Several studies on DM cups in primary THA have reported a low rate of postoperative implant instability[19-22]. Farizon[23], Philippot[12], Lautridou[24], Vielpau[21] and Boyer[25] reported their experience on the use of first generation Bousquet cups (Novae^®, Serf, Décines, France) with a 22.2 mm metal head and conventional PE. At 15 years follow-up, survivorship ranged from 81.4% to 96.3% with a dislocation rate (with large articulation) between 0% and 1%. However, these authors did not include the dislocation rate of the femoral head and the mobile PE bearing (the small articulation) that ranged from 0% to 5.2%. Causes of cup failure included aseptic loosening (1.8%-3.4%), excessive PE wear (1%-2%) and acetabular screw fracture (1%). Guyen[19], Leclercq[20] and Vielpeau[21] published series of 167, 200, and 231 primary THA patients using current DM designs with a follow-up time period of 3 to 6 years and reported a 0% dislocation rate.

Dislocation rate after revision THA ranges from 5% to 30%[26-29]. Many factors have been implicated in postoperative instability including muscular insufficiency, aggressive capsulectomy, bone loss and implant positioning problems[30]. Leiber-Wackenheim[14], Hamadouche[31], Langlais[17] and Guyen[16] reported their results on the use of DM cups in patients revised for instability after primary THA. Survivorship of the cups at a mean follow-up period of 5 years was between 94.5% and 98% with a dislocation rate of 1.1% to 5.5%. These studies suggest DM cups are a reliable treatment option for patients revised for instability after primary THA.

Femoral neck fractures (FNF) treated with osteosynthesis have an increased risk of reoperation when compared to hip arthroplasty[32]. Although THA showed better functional results than osteosynthesis in FNF treatments[33], prosthetic dislocation remains a serious problem. In a recent meta-analysis by Iorio et al[34] the mean dislocation rate was 10.7% in patients with FNF treated with THA, five times higher than THA for osteoarthritis. Adam et al[35] reported 3 dislocations (1.4%) at 9 mo follow-up in a series of 214 patients with FNF treated with DM implants. Tarasevicius et al[36] compared dislocation rates of DM cups with that of conventional cups in patients with FNF treated with THA through a posterior approach. At 1 year follow-up, there were 8 dislocations (14.3%) in the conventional THA group and no dislocations in DM group. DM cups may also be considered as an option to prevent postoperative dislocation when treating FNFs in elderly patients who are candidates for THA.

THA after tumor resection has also been associated with a high risk of dislocation due to bone loss and soft tissue compromise. Philippeau et al[37] retrospectively analysed 71 patients with bony lesions of the hip treated with a THA and DM cups. They reported 7 postoperative dislocations (9.8%). Dislocation rate was lower when abductors were preserved (3.5%) and higher when abductors were sectioned/reattached (9.5%) and when the gluteus medius muscle or nerve were resected (18%). They also reported acetabular loosening in 4 cases (5.6%).

Several studies on THAs in patients with cerebral palsy (CP) showed good results on pain relief and function outcome; however the dislocation rate in this challenging patient group is reported to be as high as 14%[38-43]. Sanders et al[44] reported on 10 hips (8 patients) with CP treated[41-43,45] with THA and a DM cup and had no dislocations after a mean follow-up of 39 mo.

The original goal of the DM cup, introduced at the end of the 1970s as an alternative to standard sockets, was to reduce the risk of THA dislocation in patients undergoing primary THA. Currently, DM cups are a well-accepted treatment option for any patient at an elevated risk for instability after primary or revision THA and in the treatment of recurrent dislocation[12,14,16,17,20,23-25,30,31,46-48]. Patients at higher risk of dislocation include patients with neuromuscular diseases, cognitive dysfunction, an American Society of Anesthesiologists score of 3 or more, and all patients older than 75 years with a history of prior hip surgery[28,49-53]. In addition, the use of DM cups is indicated in revision THA for any cause[17,30], primary THA after femoral neck fracture[35,36], and primary THA after tumor resection[37]. While several studies demonstrate a reduction in the dislocation rate in patients over 60 years, limited data is currently available on active patients younger than 50 years old and care should be taken when using DM cups in a population more prone to develop wear and osteolysis[20,21,24,48,54].

Dual mobility acetabular components are associated with some specific complications secondary to its dual articulating design. For example, intra-prosthetic dislocation or retentive failure is a complication observed exclusively with this type of implant and involves failure of the articulation between the femoral head and the PE liner. The proposed mechanism for dissociation is a result of wear of the PE liner’s retentive chamfer[25]. After dissociation, the head articulates directly with the metallic bearing surface of the acetabular shell, producing acute limb shortening and limp. Furthermore, as the shell is not designed for a metal-on-metal articulation, friction between its bearing surface and the femoral head results in rapid wear, metal ion release and surrounding soft-tissue metallosis[55]. In plain X-rays, the asymmetric position of the femoral head within the cup can be visualized which may be mistakenly attributed to “polyethylene wear”. However, the characteristic “bubble sign” which corresponds to the dislocated liner is pathognomonic of retentive failure (Figure 2). Management is dependent on the time interval from dislocation to diagnosis. If IPD is diagnosed early, before significant wear of the femoral head and acetabular shell occurs, it can be treated with simple liner exchange. In cases of late diagnosis, revision of the acetabular component, as well as femoral head exchange may be necessary due to femoral head and acetabular shell damage. Boyer et al[25] in a series of 240 hips followed for 9 years and 11 months reported a 4.1% incidence of IPD. A similar incidence (4%) was reported by Philippot et al[15] among 1960 primary THAs with a mean follow-up of 14 years. The authors recognized three distinct types of IPD: type 1, which was typically due to liner wear; type 2, which was related with arthrofibrosis blocking the liner; and type 3, which was associated with cup loosening. When comparing the two stems with different neck diameters and incidence of retentive failure, no statistically significant difference was observed. The authors note that the narrower neck was unpolished titanium (which is rougher than stainless steel used in the larger neck diameter stem) and could have counteracted the positive effects of the smaller neck size. In their series of 231 primary THAs where a second generation dual mobility cup was used, Vielpeau et al[21] reported 0% retentive failure rate at 5.2 years. In 437 hips using the original Bousquet implant design, intra-prosthetic dislocation after a mean of 16.2 years was observed in 3 hips. The authors attributed the low incidence of intra-prosthetic dislocation to the smooth, polished, and narrow femoral neck. In other studies with mid- to long-term follow-up, the incidence of retentive failure ranges from 0% to 5.2%[12,14,19,20,47,48,56]. Table 1 summarizes reported retentive failure rates in the literature.

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Figure 2 Bubble sign. AP pelvis radiograph of a patient with acute onset of left hip pain and limp. On the left, eccentric position of the femoral head within the dual mobility cup can be noted. Careful scrutiny reveals a circular radiolucent area superior to the acetabular component (arrows), which represents the dislocated polyethylene liner (“bubble sign”).

The configuration of the dual mobility cup with its two articulations and thinner liner has raised concern for accelerated PE wear and associated osteolysis. In a retrieval study of liners removed after revision surgery for infection or aseptic loosening, no difference in total volumetric polyethylene wear was noted between tripolar unconstrained cups and conventional cups with 22.2 mm heads[57]. However, greater wear was noted at the convex bearing surface of the liner. Failure rate due to accelerated polyethylene wear was 2% in a series of patients with first generation PE[25]. In the same series, age < 50 years was associated with significantly greater wear rates, apparently due to the higher activity level of these patients. Similarly, in a series of Philippot et al[48] revision rate due to PE wear was 1.6%. Combes et al[22] reported a 7% rate of osteolysis (with first-generation PE), especially in patients of younger age and those treated for sequelae of childhood hip disease.

Radiographic evaluation of PE wear can be difficult in the setting of a dual mobility component, because of the deep position of the head within the cup and the cylindrical-spherical shape of the shell itself. An eccentric femoral head implies concomitant wear of both the concave and the convex bearing surface of the liner[58]. Highly cross-linked UHMWPE and vitamin-E impregnated polyethylene has reduced volumetric wear in standard implants[13,59] and have been integrated into dual mobility implants in an effort to deal with accelerated wear issues. Bauchu et al[60] in a retrospective series of primary THAs with the POLARCUP^® component (Smith and Nephew Orthopaedics AG, Rotkreuz, Switzerland), which incorporates a highly cross-linked UHMWPE liner, reported no incidents of wear-related osteolysis. However, there are currently no independent studies of tripolar cups further supporting these findings. For all these reasons, the use of dual mobility cups should be used with caution in younger patients with high demands and increased risk of wear-related osteolysis.

Another issue with the DM cup is aseptic loosening. Loss of fixation of the original design was attributed to delamination of the plasma sprayed alumina layer[23] which led to design modifications. Some authors propose that fixation of tripolar cups, particularly second-generation implants, should always be supplemented with screws[11]. Nonetheless, as noted earlier, current shell options include monoblock cups, which take advantage of modern porous coated surfaces for enhanced osseointegration. A potential drawback of these monoblock cups is the difficulty assessing proper seating of the cup within the acetabulum which may contribute to the reported rates of aseptic loosening ranging from 0% to 8.3%[12,14,19-22,47,48,56] (Table 1).