We looked at 149 patients at our institution who received tripolar constrained liners and followed them an average of 4.2 years, where the average number of previous surgeries was 4. The indications for use of a constrained liner were recurrent dislocation in 82 of the hips and poor stability at the time of the revision procedure in 45%.
There are different types of fixation and they were about one-third, one-third, one-third in terms of revision of the whole acetabular component versus just putting a constrained liner into a compatible shell or cementing into an existing well-fixed shell in this group of patients.
Here’s the data—16 revisions (10.6%), but if you really ferret out that data only 3.3% of them—5 patients—had recurrent instability in that tripolar constrained population.
A unique problem we see with a constrained tripolar is the metal ring which goes around the polyethylene can disengage and present a problem and need for reoperation.
It’s important to differentiate the results. When you look at the literature and you talk about constrained implants, tripolar versus focally constrained, the recurrent dislocation rate in the tripolar constrained is less than 10% (Bremmer, et al. J Arthroplasty, 2003), and much, much higher in those that are focally constrained (Della Valle, J Arthroplasty, 2005); Berend, Arthroplasty, 2005; (Andersen ISRN Orthopedics, 2013; Chalmers, J Arthroplasty, 2016).
Same terminology but entirely different biomechanics.
Significantly less, so don’t mix up the two types of constrained implants when you talk about them.
At our institution, we’ve had a 96% success rate in terms of joint stability with a constrained implant for various revisions or recurrent instability.
What are the potential problems with dual mobility cups? The intra-prosthetic dislocation both I and Paul have referenced. The wear is still unclear, but probably won’t be a major problem. But when we use modular dual mobility cups, we put cobalt chrome against titanium, and I have some concern that that interface may be a problem long term just as we’ve seen with other metal-metal interfaces.
In conclusion, I still use tripolar sockets. I think they’re best with patients with severe neuromuscular disease, an absent abductor mechanism, failure of a dual mobility or when you want to avoid dissimilar metals.
I’m going to finish with a quote from Paul Lachiewicz at the 2015 Hip Society meeting, “…a cautious approach to dual mobility components for recurrent dislocation is recommended.” And I use dual mobility. And I like it as an implant, but I do think a lot of unanswered questions are still out there.
Moderator Maloney: Paul, you have one minute to rebut.
Dr. Lachiewicz: First, I now take an “audacious approach” not a “cautious approach,” based on my experience the last couple of years. Two questions, Tom. I do not understand the large number of failures. I have heard in the grapevine that at HSS you’re having more failures with these dual mobility components. I’m not quite sure why. Are these the ones with tumor prostheses? You know my friend, Stephen Jones from Cardiff, has done a meta-analysis of all the publications on constrained liners. He’s not as optimistic as you are about that. He reserves them for very select group that have almost massive destruction of the abductors.
And the other thing is, dual mobility may have three articulations, but the tripolar, I remember a paper from HSS where they said there were six different mechanisms of failure. You could pull it out in any of the different areas. And then finally, John Callaghan used to say, “I only have 3% dislocation with the constrained liner.” But if you add up his 10% loosening of acetabular components, 10% loosening of femoral components, periprosthetic fractures, the failure rate seems to skyrocket. It’s kind of how you define it.
Dr. Sculco: First, our experience with dual mobility is really quite good. I use dual mobilities both in the revision and the primary. And I use the ADM in my primary, high risk patients. I can tell you I’ve done 700-800 of them and I’ve had two dislocations. So, the dislocation rate is small in my experience. And I think in our experience in general. But I do think if you do have dislocation then this intra-prosthetic dislocation is going to be a potential issue.
