Higher cartilage wear in unipolar than bipolar hemiarthroplasties of the hip at 2 years: A randomized controlled radiostereometric study in 19 fit elderly patients with femoral neck fractures

Abstract

Background and purpose - The use of unipolar hemi-arthroplasties for femoral neck fractures is increasing in some countries due to reports of higher reoperation rates in bipolar prostheses. On the other hand, it has been proposed that bipolar hemiarthroplasties have clinical advantages and less cartilage wear than unipolar hemiarthroplasties. We compared cartilage wear between bipolar and unipolar hemiarthroplasties using radiostereometric analyses (RSA), in patients aged 70 years or older. Patients and methods - 28 ambulatory, lucid patients were randomized to treatment with a unipolar or a bipolar hemiarthroplasty for an acute femoral neck fracture. Migration of the prosthetic head into the acetabulum was measured using RSA. Secondary outcomes were Harris Hip Score (HHS), and EQ-5D scores. Patients were assessed at 3, 12. and 24 months. Results - 19 patients were available for follow-up at 2 years: mean proximal penetration was 0.83 mm in the unipolar group and 0.24 mm in the bipolar group (p = 0.01). Mean total point movement was 1.3 mm in the unipolar group and 0.95 mm in the bipolar group (p = 0.3). Median HHS was 78 (62-96) in the unipolar group and 100 (70-100) in the bipolar group (p = 0.004). Median EQ-5D Index Score was 0.73 (0.52-1.00) in the unipolar group and 1.00 (0.74-1.00) in the bipolar group (p = 0.01). Median EQ-5D VAS was 70 (50-90) in the unipolar group and 89 (70-95) in the bipolar group (p = 0.03) Interpretation - Patients with unipolar hemiarthroplasties had higher proximal cartilage wear and lower functional outcomes. Unipolar hemiarthroplasties should be used with caution in ambulatory, lucid patients.

Figure 1.

The CONSORT diagram shows the recruitment and flow of patients throughout the trial.

Figure 2.

The image on the left is a conventional radiograph showing a bipolar hemiarthroplasty of the right hip. Tantalum markers have been implanted around the acetabulum, in the anterior superior iliac spine, and in the superior pubic ramus. To the right is a radiostereometric image with tantalum markers in the pelvis (numbered) and tantalum markers in the calibration cage. The center of the outer prosthetic head has been computer-calculated using edge detection.

Figure 3.

Graph showing mean migration in the vertical plane (Y-axis) and total point motion (3D migration) of patients at 3 months, 1 year, and 2 years.

Figure 4.

The graph shows the mean difference of proximal penetration (Y-axis) and TPM between the two groups at 2 years, in millimeters. Error bars indicate 95% confidence intervals (CIs) for the mean difference. Blue area indicates zone of equivalence, defined as ±0.5 mm (Delta). The CI for the Y-axis lies above zero but exceeds the zone of equivalence, indicating a superior result. For TPM, the 95% CI includes zero and exceeds the zone of equivalence, indicating a nonsignificant result.

Figure 5.

Box plots of Harris Hip Score, EQ-5D Index Score, and EQ-5D Visual Analogue Scale (VAS) at all time intervals. Boxes represent the middle 50% of the data, with the central band representing the median. The ends of the whiskers represent the minimum and maximum of all of the data. Statistical analyses conducted using the non-parametric Mann–Whitney U test.