The usefulness of dual mobility cups in primary total hip arthroplasty patients at a risk of dislocation

In this retrospective cohort study, satisfactory clinical and radiologic results were confirmed for DM THA during a follow-up of at least 2 years. Particularly, postoperative dislocation was not observed with DM THA, although all THA procedures were performed using a posterolateral approach and the anteversion of the DM group was smaller than that of the FB group. Thus, this study supported the hypothesis that DM THA is a good option for the prevention of postoperative dislocation in patients at a high risk of dislocation after THA. The strengths of this study included the direct comparative analysis of follow-up data from patients at a high risk of dislocation and its design, in which prostheses from a single manufacturer were used in consecutive patients by the same surgeon.

Among the risk factors associated with postoperative dislocation after THA, the most difficult to predict are the patient-related factors. Neuromuscular disorder, muscle weakness, dysplastic hip, abnormal spinopelvic movement, previous hip fractures, and osteonecrosis of femoral head are well-known patient-related risk factors for dislocation^9,10,11,12. DM cups have been used to preclude dislocations in patients at risk, and various studies have elicited promising results. The risk of dislocation following THA in osteonecrosis of femoral head compared to THA for primary osteoarthritis is higher. Assi C et al. reported that the new generation of DM cup in patients with osteonecrosis of femoral head showed excellent functional early results with no major complication such as dislocation¹². THA for femoral neck fractures is often associated with a high risk of dislocation secondary to a combination of muscular insufficiency and a propensity for recurrent falls. Tarasevicius et al. described a statistically significant reduction in the dislocation rate with THA using DM as compared to THA with FB (0% vs. 10.4%) during the first postoperative year¹³. Assi CC et al. also reported similar results that the use of DM cup could significantly reduce the rate of dislocation in such a high risk population of patients with femoral neck fracture, and consequently the rate of THA revision surgery and the health cost¹⁴. Furthermore, DM cups may represent an excellent option in salvage THA performed for failed fixation of hip fractures, which is associated with a high rate of postoperative instability¹⁵. Many factors, including structural damage after removal of internal fixation and loss of bony landmarks due to trochanteric displacement, are likely to contribute to this instability. In a consecutive series of 1000 patients, Esposito et al. demonstrated that fixed spinopelvic alignment from standing to sitting caused a statistically significant increase in rate of dislocation after THA, with 92% of the patients with dislocation suffering lumbar multilevel degenerative disc disease or surgical spine fusion¹⁶. Therefore, such patients may benefit from DM THA in reducing postoperative dislocation risk. DM THA has demonstrated excellent mid-term results in patients with neurological diseases or cognitive impairment. The study by Bassiony et al. did not report any case of dislocation of the prosthesis used in hip fractures in patients with Parkinson’s disease¹⁴. However, most of these studies were limited to specific diseases or were case series without comparison with a control group. In contrast, our study matched and compared not only age and sex but also various risk factors for dislocation between the two groups, in order to exclude confounding factors as much as possible. Several studies have compared the results of DM and FB THAs in general patients, but only the verification of the degree of non-inferiority of DM THA was possible.

It is known that postoperative dislocation usually occurs within 3 months after THA, and joint laxity related to polyethylene wear is the cause of chronic dislocation. The modern DM cup has evolved considerably since the first-generation model of Bousquet in 1974. The retrieval study of polyethylene DM components by D’Apuzzo et al. showed that motion occurs at both articulations, but the motion of the femoral head relative to the inner aspect of the polyethylene head dominant, which produces more wear¹⁷. Previous studies have reported decreased dislocation rates with primary THA in patients at risk, but with an elevated risk in revision surgery compared to conventional implants. This might result in the release of polyethylene microparticles from the liner and eventually lead to aseptic loosening¹³. Polyethylene wear in the DM system affects the intraprosthetic stability. Excessive eccentricity wear of the inner bearing can lead to loss of constraint of the prosthetic femoral head within the large-diameter polyethylene liner, thus resulting in IPD. The retrieval study of 93 cases with DM system by Neri et al. demonstrated that IPD is a wear-related complication due to contact between the retaining polyethylene rim and the femoral neck¹⁸. Consequently, biomaterial advancements have replaced first-generation polyethylene with HXLPE to minimize wear due to contact with the femoral neck. Laboratory data illustrates the favorable rate of wear in the contemporary DM cups when compared to that of first-generation implants^19,20. The DM systems utilized in this series contains VEPE. VEPE is created by adding a free radical scavenger, vitamin E, to polyethylene during processing; vitamin E adequately quenches free radicals that remain after irradiation, eliminating the need for a post-irradiation heating step. Although this study did not seek to assess polyethylene wear, and the follow-up was insufficient to determine this accurately, none of the cases required reoperation for polyethylene wear or IPD. Third-generation HXLPE, such as VEPE, is considered as the most suitable polyethylene material for DM THA in terms of wear and other properties, and it can be expected to prevent dislocation in the mid to long term period.

All 4 cases of postoperative dislocation occurred in the FB group. As for the prosthetic femoral head used for FB, a prosthetic femoral head of 32 mm or larger was used in 58 cases (92%) except 5 cases using 28 mm. Although it is known that the risk of dislocation can be reduced when a head of 32 mm or larger is used compared to a 28 mm or smaller head, the occurrence of dislocation was significantly higher in the FB group than in the DM group. In other words, it can be estimated that the DM cup is an excellent implant for preventing dislocation regardless of the prosthetic femoral head size when THA is performed in patients at high risk of dislocation.

This study had some limitations. First, this was a single-center, retrospective, cohort study, despite accounting for all postoperative radiologic outcomes in our consecutive patients. Second, proper survival analyses could not been performed because of the small sample size; however, to overcome this limitation, a comparative study using 1:1 propensity score matching was conducted to improve the research design. Third, although deformed spine disease and dysplastic hip were the most common risk factors for postoperative dislocation in this study, DM cup was need for various diseases. However, because the number of disease groups was not large, analysis by disease was not performed in this study. Also, it is an obvious limitation that the patients at a risk of dislocation in this study did not include all known dislocation-risk patients. Finally, although the evaluation of the postoperative dislocation during the 2–4 years of follow-up was meaningful, this period was relatively short; hence, the long-term success and polyethylene wear in cases of DM THA using VEPE could not be evaluated. These limitations are obvious obstacles in the generalization of our results, and further multicenter prospective studies are needed to verify their authenticity. We will continue to conduct further follow-up in these patients.

In patients at a risk of dislocation after primary THA, DM cups showed more promising outcomes than did FB. This study reported no dislocation or IPD in patients who underwent primary THA using a DM system at a mean follow-up of 3.1 years, indicating that DM cups could offer the desired early hip stability. Furthermore, DM cups provided good functional results. Contemporary DM bearing with VEPE may be beneficial for patients with a high life expectancy and early compelling hip stability. Based on our findings, we recommend the use of DM cups in all patients at a high risk of dislocation.