Dislocation is one of the major complications following both primary and revision total hip replacement (THR). The incidence is estimated to 2-4% following primary THR. The incidence following revision surgery is even higher and has been reported to be approximately 8% [1Alberton G, High W, Morrey B. Dislocation after revision total hip arthroplasty. J Bone Jt Surg Am 2002; 84-A(10): 1788-92.]. Dislocation accounts for approximately one third of all revision within a five year period post-operatively and significantly affects the quality of life [2Kotwal RS, Ganapathi M, John A , et al. Outcome of treatment for dislocation after primary total hip replacement. J Bone Joint Surg Br 2009; 91(3): 321-6.]. Treatment of recurrent dislocations is a difficult challenge.

The use of constrained components is one option for treating recurrent dislocation of THRs. The mean rate of dislocations following revision surgery with a constrained component has been reported to be 10% [3Williams JT, Ragland PS, Clarke S. Constrained components for the unstable hip following total hip arthroplasty a literature review. Int Orthop 2007; 31(3): 273-7.]. Furthermore, constrained components are associated with high rates of aseptic loosening in the long term. Another treatment option for recurrent dislocations is the use of a dual-mobility cup. The dual-mobility concept was introduced in the late 1970s by Bousquet and does not, in contrast to the constrained components, restrict the range of motion in the joint articulation [4Farizon F, de Lavison R, Azoulai JJ, Bousquet G. Results with a cementless alumina-coated cup with dual mobility. Int Orthop 1998; 22: 219-24.]. Few studies investigating the effect of a dual-mobility cup on recurrent dislocations report a dislocation rate of in the range 1.7% to 5.5% following revision surgery [5Guyen O, Pibarot V, Vaz G , et al. Use of a dual mobility socket to manage total hip arthroplasty instability. Clin Orthop Relat Res 2009; 467(2): 465-72.-8Parvizi J, Morrey BF. Bipolar hip arthroplasty as a salvage treatment for instability of the hip. J Bone Joint Surg Am 2000; 82-A(8): 1132-9.].

The aim of this study was to retrospectively investigate the effect of dual-mobility cups as a treatment for recurrent dislocations of THR in a consecutive series. We focused on dislocation and implant survival following revision surgery.

Recurrent dislocation following THR is a surgical challenge and the outcome of revision surgical can be poor [11Lind M, Krarup N, Petersen LG , et al. Acetabular revision for recurrent dislocations. 2002; 73(3): 291-4.]. We found that the use of an uncemented dual mobility cup as treatment for recurrent THR dislocation in 56 consecutive patients resulted in a dislocation rate of 1.8% after a mean of 44 mounts.

Our study is retrospective and thereby limited by the enclosed limitations related to the design. The main limitation is the risk of not being able to assure 100% follow-up on all patients since the main source of data was medical records from our hospital. However, in order to optimize completeness of the data, all patients have been crosschecked with the Danish National Patients Register. This register records among other things surgical and diagnosis codes linked to a personal identification number given to all Danish citizens.

Three major causes of dislocations have previously been described [12Dorr L, Wan Z. Causes of and treatment protocol for instability of total hip replacement. Clin Orthop Relat Res 1998; 335: 144-51.]: type I by extreme hip joint position, type II by muscular or soft tissue imbalance, and type III by malposition of one or more of the components. Following revision surgery the important variable for dislocation is probably soft-tissue imbalance [1Alberton G, High W, Morrey B. Dislocation after revision total hip arthroplasty. J Bone Jt Surg Am 2002; 84-A(10): 1788-92.]. One approach to optimize implant stability and treat recurrent dislocation is revision with a constrained liner. The limitation of this approach is the limited range of motion and relative high re-dislocation rate of 10 % [3Williams JT, Ragland PS, Clarke S. Constrained components for the unstable hip following total hip arthroplasty a literature review. Int Orthop 2007; 31(3): 273-7.]. In this study, we investigated the effect of an uncemented dual mobility cup. We demonstrated a re-dislocation rate of 1.8% after a maximum follow-up period of 11 years and an improvement in Harris Hip Score from a mean of 76 to a mean of 87. Our results are comparable with other studies where dislocation rates between 1.7% and 5.5% have been reported [5Guyen O, Pibarot V, Vaz G , et al. Use of a dual mobility socket to manage total hip arthroplasty instability. Clin Orthop Relat Res 2009; 467(2): 465-72.-13Philippot R, Adam P, Reckhaus M , et al. Prevention of dislocation in total hip revision surgery using a dual mobility design. Orthop Traumatol Surg Res 2009; 95(6): 407-13.]. The stability of the dual mobility design is attributed to two factors: first, the relative large diameter head obtained by the mobile liner. Second, the relative large range of motion before impingement of the femoral neck with the metal cup [10Grazioli A, Ek ETH, Rüdiger HA. Biomechanical concept and clinical outcome of dual mobility cups. Int Orthop 2012; 36(12): 2411-8.].

Two metal-to-polyethylene articulations exist in the dual-mobility design: One between the small metal head and the liner and a second between the liner and acetabular metal cup. This dual articulation design has led to concern about increased wear rates, leading to increased implant loosening and failure [10Grazioli A, Ek ETH, Rüdiger HA. Biomechanical concept and clinical outcome of dual mobility cups. Int Orthop 2012; 36(12): 2411-8.]. In our study with 56 patients only 3 had to be revised after index surgery. Only one of these re-operations was due to aseptic loosening of the acetabular component. Another concern is disintegration between the femoral head and the liner [13Philippot R, Adam P, Reckhaus M , et al. Prevention of dislocation in total hip revision surgery using a dual mobility design. Orthop Traumatol Surg Res 2009; 95(6): 407-13.]. In a study investigating the survival of 200 cemented and 28 uncemented dual-mobility cup inserting as a treatment for recurrent dislocation only 18 cups had been revised after a median follow-up of 2 years [6Hailer NP, Weiss RJ, Stark A, Kärrholm J. Dual-mobility cups for revision due to instability are associated with a low rate of re-revisions due to dislocation 228 patients from the Swedish Hip Arthroplasty Register. Acta Orthop 2012; 83(6): 566-71.]. Few studies have investigated the survival of dual-mobility cup as a part of a primary THA. One of these few studies following 437 1. generation Bousquet cups and 231 2. generations cup reports a 5 years survival rate at respectively 95.6% and 99.6% [14Vielpeau C, Lebel B, Ardouin L , et al. The dual mobility socket concept experience with 668 cases. Int Orthop 2011; 35(2): 225-30.].

The role of the dual-mobility cups in total joint replacement still needs to be defined. Few studies indicate that dual-mobility cups might have a role in reducing dislocation when treating femoral neck fractures with a joint replacement [15Bensen AS, Jakobsen T, Krarup N. Dual mobility cup reduces dislocation and re-operation when used to treat displaced femoral neck fractures. Int Orthop 2014; 38(6): 1241-5.-16Tarasevicius S, Busevicius M, Robertsson O, Wingstrand H. Dual mobility cup reduces dislocation rate after arthroplasty for femoral neck fracture. BMC Musculoskelet Disord 2010; 11: 175.]. Other studies encourage to use dual-mobility cups as a part of primary THR [4Farizon F, de Lavison R, Azoulai JJ, Bousquet G. Results with a cementless alumina-coated cup with dual mobility. Int Orthop 1998; 22: 219-24.]. Only one national registry study investigating the implant survival but not the dislocation after revision for recurrent dislocation exists [6Hailer NP, Weiss RJ, Stark A, Kärrholm J. Dual-mobility cups for revision due to instability are associated with a low rate of re-revisions due to dislocation 228 patients from the Swedish Hip Arthroplasty Register. Acta Orthop 2012; 83(6): 566-71.].

In conclusion, this study suggests effectiveness of dual-mobility cups in the treatment of recurrent dislocations of THRs. However, we are still in need of long term follow-up register studies with focus on both dislocation rates and implant survival in order to determine the role of dual-mobility cups in both primary and revision total hip replacements.

The authors declare that they have no conflict of interest.