A thirty-year-old woman was involved in a motor-vehicle accident in which an ambulance struck the automobile on the driver's side; the patient was sitting in the front passenger seat and was wearing a seat belt at the time of impact. The force of the collision caused the pelvis to strike the console between the two front seats. The patient was admitted to a community hospital and was transferred to the trauma center at Inova Fairfax Hospital three days later. She reported pain in the left sacral and right pubic areas and numbness without pain or weakness in the left foot and leg. She also had neck pain and pain in the left forearm.
Physical examination revealed tenderness over the left sacrum and the right pubis as well as decreased sensation on the plantar aspect of the left foot. Radiographs showed a disruption of the pelvic ring with an impacted fracture of the left sacral ala in the region of the foramina (zone II of Denis et al.); a minimally displaced fracture of the left pubis; and a displaced, overriding fracture of the right pubis and ischium with a non-displaced extension into the hip joint (Fig. 1). There was internal and anterior rotation of the left hemipelvis with pivoting at the site of the zone-II fracture4 of the sacral ala. The fracture was classified as a lateral compression type-1 injury according to the system of Young and Burgess. Judet radiographs confirmed that the fracture that extended into the right hip joint was not displaced and did not involve the weight-bearing dome. Computed tomography demonstrated an impacted fracture of the left sacral ala. The patient also had a displaced fracture of the distal part of the left radial shaft.
The disruption of the pelvic ring was judged to be stable. The patient was allowed out of bed but was advised not to bear weight on the left lower extremity. She needed to use a wheelchair because of difficulty in bearing weight on the right lower extremity due to pain in the groin caused by the right pubic and ischial fractures. The fracture of the radius was treated with open reduction and internal fixation four days after the injury. Sixteen days after the injury, the patient was transferred to a rehabilitation center for four weeks.
An electromyogram of the left lower extremity, made one month after the injury, showed a left radiculopathy of the first sacral nerve root. At six weeks, radiographs demonstrated healing of the pelvic fractures, and full weight-bearing was permitted as tolerated.
Four months after the injury, the patient began to have episodic sharp pain that radiated from the groin into the medial aspect of the right thigh. The pain occurred only with weight-bearing and was more intense with twisting and turning. She continued to have mild-to-moderate pain in the left sacroiliac area and painful paresthesias in the left foot. Physical examination revealed tenderness on the right side of the groin. There was mild pain with internal and external rotation of the right hip, but the intense pain could not be reproduced. Radiographs showed apparent union of the fractures of the pelvic ring without any change from the original position. Computed tomography and magnetic resonance imaging were performed at five months to determine whether incongruity of the hip joint, avascular necrosis, or non-union was the cause of the pain. An obturator oblique radiograph (Fig. 2) and computed tomography scans (Figs. 3-A and 3-B) showed callus formation but no incongruity of the joint. Magnetic resonance images showed the obturator nerve and exuberant callus, but a diagnosis of entrapment could not be made.
Two months later, the patient noted intermittent paresthesias in the medial aspect of the right thigh and weakness in the right lower extremity. She also had persistent pain and a burning sensation in the left sacroiliac area and the left foot. One year after the injury, physical examination revealed atrophy of the right thigh, hypoesthesia in the medial aspect of the right thigh, and an absent adductor reflex on the right side. An electromyogram of the right lower extremity showed abnormal insertional activity; diffuse, spontaneous fibrillation potentials; and polyphasic motor-unit action potentials in the adductor longus, adductor magnus, and gracilis muscles. The electromyographic examination had to be discontinued because of pain, so the recruitment patterns could not be evaluated. Other muscles supplied by the second, third, and fourth lumbar nerve roots were normal.
Fourteen months after the injury, the right obturator nerve was explored through a right Pfannenstiel or Stoppa approach3. After the right pubis had been exposed medial to the femoral vessels, the obturator nerve was identified inside the pelvis and traced to the obturator canal. The canal was opened by incision of the overlying membrane, but no obvious impingement was found. Therefore, the dissection was carried anterior to the pubis to the exit of the nerve from the obturator canal. Entrapment of the nerve was due to the inferior displacement of the fracture and subsequent callus formation, which had resulted in tension on the nerve as it passed anteriorly. During the procedure, electrical stimulation of the obturator nerve proximal to the obturator canal resulted in contraction of the adductor muscles. The obturator nerve was decompressed by removal of a two-centimeter-wide full-thickness section of the right pubis (Fig. 4).
Three years after the procedure, the patient had had a complete recovery of the motor and sensory function of the obturator nerve and had no paresthesias in the right thigh. She had residual tenderness over the pubis, mild paresthesias but no pain in the left foot, and pain with associated tenderness in both sacroiliac joints.
The obturator nerve supplies motor fibers to the adductors of the thigh and carries sensory fibers to the hip joint and to a small area of skin on the medial aspect of the thigh. The obturator foramen is covered by the obturator membrane, a layer of fascia that almost completely closes the foramen, leaving only a narrow opening in the superolateral aspect of the foramen—the obturator canal—through which the artery and nerve pass. The obturator muscles arise partly from the obturator membrane. The neurovascular bundle runs anteriorly and laterally in a groove on the caudal aspect of the superior pubic ramus as it passes through the obturator canal1.
Injury or entrapment of the obturator nerve rarely occurs in association with a fracture of the pelvic ring; only nine such cases were found in the literature. Patterson and Morton reported two such injuries in a group of sixteen patients who had neurological complications after a pelvic fracture. One of these injuries was diagnosed on the basis of an abnormal electromyogram, and the other was diagnosed late (at three months) but was not confirmed with electromyography. Both injuries were left untreated. The outcomes were not reported, and the authors did not indicate whether the injuries were anterior or posterior. Pohlemann et al. reported on one patient who had an injury of the obturator nerve in association with a fracture of the anterior part of the pelvic ring, but they did not describe the end result. Other authors have described six instances in which an injury of the obturator nerve occurred in association with a fracture of the posterior part of the pelvic ring7,13.
Lam, in 1936, described six nerve injuries in association with eighteen fractures of the pelvic ring, but none of these injuries involved the obturator nerve. He concluded that "paralysis of the obturator nerve is a rare complication of fractures of the anterior pelvic ring." Other reports on nerve injuries associated with fractures of the pelvic ring have contained no mention of involvement of the obturator nerve2,5,9,18,19.
Obturator neuritis can cause pain in the medial aspect of the thigh, which may be accompanied by objective findings such as atrophy or spasm of the adductor muscles10, absence of the adductor reflex6, and electromyographic abnormalities. Compression or irritation of the obturator nerve within the obturator canal has been reported in association with a number of causes, including obturator hernia8; aneurysm of the hypogastric artery10; osteitis pubis11; and iatrogenic injury due to fixation of an acetabular fracture3,14, total hip arthroplasty15,20, or gynecological procedures22. Fracture of the anterior part of the pelvic ring should be included in this list.
In my patient, fracture of the superior pubic ramus and subsequent callus formation placed the obturator nerve under tension. Decompression of the nerve was accomplished by removal of the overlying section of the pubis.