Significantly Displaced Femoral Head Component in a Dissociated Bipolar Hip Hemiarthroplasty

Introduction

Hemiarthroplasty of the hip for subcapital neck of femur fractures is a commonly performed procedure. Dislocation of a hemiarthroplasty from the native acetabulum is a recognized, but uncommon, complication with a reported incidence of 1.2% to 3.4%1, 2. Bipolar hip prosthesis dissociation, in which the femoral stem and head components separate, is a rare complication. We present a case of dissociation of a hemiarthroplasty with a significantly displaced femoral head component.

Case report

An 84‐year‐old woman presented to our emergency department with left sided hip pain after a simple fall. One month previously, she had undergone bipolar hemiarthroplasty (Charnley round back femoral component and Hastings femoral head component) for a subcapital neck of femur fracture on the same side. Plain film radiographs showed an anterior dislocation and dissociation of the hemiarthroplasty components with a significantly displaced femoral component (Fig. 1). The patient was taken to the operating theater where she underwent revision hemiarthroplasty under a spinal anaesthetic.

Figure 1.

Anteroposterior and lateral radiographs of the left hip demonstrating a dissociation of left hip hemiarthroplasty with a significantly displaced femoral head component.

Intra‐operatively we found the femoral head component in the subcutaneous tissues; it had been extruded through the original capsule, abductor repair and fascia lata. There was no evidence of fracture of the acetabulum and the femoral component remained well fixed. We replaced the femoral head component with a Hastings implant of the same size, after confirming that it was the appropriate size. We reduced the hip and confirmed it to be stable. We carefully repaired the abductors in layers and the defect in the fascia lata. Post‐operative radiographs (Fig. 2) were satisfactory and recovery was uneventful. The patient was mobilized within 48 hrs of surgery. At 6‐month follow‐up, the patient was independently mobile with a stiff, but pain‐free, range of motion in her hip.

Figure 2.

Postoperative radiograph demonstrating the revision hip hemiarthroplasty.

Discussion

Hemiarthroplasty of the hip for a subcapital neck of femur fracture is a common procedure. Complications of this procedure are uncommon. There is no significant difference in the complication rates of unipolar and bipolar hemiarthroplasty3, 4. Dissociation of a hemiarthroplasty is a rare complication5, 6. Gergiou et al. have reported a series of five cases, which is one of the largest published to date7.

There are several postulated reasons for occurrence of dissociation. These include locking of the femoral head component against the acetabular rim during dislocation, together with inaccurate component sizing7. This is the most likely mechanism in our patient. Dissociation is also reportedly a complication of closed reduction of a dislocated hemiarthroplasty. This is probably caused by locking of the femoral head component on the posterior wall resulting in a “bottle‐opener” effect while traction is being applied to the limb6, 8. To minimize the likelihood of this complication, closed reduction should only be attempted in theatre with fluoroscopic guidance. Implant failure due to excess wear of the polyethylene liner and locking ring mechanism is another reported cause of dissociation9.

The management of a dissociated hemiarthroplasty is open reduction. The acetabulum should be inspected for any fractures or defects and the femoral stem assessed for loosening. Previous X‐ray films should be carefully evaluated to identify any alignment issues that may have predisposed to dislocation. Because the stem was well fixed and in good alignment in our patient, we only replaced the Hastings femoral head component. However, surgeons should be prepared to revise all components in order to achieve stability.

In most cases, the dissociated femoral head component remains within the acetabulum or close to it. In our patient, this component had been significantly displaced. It was found in the subcutaneous tissues, superficial to the fascia lata, necessitating prompt operative intervention and careful repair of the disrupted soft tissues. To the best of our knowledge, such severe displacement has not been previously reported.