Dislocation after hemiarthroplasty for femoral neck fracture is a rare event. Two hundred nineteen patients underwent hemiarthroplasty for femoral neck fracture all with a modern stem and modular unipolar prosthesis at our institution over a three year period. Eleven patients (4.1%) suffered a dislocation. Epidemiologic, technical and anatomic-radiographic factors were reviewed to assess risk factors for dislocation. Our findings indicate that. Mental disease was a significant risk factor. Femoral neck offset and center edge angle of the acetabulum were also contributing factors for dislocation.

To determine the risk factors for dislocation after hemiarthroplasty with a modern stem and modular unipolar prosthesis for femoral neck fracture.

A retrospective review was undertaken at our institution from January 2000 to December 2003. We reviewed factors including gender, age, time of dislocation, medical comorbidities, side of operation, surgical approaches and radiographical data including femoral neck offset, contra lateral femoral neck offset, center edge angle, residual femoral neck, prosthesis femoral neck shaft angle and acetabular index.

Hemiarthroplasty for femoral neck fracture was pre-formed on two hundred and nineteen patients all with a modular unipolar prosthesis. Eleven patients (4.1%) had at least one dislocation during a one year follow-up. The average time of dislocation following surgery was 19.3 days (std 12.7days). There were no significant difference for gender, age, surgical approach, residual femoral neck, and prosthesis femoral neck shaft angle. There was a statistical significance of mental disease in 55% of dislocated hemiarthroplasty compared to 21% of nondislocated hemiarthroplasty but other comorbidities were not a factor. Median femoral neck offset of nondislocated hemiarthroplasty measured 51mm versus dislocated hemiarthroplasty value of 47mm, a difference of 4 mm (P=0.043). Center edge angle of nondisplaced hemiarthroplasty measured median thirty-six degrees versus a median of thirty-two degrees for dislocated hemiarthroplasty, a difference of four degrees (P=0.006).

The factors predicting dislocation are likely multifactorial. Our findings indicate that Mental disease was a significant risk factor. Femoral neck offset and center edge angle of the acetabulum are contributing factors for dislocation.