Dual Mobility Does Not Eliminate Dislocation: Mid-Term Outcomes in Primary Total Hip Arthroplasty Using Dual-Mobility Cup

Introduction Dual mobility cups (DMCs) were developed to enhance stability and reduce dislocation after total hip arthroplasty (THA). The uncemented monoblock M2a-Magnum cup combined with the Active Articulation E1 DMC (Zimmer Biomet) accommodates a femoral head 6 mm smaller than the cup diameter, theoretically improving dislocation resistance. This study evaluated the mid-term outcomes of this construct in primary THA. Methods Between June 2014 and April 2020, 210 hips in 188 patients underwent primary THA using this implant across two institutions. Of these, 168 hips (148 patients; follow-up rate: 78%) were reviewed ≥5 years postoperatively (mean 7.2 years, range 5.0–10.4). The mean age was 68.7 years (33–90), with 61 men and 96 women; the mean BMI was 23.5 ± 3.5 kg/m 2 . Diagnoses included osteoarthritis (120), osteonecrosis of the femoral head (29), traumatic osteonecrosis (6), rapidly destructive coxopathy (5), rheumatoid arthritis (4), and fractures (13). Clinical outcomes were assessed using the Japanese Orthopaedic Association (JOA) hip score. Kaplan-Meier survival analysis was performed with dislocation as the endpoint. Radiographic evaluation included cup orientation and radiolucent lines (RLLs). Dislocation cases were further examined with range-of-motion (ROM) simulation using Zed Hip software. Results The mean JOA score improved from 54.6 preoperatively to 88.9 at final follow-up. No implant loosening or significant RLLs were detected, although 11.9% of cups were oriented outside the Lewinnek safe zone. Complications included one intraprosthetic dislocation at 4.5 years requiring revision and one dislocation at 2.7 years, successfully managed with closed reductions, though a 7 mm offset discrepancy was noted. ROM simulation demonstrated implant-to-implant and bone-to-bone impingement, respectively. Dislocation-free rate was 98.8% at final follow-up. Conclusion The M2a-Magnum with Active Articulation DMC demonstrated excellent mid-term outcomes and a low dislocation rate. However, malpositioning contributed to instability, emphasizing the importance of precise implant positioning to maximize the stability benefits of DMCs. Key words: dual mobility cups (DMCs), total hip arthroplasty (THA), dislocation, implant positioning