Dissociation of a Bipolar Prosthesis after Right Hip Hemiarthroplasty

Biomechanical and clinical assessment of dissociation in bipolar hip hemiarthroplasty

BACKGROUND

Dissociation is a special type of dislocation that is rarely seen in bipolar hemiarthroplasty.

OBJECTIVE

To investigate the clinical and biomechanical causes of dissociative dislocation of acetabular cup components in the hips of patients after bipolar hemiarthroplasty (BHA).

METHODS

BHA heads were divided into three groups according to their design. Cam-out and pull-out biomechanical tests were conducted to investigate the separation strengths of the BHA heads.

RESULTS

Among the 1684 BHA surgeries performed, the revision surgery rate was 4% (68 hips) and dissociation rate was 0.8% (15 hips). According to the cam-out test result, the highest values in the parameters ultimate force (Fmax) ultimate torque (T), vertical displacement at maximum force (δm), rotation at maximum force (ϕm), maximum principal strain (Ɛmax), minimum principal strain (Ɛmin), average normal strain (Ɛav), maximum shear strain (γmax), uniaxial normal strain (Ɛun) were detected in Type 2 BHA head. The pull-out values of the implants were sufficient to prevent the distraction force that may occur on the implant due to manual traction applied to the hip dislocations. However, in closed reduction maneuvers, cam-out-shaped deformation forces may cause dissociation in Types 1 and 3, but not in Type 2 BHA heads. According to the pull-out test results, while high values were detected in the parameters Fmax and stiffness (k) in the Type 2 BHA head, δm and maximum force (E) parameters were found to be high in the Type 1 BHA head. In the cam-out test, a strong positive relationship was found between the thickness and width of the polyethylene locking ring and Fmax, T, δm, ϕm, Ɛmax, γmax, Ɛun.

CONCLUSION

Better BHA head designs and polyethylene designs may help resolve the rare problem of dissociation, which almost always leads to resurgical procedures.

Be aware of the "O" sign in the bipolar cup dissociation during closed reduction of bipolar dislocation: A case report

RATIONALE

Bipolar cup dissociation following hip hemiarthroplasty is a rare complication of which only a few cases have been reported, and it usually requires revision surgery because of difficulties in closed reduction.

PATIENT CONCERNS

We report the case of a 57-year-old man who underwent bipolar hemiarthroplasty 2 months ago caused by a left femoral neck fracture. Postoperatively, bipolar dislocation occurred thrice, and the patient showed bipolar cup dissociation during closed reduction maneuver at the recent visit. At the time of this event, no consideration was given to the shape of the prosthesis on the radiograph.

DIAGNOSES

The patient was diagnosed with early bipolar cup dissociation.

INTERVENTIONS

The patient underwent revision surgery to replace and reassemble the femoral head component.

OUTCOMES

No further dislocation occurred following the surgery.

LESSONS

To avoid dissociation of the components during closed reduction, it would be helpful to have knowledge of the "O" sign, a concentric circle shape of the prosthesis on the radiograph.

LEVEL OF EVIDENCE

Level V, case report.

Dislocation and dissociation of bipolar hip hemiarthroplasty

Hemiarthroplasty of the hip is a commonly performed procedure for subcapital neck of femur fractures. Dislocation of the hemiarthroplasty is a recognised but uncommon complication. Dislocations can be complicated by the uncoupling or dissociation of the femoral head component from the femoral stem. Dissociations are rare and require open reduction. We present a case of a patient with a hip hemiarthroplasty dislocation and dissociation subsequent to a fall.