Larger effective joint space in a THR? Also Dave Langton has quite nicely demonstrated that it may well be the trunion wear that produces more debris than the joint surface, which of course a resurfacing would not have.

For large metal-metal head, I agree that trunion was the main cause of trouble. For resurfacing, cup design of the ASR system was less than optimal (not enough coverage, high polar peek pressure

My failure rate, 5 out of 65 cases, 7.6% after 5 to 6 years, all were cup failure . No ALVAL, no pseudotumor, all metalosis between a loosen cup and the acetabular host bone, all solved with Pinnacle cup revision. I blame the cup design for ASR XL failure. Why the difference with resurfacing?. Maybe the heavy bulky head.

It is the taper head junction in ASR XL MOM. ASR resurfacing has no taper junction.

We are now in the process in studying failure mecanisms in Biomet magnum M2A THA. Edge loading could well be more important than trunion problems in this implant.

More susceptible to edge wear due to design (arc of coverage). Low clearance - fine if always getting optimum lubrication regime, not probably the case when going from stop start. Short modified trunnion of the corail (AMT) short / grooved.

All these I suspect have tipped this bearing surface to most susceptible in LHMOMTHR setting but due to common areas such as large head / increase torqque and modified trunnions over the years has let this to also now becoming a generic problem for all LHMOMTHR regardless of brand / combination.

Biomet magnums are going to be very interesting as additional interface with large sleeve and head on type 1 bimetric taper.

I agree with others. The problem in ASR XL is the trunion and the increased torque forces related to the large metal head over 36mm.. In the past with small diameter metal head there was no this kind of problem. Moreover the ASR cap during the press-fit process deform itself with head size up. This is true for ASR XL and resurfacing.

Larger head diameter gives a better range of motion, but leads to a more expressed wear process. It is well recognized that debris particles are related to loosening and failure of the prosthesis.

There is a lot of discussion as to the location of the wear components and most are trying to avoid naming the bearing surface. The ASR THA has a technically challenging acetabular component, as such surgeons struggle for good placement. If you look at the design studies it can be seen that poor implant placement is largely responsible for the failures. There are quite a number of implants that have large heads MOM with tapers that do not display the same behavior as the Depuy ASR, so it would be wise to hold some scepticism about the wear coming from the trunion junction. The resurfacing revision rate is still excessive when compared to THA revision rate. It would also be important to consider the long term effects of the resurfacing on future implants as they dramatically reduce the life of the revision THA.

Lance, would you say then that if you examined ASR XLs which had failed due to a tissue reaction and compared them to failed ASR resurfacings which had failed for the same reason you would find similar bearing surface wear rates?

On your point about other large diameter THRs not having problems - which ones are you specifically referring to?

Assuming that the acetabular cups are the same then the wear will be the same, probably more in the resurfacing cohort as they are typically younger and more active. In a resurfacing scenario the extent of acetabular damage is probably significantly less than most THA patients aiding in cup placement.

If you look at the Australian Orthopaedic Associations special report (attached) on MOM bearings there is list on page 10 Table MM7 detailing the 7 year revision rates. There are six combination which are less than 10% at 7 years, with the ASR at 39.1% at 7 years.

https://aoanjrr.dmac.adelaide.edu.au/documents/10180/127369/Metal%20on%20Metal%20Total%20Conventional%20Hip%20Arthroplasty

As far as resurfacing prostheses are concerned, I think the good early results should be subjected to a longer follow-up. In the 70's and 80's we had some experience with the Wagner prosthesis (cup prosthesis covering the head) in younger patiens which had the same resurfacing principle as the contemporary ones. Unfortunately, they all failed in 5-6 years. There is another prototype of resurficing introduced in the 40's by Smith-Petersen. The results had been detrimental. This ondulating curve of introducing old "forgotten" principles is a fact in medical practice, but not always successful.

Lance, if you take patients who have a significant soft tissue injury - an ASR XL patients bearing surface wear is ONE THIRD that of an ASR resurfacing patient. In your opinion, why is that?

I don't agree with using registry data on these matters. New problems take time for surgeons to understand and treat. New problems are falsely represented in registry reports.

To me it seems of great importance to go into the details of the composition of the metal alloys used in the different products. In Austria and in Germany there were big problems with respect to the carbon content of the alloy. The first alloy - a so called high carbon alloy with 0,2% carbon had good tribological simulator results but there were some cases of grade IV hypersensitivity reactions around the prostheses which had to be revised. The second alloy a so called low carbide alloy with 0,1% carbon content and even less had also good simulator parameters but failed very early due to grade IV hypersensitifvity reactions too but in a number of about 20% of the primary implants in the first years after implantation already. So perhaps you can explain the phenomenon of different failure rates with this difference in the composition of the alloy. ( See also our publication on adverse tissue reactions of M-O-M prostheses)

Sincerely

Max Boehler, MD, Prof. of Orthopedic Surgery

David I would definitely agree that registry data should be taken in context!

With regards to the wear I don't have a good answer, apart from some more questions.

How was the wear measured? In the retrieved implants I have seen very sophisticated techniques are required to even see the wear patterns, let alone measure any volumes. If you have some references that would be relevant please let me know.

Probably some of the reason will be patient/cohort related factors such as stiffer joints, more limited range of motion. Is there a difference in initial tension of the surrounding tissues? I know different surgeons prefer differing amounts of muscle/tendon tension to prevent dislocation and tendonitis.

It definitely is a complex problem!

Hi lance

I use a CMM. Volumetric wear is accurate to 0.5 microns.

For references look up Nargol AVF for out stuff but also redlux are super super accurate.

To be honest - I really believe its taper wear all the way. Taper debris is nasty stuff!

Even if the measurement of wear is inaccurate it's the same technique being used to evaluate the ASRXLs and the resurfacings so measurement technique can't really explain this difference. Plus the blood metal ion levels (again in terms of spread - lower in the thr group).

We have looked at over 200 in each group and the results just get more convincing by the week.