Is the DePuy ASR™ cup too shallow and does it therefore mean that the surgeon has to be more accurate in his cup insertion to prevent edge loading?

The depth of a one-piece acetabular cup is a balance between range of motion (ROM) and coverage. Full hemisphere cups have greater coverage and would be less susceptible to edge effects but may have an increased risk of impingement and partial subluxation which would also lead to increased wear.   Sub-hemispherical cups with a reduced coverage improve ROM but may result in edge effects if implanted in the open position. There is no hard evidence to indicate that either design option is a problem. A computer study highlights the large posterior prosthetic wall associated with surface replacement1. This effect will be exacerbated with full hemisphere cups.

Is the DePuy ASR™ acetabular component too thin and will it deform and cause binding of the bearing?

DePuy did a substantial amount of design and development work to establish the correct thickness and the correct clearance for the DePuyASR™ system. This data has recently been published in the peer-reviewed ‘Proceedings of the Institution of Mechanical Engineers’²

It has also been reported that the DePuy ASR™ has already had two major design changes; one for the cup to increase the thickness, the other for the cup inserter to change from a suction device to the rim inserter design

Since its launch in July 2003, the DePuy ASR™ system has undergone no design changes to either its acetabular cup or its acetabular cup inserter. The presentations at the Learning Centres and the publications mentioned in the last query describe the prototype development process and development history prior to the release of the first implanted product; there have been no design modifications since then.

The cup inserter has never been a ‘suction’ type of design. The design has always been a rim type of connection and again there have been no modifications since the first implantation in July 03. The development history for DePuy ASR™ has been documented in the ‘Proceedings of the Institution of Mechanical Engineers’³

Are the clearances of the DePuy ASR™ bearing dangerously small?

The clearances for the ASR have been validated during the development of the DePuy ASR™ with a significant amount of hip simulator testing. This work has been summarised in the peer-reviewed Journal of Arthroplasty4.   Furthermore there have been no reported cases of component seizure either indirectly or through our complaints procedure. The ASR was first used clinically in July 2003, since then more than 13,000 DePuy ASR™ and DePuy ASR™ XL heads have been implanted.

It has been reported that a picture taken during the hip simulator testing (carried out to check design validity) were rigged and a ‘cheap stunt’ to exaggerate the positive effect of the DePuy ASR™ bearing.

The picture taken and published in the Journal of Arthroplasty in December 2004⁵ is exactly as stated in the paper. The results were reproducible across all the tests we conducted. In each test we carried out, the high clearance bearings produced more wear debris than the low clearance bearings. The photographs are a simple visualization of this result and were in no way enhanced or altered to attempt to exaggerate the results.

Does the heat treatment of the metal increase both the rate of implant wear and failure rate?

Work from various authors has established that if the bearings are designed correctly, the material type (within the general type: High Carbon Cobalt Chrome Molybdenum alloy) is of secondary importance to the design of the bearing^6,7.

A fracture of a DePuy ASR™ central pin of the Femoral Component has been presented by Mr. D McMinn (BHR design surgeon)

There have been no reports of a fracture of the DePuy ASR™ femoral implant pin either indirectly or through our complaints procedure. The X-rays and retrieval that Mr. McMinn shows on his website are of a Wright Medical; Conserve Plus femoral implant pin fracture.

What is the data from the Australian registry showing for various manufacturers?

The relevant part of the Australian ‘05 Registry is Table H34: Resurfacing Hip systems requiring revision, The table lists device; number revised; total number revised; observed "component" years; revisions per 100 observed "component" years; and the exact 95% confidence interval of revisions per 100 observed "component" years.

If one compares the observed component years, this takes into account how long the implant has been in the body. Newer to Market type implants (e.g. DePuy ASR™, Zimmer’s Durom) are showing a failure rate 4-5 times higher than older type devises (e.g. Smith and Nephew’s; BHR). Why is this? We know that failure of a resurfacing implant is an early type of failure (femoral neck fracture happens within the first six months), so it would be more relevant to look at the straight revision rate. Therefore the data from the same table looks like this:

5 revisions out of 206, or 2.4%, for the DePuy ASR™;

93 revisions out of 4640, or 2.0%, for the S&N; BHR.

To test for significance do a Chi-square test:

Does device type affect revision rate? p = 0.673. At the conventional significance level of 0.05 for p, device type is nowhere near significant as an influence on revision rate, however the sample size used for this calculation is not large enough to demonstrate a difference either way.

Is the DePuy ASR™ instrumentation primitive and does the system rely on eyeballing to achieve position?

The instrumentation for DePuy ASR™ has a variety of options for positioning of components. These range from simple handheld jigs, through rigidly attached jigs with variable orientation and positioning features to a computer navigation and planning system. A full explanation of the instrumentation has been published in a peer-reviewed journal8.

The DePuy ASR™ cementing technique has been criticised for possibly being too thick, therefore requiring the surgeon to hold the prosthesis steady as the cement sets.

The manual application of cement onto the femoral head allows an easier less time pressured application of the cement, ensures an even cement mantle around the femoral head, and importantly ensures that there is no requirement for high impaction forces to get the femoral head down to its final seating position. The high viscosity cement also ensures that there is no contamination of the bearing surface with cement, as in its high viscosity state it will not adhere to the bearing surface.

Secondly, the fit between the implant and the bone, due to the 3-degree taper can vary by only a small amount, in fact it is ranges from size to size to about 0.5mm maximum clearance. Therefore there is no need for the surgeon to hold the implant steady as the cement sets.

Can the DePuy ASR™ cementing technique also cause possible air embolisms?

The ASR cementing technique allows the surgeon to use a vacuum cannula in the lesser trochanter. This device reduces the possibility of air being trapped in the femoral head that might lead to an air embolism in exactly the same way as the S&N, BHR and other devices. The cementing technique has recently been published in the peer-reviewed Proceedings of the Institution of Mechanical Engineers⁹.

References

1. The impact of Arthroplasty on Hip Motion: Resurfacing vs. THR vs. the Intact Joint. S Doherty, M Thompson, M Ursey, S Muirhead-Allwood, P Noble 52nd Annual Meeting of the ORS, Chicago March 2006, Paper 0526

2. Proceedings of the Institution of Mechanical Engineers [H]. 2006;220(2) Deformation of press-fitted metallic resurfacing cups. Part 1: experimental simulation Z M Jin, S Meakins, M M Morlock, P Parsons, C Hardaker, M Flett, G Isaac p. 299 – 309. Deformation of press-fitted metallic resurfacing cups. Part 2: finite element simulation A Yew, Z M Jin, A Donn, M M Morlock, G Isaac p. 311 - 319

3. Proceedings of the Institution of Mechanical Engineers [H]. 2006;220(2) Development rationale for an articular surface replacement: a science-based evolution. G H Isaac, T Siebel, T P Schmalzried, A G Cobb, T O'Sullivan, R D Oakeshott, M Flett, T P Vail p. 253 - 268

4. Journal of Arthroplasty. 2004 Dec;19 A hip joint simulator study of the performance of metal-on-metal joints: Part II: design Dowson D, Hardaker C, Flett M, Isaac GH.

5. A hip joint simulator study of the performance of metal-on-metal joints: Part I: the role of materials. Dowson D, Hardaker C, Flett M, Isaac GH. - J Arthroplasty. 2004 Dec;19(8 Suppl 3):124-30.

6. Proc Inst Mech Eng [H]. 2006;220(2) 'Severe' wear challenge to 'as-cast' and 'double heat-treated' large-diameter metal-on-metal hip bearings J G Bowsher, J Nevelos, P A Williams, J C Shelton

7. A hip joint simulator study of the performance of metal-on-metal joints: Part I: the role of materials. J Arthroplasty. 2004 Dec;19(8 Suppl 3):118-23Dowson D, Hardaker C, Flett M, Isaac GH.

8. Ref: Proc Inst Mech Eng [H]. 2006;220(2)Development rationale for an articular surface replacement: a science-based evolution. G H Isaac, T Siebel, T P Schmalzried, A G Cobb, T O'Sullivan, R D Oakeshott, M Flett, T P Vail p. 253 – 268

9. Proceedings of the Institution of Mechanical Engineers [H]. 2006;220(2) Cementing techniques in hip resurfacing M Chandler, R S Z Kowalski, N D Watkins, A Briscoe, A M R New p. 321 - 331

* ASR = Articular Surface Replacement