6. LOWER ODDS OF DEBRIS FAILURE

Hip resurfacing now has a lower chance of debris failure than total hip replacement.

Excess wear or corrosion debris can cause failure due to tissue irritation. In HRA, the problem is wear due to a malpositioned cup (risk 0/4000 in the last 12 years). In THR, the problem is corrosion of the trunion of the head/neck connector (risk 1-5% by 10 years).

The last wear failure I experienced was a hip resurfacing arthroplasty (HRA) implanted in 2009, over 4000 cases ago. HRA does not have a trunion to corrode. In total hip replacement (THR), wear failures have been extremely rare in the last 10 years with the use of improved plastics (antioxidant stabilized crosslinked polyethylene), But failures due to trunion corrosion in THR have become more common, resulting in 1-5% failures by 10 years. Both wear failures in HRA and trunion corrosion failures in THR are mainly caused by extreme tissue irritation of the resultant debris.

The reason many hip replacement experts cite to claim THR is superior to HRA is because of failures due to inflammatory tissue reactions to debris. This process has many names such as ARMD (Adverse Reaction to Metal Debris), ALTR (Adverse Local Tissue Reaction), ALVAL ( Acute Lymphocytic Vasculitis Associated Lesion), “ion problems”, “pseudotumor”(false tumor), or “metal allergy”. The terminology is confusing and even scary; these processes are not fully understood by joint surgeons and are often used as a scare tactic to persuade patients to avoid metal-on-metal hip resurfacing. But the problem of inflammatory tissue reaction is now more common in THR than in my HRA. There are several complex and intertwined issues at play here; I will try to explain them.

Any artificial joint replacement is subject to failure due to corrosion, or excess wear. All corrosion/wear debris has the opportunity to cause soft tissue or bone irritation and damage as well as pain. A small amount of debris is well tolerated by most people. If the amount of debris gets too large, soft tissue irritation, muscle damage, and bone loss can occur. How does debris cause tissue irritation?

Allergy is not a cause of failure. It is likely that some people are more sensitive to debris than others, but “allergy” to internal metal or plastic particles does not seem to occur. For example, skin rashes to nickel touching the skin (cheap jewelry) occur in about 20% of the population and can be verified by a skin patch test. People who have a positive skin patch to nickel are not more likely to suffer residual unexplained pain or frank implant failure in joint replacement surgery. Cobalt chrome alloy contains a trace amount of nickel and there is a pervasive belief that implants made of this alloy can result in failure in “allergic” patients. Skin patch studies refute this hypothesis, but it persists. Others claim that a Lymphocyte Transformation Test (LTT) is a better way to asses allergy than skin patch tests. There is no validation of this blood test for this purpose. We attempted to validate this hypothesis and found the LTT done before surgery to be unable to predict patients with unexplained pain or implant failure. Allergy has never been demonstrated as a plausible cause for failure by any scientific study. My interpretation of the problem is as follows.

Joint replacements don’t recreate totally normal joints. Most patients with really bad joints feel vastly improved. But having some residual symptoms is “normal”. Certain failure modes or causes of residual pain are understood; these causes of failure are “explained” and can sometimes be corrected or improved with further surgery. Causes for residual pain after joint replacement continue to be discovered, but many are elusive and may be discovered with time. Unexplained residual pain is seen in 30% of the total knee, 20% of the total hip, and 2% of my HRA patients. When a patient has “unexplained” pain, “allergy” is often used to explain the cause. One danger is that some surgeons recommend risky revision surgery to resolve the unproven “allergy” when no evidence exists that this will work.

Some causes of residual pain are inherent in the operation. For example, residual thigh pain in stemmed THR is a major problem for many patients who wish to participate in high-impact activities and a less common problem for those satisfied with regular activities of daily living (ADL). It is caused by abnormal stress transfer from a stiff THR stem to a more flexible surrounding femoral bone. There is no solution to this problem. The prevention is to perform an HRA instead. 30% of THR patients experience at least some thigh pain, and 3-5% have moderate to severe thigh pain. Impact sports are limited by this problem

Metallosis is a real problem. Unlike “allergy”, there is a strong body of evidence that helps us understand the cause, solution, and even prevention of this problem. Metallosis can also be called adverse wear-related failure (AWRF). It is caused by overloading the hip with cobalt-chrome wear debris. Again, a small amount of debris is well-tolerated, but excessive debris irritates the tissue causing pain and it can gradually damage muscle and bone. This requires revision surgery. If done by an expert, the success of the revision is similar to any other revision. We have published a 96% 8-year implant survivorship for HRA revision for any cause. Permanent severe muscle damage crippling the patient has been reported by some centers for AWRF revision. I would suggest their high failure rate is due to inexpert surgical technique.

Cobalt-chrome (CC) metal-on-metal (MoM) bearings exhibit a very low wear rate when tested with fluid lubrication in the lab. In fact, the wear rate is lower for MoM than for metal or ceramic on plastic (MoP, CoP). Only ceramic on ceramic (CoC) sheds less wear debris. All of these bearings shed so little wear debris that tissue irritation should never occur. If the wear rate were the only consideration, CoC would be the ideal bearing choice. But many other factors (beyond the scope of this article) are also at play. Why then do we see metallosis with MoM bearings?

The reason, first discovered by Koen DeSmet and then further elucidated by us, is that edge loading causes excess wear. When MoM implants are “ideally” positioned, extremely low wear results – the implant should never “wear out” in a hundred years. If the socket component is placed too steeply or too anteverted (forward tilt), edge loading can occur. Edge loading occurs when the bearing “contact patch” is too close to the edge of the socket. The main point of contact between the head and socket and its patch of greatest wear in the standing position is called the “contact patch”. The degree of socket tilt (inclination and anteversion) and the tilt of the pelvis in the standing position influence the position of the “contact patch”. If the socket is placed so that the contact patch is too close to the edge of the socket, edge loading is said to occur. When a bearing is edge-loaded, the normal fluid film that the bearing runs on escapes, and the wear rate goes through the roof. Although CC bearings have a lower ideal wear rate than all other bearings except CoC, they are much worse during edge-loading conditions. But other problems are caused by edge-loading: loud squeaking in the CoC and liner breakage in MoP bearings.

Edge-loading is more common when the acetabular coverage arc (depth) is lower. One brand (Depuy ASR) of metal on metal THR and HRA was designed with a much lower coverage arc than all other brands and was therefore much more likely to be implanted in a position to result in edge loading. This was the main reason that the DePuy ASR had a disastrous 30-50% five-year failure rate. Small implant sizes in all brands have lower coverage arcs than larger bearing sizes; this explains the higher AWRF rate in women.

This problem has 3 possible solutions. Taking smaller implant sizes off the market (BHR), changing socket design to equalize coverage arc on all implants to 165º (Conserve plus), or learning to implant all sizes in a safe position to avoid edge-loading. The latter has been my solution.

After Koen DeSmet noticed that AWRF in HRA occurred more commonly in women with steep cup positions, we began to study this further. In 2013, we published a paper based on an analysis of over 700 cases. We were able to calculate a “safe zone” for socket position which avoided AWRF. This safe zone was named RAIL (Relative acetabular Inclination Limit). A “safe zone” to avoid edge loading exists for all implants, but the size of the zone increases with increasing bearing size. While we were working on this paper, we also worked on techniques to make sure that the socket component could be placed within this safe zone.

By 2009, we were able to develop an intraoperative x-ray technique called NSIOR (normalized to standing intraoperative radiograph) that allowed us to be certain that every single patient (including for smaller bearing size) could be implanted within the safe zone. The last socket that was too steep was in 2009. No further AWRFs have occurred in the 13 years since that time; I have performed over 4000 HRA since then. We then followed up with another study published in 2019 verifying that 100% of 2500 consecutive cases could be placed within RAIL and that this would in fact prevent AWRF. This study “validated” the RAIL safe zone that we had created previously with an entirely new set of patients.

Problem solved. AWRF no longer occurs in HRA if the RAIL safe zone is respected. Meanwhile, trunionosis in THR is a cause for failure in 1-5% of cases by 10 years and no solution is in sight.