One of those studies came from Tom Sculco’s place and his son was a co-author (DeMartino, et al., Bone Joint Journal, 2017). They reviewed 59 articles and came to the conclusion that a dual mobility implant is certainly preferred to anything else in terms of the rates of dislocation. In primary less than 1%, revision 3%.
There is also a second review from Rush (Darrith, et al., Bone Joint Journal, 2018) and a third one with a small number of articles (Pitukanotai, et al., Eur J Orthop Surg, 2018).
Can dual mobilities fail? Of course. Anything can fail. This can also dislocate, but there are two particular ways it can dislocate. The large polyethylene “head” can come out of the metal shell. Or you can actually have a pull out of the small either metal or ceramic ball from the polyethylene.
We’ve seen one or two cases of early acute disassociation (Klement, et al., Am J Orthop, 2017). There have been several case reports. What is this due to?
We think the patients have a skirted neck. It could also be due to a closed reduction maneuver that a lot of these patients have when they try to have a dislocation reduced in the ER.
The chronic intra-prosthetic dislocation is really a phenomenon of late wear of conventional polyethylene (Hamadouche, et al., Clin Orthop, 2012). I’ve spoken to Moussa Hamadouche…they have not seen this in France with the newer polyethylenes.
Our conclusion is that dual mobility is now the standard for recurrent dislocation and high-risk primaries. It gives you better range of motion. Will these always work when the abductors are deficient? And there may be a limit to these. We don’t know what it is. We think polyethylene wear and intra-prosthetic dislocation are very rare phenomena. We certainly need more data and longer follow-up on the newer U.S. designs.
Dr. Sculco: Hip dislocation tends to be a traumatic and difficult problem to deal with. If you look at the literature, it’s now really the most common cause of revision hip replacement in the United States (22.5%).
The treatment for recurrent dislocation has evolved. Jumbo or bipolar heads are not used very much now. We are looking at dual mobility liners. And then constrained liners.
In the paper that Paul alluded to, we looked at 59 studies, over 17,000 dual mobilities. And you see, in fact, that dislocation after dual mobility is uncommon—1% in the primaries and 3% in revisions.
There is this unique type of dislocation that does occur in dual mobilities where the head is eccentric in the socket after reduction. And the reason for that is the polyethylene liner is, in fact, outside the socket. A CT scan shows it beautifully.
So in the reduction of a dislocated dual mobility, an intra-prosthetic dislocation may occur. At our institution, we’ve had four. It’s something to think about if one has a dislocation with a dual mobility that it may convert to an intra-prosthetic dislocation with reduction.
Constrained liners…you have to realize that there are different types of constrained liners. Constrained liners have gotten a bad rep because often they’re confused with the two different types of constrained liners available.
There are the constrained tripolar and the focal constraint designs. The latter adds a ring to the polyethylene in some way to try to improve the constraint of the socket and those have not done as well. I think it’s given constrained liners a bad rep.
