Let’s now look at the non-constrained dual mobility. I think it’s interesting, and this dates back to 1974 in a study by Bousquet and Rambert, and they wanted to combine the low friction, small head popularized by Charnley with a very large diameter of McKee and Farrar. The goal of dual articulation was to achieve the greatest possible range of motion in a stable environment in addition to reducing wear (DeMartino, et al., WJO, 2014).
An excellent study by the Mayo Clinic, which won the Otto Aufranc Award at the Hip Society, looked at non-constrained dual mobility group and compared it to large femoral heads. They found a lower dislocation rate of 3% compared to 10%. But they also had a lower revision for dislocation of 1% compared to 6%. Finally reoperation for any cause was 6% compared to 15%. All statistically significant (Hartzler, et al., CORR, 2018).
There are some problems with dual mobility. Older designs have up to a 5% risk of intraprosthetic dislocation displacement. Modern designs, it’s much lower—0-2%. Dual mobility should not be a substitute for a malpositioned component.
There is also a slight risk of damage and wear of the dual mobility. At Doug’s institution they found predominant motion was in the inner bearing area and there was damage to the surface, but its significance is completely unknown (D’Apuzzo, et al, JOA, 2016).
Finally, a wear study found that median annual wear of the dual mobility is as low as the wear of cemented polyethylene liners and lower than cementless liners. Keep in mind that these were not highly-crosslinked polyethylene liners (Boyer, et al., SICOT, 2018)
To summarize, the short- and mid-term results strongly support the use of the dual mobility construct in patients at high risk for dislocation and those undergoing revision. Intraprosthetic dislocation and material wear are risks associated with this construct in our young, more active patients.
Dr. Padgett: Kevin, I think you’re losing it.
We know instability is a problem (Bozic, et al., JBJS, 2009). We know that it represents a big burden in the revision realm. And we’re certainly concerned. We know at this point that there are lots of factors associated with instability. Patient factors, surgeon factors and implant factors. But all these, again, are associations.
We’ve overreacted. The tendency is to constrain and I think that is wrong.
Kevin’s given a little background about the dual mobility designed in the 1970s. Our group has demonstrated that it works from a computational standpoint. And the clinical track record in Europe has been excellent with increasing adoption in the United States.
In the designer series, 16-year follow-up; 1% dislocation and no described lysis (Vielpeau, et al., International Ortho, 2011). Certainly, that gives us some enthusiasm to essentially use this device, especially perhaps, in high-risk groups.
Our understanding of the mechanism of instability has changed…dramatically changed. What we assumed previously was that alignment was static. We used AP and lateral radiographs to determine radiographic success. And, our surrogate for clinical success in terms of socket position was always stability.
It was all about the Lewinnek safe zone and hitting the target. I would submit that when we revisited that history about hitting that safe zone we found a 2% dislocation rate, but that the rate of dislocation was the same whether you were in or out of that static safe zone.
Why is this? There must be something going on. Static assessment of implant position is not indicative of what happens in daily life. We started thinking that pelvic motion might, in fact, influence functional position. And I’d like to thank Larry Dorr who has really been the champion for this whole concept and has been a big mentor of mine.
