator nerve6. However, the presence and subsequent progression of a positive Tinel sign distal to the inguinal ligament confirmed axonal growth into the extra-anatomically placed nerve graft. Also, weakness of the adductors, which had been unchanged for more than one year despite physical therapy, improved only after nerve-grafting.

*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. No funds were received in support of this study.

Investigation performed at the Division of Hand and Peripheral Nerve Surgery, Department of Surgery, University Clinic of Basel, Kantonsspital, Basel

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