Dual mobility THA yields similar dislocation, revision risks vs. metal, ceramic heads

Many would agree that dual mobility hips have helped reduce the risk of dislocation, and many surgeons appreciate the fact that dual mobility hips are more stable than traditional, fixed-bearing hips. There are many studies in the literature, both in primary hip replacement and in revision hip replacement, where dual mobility hips are used, and the dislocation rates are extremely low. There is consensus that dual mobility hips are used for patients who are at higher risk for instability, because dual mobility implants have solved the instability problem.

This article in the Journal of Arthroplasty took a cohort of patients who are heterogeneous across one insurance carrier and looked at the risk of revision, and they imply there is no difference between a dual mobility hip or a fixed-bearing hip with metal-on-highly crosslinked polyethylene or a fixed-bearing hip with ceramic-on-highly crosslinked polyethylene. The problem is, because dual mobility hips are more stable, surgeons have used dual mobility hips during the past 10 years in patients who are at higher risk for dislocation. In a retrospective study like this, there is selection bias that has already occurred, because the dual mobility hip cohort are not patients for which they randomly decided to use a dual mobility implant. Those are patients that one would assume are at higher risk for dislocation. In a study like this, they are not taking all comers and randomly assigning patients to a fixed-bearing hip or a dual mobility hip. Therefore, when you look at the data, you have to understand that you have taken all the high-risk patients who would have had a higher risk for dislocation, and you put in a dual mobility hip. It’s an assumption that you have to make.

The second thing is that the most common dual mobility implant used in this study that overshadows every other company is the G7 from Zimmer Biomet. Almost 1,900 out of 2,200 patients received one particular company’s dual mobility hip. That is about 86% or 87% of the dual mobility hips in this study. What we are looking at here is not dual mobility hips across all the manufacturers. What we are looking at is mainly one company’s dual mobility hip, namely Zimmer Biomet, and you have to be careful, because what this study is showing to me is that the G7 dual mobility hip does not have an advantage compared with fixed-bearing hips that are metal-on-highly crosslinked polyethylene or ceramic-on-highly crosslinked polyethylene. That does not mean that all dual mobility hips do not have an advantage.

The other thing this study does not include is any radiographic analysis of implant positioning. Implants can dislocate if not put in properly, and the fact that they had a 2.1% incidence of dislocation in their dual mobility cohort – we do not know if those implants were malpositioned and we do not know if the leg lengths were equivalent. We do not know radiographically if there were risk factors in the dual mobility group compared to the fixed-bearing groups, because there is no radiographic analysis that was done in the study.

I think the takeaway for me is that I would be cautious interpreting their data as a generalization for all dual mobility hip replacements and, more specifically, that insofar as their data includes the G7 as approximately 87% of the dual mobility implants that that particular dual mobility implant may not have any advantage compared to fixed-bearing, but I would not lump all dual mobility hips into that conclusion.