Unexplained Pain May Be Elevated Serum Metal Levels From Metal/Plastic!!
Question: What presents like a failed metal/metal implant, but is actually metal/plastic? Craig Della Valle, M.D., wanted to know. Dr. Della Valle, an orthopedic surgeon at Midwest Orthopaedics at Rush and associate professor at Rush University Medical Center in Chicago, tells OTW, “My colleagues and I began to see a spate of patients with pain that we couldn’t explain. As we evaluated them we figured out that the problem was a corrosion reaction between the modular head and stem. We figured that our colleagues around the country must be getting these cases as well…and that most of them probably didn’t know what they were seeing.”
“These patients present like cases of failed metal/metal, but they have a metal/plastic construct. A couple of years ago we reported on ten cases and then recommended that these patients be treated with a ceramic femoral head and a titanium taper sleeve adapter; we thought that removing the cobalt from the metal head would solve the problem. We have now treated 31 patients using this protocol, namely, changing them from a cobalt chromium head to a ceramic head with a titanium taper sleeve adapter. We had two recurrences and both were early in our experience where we used a new cobalt chrome head instead of ceramic one. The balance of the patients have shown not only that they are doing well clinically, but that they have reductions in their serum metal levels to <1 part per billion, which is what we expect for a well functioning metal on poly bearing.”
Those at Rush arguably have the most experience with such cases. Thus, says Dr. Della Valle, “You should look for patients who present with postoperative pain where you have ruled out infection and loosening and you are unsure of the diagnosis. In general, we recommend getting the serum metal levels checked. Most surgeons who see someone with a metal/plastic construct are not going to think that it would make sense to get serum metal levels but that’s exactly what you need to do. When it comes to interpretation, things get tricky. Guidelines from the Medicines and Healthcare Products Regulatory Agency in England indicate that serum metal levels should be less than 7 parts per billion—and that number is stuck in everyone’s minds from metal on metal bearings.”
“Our work shows that in metal/poly bearings it should be below 1 part per billion…above that is abnormal. If a patient has metal/poly serum with a cobalt level of 2 a surgeon may think, ‘Well, it’s only 2.’ In fact, that is highly abnormal! The third piece of the puzzle is that cobalt will be elevated over chromium. If the cobalt level is 7 and the chromium 0.2 then what you are seeing is probably a corrosion reaction. I would say that without even seeing the patient. We recommend getting a metal artifact reduction sequence MRI to confirm the diagnosis. You may find that the MRI shows an adverse local tissue reaction similar to what you would expect from a metal on metal bearing that has failed.”
Pre-operative Hemoglobin Protects Against Infection in TJA
Accuracy is everything in research. To that end, researchers from The Rothman Institute set out to determine risk factors for surgical site infection after total joint arthroplasty (TJA) using the most reliable data source possible: National Healthcare Safety Network (NHSN) surveillance program of the Centers for Disease Control and Prevention (CDC). Mohammad Reza Rasouli, M.D., a former research fellow at Rothman who is now a surgery intern at Mercy Health System, tells OTW, ”While surgical site infection is rare after TJA, it is particularly devastating. We set out to identify patient related risk factors for surgical site infection following primary and revision TJA. We used our database, as well as data reported from our institute through the CDC’s NHSN surveillance program.”
“The CDC data is considered to be more reliable because infections can be captured through multiple paths in addition to infection surveillance staff. For example, even if someone is admitted to another hospital for surgical site infection a year later than the index TJA that gets captured. This is much more than a retrospective chart review.”
“We reviewed more than 6, 100 hip and knee surgeries from 2010-2012(either primary or revision cases). We found that the rate of infection was low at 1.3% (80 cases), which is similar to other studies. The highest rate of surgical site infection was in revision total knee arthroplasty, followed by revision total hip arthroplasty. Based on suggestion of the biostatistician we worked with, only few variables that were most likely to affect the rate of infection (age, gender, type of surgery (revision or primary), comorbidities, and preoperative hemoglobin level) were selected. A higher Charlson Comorbidity Index was predictive of surgical site infection, as was the male sex.
In the one year that we followed cases we found that the higher the number of comorbidities the higher the rate of infection. The most interesting finding was that a higher pre-operative hemoglobin level was protective against infection. This is promising, because it is a modifiable risk factor.”
Tibial Tunnel Placement: A Bit More Clarity
In ACL (anterior cruciate ligament) reconstruction, should you place the tibial tunnel posteriorly or anteriorly? If anteriorly, how far? Who knows? Mark D. Miller, M.D., the S. Ward Casscells Professor of Orthopaedic Surgery at the University of Virginia, is working on some answers. He tells OTW, “While a lot of research has focused on femoral tunnel placement, not much has been published on tibial tunnel placement in anterior cruciate ligament reconstructions. In fact, one recent multi-center study suggested that tibial tunnel placement is acceptable anywhere between 30% and 55% of the way across the tibia. Last week we completed a cadaveric study looking at impingement with anterior tibial tunnel placement. In the old (trans-tibial) technique for ACL reconstruction, we drilled the femoral tunnel through the tibial tunnel; this usually resulted in more vertical femoral tunnel placement. The problem was that this also often led to roof impingement, especially with anterior tibial tunnels. We decided to look at independent femoral tunnel drilling to see if it also leads to this problem.”
“Everybody was placing the tibial tunnel posteriorly in order to avoid roof impingement, but it may not be the best place because studies (although limited) suggest that anterior placement is both biomechanically and clinically superior. It turns out that with this independent femoral drilling you get less roof impingement…so you CAN put it more anteriorly.”
Dr. Steve Howell looked at two dimensional models and he advocated the use of hyper-extension radiographs to determine whether the tibial tunnel impinged on the roof of the femur. We utilized 3D models and drilled more anteriorly and then passed a Goretex graft into the tunnels so we could look for impingement on 3D CT scans. We compared trans-tibial and independent tunnel drilling. The 3D model verified that there was less impingement with independent tunnel drilling, and that we can now safely drill tunnels more anteriorly.”
“The next phase will be a clinical study to be carried out at the University of Virginia and the University of Kentucky where we will randomize patients into anterior versus posterior tibial tunnel placement. We will drill two tibial guide pins before drilling the tibial tunnel—one will be less than 35% of the way across the tibial plateau and the other will be greater 35% of the way across. After guide pin drilling, we will open a randomized envelope and determine which pin to drill over for the tibial tunnel. We will attempt to get at least 100 subjects in each group.”
“There are some who advocate for more posterior tibial tunnel placement…but there are several biomechanical studies and one nonrandomized clinical study that found that anterior tibial tunnel placement is superior. Our overarching goal is to provide better guidelines for surgeons. We may end up at a point where the best approach is to use intraoperative fluoroscopy to verify tunnel placement before drilling.”
