This Simple Question Can Be Challenging for a Surgeon
One of the most challenging decisions a surgeon can make when revising a patient with a Stryker Accolade TMZF stem that has failed due to metal wear disease (corrosion and fretting) is whether to leave a well-ingrown stem in place. This would also be true in any trunnion failure case regardless of manufacturer.
Currently, the orthopedic and scientific community is trying to grasp the scope and danger associated with corrosion and fretting failures at the femoral head and stem interface or trunnion. Every manufacturer has thousands if not hundreds of thousands of titanium stems with metal femoral heads implanted in patients so this is a huge issue.
Why the sudden uptick in failures? I believe two factors are relevant. First, over the past ten years manufacturers have designed and released larger and larger metal heads. They have done so under the guise of providing surgeons with an option that will lessen the risk of dislocation. Since the true risk of dislocation in competent hands is less than 2%, this is another example of industry creating a solution to a problem that does not exist. It only provides yet another marketing advantage. At the same time, trunnions and neck profiles have gotten smaller and smaller under the guise of lessening the risk of impingement. Again, a risk that is extremely low in competent hands. Skilled surgeons tell me that if an implant is placed in correct, anatomic alignment the hip will not dislocate or impinge.
Second, since most of these devices are not “recalled” many surgeons have simply failed to make the appropriate diagnosis leading to under reporting of adverse events. Metal wear disease is a relatively new concept in orthopedics. As such, non-recalled devices don’t raise eyebrows when patients return complaining of pain and limited mobility. I’ve seen surgeons follow such patients for years giving repeated steroid injections, aspirating the hip, suspecting infection and sending patients back to rehab without ever making the correct diagnosis. So, simple lack of recognition has led to under-reporting. But, more recently the medical literature has started to address this situation. Stryker recently recalled thousands of metal heads. The issue is becoming more prominent leading to quicker, more accurate diagnosis. The issue has simply risen to the top.
When Trunnions Get Smaller and Heads Get Larger
What is the result of smaller trunnions, lower profile necks and larger heads? Well, if you were to pose that question to Sir John Charnley over 50 years ago he would say that larger heads increase the rotational forces on the head (torque) which is directly transferred to the head/neck interface (trunnion). As trunnions get smaller, the larger forces created by larger heads are borne by smaller surface contact. The result is more motion at the interface between head and stem. Motion = fretting or wearing away of the metal (actually the passive layer of oxides that form on the metal) and corrosion occurs. The additional factor of smaller neck profiles exaggerates the motion and simply makes matters worse. The laws of physics did not change over the past 50 years! Why ignore the sage wisdom of your forefathers?
When a titanium stem and metal head (most frequently a chromium/cobalt alloy) move upon one another and corrosion occurs there is actual material loss from both the chrome/cobalt head and the titanium stem. It’s typically microscopic and almost undetectable to the eye. But, the amount of chromium and cobalt that are given off during the corrosion process is substantial and biologically devastating to tissue and bone. Surgeons have taken to calling this failure, “metallosis.”
What Is Metallosis?
Metallosis is simply toxicity to tissue and bone when exposed to chromium and cobalt. Tissue, muscle and bone die. Frequently abnormal masses of tissue grow and these are referred to as pseudotumors. Large collections consisting of milky, purulent appearing fluid can also occur and are often mistaken for infection. If allowed to progress, metallosis can destroy very important muscles as well as bone.
The surgical “fix” for metal wear failure is to remove the offending device and replace it. But, there is substantial debate among surgeons as to what should be removed. Many doctors are simply removing the metal head and replacing it with a ceramic head, leaving the well ingrown femoral stem in place. That would include the potentially worn trunnion upon which the new head is placed. Others are removing both the stem and head out of concern that an “eyeball test” of the trunnion is sufficient.
Surgeons are very hesitant to remove a well ingrown stem. The procedure is time consuming and frequently requires cutting a large window in the femur followed by placing several cables to hold the bone together. Rehabilitation time is greatly increased. Complications are not uncommon. So, if you can get away with leaving the stem, the temptation is to do so. But, at what cost?
I actually attended an American Society for Testing Materials (ASTM) subcommittee on Medical and Surgical Materials meeting devoted entirely to this topic. At that meeting there was substantial disagreement over whether an intraoperative “eyeball test” of a used trunnion was sufficient to insure integrity. I have also spoken with numerous surgeons who have expressed great concern over this issue. They have no idea what the long-term consequence is when a new head is impacted on a worn, used trunnion. It scares them.
L-Fit V40 Head Revisions
Finally, this week I had the opportunity to review a revising surgeon’s pre-operative office note. The surgeon is well-known and was revising a failed Stryker Accolade TMZF stem that had been implanted with Stryker’s L-Fit V40 (Low Friction Ion Treatment) chrome/cobalt head. This surgeon has been a Stryker consultant in the past and has worked on design teams creating new Stryker products. The head was one Stryker has recently recalled (36mm +5). All of the appropriate pre-revision work-up indicated the devices had failed due to metal wear. Even before he performed the surgery, this Stryker consultant planned to remove the stem. In the final pre-operative office note he said, “I would not sleeve this and convert it over to ceramic. There is too much damage to the trunnion. This is a small trunnion and I do not think sleeving these is reasonable.” He made the comment about too much trunnion damage before surgery, well before he even saw it.
This is the first time I have seen a surgeon commit to writing the thought process I have heard so many voice and what some advocated at the ASTM conference. It confirms my suspicion that thought processes are changing and more surgeons are going to be unwilling to leave a worn trunnion/stem in place. Of course, the risks of leaving a worn trunnion behind will not be known until well down the road when and if they fail.