CORR Insights®: Can Dislocation of a Constrained Liner Be Salvaged With Dual-mobility Constructs in Revision THA?

Where Are We Now? Two factors affect hip stability during movement: (1) Those things contributing to pushing the head out of its socket (impingement, malposition, large ROM, neck offset, and osteophytes) and (2) those things contributing to holding the head in its socket (component alignment, capsular integrity, passive and active muscular forces). Constrained cups are susceptible to movements that contribute to pushing the head out of its socket such that at the extremes of motion, the constraining elements can be overcome. Dual mobility hips, by contrast, are more susceptible to problems with the soft tissues, such that when all soft-tissue stabilizers have been compromised, nothing but constraint will hold the hip together. In the current study, Chalmers and colleagues have identified an interesting group of patients—those undergoing conversion of constrained liners to dual-mobility constructs in revision THA. Surgeons have three options when dealing with patients who have dislocated their constrained liners: (1) Try constrained again (preferably with a different design), (2) revision to dual mobility, or (3) resection arthroplasty. For this specific patient population, the authors make a strong case for the use of dual-mobility cups by noting that an important advantage of dual-mobility liners over constrained liners is that the former can be closed reduced, while the latter generally cannot. To some degree, though, this represents a planned failure, an acknowledgement that the situation is desperate, and so the option chosen should be easiest one to manage when it fails. While the authors have demonstrated that revising failed constrained implants to dual mobility works most of the time, and while I agree with the authors that a dislocation of a dual mobility hip is preferable to a failure of a constrained implant, it is important to note that dual-mobility designs also can have severe complications that are not manageable without open surgery. Although uncommon, dual-mobility hip designs can dissociate [1]. We don’t, in fact, know what the “best” approach is here, and it is important that the information in the present study is not misconstrued to condemn the use of constrained implants in the future. Where Do We Need To Go? The findings by Chalmers and colleagues raise a number of questions. Are there some designs that are destined to fail or is the concept of locking the hip together just a bad one, regardless of the design? All constrained cup designs are not the same, and the Osteonics Tripolar cup (now called the Trident® Constrained Acetabular Insert [Stryker Orthopaedics, Mahwah, NJ USA] showed results (94% at 10 years) that were as good or better than all current designs [2]. Additionally, approximately half of the dislocations were caused by surgical issues such as malposition and inadequate seating or fixation [2]. Its infrequent use makes it difficult for surgeons to surmount the learning curve. Potentially, future design modifications could improve upon the current technical issues. Additionally, how well do patients accept recurrent hip instability as an endpoint? The authors introduce a novel concept in management of recurrent instability of the hip; acceptance, by the surgeon and patient, of future dislocation episodes. Future studies should help us to know more about patients’ preferences; specifically, which health state is superior—that of the patient with multiple, ongoing, recurrent dislocations, or the patient who has undergone resection arthroplasty? Future studies should also examine patient tolerance. Are these patients willing to make the adjustments in activities to minimize instability and tolerate an occasional dislocation? The answer probably will lie in how easily the hip dislocates (does it dislocate getting out of a chair or only with yoga?), whether a brace is effective, and whether it is tolerable to wear. Can the patient be taught how to avoid dislocations? Frequency also matters; there is a big difference between once-a-year dislocations and once-a-week dislocations. Further, will a larger series or additional case reports continue to support this approach? While both constrained implants and dual mobility have been available since the 1970s, the usage of dual mobility designs is trending upward. Potentially, as constrained implant designs improve and longer followup of dual mobility uncovers technical or design issues, we may see studies in the opposite direction (that is, revisions from dual mobility to constrained). To accept recurrent instability as an endpoint, we need to demonstrate that for some patients, revision with dual-mobility is superior to resection arthroplasty. Otherwise, recurrent instability needs to be viewed as a failure. How Do We Get There? Short of bioengineering ligament/capsule augmentation of soft-tissue deficiency; researchers can either improve upon current options or better understand the ramifications of what we have. It may be that dual-mobility works better for impingement, while constrained liners work better for soft-tissue deficiencies. It would be interesting to study a patient population where the hip remains unstable, but is considered inoperable short of a resection arthroplasty. This type of study would help answer the questions posed in the previous section, but would likely require the involvement of multiple institutions in order to enroll enough patients to control match to resection arthroplasty. While large, randomized studies are preferable, it would be difficult to improve the methodology of this study on such a unique patient population.