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OASIS.

Source: AAOS 2012 Annual Meeting

Date/Time:Tuesday, Feb 07, 2012, 8:00 AM – 6:00 PM
Poster Hall/ Multimedia/ Scientific Exhibits: Tuesday, Feb 7 – Saturday, Feb 11, 2012, 8:00 AM – 6:00 PM

Presentation Number:P005

Title:

Retrieval Analysis of 130 Metal-on-Metal Hips Showed that the ASR is Higher Wearing when Compared to the BHR

Classification:Adult Reconstruction HipKeywords:Bearing Surfaces

Author(s):

Ashley Matthies, BSc, London, United Kingdom
Johann Henckel, BM, London, United Kingdom
Kevin Ilo, MBBS, London, United Kingdom
John Skinner, FRCS, London, United Kingdom
Alister Hart, FRCS, London, United Kingdom

Abstract:

INTRODUCTION 

The National Joint Registry for England and Wales report a five-year clinical failure rate of 12% for the DePuy ASR. This compares to 4.3% for the Smith and Nephew BHR. Subsequently this has led to the recall of the ASR resurfacing and XL hip systems. However, the mechanisms responsible for the high failure rate of the ASR compared to other current generation metal-on-metal (MOM) hip arthroplasties remain unclear. We compared clinical data and wear performance for a large series of retrieved ASR and BHR hips.

METHODS

This was a well-powered study of 130 consecutively revised large diameter MOM hip arthroplasties, comparing the DePuy ASR (n = 66) with the Smith and Nephew BHR (n = 64). Pre-, intra- and post-operative clinical data was collected prospectively for all patients to establish the clinical cause of failure. This included pre-revision radiographs and blood metal ion analysis. Linear wear rates were measured for all explanted components using a roundness measuring machine according to a published protocol.

RESULTS

The two groups had comparable clinical variables (Table 1), including the clinical cause of failure. The ASR group demonstrated higher levels of whole blood cobalt and chromium, although this trend was not statistically significant. Wear analysis (Table 2) showed that the acetabular components of the ASR, when compared to the BHR, were significantly higher wearing (p = 0.03) and more likely to be edge worn (p < 0.01). However, there was no difference in the femoral head wear rate (p = 0.14).

DISCUSSION AND CONCLUSION

We attribute our findings to specific design differences between the two prostheses. The ASR has a lower clearance and reduced cup articular arc angle when compared to the BHR. Both of these factors increase the likelihood of edge contact and are likely to lead to increased wear, particularly of the cup. This may explain why the ASR appears to be more sensitive to sub-optimal position.

Table 1. Summary of patient and clinical data for the ASR and BHR hips in this study.

ASR BHR
Number of hips 66 64
Female / Male 42 Females / 24 Males 42 Females / 22 Males
Age at primary 56 (23 to 78) 56 (23 to 68)
Time implanted (months) 35 (7 to 59) 49 (10 to 121)
Femoral diameter (mm) 47 (35 to 55) 46 (38 to 58)
Cup inclination (°) 51 (15 to 82) 51 (24 to 73)
Cup version (°) 15 (-8 to 48) 23 (-47 to 50)
WB Chromium (ppb) 9.8 (0.2 to 119.0) 4.8 (0.4 to 183.0)
WB Cobalt (ppb) 13.5 (0.5 to 167.0) 10.2 (0.0 to 167.0)
Cause of failure:
1.
Unexplained pain
2.
Acetabular loosening
3.
Femoral Loosening
4.
Infection
5.
Fracture
6.
Malalignment
7.
Component mis		 46
10
3
1
2
4
0		 43
6
5
3
1
5
1

Table 2. Comparison of linear wear rates between the ASR and BHR hips.

 
ASR BHR P
Acetabular Cup: 
Linear wear depth (μm)		 21.99 (1.3 to 651.8) 14.9 (2.0 to 740.4) p = 0.646
Acetabular Cup: 
Linear wear rate (μm/yr)		 9.2 (0.0 to 245.6) 4.2 (0.0 to 153.8) p = 0.032
Femoral Head:
Linear wear depth (μm)		 13.14 (0.0 to 315.3) 15.07 (1.5 to 234.4) p = 0.779
Femoral Head:
Linear wear rate (μm/yr)		 6.0 (0.0 to 84.7) 3.5 (0.7 to 52.4) p = 0.143
Edge Loading 85% (n = 56) 63% (n = 40) p = 0.005
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