HR Issues

The controversy regarding adverse wear in metal-metal bearings

Thomas P. Gross , MD 3/5/2010

I have used over 3000 metal bearings in primary total hip and hip resurfacing as well as revision surgery. I have revised 2 for adverse wear 7 years after implantation. I know that most other high volume hip resurfacing surgeons have a similar experience. The revisions were straightforward and the patient enjoyed the same rapid and complete recovery as if she had a primary hip replacement.

Currently less than 5% of my practice involves revision surgery. However, I have revised over 100 metal plastic replacements for excess wear. Furthermore significant wear related damage to the tissues is seen in virtually all metal plastic hip replacement or knee replacement revised for other causes.

A surgical group that has seen a surprisingly large number of wear‐related failures of metal bearing implants has coined the term "pseudotumor" when an inflammatory soft tissue mass is seen around the hip of a metal bearing implant. However, this inflammatory soft tissue reaction to metal wear debris is not much different than the inflammatory reaction that we have seen with plastic wear debris for many years.

All artificial bearing implants give off wear particles. The question is, which type of wear debris is best tolerated by the body? During the last 20 years of joint replacement polyethylene osteolysis (bone destruction caused by plastic wear debris) has been a major problem. But anyone who has revised total joints is also aware that polyethelene debris also is always associated with large amounts of soft tissue reaction around the joint. Polyethelene has been improved, and metal bearings have been developed. Both give off much less wear debris than the old polyethelene implants. The question is which results in less wear related damage? At this point we do not yet have the answer. Adverse wear reaction is a serious problem, but fortunately it is very rare.

Lets put this into perspective. The most common reason resulting in revision of total hip replacements in the US is hip instability (recurrent dislocation). 20% of all hip revisions are done for this reason. This is far more common than adverse wear reaction. Hip instability is a very disabling condition that occurs in 3‐5 % of hip replacements. The rate of instability for large head metal bearings is less than 1/2 %. Larger bearings are the solution for this problem. Large head metal bearings (resurfacing and total hip) are currently the only ones that allow reconstructing the hip in a biomechanically normal fashion to avoid instability. Proponents of plastic and ceramic bearings realize this and have made their bearings thinner recently to allow larger heads to be inserted (32‐36mm). This has made them more stable, but 32‐36mm does not yet approximate normal femoral head sizes in the average female (48mm) and average male (52mm) patients. These larger head (32‐36mm) implants for plastic and ceramic bearings have only been in use for a few years and it is not yet clear if these bearings will break at a higher rate because they are thinner. I would not recommend impact sports on thin plastic and ceramic bearings. Anatomic sizing that matches the patient's own size is only possible with large metal head designs. These are stable and can tolerate repetitive full impact without breaking. Wear rates are not significantly increased by running.

In the last few years we have learned that these rare cases of adverse wear in metal bearings are related to three factors: steep acetabular inclination greater than 55 degrees, small component sizes, certain component designs with an extremely shallow arc of coverage. At this point it is still only a very tiny percentage of patients with cup inclination angles above 55 degrees that have had wear problems. If a patient with an inclination angle above 55 degrees develops symptoms years after surgery, I would first check metal levels and an MRI. If the levels were high or a soft tissue mass developed I would recommend revision. So far this has happened twice in my practice.

More important, however, is prevention of this adverse wear complication. Since this information about cup inclination has become available several years ago we developed and tested a protocol for measuring the inclination by XR during the operation. The paper reporting this technique will be published in CORR this year. Using this technique in every case, I now have had no cups implanted with inclination greater than 55 degrees since 10/ 2007. We expect that this technique will completely eliminate this rare cause of failure in metal bearing hip implants: adverse wear reaction.

Knowing the cause of resurfacing failure can ensure successful conversion to THR by Edwin Su, MD
The shell can be retained in cases involving femoral neck fracture, femoral loosening or impingement.

Read Complete Article by clicking here

March 2010

Causes of failure

"The cause of failure must be carefully assessed prior to the conversion surgery in order to ensure an optimal THR outcome," Su said. He noted that femoral neck fracture is the primary cause of short-term failure in resurfacing procedures. He theorized that the rate of these fractures could be reduced with improved surgical techniques, careful patient selection and preoperative evaluation of bone quality. Inadequate acetabular fixation or the so-called "slipped cup" is another cause of early failure, which may also be related to surgical technique.

"The greatest cause of a mid-term failure is femoral component loosening and osteonecrosis probably plays a role in this," Su said. "I think that component malposition is going to play a large role in these mid-term failures as well." He noted that mid-term investigations of patients with acetabular component malpositioning revealed painful metal reactivity requiring revision.

Other causes of failure include metal hypersensitivity and unexplained pain due to impingement, undetected stress fractures or pseudotumors...

...Shell retention or full revision?

In planning conversion procedures, surgeons have the option of retaining the shell from the hip resurfacing.

"I think this is acceptable for a well-positioned, well-fixed and undamaged shell," Su said. "It is applicable in situations such as, a femoral neck fracture and in a femoral loosening...

..."A full revision is necessary when there is component malposition of chronic duration because there will be damage to the metal components," Su said. "It is also best when there is a question of metal hypersensitivity."

Reference:

• Su E. Surface replacement conversion: Assuring an optimal THR outcome. Paper #44. Presented at the 26th Annual Current Concepts in Joint Replacement Winter Meeting. Dec. 9-12, 2009. Orlando, Fla.
• Edwin P. Su, MD, can be reached at Hospital for Special Surgery, 535 East 70th Street, New York, New York 10021; 212-606-1128;

The status of DePuy Orthopaedics’ ASR platform as Feb. 2010

Last fall of 2009, DePuy decided to discontinue ASR® XL Acetabular Head System and DePuy ASR® Hip Resurfacing Platform (not available in the U.S.) worldwide. As a result of declining demand for the ASR platform and other market factors, DePuy is in the process of phasing out this platform to focus on the development of next generation hip replacement and resurfacing technologies that best meet the needs of surgeons and patients.

DePuy wants to assure patients who have been treated with a device from the ASR platform that there will be options available to them in the future should they need a revision:

· If a patient who had received the DePuy ASR® XL Acetabular Head System for total hip replacement requires a revision surgery, the acetabular component could be revised with the Pinnacle Hip Solutions platform, which would be compatible with an existing well-fixed femoral stem.

· As with any hemi-resurfacing prosthesis, including the DePuy ASR® hemi arthroplasty, a patient requiring a revision procedure would generally be treated with a total hip replacement.

· For patients outside the U.S. treated with DePuy ASR® Hip Resurfacing (not commercially available in the U.S.), DePuy intends to maintain an inventory of ASR XL heads outside the U.S. for use on compatible DePuy femoral stems. This will allow surgeons outside the U.S. the option of retaining a well-fixed ASR Cup when appropriate as part of the revision procedure.

Lorie Gawreluk
Vice President, Worldwide Communications
DePuy, Inc.

WHAT ABOUT NAVIGATION

Today navigation is still a tool that is not easy to use and that needs a certain learning curve as resurfacing itself also has.

So it is not a useful tool today for resurfacing beginners, where it should be! It would be nice if it would be a help at the start of the learning curve.

So can somebody with experience use it or should they use it?

It is like doing a certain approach and having experience with it, so it feels better and confident.

Most of the experienced surgeons do feel they do not need it. MAYBE it could help.

BUT there are some things that have to be cleared out still today:

*there is no correlation in most of the systems between head and cup.
*Most of the systems only look to the head, and nobody can tell us today what is now the best place to put the implant
*It would be the best to use it for the cup because there we have the most failures!

BUT AGAIN the most problems will be with females, that easily have twisted pelvis on the table and smaller sizes, and it is not sure it will have a big influence here.

If it is a system with preop CT of the pelvis to do the acetabulum, the pictures are taken in SUPINE (lying down position!). The patients walk and run on their hips, they do not lie on them, and that can make a complete difference!

So we are not there yet, if something could help me to do better surgery it would be navigation, but as it is today, it is not a 100% proven project. I have today so designed instruments that I call it navigation without navigation; of course in other sites navigation really could help!

I do not know if the 7 malpositioned cups in my series of 3000 would have benefited with navigation, possibly yes, but maybe would have had others where then the placement was worse?

It is the future?, maybe, but not there yet at present for everybody. That is why not everybody is using it, not just because we would be to lazy, to old, to stubborn or whatever.

If it would be used tomorrow in all cases from the start, the worry is also there, that if the navigation fails we do not know anymore what to do. All these facts should not be used for marketing or publicity issues but left to the orthopaedic community to make it better, try it and try to succeed better, what prof.Cobb, myself and all others I think try to do.

KOEN

koen de smet

hipsurgeon amc gent krijgslaan 181 9000 GENT BELGIUM +3292525903 www.heup.be

anca clinic roma valle giulia ROMA ITALY www.ancaclinic.it

There is some data indicating that navigation can improve the accuracy of femoral component placement in hip resurfacing.  The real issue is "compared to what?"  For an inexperienced surgeon, navigation may help him avoid component positioning problems that have been associated with "the learning curve".  However, for an experienced surgeon, who has an established mechanical alignment system with a high success rate - it is difficult to demonstrate an advantage to him with a navigation system.  Further, the registration process takes a little time – so the cost-to-benefit ratio for the experienced surgeon in a busy O.R. is questionable.  The navigation systems are not hard to learn to use, even for old guys like me!

A bigger challenge is acetabular component positioning.  This is true for total hips, and even more important for resurfacing.  There are 2 parts to the acetabular positioning problem.  The first is identifying the desired position for that patient and the second is putting the cup in that position.  Keeping the pelvis in one position and finding accurate pelvic/acetabular landmarks can be challenging.  The lateral opening angle is the easier part.  Most surgeons today agree that between 40 and 50 degrees is desirable.  Version is more complicated because the desired acetabular version is dependent on femoral version.  Acceptable version is also related to the lateral opening angle and the resultant bearing contact area.  Again, the issue is experience.

If I have any doubt about component positioning, I get an intra-operative x-ray.  Admittedly, there can be some challenges to getting a good intra-operative view.   For what it’s worth, we did an x-ray review of my first 500 resurfacings (minimum 1 year follow-up).  I have never had a femoral neck fracture and all sockets are below 50 degrees lateral opening.

Best wishes.

Thomas P. Schmalzried, M.D.

Computerized navigation has been around for a long time, in total hips, total knees, and now hip resurfacing. A lot of surgeons, including me, have tried it out and not seen an advantage in all but very exceptional cases. Yet other surgeons use it on every case.
 
This is what I think about computerized navigation: It is a tool which can narrow the "bell-curve" of component position, but the curve still has some spread. That helps a surgeon avoid "outliers", or badly misplaced components. Navigation does not make component position the exact same every time, but it helps avoid those outliers. (If it was the exact same every time there would be no bell-curve at all.)
 
So, if a surgeon has no outliers, in other words if he is doing a good job of keeping his personal bell-curve narrow, there is no advantage to using computer navigation. Alternatively, if a surgeon thinks he might accidentally misalign a component so much that it would be considered an outlier, the computer may prevent that.
 
Like any computer, what comes out depends on what went in. Registering the anatomy (which tells the computer where everything is) at the beginning of a computer-navigated operation is not at all an exact science, but depends upon knowledge and experience. It's the same with mechanical alignment jigs. With either method, one should hope that the surgeon is ready to adjust the verdict of the computer or the jig to place the component accurately in the bones which are clearly visible.
 
Are there any downsides to using a computer? Well, there is the extra time involved, which prolongs the surgery (think infections, blood clots). There is extra expense. There is often one more person in the OR, and more traffic in the OR can lead to infection. There is the possibility of surgical complacency if the doctor believes in the infallibility of computers.
 
I have heard this discussed at resurfacing meetings, and people whom I respect more than any others in this field have tried navigation and declared it "useless", and a "waste of time". While unwilling to go quite that far, it does make me think I am fine in continuing with mechanical jigs.
 
Your question about doctors not having 100% "retention" due to component malposition requires a reply. Personally, I have not had any failures in almost 600 resurfacings due to component malposition. I have 1 femoral neck fracture due to leg presses 8 weeks after surgery, and one pelvis fracture resulting from trauma 2 years after resurfacing. That's it. But malposition is an important cause of fracture, wear-related failure, and possibly pseudotumors as well, so should be avoided.
 
Any surgeon "young" enough to learn hip resurfacing is certainly young enough to learn the much easier task of computer navigation, so people who consider someone too "old" to learn navigation are being silly.
 
Similarly, a patient who would choose his surgeon based upon their use of computer navigation is badly misguided. There are many much more important issues to consider.
 
Having said all this, I wouldn't be surprised if at some point in the future surgical navigation becomes more accurate, easier, cheaper, and quicker. Robots will substitute for doctors. Surgeons will look back on the old days and shake their heads in amazement that we used to do all this by hand.
 
Peter Brooks MD, FRCS(C)
Cleveland Clinic

Using computer navigation in  joint surgery is a double edged weapon. While potentially it can reduce the number of outliers,  it can also cause tremendous deviations and absurd placements which would never be done with conventional jigs. I have seen many examples of this done elsewhere and referred to me for revision surgery.

Generally the input to the computer is made by a technique known as bone morphing where the surgeon uses pointer probes to point out the various bony landmarks to the computer. If the surgeon makes an error in this step then it obviously carries on in all further steps leading to a faulty placement. To argue that it removes human  error is most irrational.

We have the brainlab navigation ( market leader in navigation)  in our unit since 2007 and I did a series of cases at that time ( about 80 cases) . I have to say that the femoral cap placement was inferior to my placement with traditional jigs. However I found it useful when one had distorted anatomy as in previous prox. femoral osteotomy. I still use it for such cases.
 
There are many reasons in my opinion by which the conventional jig is far superior to the navigation in hip resurfacing.
 
1. bone morphing with the pointer probes damages the neck capsule which I protect passionately during hip resurfacing surgery and which I am sure is one of the key elements for my success rate.

2. I  use navigation routinely  during my Total knee replacements as the aim of the TKR surgery is to allign the components to the hip and ankle which are not visible in the surgical wound. In contrast in hip surgery the goal is not to align hip component to the spine , pelvis or knee/ ankle. The aim is to align components to local landmarks in the surgical wound, the location of which is given to the computer by the surgeon.  Then the computer gives back the same information which the surgeon offered in the first place. ( this is unlike the TKR where the computer picks up the hip on merely moving the hip and not morphing). Arguments that the computer increases accuracy in hip surgery is frankly absurd and have to be dismissed as marketing techniques.
 
3. The concept of incorporating the  combined anterversion is now the key in operating on FAI ( Femoro- acetabular impingement) which is the pathology in over 95 % of male patients having primary osteoarthritis. This is a dynamic assessment and can be done only with a jig using a lat cortex pin and cannot be done with navigation.
 
Having said all of the above  one must make a distinction between what Prof. Cobb uses and what others use.

Prof . Cobb is the  only one to my knowledge who uses a CT based navigation. The CT gives information which the surgeon cannot access unlike imageless navigation with all other surgeons which depends on surgeon's input based on bone morphing that  defeats the whole purpose of navigation.

In conclusion I would like to say that imageless navigation has very limited role in hip arthroplasty ( eg previosely operated cases) and is an excellent tool in Knee arthroplasty.
CT based navigation for hips which is still not available commercially ( which prof. Cobb uses) may have a significant role in hip arthroplasty. This has to be balanced with the radiation dose for routine CT to be applied universally( approx 30 -50 conventional x-ray dose )
 
wishing you the very best
 
with best regards
vijay bose
chennai

Mr. Bloomfield responds to the The Times Article: "Is hip resurfacing the best solution for arthritis?"

Let's start at the beginning!

Fact No. 1 : Nothing is as good as nature's own. Nothing can ever replicate the perfection of your native, original hip - before it became diseased. One day, maybe we can grow you a new one, then this debate will be irrelevant. Everything else is a compromise. Some compromises are better than others, and it depends on the individual patient, their activity or age, as well as the experience of the surgeon and the quality of components used.

Fact No. 2: However you 'spin' it, Conventional total hip replacement or THR is effectively an amputation of the head & neck of the femur. No if's and's or but's. Once it is gone, that's it, no going back. So, even if hip resurfacing [I call it BHR as I only use the Birmingham device] has a SLIGHTLY higher failure rate than THR, it is still worth thinking about the preservation of your femoral head & neck. The younger or more active you are, the more important this thought becomes.

Fact No. 3: The article only looks at revision rates when comparing BHR to THR. It says nothing about other, more subtle problems with THR like dislocation. OK, dislocation maybe rare with THR and almost unknown with BHR, but it is still a great concern in the early recovery phase. The fear of dislocation with THR drives the rehabilitation in the first few weeks and greatly restricts the advice the surgeon can give patients. Patients have to be given guidance to avoid dislocation which is often more onerous than is strictly required so that everyone can 'cover their backsides' so to speak. With BHR, my team is now [or should be!] telling MOST patients there are no special or onerous restrictions. Patients can sleep on their sides. They do not need raised toilet seats at home. They do not need to worry about dislocation because it is almost impossible. It allows the patient to recover full range of motion earlier and more safely. Unless there are concerns about bone quality, patients can be told to get back to activities of daily living as fast as their body allows. The only thing we have to be a bit cautious about is high impact stuff like running or jogging, football, rugby, skiing and the like. These can be allowed after the 3 or 4 month x-ray and if surgeon is happy that the danger of neck of femur fracture has passed.

The other, very subtle and impossible to quantify downside of THR is that surgical invasion of the femoral medullary canal forces marrow contents into the bloodstream. The bone marrow of the long bones is where your body makes all your blood cells. Red ones, white ones and platelets. It is why dogs love the marrow of a bone so much - it is rich in fat and protein. Forcing this marrow fat, rich in immature blood cells and other proteins, triggers an inflammatory cascade in the leg around the whole length of the femur and in the lungs which filter the globules before they would enter the circulation to the brain or other major organs. When severe, this phenomenon is called fat embolism. BHR dramatically reduces this embolisation phenomenon and is why I feel quite happy doing bilateral BHR when the patient has bilateral disease, but I would be very, very careful or wary of bilateral THR on the same day. In fact I tried bilateral THR several times before BHR came along and had lots of trouble. Done over 30 cases of bilateral BHR now and never regretted it. A truly astonishing operation as patients take only one or 2 more days to go home as compared with a single side BHR. i.e the recovery time is not doubled.

Fact No. 4: Some of us have always instinctively realized this, but BHR is exquisitely sensitive to accurate component positioning, and the exact metallurgy/manufacture of the components. THR can be put in quite sloppily and still work. At least for more than the 3 years the Times article is looking at. The figures in the UK National Registry are for all surgeons, using all the currently available hip resurfacing prostheses in varying mix. One should look ONLY at high volume, experienced surgeons to get the true picture. I wish I had the time and energy to look in detail at my own series, but it is certainly less than 4% failure at 3 years! The other trouble is that McMinn has already published large, detailed series so does the world need yet another one? McMinn's own figures, particularly in the under 55's are so good, many thought he must have fabricated them. I think less than 1% 'failure' at 5 years, not 3 years. This is the problem with raw statistics: they are so easily used like a drunk man uses a lamppost - more for support than illumination.

So much of the 'failure' we are looking at is due to poor surgery, poor prostheses or a combination of both. Women are only more at risk because their hips tend to be smaller, therefore the precise positioning of components is more critical. Women also tend to naturally have slightly weaker or less dense bone than men, so their cups may not integrate as planned or they may fracture through the neck of the femur. Apart from that, I personally don't believe there is any great gender difference.

Fact No. 5 ALVAL or metal ion 'allergy' is very, very rare. Irritation from excessive metal wear from poorly positioned or poorly manufactured prostheses accounts for the vast majority of the so-called ALVAL being reported. It sounds to me like Andrea had excessive metal wear leading to predictable irritation, fluid accumulation around the hip, and pain. Andrea, I do not think you had true ALVAL. Indeed your surgeons tend to confirm this as they did not find the masses of inflammatory tissues and destruction that would have been present if you had true ALVAL. The Melissa test is useless for predicting who will get ALVAL. The Melissa test has been used to justify large scale extraction of dental fillings from people, particularly in Scandinavia, on the basis that allergy to the metal in the fillings was making these people ill. Mass hysteria on a quite fascinating scale, and remember for very tidy profit. ALVAL is not confined to BHR. It is a problem with any metal-on-metal bearing couple. If ALVAL is used as a reason to discredit BHR, then all metal on metal bearings would have to be suspect. Which would leave only metal or ceramic on polyethylene, or ceramic on ceramic.

So lets look at metal or ceramic on polyethylene. Polyethylene is basically like hardened wax. Soft and slippery. Under pressure and when heated, it deforms or flows, just like melting wax. You can make the wax a bit harder, but it is still wax. There are constantly new or improved polys on the market. We have been here before. Let's look at Hylamer, a trade name from De Puy:

Hylamer polyethylene was introduced in the 1990s as an alternative to conventional polyethylene. Its chemical and physical properties, and especially its high crystallinity, were claimed to improve resistance to wear. Initially Hylamer devices were sterilized by gamma radiation in air, then the technique was changed and gamma radiation was performed in the absence of oxygen. Clinical experience has shown the early loosening of some devices made from Hylamer.

The text understates the problem. Hylamer was an unmitigated disaster and has long ago been withdrawn. So I don't trust poly in any shape or form FOR YOUNG ACTIVE PATIENTS. I still use it for the more elderly and sedentary. It still works perfectly well in this group.

What about ceramic-ceramic? This is the best alternative if you cannot have metal-metal for any reason. BUT some ceramic hips squeak. So loudly they can be heard across a room full of people. Ceramic is brittle and although ceramic fracture is now rare, it still happens and is under-reported. Ceramic ages or oxidises in the body and this can then lead to higher wear rates as the ceramic surfaces lose their shine or surface finish. Finally ceramic-ceramic is a very 'hard' bearing couple with no 'give' or shock absorption. BHR will, in most situations, have a thin film of fluid which can be displaced to absorb shocks at bearing interface.

So, in summary: Yes, BHR will likely ALWAYS have a very slightly higher revision rate than THR at 3 or 5 years, when comparing like for like in terms of young active patients. But the increased risk should be of the order of 1% or less, in the hands of an experienced surgeon. Not the 7 to 14 times quoted. It is the 30 or 40 year comparative results that will tell a different tale!

BHR revision, if ever unfortunately required, will always be easier than THR revision. Pity the poor patient whose THR fails early, or even later, particularly if the femoral side needs to be redone - their surgeon has a much tougher job on his/her hands. And abandoning BHR in favour of THR would mean abandoning all the more subtle advantages of an anatomical-sized component sitting on top of your own preserved femur.

We need to focus on precise surgery, good patient selection, the very best metallurgy and manufacture, not scare ourselves into abandoning the most revolutionary development in the field of hip arthroplasty in the last 50 years.

Mark

Surgery hits hip pocket

Click Here to Read Complete Article

By Sue Dunlevy From: The Daily Telegraph January 04, 2010

ONE in every 12 hip and knee replacements need corrective operations and new surgical techniques are more susceptible to problems, according to Australia's largest health insurer. Medibank Private has reviewed its data on the 3990 hip replacements and 4860 knee replacements it paid for in 2008 and found "on average surgeons perform revisions on 8.3 per cent of their total procedures"...

...The National Joint Replacement Registry, which is studying the reliability of hip and knee replacements, has found newer joint replacements that are cementless or hybrid are more likely to need further surgery than the older cemented replacements...

...And research found the more reliable cement joint replacements are used in just 23 per cent of hip replacement operations...

...The latest report from the National Joint Replacement Registry found that three types of hip replacements - the ASR, Durom and Recap hip replacements - had more than twice the risk of revision of other resurfacing prostheses. Hip replacements with smaller femoral head sizes are also more likely to be revised.

The Allegretto knee, one of the most common knee-replacements used in Australia, has a 10 per cent revision rate at 2.5 years, which is considerably greater than other similar prostheses.

 

Read Complete Article Here By Robert Trace
September 17, 2009

MANCHESTER - Researchers here reported that adverse soft tissue reactions following metal-on-metal hip arthroplasty are typically due to increased wear of the bearing surfaces, and patients with smaller femoral heads may be particularly susceptible to these complications.

In an independent center study, David Langton, FRCS, and colleagues in the Joint Replacement Unit at the University Hospital of North Tees in Stockton, England, reviewed 155 Birmingham Hip resurfacings (BHR, Smith & Nephew) performed between 2002 and 2009 (mean follow-up, 60 months). They also studied 420 articular surface replacements (ASRs) and 75 total hip replacements using ASR XL implants (both DePuy Orthopaedics) with S-ROM stems (DePuy Orthopaedics) with a mean follow-up of 35 months...

...There were 17 failures of this nature in patients with ASR implants (3.5%) and no failures in the BHR group...

...Patients who had adverse reactions to metal debris (ARMD) had a mean femoral size of 45 mm, a mean acetabular angle of 27° and a mean inclination angle of 53°. Among the asymptomatic patients, those numbers were 49 mm, 20° and 48°, respectively...

..."I think we can say that the most important points are that increased wear causes more complications, and all hip resurfacing systems are not the same," Langton said. "Also, we found that it is an issue of joint size and orientation, rather than an issue of gender. And size does matter because men with femoral components less than 49 mm have a 10% incidence of ARMD."

Reference:
Langton D, Jameson S, Joyce T, et al. The incidence of adverse reactions to metal debris (ARMD) following hip resurfacing with the articular surface replacement (ASR) and Birmingham Hip Resurfacing systems (BHR). Presented at the British Orthopaedic Association Annual Congress 2009. Sept. 15-18, 2009. Manchester.