Symptomatic or clinically detectable peripheral neuropathy after a total hip arthroplasty is rare, with a prevalence of 0.3 to 1.0 per cent according to one report; however, the total number of hip replacements performed during the period of the study was not provided9. Injury to the obturator, sciatic, or femoral nerve can occur during or after the procedure. In a prospective study of thirty hip arthroplasties in twenty-eight patients, some degree of nerve damage was detectable electromyographically in twenty-one patients (75 per cent)10.
The principal risk factors for neural injury include revision total hip arthroplasty, limb-lengthening, anticoagulation, female gender, and vascular insufficiency4,10. Delayed palsies of the sciatic nerve are usually due to protruded cement or fragments of broken wire1,3,5. However, to our knowledge, delayed sciatica secondary to protrusion of a cemented acetabular cup that had migrated superiorly has not been reported previously.
We report the case of a patient who had sciatic neuropathy caused by intrapelvic migration of the acetabular cup, and we describe the clinical importance of differentiating this condition from coexisting conditions of the lumbar spine.
*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.
†Department of Orthopedic Surgery, Baskent University Hospital, Oran Mefa Park, Sitefi Soght, Blok 31, Ankara, Turkey.
‡Department of Orthopedic Surgery, Baylor College of Medicine, 6560 Fannin, Suite 1900, Houston, Texas 77030.
A sixty-nine-year-old woman sustained a fracture of the left femoral neck in 1979. Treatment with open reduction and internal fixation was complicated by aseptic failure of fixation and avascular necrosis. A left total hip arthroplasty, also performed in 1979, failed without infection as well. In 1990, a revision was done with insertion of a cup with cement and titanium wire mesh. After the revision the pain in the hip persisted, and at twelve months the patient began to have symptoms consistent with sciatica of the left lower extremity. This radiculopathy prompted a workup of the low back. Spinal stenosis was diagnosed with use of myelography and was thought to be the cause of the pain in the lower limb. A lumbar laminectomy and decompression was performed at the level of the stenosis, but the pain was not relieved.
Medial migration of the acetabular cup was noted incidentally on radiographs of the lumbar spine that had been made during the workup of the low back. The patient then was referred to the senior one of us (H. S. T.) for a repeat revision of the acetabular component. At the time of presentation, radiographs of the pelvis were assessed for loss of central acetabular bone. The radiographs demonstrated a grade-V acetabular defect, according to the system of Jasty and Harris, with proximal and medial displacement of the acetabular cup past the Köhler line (Fig. 1). An angiogram was performed to rule out an aneurysm or vascular compression.
Physical examination revealed normal lumbar lordosis with a well healed scar at the site of the midline incision. There was no evidence of paraspinous spasm or tenderness. The patient had decreased sensation to pinprick and light touch in the lateral aspect of the calf and the dorsum of the foot on the left side. Motor function, deep tendon reflexes, and straight-leg raising were normal and symmetrical. The patient had pain with motion of the left hip and four centimeters of shortening of the left lower limb. Motion of the hip was limited to 60 degrees of flexion, 20 degrees of abduction, and 0 degrees of external rotation.
The first stage of the operation consisted of removal of the acetabular cup through an ilioinguinal intrapelvic approach. The component and the cement were found to be encapsulated by dense scar tissue adherent to the bladder, vagina, and iliac vessels as well as the obturator and sciatic nerves. The obturator nerve was displaced superiorly and laterally proximal to the dense scar tissue surrounding the cup and the cement mantle. The sciatic nerve was visibly flattened at the superior sciatic notch as it exited the pelvis (Fig. 2). The cup, wire mesh, and cement were removed after dissection of scar tissue around the mantle en bloc, without additional injury to visceral or neurovascular structures, and a large residual cavity was noted in the central-superior aspect of the acetabulum. After closure of the anterior wound, the patient was placed in the lateral decubitus position and the acetabulum was reconstructed through a posterolateral approach with use of a femoral head allograft and insertion of an acetabular cup without cement (Fig. 3).
Postoperatively, the patient was managed with balanced suspension for three weeks, followed by uneventful progression to walking. The preoperative radiculopathy and numbness resolved both immediately and completely.
Medial migration and early loosening of cemented cups is more prevalent in the presence of medial defects of the acetabular wall. At an average of 6.8 years after the arthroplasty, Jasty and Harris reported loosening of a cemented cup in three of twenty-one patients who had grade-IV acetabular deficiency and in three of four who had grade-V deficiency. Although most medially and proximally migrated cups can be removed with use of routine approaches to the hip, severely displaced cups should be removed through an intrapelvic approach to avoid uncontrollable bleeding, nerve palsy, vesicoacetabular fistula, aneurysm, or arteriovenous fistula6,7. The retroperitoneal, Rutherford-Harrison, and ilioinguinal approaches permit direct visualization of vital pelvic structures. (For the Rutherford-Harrison approach, an incision is made two fingerbreadths superior to the pubis and is extended laterally to include the anterior superior iliac spine and the anterior portion of the iliac crest. The underlying external and internal oblique aponeuroses are incised in line with the skin incision, and the iliopsoas with the femoral nerve, the femoral artery and vein, and the spermatic cord or round ligament are mobilized and enveloped by Penrose drains to facilitate their retraction. The abdominal contents within the peritoneum can then be reflected cephalad, and the pubic ramus, the dome of the acetabulum, and the inner pelvic brim can be exposed subperiosteally.)
Among the numerous etiologies of nerve palsy after total hip arthroplasty, the most common is revision total hip arthroplasty, followed by limb-lengthening, anticoagulation, female gender, and vascular insufficiency2,4,9,10. However, neuropathy due to these factors generally is clinically evident early. Delayed lesions of the sciatic nerve also may occur secondary to compression by protruding bone cement or trochanteric wires. This rare, delayed sciatica may not occur until as long as several years after the operation5.
In our patient, the sciatica was not evident until twelve months after the revision, and it corresponded with increasing pain in the hip. This suggests that the peripheral neuropathy was more likely due to intrapelvic migration of the cup than to the spinal stenosis detected with myelography. In the presence of late-onset sciatica, a pre-existing spinal abnormality may lead to incorrect diagnostic or therapeutic measures, as it did in our patient. A lack of appreciation of the proximity of the displaced cup to the sciatic nerve resulted in an operation to decompress the lumbar spine without symptomatic relief. Only after removal of the intrapelvic acetabular component was the pain immediately and completely relieved. The case of this patient clearly illustrates that the risks associated with a centrally migrated acetabular component include sciatica, nerve compression, and neuropathy. A delayed radiculopathy after a hip arthroplasty, especially a total hip revision performed with cement, should alert the clinician to consider migration of a component, mesh, or wire as the cause. Because of the diagnostic challenge posed by such patients, a thorough workup of both the hip and the lumbar spine is warranted before operative intervention.