Dual mobility is a compelling option—its big advantage is its decreased risk of instability. We’ve used it for patients at high risk in primary total hips. We’ve used it in revisions to specifically treat instability as well as in lieu of a constrained liner, we also have a series using it as a salvage operation for failed monobloc metal-metal cups.
There are concerns out there regarding overall durability and intra-prosthetic dislocation. But I think with the contemporary designs, the risk of that should be lower, but I tell you we do need to follow these closely.
Dr. Sculco: I want to talk about the role of constrained sockets in hip instability and as Craig pointed out, there is no more traumatic event for the patient than having a hip dislocation and they’re frantic when this happens.
In the United States, dislocations are currently the most common cause of revision hip replacement surgery. There are lots and lots of causes, as Craig pointed out, for hip instability and even though you put your implant in perfectly, the hip can still dislocate, which is a very frustrating problem.
In terms of treatment, we have three major treatments available for the patient with a chronic hip dislocation: jumbo or bipolar heads, dual mobility liners, and the constrained liner.
I think using larger heads gets you some increased stability but not a lot, and I don’t think that’s a great option at this point in time if you really have a patient with persistent hip instability. With the larger heads, yes, you may get less prosthetic impingement, but you can also get more bony impingement which leads to instability.
I like dual mobility. I’m going to be an advocate for constrained sockets, but I do believe there is a great place for dual mobility.
A recent paper we published in the Bone and Joint Journal was a meta-analysis looking at 59 different studies comprising 18,000 hip replacements. The dislocation rate with dual mobility for the primary THAs was 0.9% and for the revision THAs was 3.0%, so it’s a very good device for these problems.
What are the potential disadvantages of dual mobility? Well, if you use the ADM system, the non-modular system, you can’t visualize the floor because it’s a monobloc—that can be a problem in terms of positioning. It’s cobalt chrome and that can be a little harder to seat because it’s a rigid metal.
Intra-prosthetic dislocation we talked of, the wear we really don’t know a lot about and we’ll know more in the future. I think with highly crosslinked polyethylene it will be better. The thing that concerns me about the MDM, so called the modular dual mobility, is we’re putting cobalt chrome against titanium, and we’ve been burned a lot with that in other systems, in other areas, and I have concerns that it may be a problem with the modular dual mobility system.
So, what about constrained liners? I think it’s important to realize there are two different categories of constrained liners, and that’s where I think some of the bad data—and the bad names for constrained liners—has come from. You have the focal constraint designs with a locking ring and the constrained tripolar which is the more conventional and time-proven device. And they’re very different.
We looked at 149 patients who had tripolar constrained devices and followed them for over 4 years. Complicated patients, 3.8 previous operations in this series. Primarily female patients and the reason that the procedure was done. Recurrent dislocation: 55%. Poor stability at revision: 45%. The tripolar systems were used differently: a new shell and socket in 38%, a liner into a compatible shell 35%, and a cemented tripolar into an existing shell, 27%.
This is the data: 16 revisions in a little over 4 years, complicated group of patients. Only 3.3% of patients had recurrent dislocation. A problem with the tripolar systems is that the ring that goes around the polyethylene, a certain number of them will break.
