(AWR in HRA 1% @ 10 years)

All artificial implants wear and release debris into your body. If the level of debris generation is low, the body can tolerate it well for many years. There are many different bearing surface options. In HRA only cobalt-chrome metal on metal bearings work. There is only one bearing surface and no additional junctions that can wear or corrode.

With stemmed THR there are four bearing options:

• Metal on plastic (polyethelene)
• Ceramic on plastic
• Ceramic on ceramic
• Metal on metal
• Metal on ceramic

The most commonly used are listed first. In addition to the bearing, wear and corrosion can occur at numerous implant junctions in stemmed THR. These include: liner on socket, ball on stem trunion, and modular junctions.

In the past, plastic (polyethelene) wear was a major problem particularly in younger more active patients. Once produced, plastic debris has no way to exit the body. It accumulates around the implant and travels along the lymph channels but it never leaves the body. These bearings carried a 30% failure rate by 8 years in patients under age 50. If a large amount of plastic debris accumulates around the hip, the body’s immune system reacts against it and destroys bone. The result is often extensive bone destruction, which is called osteolysis. There is no way to measure the amount of plastic particles released into the body. Improvements in plastics (cross-linked polyethelene) appear to have solved this problem. The plastic wear rate has been reduced dramatically. But the down side is that these plastics are more brittle, increasing the risk of implant (liner) breakage with impact activity. As the new plastic liners have become more wear tolerant, we have moved into larger bearing sizes to improve stability. In this situation, the liner becomes thinner and breakage becomes a greater concern. These are not suitable for impact activity.

Ceramic on ceramic bearings produce the most benign debris. Cases of adverse reaction to ceramic debris are exceedingly rare. If an implant was chosen based primarily on debris generation, ceramic on ceramic would be the clear favorite. The amount of ceramic debris deposited in the body cannot be measured. Unfortunately these cannot be used for resurfacing. Ceramic fracture is also an exceedingly rare event with current ceramic bearings. But as ceramic bearings are made larger to improve stability, the liner becomes thinner and fracture may become a problem, especially with impact activity. The main problem currently is that larger bearing sizes are not available, making instability a persistent problem.

Metal on metal bearings are unique because very thin one-piece acetabular components can be manufactured in which there is a porous bone ingrowth surface on the outside, and a polished bearing surface on the inside. This allows us to reconstruct the hip of each individual patient with its natural bearing size, providing normal hip stability. This also allows us to preserve the femoral head for resurfacing. Alternatively, a large metal head can be placed into this acetabular component and attached to any standard total hip stem. This results in a very stable hip joint. All metal on metal bearings are made of cobalt chrome. The wear products are cobalt, chromium and traces of nickel and molybdenum, which are also present in the alloys used. In a well functioning metal on metal bearing the amount of wear debris generated is very small and is well tolerated by the body. It is deposited in the tissues around the hip. It is then absorbed into the bloodstream and excreted in the urine. A level can be measured in the blood or urine. I have used over 4000 metal bearings; the vast majority are well tolerated by the body.

If a well-designed metal bearing is implanted in the correct orientation, the wear rate is low and the small amount of wear debris is well tolerated. Numerous large long term studies have been done showing that these particles do not cause cancer. There are a handful of individual case reports of systemic metal toxicity. Most have these have not been in cases of metal bearings. None of the cases that I have treated for adverse wear reaction (AWR) have had any signs of systemic toxicity, despite extremely high metal ion levels being measured.

There is no evidence that the metal ions released form bearing wear cause cancer or kidney damage, and even systemic reactions are rare in cases of excessive bearing wear.

Adverse Wear Reaction (AWR)

Local inflammatory tissue reaction to excess metal wear debris is the problem. I have called this an adverse wear reaction (AWR). In plastic bearings, excess debris tends to cause more bone destruction and less soft tissue inflammation, in metal bearings there tends to be more soft tissue inflammation and less bone destruction. In cases of AWR to metal debris there is fluid collection, metallic staining of tissues, and production of inflammatory masses (not cancer). If severe reactions are not treated for years, muscle destruction can occur. But I have not yet seen any case of muscle damage.

We now recommend measuring blood metal ion levels on all patients at 2 years after surgery to assess their wear rate. We have found that this is an extremely helpful test to discover patients whose wear rate is excessive. Using blood ion level testing, we have found some patients who have an early AWR that is not yet symptomatic and I have revised them. On the other hand, we have found a few cases with high ion levels without AWR and we are monitoring them with yearly levels.

We have done extensive studies on AWR. Based on detailed studies of hundreds of patients with accurate x-rays and ion levels, we have been able to develop a “safe zone” for placing acetabular components to prevent AWR. This paper will be published in Journal of arthroplasty in 2013. This safe zone is based on bearing size. Although safe zones have been published previously for plastic bearings (to prevent dislocations), they are very unreliable. Our safe zone reliably prevents both AWR and dislocation with >99% accuracy. This has never before been achieved for any type of hip replacement type.

The cause of the AWR in metal bearings is now fairly well understood by most hip resurfacing experts. I will try to summarize the problem briefly. AWR occurs when a metal bearing exhibits an abnormal wear pattern called edge loading. When this occurs, the natural fluid lubrication film breaks down and an extremely high wear rate ensues. A large amount of metal is produced and deposited in the surrounding tissue leading to the AWR. Elevated metal levels will be measured in the blood. There are 2 factors that will result in edge loading; an acetabular component that is shallow by design, or a component that is placed in too steep of a position (High inclination angle as measured on a standing pelvis x-ray).

The recalled Depuy ASR implant had an excessively shallow design, this is one reason it had a 30% failure rate within 5 years. It is a quirk in design that in virtually every resurfacing system on the market, the smaller bearing sizes have shallower acetabular components than the larger sizes for the same brand. This is the primary reason why smaller bearing sizes carry a higher risk of AWR than larger bearing sizes in every system. Women tend to have smaller bones and typically require the smaller sizes; therefore they have higher rates of AWR. AWR occurs when an acetabular component is implanted too steeply. The more steep the acetabular inclination angle (AIA), the less well the head is “captured” and the more likely that edge loading and AWR will develop.

In our studies, no components with AIA< 50º on standing pelvis x-ray have developed AWR, while about 5% of those with AIA>50º have had this problem. We have also had no cases of AWR in bearing sizes >48mm. But the AWR can be avoided in smaller sizes, simply by implanting them with lower inclination angles. This allows the shallower component to function well without edge loading. Our paper on safe zones makes this very clear.

In addition to the fiasco with the Depuy ASR (total hip and resurfacing) recalled in 2010, there was a very disturbing report published by the Oxford Group in the JBJS British Journal in 2008. This was the first major clinical series highlighting the problem of AWR. This group found a 4% @ 8 years rate of AWR in a series of 1400 cases while using proven well designed implants. They called these failures “pseudotumors” and speculated that they may be severe allergic reactions to metal. Most disturbingly, they reported that these failures were not related to steeply implanted acetabular components. We have now published a larger report of 2600 cases with a 1% rate of AWR in 10 years in the journal hip international 2013. Our data clearly indicate that AWR is caused by steep component placement, particularly in smaller implant sizes. All 8 of our cases had AIA>50º and bearing size

To make matters worse, the Oxford Group soon published a report on the results of their revision surgery for “pseudotumors”. It showed disastrous outcomes for these patients. I have not yet published my outcomes for revisions of AWR (my own 8 cases plus others referred to me from elsewhere), but can assure you that my results again are diametrically opposed to their report. I have not even transfused any of my cases. The revision surgery for AWR is actually quite straightforward and carries only a slightly higher risk than the original operation.

The combination of the DePuy ASR disaster and the poor results with good implants from the Oxford Group, have scared many surgeons away from large metal bearings for stemmed THR and resurfacing. This is a shame, because we now know that the problem is virtually completely avoidable with proper implant choice and placement. These large metal bearings are the only ones that provide a truly stable hip joint and avoid the most common problem in hip replacement: recurrent dislocation.

Metal Ion Testing

We have made a concerted effort to obtain metal ion testing on all of our patients who are at least 2 years postoperative. It is important to recognize that metal ions found in the blood are not in and of themselves a problem. We use blood ion levels in two ways. First, they are used a screening test to see if AWR are developing in asymptomatic patients. We have found occasional cases where this is true. Secondly, in patients who are having pain, a low ion level is very good evidence to rule out an AWR. If a metal ion level is high we then perform either a metal suppression CT or MRI scan to see if a substantial fluid collection or soft tissue mass (not cancer) is present.

Small amounts of fluid can be normal around well-functioning implants (A hip replacement or resurfacing is not a normal hip). A large collection together with a high ion level is diagnostic of AWR. This is then confirmed at the time of revision surgery with extensive metal staining of the tissues. If the metal level is high but there is no fluid collection we recommend yearly monitoring with ion levels. We are not certain what the long-term outcome will be in these cases, but we don’t think it is dangerous to observe these patients. Sometimes metal levels can be elevated when an implant is loose (this is symptomatic).

There are no accepted guidelines regarding what constitutes an acceptable ion level. We have extensive experience and have developed our own internal guidelines. We consider any level below 10ug/L safe and would only recommend further testing in a significantly symptomatic patient. Patients who have bilateral implants or even joint implants elsewhere in the body may have higher levels, as could patients with kidney dysfunction, or those who are taking vitamins or supplements (they may contain chromium even without listing them).

Normal levels as reported by the labs (for patients without implants) are below 2ug/L. As we are getting better at placing these components, we are seeing a trend of more patients with very low levels, even many in the normal range. There is a period of “run in wear” with these bearings that lasts 1-2 years. Ion levels are somewhat higher during this time. This is why we have chosen 2 years as our starting point for testing. Long-term studies have shown that there is a general trend of ever declining levels out to 10 years.

One study has shown that ion levels for total knee replacement are similar as for a well-functioning metal on metal hip replacement. Knee replacements have a cobalt chrome femoral component that articulates on a plastic liner; they are not metal on metal bearings. About 300,000 of these are done annually in the US. There is no evidence that mildly elevated metal ion levels have caused any clinical problem in TKR over the last 2 decades. This is further evidence that mildly elevated metal ion levels are not harmful.

In conclusion, ion levels will be elevated in HRA, but there is no evidence that this harms the body. Measuring ion levels is primarily useful in diagnosing local (around the hip) adverse wear reactions (AWF).