“Dual mobility does not show equivalence in wear, fixation, midterm results, complications, or cost. It is not an option for primary total hip, ” says Steve MacDonald. “It provides excellent stability and ROM…and it can reduce your dislocation rate to less than 1%, ” counters Edwin Su.
This week’s Orthopaedic Crossfire® debate was part of a landmark event, the first Brazilian CCJR meeting. The event, which took place in September 2014, was held in Iguassu Falls. This week’s Orthopaedic Crossfire® debate is “Dual Mobility in Primary THA: Yet to be Justified.” For the proposition is Steven J. MacDonald, M.D., F.R.C.S.(C) from the University of Western Ontario. Against the proposition is Edwin Su, M.D. of Hospital for Special Surgery (HSS). Moderating is Daniel J. Berry, M.D. of the Mayo Clinic.
Dr. MacDonald: “The original idea of dual mobility was a monoblock shell with a large poly interface that was smaller (usually 26mm or 28mm), and articulated with a stem. In 2014 we have choices. We have that construct, as well as a modular option with a modular shell.”
“Do these cups make sense? In a differential hardness bearing, the harder surfaces are normally articulating against the softer surfaces. Why is that? Because if you reverse it—which has been done in the lab—the wear rates are much higher. This has been shown time and again.”
“So let’s review the published basic science on this construct…OK, we’re done! There is only one published study on wear data—dual mobility wore 75% less than single. Don’t read the article, though, because it’s actually highly crosslinked poly versus conventional poly. So it’s a completely biased review.”
“We have advertised data that shows a 94% reduction in wear. That’s if you put your cup in at 65 degrees and you’re comparing it with metal-metal. That’s pretty biased data.”
“If you look at the design itself, the poly is free to go into a more closed position, which could theoretically increase edge loading and poly failure…but no retrievals have shown that. We have two articulating poly surfaces, so the potential is there for increased wear and osteolysis. And the claim is for improved range of motion (ROM) and stability. But once you go beyond 36mm/38mm you don’t get improved ROM or stability. So given our current constructs and published data, we don’t have to go to larger heads. We can achieve that same ROM and stability with current implants with registry published data.”
“Monoblock shells in registries have not done well; they have shown a propensity to not ingrow. The technique’s manual recommends these for osteoarthritis, rheumatoid arthritis, revisions, and patients with a dislocation risk. That could be everyone.”
“Let’s assume increased stability is the goal. One series of dual mobility implants showed about a 0.5% dislocation rate. Other authors have shown the same 0.5% dislocation rate, but current Medicare data say it’s about 2%. There are two recent reports with dual mobility. One paper—on primary hips—came out less than two years ago. It involved 168 hips at a mean of six years; they had a 2.4% intraprosthetic dislocation rate. That’s even a bit higher than the Medicare data. And in this construct they used the longest ball option and ended up with a 7.5% dislocation rate.”
“The other paper involved almost 2, 000 dual mobility implants; they had nearly a 4% incidence of intraprosthetic dislocation. There were enough of them that they had three different types. There are several reports of small series, but they’re often from the same centers and same authors. And in one series it selected patients at high risk for dislocation, which is not what’s being done, particularly in the U.S.”
“In my opinion this dual mobility should be applied to ones we don’t already have good answers for. Paul Lachiewicz said the same thing: ‘The greatest utility may be to manage recurrent instability in revision total hip.’ He is right. There are two very good papers showing that in revision applications for patients with recurrent instability, they do quite well. In this series, 5% were re-revised for instability.”
“Any new implant should show equivalence in wear, fixation, midterm results, complications, and cost. This construct does not do that. In my opinion it is an option for revision total hip for instability, not primary total hip.”
Dr. Su: “The dual mobility bearing can impart improved hip stability because the larger diameter ball has been shown to produce a greater jump distance. It provides excellent ROM and reduced impingement because of the greater head/neck ratio. Prosthesis dislocation is a leading cause of revision in hip replacement; I believe it’s on the rise because our patients are more active and younger. So dual mobility can help combat instability, and I believe that it’s preferable to constrained liners (which can lead to increased stresses and have multiple ways of failing).”
“I believe the indications for the dual mobility in a primary hip setting are for patients at increased risk of dislocation such as the elderly female or a patient with neuromuscular disease who can’t control and stabilize their hip. It would also apply to a revision situation, acute hip fractures, those with chronic steroid use, patients with hyperlaxity, etc.”
“The current design of the anatomic dual mobility cup includes an anatomic laterality design, has a cutout for the iliopsoas tendon, and reduces impingement of the metal neck upon the implant. There is a large head size with a hydroxyapatite coating for fixation, and there is a 3mm thickness (so a 6mm difference between the cup and the head diameters).”
“There’s also the modular dual mobility which allows for screw placement in a revision or dysplastic setting. A metal liner will engage the acetabular shell; therefore you will have a thicker construct. In terms of the head, it’s a larger diameter poly head…the inner head is constrained within this outer head. It can be ceramic or cobalt chrome. It’s highly crosslinked poly and the head is snapped onto the back table, and with this device you can press the head into the outer head and make sure that it’s engaged.”
“Some of the positioning advantages because of this increased stability: the dual mobility cup can be placed more anatomically with respect to anteversion. And there’s no need to add extra anteversion in order to make the hip more stable. However, the acetabular floor is not visualized with this solid cup and it can be difficult to insert. You can’t put in screws with the anatomic version, and intraprosthetic dislocation of the inner head from the outer head.”
“As for stability, I think that the jump distance is related to dislocation and the larger head size will provide a greater jump distance and impart stability. The larger head sizes have an increased resistance to dislocation. In terms of wear, a basic science simulator study by Stryker using that company’s dual mobility liner looked at wear at the different bearing surfaces, a micro separation model as well as a third body particle group. They found that the micro separation group was the one that had the most increased wear; overall they found that the bearing had a high tolerance for wear in this simulator.”
“In a study that looked at ROM prior to impingement with the dual mobility anatomic cup the ROM was greater because of that cutout. One study with the dual mobility found no dislocations in 10 years; one socket was revised for loosening, however it was a smooth type socket. Another study had 22 years of follow-up with no dislocations; there were, however, intraprosthetic dislocations. It’s generally considered to be from impingement…the stem they used had a very thick neck and they used skirted heads that were impinging and creating wear.”
“In Dr. Tom Sculco’s experience with over 400 dual mobility cups there were no dislocations. So it’s definitely a more stable joint that provides excellent stability and ROM…and it can reduce your dislocation rate to less than 1%. I would consider it for patients who are at risk for instability.”
Moderator Berry: “Ed, in the U.S. there are a fair number of people using these on a routine basis. Are you advocating for that?”
Dr. Su: “I’m only advocating it for use in higher risk patients.”
Moderator Berry: “Steve, you argued that there may be a role in high risk revisions. Is there such a thing as a high risk primary where you might see a role?”
Dr. MacDonald: “We all know that there are high risk primaries. The point is that there’s no data to suggest that the dislocation rate with this construct will be any less than if we use a 36mm head…which we can get to now.”
Moderator Berry: “Ed, your response?”
Dr. Su: “I’d argue that larger head size with a dual mobility bearing should reduce the dislocation rate. In a revision setting the dual mobility is better than a 36mm on poly then why not in a primary setting?”
Dr. MacDonald: “I didn’t say it was better in a revision. I said there are at least two published reports showing around a 5% re-dislocation rate. There’s nothing in primaries.”
Moderator Berry: “Ed, what is the cause of intraprosthetic dislocation in most cases?”
Dr. Su: “The retaining mechanism on the outer head fails, and the smaller head pulls out of the outer head. Typically, I believe it’s associated with wear of the locking mechanism.”
Dr. MacDonald: “It seems to be an impingement-induced wear over time. That being said, studies show that the mean time for that is around 5-6 years.”
Moderator Berry: “So if the dual mobility helps with dislocation then it’s probably going to be in the early phase.”
Dr. MacDonald: “Dislocation isn’t a one time phenomenon. I’m concerned that we’re going to see this slow, steady rise as you get impingement in this induced failure.”
Dr. Su: “The promise of the highly crosslinked poly is that it will be more resistant to that wear. However, as Steve said, it’s an impingement phenomenon and we know that highly crosslinked poly isn’t great with impingement either. We need to be careful.”
Moderator Berry: “Thank you, gentlemen.”
Please visit www.CCJR.com to register for the 2015 CCJR Spring Meeting, May 17 – 20 in Las Vegas.
