“Dual mobility is an option for revision THA [total hip arthroplasty] for instability, but not for primary THA, ” states Steve MacDonald. Not exactly, says David Stulberg, “There is evidence showing that for revisions and primary the dislocation rate was significantly lower than rates for non dual mobility liners.”
This week’s Orthopaedic Crossfire® debate is “The Dual Mobility Poly Liner: 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 S. David Stulberg, M.D. from Northwestern University in Chicago. Moderating is Robert T. Trousdale, M.D. from Mayo Clinic in Rochester, Minnesota.
Dr. MacDonald: “The original idea of dual mobility was a monoblock shell, a large poly articulation, a smaller head, and then a femoral component. In 2013 there are two options: the original ADM [acetabular dual mobility] and the MDM [modular dual mobility], which is the modular version.”
“Does it make sense? In a differentiated hardness bearing, the harder surface is normally articulating against the softer surface. So you have a metal or ceramic head articulating with a softer surface on the bottom. If you switch that around and make it soft-on-hard, it escalates the wear. So from a biomechanical point of view it doesn’t make a lot of sense.”
“Looking at the published basic science on this construct, I could only find one article (Journal of Arthroplasty, 2013, Loving et al.). It said that the dual mobility liner actually wore 75% less than a single articulating bearing. It makes sense if the dual mobility is highly cross-linked and is being compared to a conventional gamma/inert poly. The study was not ideally designed.”
“We do have advertisements in the literature that show a 94% reduction in wear with a 65 degree shell. So if you put your shells in like Rob Trousdale maybe you should use this construct. If you look at the design, the poly is free to go into a more closed position, but the potential is for edge loading and poly failure. And we have two articulating poly surfaces now, so there is a cumulative risk of increased wear and osteolysis. The claim is for improved ROM [range of motion] and stability, but we know from cadaver work that hip ROM peaks at around 36-38mm. Using larger and larger heads doesn’t give you ROM because you just reproduce bone on bone impingement.”
“We can achieve that same ROM and stability with current implants that have registry published data. So if you look at the design you have a monoblock shell. In the Australian registry monoblock shells have a higher revision rate; fair enough, most are with metal-metal constructs.”
“So who is this being recommended for? It is unclear. In the technique manual it says, ‘OA, RA [osteoarthritis, rheumatoid arthritis], revisions, and patients with a dislocation risk.’ That could be every patient! So let’s assume that increased stability is the primary goal. David Stulberg has published an article (Orthopedics, 2010) looking at several series, and found a dislocation rate of 0.4%. There are similar published dislocation rates with conventional THA.”
“There is a recent report by Remi Philippot of a new complication (CORR, 2013). A large series showed a 0.4% incidence of intraprosthetic dislocation (ball from poly). A 2012 article (CORR, Hamadouche) showed a 7.5% incidence of intraprosthetic dislocation. In these constructs they had larger heads, longer balls, no skirts, and they were creating impingement.”
“So which patients should a dual mobility implant be used for? How about ones for which we don’t already have a good answer? Lachiewicz wrote an excellent review article (JAAOS, 2012) saying, ‘Caution is advised in the routine use of dual mobility in primary and revision THA. The greatest utility may be to manage recurrent instability in revision THA.’ A study by Hailer et al. (Acta Orthopaedica, 2012) found—using Swedish registry data—the exact same thing. They studied 228 revisions for instability using dual mobility. There were 8% revised for any reason, but only re-revised for instability.”
“So conceptually there are challenges with the construct…and any new implant must show equivalence in wear, fixation, mid-term results, complications, and cost. We’re not there yet with dual mobility. Today it is an option for revision THA for instability, but not for primary THA.”
Dr. Stulberg: “While we’ll be discussing the femoral head, there are many causes for recurrent dislocation. So the rate of dislocation decreases with the use of larger head sizes. The problem is that it’s unclear whether these large heads have an adverse impact on polyethylene; the impact of using large heads in patients at risk is also unclear. You’ve also heard the concern about using large heads on the Morse taper. So is there a role for dual mobility when you’re concerned about instability?”
“In the 1970s the French orthopedic community was beginning to worry about the issue of hip stability and poly wear. They were especially concerned about what they thought were causes of instability, things such as underlying diagnosis and cognitive dysfunction. But over the years the French have published many series where the dislocation rate in primary THA is very low (<0.4%)—lower than those published in general. The dislocation rate in revisions is very low (2.1%), much lower than those published in the same time period—before non dual mobility.”
“The survival rate for a non-highly cross-linked poly dual mobility cup was surprisingly long (Leclerc, 1999, 94% at 15 years; Philippot, 2006, 95% at 10 years, etc.). When the French looked at this again last year (CORR, Delauney, 2012), they compared their registry data on dislocation to the dislocation rate in other countries. Use of the dual mobility cup in France is 30% or more; their dislocation rate is lower (Australia, 27.2%; New Zealand, 30.6%; Delauney study, 10.4%). When they compared the use of the dual mobility cup in France to other countries, this same data tended to flush itself out.”
“But there are other ways of using dual mobility cups, such as with a cup cage construct. It’s an interesting opportunity in situations with acetabular deficiency, one that has been associated with a very low dislocation rate (CORR, Civinini et al., 2012). When Mike Mont reviewed all of the series—not just the French—he found that in both primary and revisions, the dislocation rate was significantly lower than rates for non dual mobility liners.”
“But is the dual mobility cup just another large femoral head? On this we have more company data than non-biased data. Because of the construct, a dual mobility cup, for example, in a 54mm outer diameter shell, has a greater ROM than the largest femoral head you can put in that same 54mm outer diameter shell. And the ‘jumping distance’ for that construct is greater, so they behave differently than femoral heads of equivalent size.”
“What’s more interesting is whether you can improve wear by using this construct. Wear seems to be better when you compare it to conventional poly; when you use dual mobility cups with highly cross-linked poly the wear is even less. Then it comes down to this: if you have an unstable hip or an at risk patient, you want to use a large head and you have a relatively small acetabulum, are you going to use a large head with thin poly or will you use a dual mobility cup? There is an option. If you take this modular dual mobility operation and put the shell in, you can decide on the table what you want to do. This seems to be a reasonable approach.”
“So the dual mobility cup offers a safe, effective, versatile solution for hip instability.”
Moderator Trousdale: “So you agree in that in a routine primary total hip a dual mobility should not be utilized. Who would you use this with in a primary? ”
Dr. MacDonald: “No one…I haven’t used it.”
Dr. Stulberg: “I would consider it in patients with dysplasia or large patients who have small outer diameter acetabular shells…where you may want the option of a large head, particularly in a young patient where you’re concerned about poly wear.”
Moderator Trousdale: “In a revision setting, Steve?”
Dr. MacDonald: “I’d use it in a case where other options have failed. In terms of all those primaries, for years we have been doing other things for those patients. There are other ways around it than having to go to a construct that doesn’t have broad spectrum registry data.”
Dr. Stulberg: “I’d like to have it in a situation where you have a revision and a recurrent dislocator, in particular a dislocator with significant acetabular bone loss. You might want to do a major acetabular reconstruction and in addition provide stability and not be constrained by the cage.”
Moderator Trousdale: “Steven, I presume you’re using constrained liners in that situation. Are those a better option than a dual mobility?”
Dr. MacDonald: “I don’t think we know the answer to that. The problem with a constrained liner in a younger patient is the cumulative risk of wear and perhaps increased stress and loosening. It depends on the surgeon and what they are used to.”
Moderator Trousdale: “David, is wear a real concern for these patients with a high risk of instability?”
Dr. Stulberg: “In the impingement situation it might be. If you have soft tissue impingement with these devices they can dissociate.”
Moderator Trousdale: “Steve, have patients failed from wear with the dual mobility?”
Dr. MacDonald: “No.”
Moderator Trousdale: “So it may be a theoretical concern?”
Dr. MacDonald: “Definitely. When you have small series of things they may look good, then we start seeing thousands and all of a sudden it doesn’t look so good.”
Dr. Stulberg: “Because of the way this technology is being ‘teed up, ’ we need to think about in a patient with a 52mm outer diameter shell in which you’re worried about stability then you can use a 36mm head with thin poly…or an equivalent or slightly larger head with a poly ball. At this point we still have to wait and see which one wins.”
Moderator Trousdale: “David, what is the etiology of an intraprosthetic dislocation with a dual mobility?”
Dr. Stulberg: “The predominant one is where you have impingement, so if that outer bearing isn’t moving so that the inner bearing starts to flex up and the outer bearing doesn’t have the opportunity to rotate, that’s where most dissociations occur.”
Moderator Trousdale: “How do you solve intraprosthetic dislocation?”
Dr. Stulberg: “It’s a theoretical question for me at the moment, but I would likely flip it and do a fixed head.”
Dr. MacDonald: “Not for long!”
Moderator Trousdale: “Steve, the modular dual mobility construct has a titanium shell and a cobalt chrome insert with a taper. Should we be concerned about corrosion?”
Dr. MacDonald: “We’re just exploring these things…that’s why I say that just because we don’t have data doesn’t mean it’s good.”
Moderator Trousdale: “Thank you.”
Please visit www.CCJR.com to register for the 2014 CCJR Spring Meeting, May 18 – 21 in Las Vegas, Nevada.
