Background. Published simulator studies for metal/UHMWPE bearings couples showed that increasing the femoral head diameter by 1 mm increases wear by approximately 10% due to increased contact area. Therefore, there are concerns about increased wear with dual mobility hip bearings. Purpose of the study. The purpose of the study was to compare wear from dual mobility hip bearings to that with traditional fixed bearings. In addition, for the dual mobility bearings, the effect of femoral head material type on the liner wear was also evaluated. Methods. The bearings selected for the study are listed in Table 1. Prior to the start of the test all liners were soaked in lubricant for 48 hours. Hip testing was performed on a Shore Western Orbital Bearing machine in the anatomically oriented position. A simulated gait profile (synchronized at +/-23° biaxial rocking motion) with a minimum/maximum 200/2000N force was applied to the bearings at frequency of 1Hz. The lubricant used for the testing was 25% bovine serum with 0.2 % sodium azide, 20 mMol EDTA and distilled water. The test was interrupted at regular intervals for gravimetric assessment of wear amount. Findings of Study. Figure 1 shows total wear at 3 Mc and wear rates (determined from the slope of the linear regression) for all the groups. At 3 Mc, dual mobility bearings with stainless steel femoral head demonstrated 5% lower wear rate than those articulated against CoCrMo femoral heads. However, there was no statistically significant difference in the observed wear rate due to the femoral head material type. The results from the study also exhibited lower wear and wear rate for dual mobility bearings compared to fixed bearings. Dual mobility bearings with CoCrMo femoral head and stainless steel femoral head demonstrated 17% and 21% lower wear rate when compared to fixed bearings. Although dual mobility bearings possess greater contact area (due to the contact between head-liner and liner-shell compared to only head-liner in fixed bearings), no such increased trend in wear was observed. Conclusions. Dual mobility hip bearings are designed to reduce the risk of dislocation and allow for increased range of motion thus improving joint function and stability. The results from the study demonstrate that dual mobility bearings have comparable wear properties when compared to fixed bearings. For figure/table, please contact authors directly

Dislocation and instability remain leading cause of failure following THA. We present a single-surgeon 10-year experience with use of Dual Mobility (DM) bearings in Primary and Revision THA using posterior approach. 127 DM bearings were implanted between September 2006 – September 2016; 102 in high-risk primary THA's and 25 revision THA's for either treatment or prevention of instability. Selection for DM bearing followed individual patient risk assessment. Criteria for use of DM bearing were presence of multiple risk factors. Mean age was 72.9 years. 100 Mono-block DM implants, 22 Modular DM implants and 5 custom-made DM devices were implanted. Revision cohort included those used in conjunction with a cage or porous metal augments. 2 dislocations (1.6%) were observed, both in the Revision group, 1 was recurrent requiring revision to constrained liner. Primary group had 2 revisions; 1 peri-prosthetic fracture and 1 deep infection. No DM bearing specific complications were observed. A constructed life table calculated survival function with endpoint set as revision for any reason demonstrated a cumulative survival of 94% at 7.4 years. In high-risk patients, DM bearings are successful at preventing and treating dislocation in THA. Primary cohort in this study all had multiple risk factors for instability but no dislocations or bearing specific complications were observed. Dislocations observed in Revision group were associated with major soft tissue deficiency. This study adds to the promising results already reported with DM THA articulations and should be considered for patients at risk of dislocation or instability. Runner Up – Best Paper Award

Dislocation remains among the most common complications of, and reasons for, revision of both primary and revision total hip arthroplasties in the United States. We have advocated identifying the primary cause of instability to plan appropriate treatment (Wera, Della Valle, et al., JOA 2012). Once implant position, leg length, and offset have been optimised and sources of impingement have been removed, the surgeon can opt for a large femoral head, a dual mobility articulation or a constrained liner. Given the limitations of constrained liners, we have looked to dual mobility articulations as an alternative, including its use in patients with abductor deficiency. We retrospectively compared a consecutive series of revision THA that were at high risk for instability and treated with either a constrained liner or a dual mobility articulation. At a minimum of two years, there were ten dislocations in the constrained group (10/43 or 23.3%) compared to three in the dual-mobility group (3/36 or 8.3%; p = 0.06). With repeat revision for instability as an endpoint, the failure rate was 23% for the constrained group and 5.5% for the dual mobility group (p = 0.03). We have also performed a systematic review of the published literature on the use of dual mobility in revision THA. Of the 3,088 hips reviewed, the dislocation rate was 2.2%, the risk of intraprosthetic dislocation was 0.3% and overall survivorship was 96.6% at 5 years. Dual mobility articulations offer anatomic sized femoral heads that greatly increase jump distance, without many of the negatives of a constrained liner. While dual mobility is associated with its own concerns and problems (including intraprosthetic dislocation and wear) our initial results suggest that they are a viable alternative to a constrained liner, even in the most challenging situations

Dual mobility components for total hip arthroplasty provide for an additional articular surface, with the goals of improving range of motion, jump distance, and overall stability of the prosthetic hip joint. A large polyethylene head articulates with a polished metal acetabular component, and an additional smaller metal head is snap-fit into the large polyethylene. New components have been released for use in North America over the past four years. In some European centers, these components are routinely used for primary total hip arthroplasty. Some surgeons in USA suggest routine use in primary hip arthroplasty. However, their greatest utility is to manage recurrent dislocation in the setting of revision total hip arthroplasty. Recent biomechanical data suggests that, in a 3D CT scan-cadaver hip model, there is no difference in range of motion between a 36mm head and an ADM dual mobility component sizes 50–56mm. There is little wear data on dual mobility components, except from one implant manufacturer. It is feared that there is a “3rd articulation” in dual mobility components—the routine impingement of the femoral neck against the polyethylene femoral head. Several retrospective series have shown satisfactory results for these dual mobility components at short- to medium-term follow-up times. There are important concerns with polyethylene wear, late intra-prosthetic dislocation, and the lack of long-term follow-up data. Big femoral heads (36mm and 40mm) articulating with highly cross-linked, e-beam, remelted, polyethylene are a better choice in primary total hip arthroplasty, to decrease the frequency of dislocation in “high risk” patients. Although the risk of early dislocation was 4% in “high risk” patients, there was no recurrence, no revision, and no late first dislocation. Until further long-term results are available, caution is advised in the routine use of dual mobility components in primary total hip arthroplasty

Dual mobility components for total hip arthroplasty provide for an additional articular surface, with the goals of improving range of motion, jump distance, and overall stability of the prosthetic hip joint. A large polyethylene head articulates with a polished metal acetabular component, and an additional smaller metal head is snap-fit into the large polyethylene. The first such device was introduced for primary total hip arthroplasty by Bousquet in the 1970s, thus, the “French connection”. Dual mobility components have been released for use in North America over the past five years. In some European centers, these components are routinely used for primary total hip arthroplasty. However, their greatest utility may be to manage recurrent dislocation in the setting of revision total hip arthroplasty. Several retrospective series and the Swedish hip registry have shown satisfactory results for this indication at short- to medium-term follow-up times. However, there are important concerns with polyethylene wear, late intraprosthetic dislocation, and the lack of long-term follow-up data. These components are an important option in the treatment of recurrent dislocation in younger patients, revision of failed metal-metal resurfacing, and salvage of failed constrained liners. There are more recent concerns of possible iliopsoas tendinitis, elevated metal levels with one design, and acute early intraprosthetic dislocation following attempted closed reduction. However, a dual mobility component may now be the preferred solution in revision surgery for recurrent hip dislocation

Treatment of recurrent dislocation: approximately: 1/3 of failures (probably higher in the absence of a clear curable cause). In the US: most popular treatment option: constrained liners with high redislocation and loosening rates in most reports. Several interfaces leading to various modes of failures. In Europe: dual mobility cups (or tripolar unconstrained): first design Gilles Bousquet 1976 (Saint Etienne, France), consisting of a metal shell with a highly polished inner surface articulating with a mobile polyethylene insert (large articulation). The femoral head is captured into the polyethylene (small articulation) using a snap fit type mechanism leading to a large effective unconstrained head inside the metal cup. With dual mobility, most of the movements occur in the small articulation therefore limiting wear from the large polyethylene on metal articulation. Contemporary designs include: CoCr metal cup for improved friction, outer shell coated with titanium and hydroxyapatite, possible use of screws to enhance primary stability (revision), cemented version in case of major bone defect requiring bone reconstruction. Increased stability obtained through an ultra-large diameter effective femoral head increasing the jumping distance. Dual mobility in revision for recurrent dislocation provided hip stability in more than 94% of the cases with less than 3% presenting redislocation up to 13-year follow-up. A series from the UK concerning 115 revisions including 29 revisions for recurrent dislocation reported 2% dislocation in the global series and 7% re-dislocation in patients revised for instability. A recent report of the Swedish hip arthroplasty register including 228 patients revised for recurrent dislocation showed 99% survival with revision for dislocation as the endpoint and 93% with revision for any reason as the endpoint. One specific complication of dual mobility sockets: intra-prosthetic dislocation (ie: dislocation at the small articulation): often asymptomatic or slight discomfort, eccentration of the neck on AP radiograph, related to wear and fatigue of the polyethylene rim at the capturing are through aggressive stem neck to mobile polyethylene insert contact (3rd articulation). Risk factors include: large and aggressive femoral neck design implants, small head/neck ratio, skirted heads, major fibrosis and periprosthetic ossifications. Current (over ?) use in France: 30% of primary THA, 60% in revision THA. Proposed (reasonable) indications: primary THA at high risk for dislocation, revision THA for instability and/or in case of abductors deficiency, Undisputed indication: recurrent dislocation

Dual mobility components for total hip arthroplasty provide for an additional articular surface, with the goals of improving range of motion, jump distance, and overall stability of the prosthetic hip joint. A large polyethylene head articulates with a polished metal acetabular component, and an additional smaller metal or ceramic head is snap-fit into the large polyethylene. In some European centers, these components are routinely used for primary total hip arthroplasty. However, their greatest utility will be to prevent and manage recurrent dislocation in the setting of revision total hip arthroplasty. Several retrospective series have shown satisfactory results for this indication at medium-term follow-up times. The author has used dual mobility components on two occasions to salvage a failed constrained liner. At least one center reports that dual mobility outperforms 40mm femoral heads in revision arthroplasty. Modular dual mobility components, with screw fixation, are the author's first choice for the treatment of recurrent dislocation, revision of failed metal-metal resurfacing, total hips, unipolar arthroplasties, and salvage of failed constrained liners. There are concerns of elevated metal levels with one design, and acute early intra-prosthetic dissociation following attempted closed reduction. Total hip surgeons no longer cement Charnley acetabular components, use conventional polyethylene, autologous blood donation, or a drain; now constrained components join these obsolete techniques! In 2017, a dual mobility component, rather than a constrained liner, is the preferred solution in revision surgery to prevent and manage recurrent dislocation

Dual mobility components for total hip arthroplasty provide for an additional articular surface, with the goals of improving range of motion, jump distance, and overall stability of the prosthetic hip joint. A large polyethylene head articulates with a polished metal acetabular component, and an additional smaller metal or ceramic head is snap-fit into the large polyethylene. In some European centers, these components are routinely used for primary total hip arthroplasty. However, their greatest utility will be to prevent and manage recurrent dislocation in the setting of revision total hip arthroplasty. Several retrospective series have shown satisfactory results for this indication at medium-term follow-up times. The author has used dual mobility components on two occasions to salvage a failed constrained liner. At least one center reports that dual mobility outperforms 40mm femoral heads in revision arthroplasty. Modular dual mobility components, with screw fixation, are the author's first choice for the treatment of recurrent dislocation, revision of failed metal-on-metal resurfacing or total hips, unipolar arthroplasties, and salvage of failed constrained liners. There are concerns of elevated metal levels with one design, and acute early intra-prosthetic dissociation following attempted closed reduction. Total hip surgeons no longer use conventional polyethylene, autologous blood donation, or a hemovac drain; now constrained components join these obsolete techniques! In 2018, a dual mobility component, rather than a constrained liner, is the preferred solution in revision surgery to prevent and manage recurrent dislocation

Dual mobility components for total hip arthroplasty provide for an additional articular surface, with the goals of improving range of motion, jump distance, and overall stability of the prosthetic hip joint. A large polyethylene head articulates with a polished metal acetabular component, and an additional smaller metal or ceramic head is snap-fit into the large polyethylene. New components have been released for use in North America over the past eight years and additional modular designs will be forthcoming. In some European centers, these components are routinely used for primary total hip arthroplasty. However, their greatest utility may be to prevent and manage recurrent dislocation in the setting of revision total hip arthroplasty. Several retrospective series have shown satisfactory results for this indication at medium-term follow-up times. The author has used dual mobility components on two occasions to salvage a failed constrained liner. However, at least one center reported failure of dual mobility if the abductor mechanism is absent. There are important concerns with dual mobility, including late polyethylene wear causing intra-prosthetic dislocation, and the lack of long-term follow-up data with most designs. Modular dual mobility components, with screw fixation, are the author's first choice for the treatment of recurrent dislocation in younger patients, revision of failed metal-metal resurfacing, total hips, large head unipolar arthroplasties, and salvage of failed constrained liners. There are more recent concerns of iliopsoas tendonitis, elevated metal levels with one design, and acute early intra-prosthetic dissociation following attempted closed reduction. However, in 2016, a dual mobility component, rather than a constrained liner, may be the preferred solution in revision surgery to prevent and manage recurrent dislocation

Constrained liners are a tantalizing solution to both prevent and treat instability, as they markedly increase the force needed for a dislocation to occur. They have, however, several important negatives that the surgeon must consider before entertaining their use including: Increased stresses at the implant bone interface which can increase the risk of loosening or cause catastrophic failure in the early post-operative period; Decreased range of motion with a greater risk of impingement; and Usually require an open reduction if they dislocate or otherwise fail. Given the limitations of constrained liners, we have looked to dual mobility articulations as an alternative to constrained liners in the past five years in our practice, including patients with abductor deficiency. We retrospectively compared a consecutive series of revision THA that were at high risk for instability and treated with either a constrained liner or a dual mobility articulation. Indications for both groups included abductor insufficiency, revision for instability, or inadequate intra-operative stability when trialing. Forty-three hips were reviewed in the constrained group (mean follow-up 3.4 years) and thirty-six in the dual-mobility group (mean follow-up 2.4 years). The rate of failure was compared using a Fisher's exact test with a p-value of < 0.05 considered significant. At a minimum of two years, there were 10 dislocations in the constrained group (10/43 or 23.3%) compared to 3 in the dual-mobility group (3/36 or 8.3%; p = 0.06). There were 15 repeat revisions in the constrained group (10 for instability, 4 for infection, and 1 broken locking mechanism) compared to 4 in the dual mobility group (2 mechanical failures of cemented dual mobility liners with dislocation and 2 for infection); 34.9% vs. 11.1% (p = 0.01). With repeat revision for instability as an endpoint, the failure rate was 23% for the constrained group and 5.5% for the dual mobility group (p = 0.03). Mean Harris Hip Score (HHS) improved from 45 to 76 points in the constrained liner group, and from 46 to 89 points in the dual-mobility group. Dual mobility articulations offer anatomic sized femoral heads that greatly increase jump distance, without many of the negatives of a constrained liner. While dual mobility is associated with its own concerns and problems (including intra-prosthetic dislocation and wear) our initial results suggest that they are a viable alternative to a constrained liner, even in the most challenging situations

In primary total hip replacements there are numerous options available for providing hip stability in difficult situations (i.e. Down's syndrome, Parkinson's disease). However, in the revision situation in general and in revision for recurrent dislocation specifically, it is important to have all options available including dual mobility constrained liners in order to optimise the potential for hip stability as well as function of the arthroplasty. Even with the newer options, available dislocation rates of higher than 5% have been reported in the first two years following revision surgery at institutions where high volumes of revision surgery are performed. Because of the deficient abductors, other soft tissue laxity and the requirement for large diameter cups, revision cases will always have more potential for dislocation. In these situations in the lower demand patient and where, a complex acetabular reconstruction that requires time for ingrowth before optimal implant bone stability to occur isn't present, dual mobility with constraint has provided excellent success in terms of preventing dislocation and maintaining implant construct fixation to bone at intermediate term follow-up. Hence in these situations dual mobility with constraint remains the option we utilise. We are also confident in using this device in cases with instability or laxity where there is a secure well-positioned acetabular shell. We cement a dual mobility constrained liner in these situations using the technique described below. Present indication for dual mobility constrained liners: low demand patient, large outer diameter cups, instability with well-fixed shells that are adequately positioned, abductor muscle deficiency or soft tissue laxity, multiple operations for instability. Technique of cementing liner into shell: score acetabular shell if no holes, score liner in spider web configuration, all one or two millimeters of cement mantle. Results: Constrained Dual Mobility Liner – For Dislocation: 56 Hips, 10 year average follow-up, 7% failure of device, 5% femoral loosening, 4% acetabular loosening. For Difficult Revisions: 101 hips, 10 year average follow-up, 6% failure of device, 4% femoral loosening, 4% acetabular loosening. Cementing Liner into Shell: 31 hips, 3.6 year average follow-up (2–10 years), 2 of 31 failures

Background:. Dual mobility components in total hip arthroplasty have been successfully in use in Europe for greater than 25 years. However, these implants have only recently obtained FDA approval and acceptance among North American arthroplasty surgeons. Both decreased dislocation rate and decreased wear rates have been proposed benefits of dual mobility components. These components have been used for primary total hip arthroplasty in patients at high risk for dislocation, total hip arthroplasty in the setting of femoral neck fracture, revision for hip instability, and revision for large metal-on-metal (MoM) hip articulation. The literature for the North American experience is lacking. Purpose:. We report indications, short term outcomes, and complications of a series of subjects who received dual mobility outcomes at one institution. Study Design:. Consecutive subjects who received dual mobility total hip arthroplasty components from February 2010 and April 2013 were identified. Charts were retrospectively reviewed for surgical indications, comorbidities, component sizes, and perioperative complications including infection, dislocation, mechanical failure, and reoperation. Results:. 86 hips in 83 subjects underwent total hip arthroplasty or revision total hip arthroplasty using dual mobility components. There were 56 primary total hips and 30 revision total hips. Indications included small acetabular components in the setting of AVN (13 hips), DDH (12 hips) or severe inflammatory arthritis (5 hips), femoral neck fracture (5 hips), intraoperative instability (6 hips), recurrent postoperative instability (5 hips), and revision of large MoM articulations in the setting of failed hip resurfacing (10 hips) or failed MoM total hip arthroplasty (6 hips). Mean follow up was 1 year (3 months to 3.3 years). There were no complications in the primary total hip group. In the revision total hip group, only one hip dislocated and this was in a patient with familial dysautonomia and insensitivity to pain. One subject underwent reoperation for acute prosthetic joint infection. No other complications were encountered. Overall dislocation rate was 1.1% and overall complication rate was 2.2%. Conclusions:. These results closely mirror that of the European literature. Dual mobility articulations in total hip arthroplasty have a low short term complication rate in this cohort and provide a simple solution to difficult cases. Indications for these implants include primary and revision total hip arthroplasty in patients at high risk for instability and revision of large MoM implants including hip resurfacing

In primary total hip replacements there are numerous options available for providing hip stability in difficult situations i.e. Down's syndrome, Parkinson's disease. However, in the revision situation, in general, and in revision for recurrent dislocation situations specifically, it is important to have all options available including dual mobility constrained liners in order to optimise the potential for hip stability as well as function of the arthroplasty. Even with the newer options available dislocation rates of higher than 5% have been reported in the first two years following revision surgery at institutions where high volumes of revision surgery are performed [Della Valle, Sporer, Paprosky unpublished data]. Because of the deficient abductors, other soft tissue laxity and the requirement for large diameter cups, revision cases will always have more potential for dislocation. In these situations in the lower demand patient and where, a complex acetabular reconstruction that requires time for ingrowth before optimal implant bone stability to occur isn't present, dual mobility with constraint has provided excellent success in terms of preventing dislocation and maintaining implant construct fixation to bone at intermediate term follow-up. Hence in these situations dual mobility with constraint remains the option we utilise. We are also confident in using this device in cases with instability or laxity where there is a secure well-positioned acetabular shell. We cement a dual mobility constrained liner in these situations using the technique described below. Present indication for dual mobility constrained liners: low demand patient, abductor muscle deficiency or soft tissue laxity, large outer diameter cups, multiple operations for instability, and instability with well-fixed shells that are adequately positioned. Technique of cementing liner into shell: score acetabular shell if no holes; score liner in spider web configuration; all one or two millimeters of cement mantle. Results. Constrained Dual Mobility Liner. For Dislocation: 56 Hips 10 yr average f/u, 7% failure of device, 5% femoral loosening, 4% acetabular loosening. For Difficult Revisions: 101 hips 10 yr average f/u, 6% failure of device, 4% femoral loosening, 4% acetabular loosening. Cementing Liner into Shell: 31 hips 3.6 yr average f/u (2–10 years), 2 of 31 failures

Introduction. Reoperations to manage unstable total hip arthroplasty are reported with a high failure rate. The dual mobility cup (figure 1) (mobile polyethylene component between the prosthetic head and the outer metal shell) is a useful option in such cases. The purpose of this retrospective study was to assess the clinical and radiologic features associated with the dual mobility cup. Materials and Methods. Fifty one unstable total hip arthroplasties (32 females, 19 males) were revised using a dual mobility socket at our institution between March 2000 and February 2005. Mean age at reoperation was 67 year old (range, 35 to 98). The outcome of the revision procedure was assessed using the Harris Hip Score, and complications were determined by detailed review of the patient's records. Anteroposterior and lateral radiographs of the involved joint were reviewed to assess the position of the prosthesis and to look for osteolysis and signs of loosening of the implant. Results. Mean follow-up was 4 years (range, 2 to 6.7). At last review 4 patients had died and one was lost to follow up. Postoperatively there was a significant improvement of the Harris Hip Score. Fifty patients (98%) had no further episodes of dislocation. There were 3 revisions for deep infection, and 2 for dissociation of the bipolar component. No cup required a revision for aseptic loosening. No radiolucent lines around the components and no osteolysis were observed at latest follow up. Conclusion. The dual mobility cup is a highly effective option to manage unstable total hip arthroplasty. Unlike constrained devices, such components provide encouraging radiologic results regarding the potential for loosening and osteolysis. Longer follow up is needed to confirm these results

Dislocation is a particular problem after total hip replacement in femoral neck fractures and elderly, especially female, patients. The increased rate of dislocation in this population is probably due to significant ligamentous laxity in these patients and poor coordination and proprioception. Another population of patients with increased propensity for dislocation is the revision hip replacement patient. Current dislocation rates in these patients can approach 10% with conventional implant systems. The Dual Mobility total hip system is composed of a cobalt chrome acetabular shell that has a grit blasted, beaded and/or hydroxyapatite coating to improve bone ingrowth. The polyethylene liner is highly crosslinked polyethylene and fits congruently into the cobalt chrome shell and acts like a large femoral head (usually > 40 mm). The femoral head attached to the trunnion is usually 28 mm. The femoral head snaps into the polyethylene liner to acts as a second protection against dislocation. Indications for the Dual Mobility socket are in the high risk for dislocation patient and particularly in elderly, female patients. It is also indicated in patients with neuromuscular disease who are at more risk to dislocate. To date 237 dual mobility cups have been performed with an average age of 79 and 207 of the procedures in women. The follow up extends to 5.6 years with an average of 3.5. There has been 1 dislocation which occurred after a traumatic event. There have been no mechanical failures, no infections and no other revisions in this series. Interprosthetic dislocation has been reported in long term follow up and there was, in this series, when reduction was performed on the only liner dislocation. Pain relief has been no different than conventional hip replacement and range of motion is unchanged as well

Although the incidence of total hip dislocation has decreased, it still remains a major problem particularly if recurrent. The actual incidence is around 1–2% but it has been documented as the leading cause for hip revision in the United States. In patients with recurrent hip dislocation, technical issues of leg length inequality, incorrect offset, and poor implant position should be addressed surgically and the abnormality corrected. In patients with recurrent hip dislocation, the articulation is preferably converted to a more stable articulation, with constrained sockets and dual mobility being the choices. In my experience, dual mobility articulations remain an excellent option for recurrent hip dislocation and its use is increasing significantly. It provides improved hip stability and data have demonstrated good success with recurrent hip dislocation. However, with use of the modular variety of dual mobility which is needed for acetabular cup fixation with screw augmentation, dissimilar metals are placed in contact (titanium socket and cobalt chrome liner insert) which potentially can pose a fretting or corrosion problem in longer term outcomes. Constrained sockets of the tripolar configuration provide another option which is useful in those patients with severe abductor dysfunction or insufficiency. Constrained sockets can also be cemented into the existing shell in cases where there is a well-fixed cup and cup removal may lead to significant bone loss and need for complex acetabular reconstruction. It is important to remember that there are two types of constrained sockets, tripolar and focal constraint. Results with the tripolar constrained socket have been significantly better than the focal constraint variety which adds a polyethylene rim piece to the liner. In a mid-term follow up (2–9 years) of 116 constrained tripolar sockets, recurrent dislocation was only 3.3%. In papers reporting on focal constrained sockets, recurrent dislocation was in the 9–29% range. There continues to be a role for constrained sockets and selection of implant type has made a difference in ultimate outcome

Background. Instability and dislocation are some of the most important postoperative complications and potential causes of failure that dual mobility total hip arthroplasty (THA) systems continue to address. Studies have shown that increasing the relative head size provides patients implanted with smaller and larger cups increased stability, greater ROM and a lesser incidence of impingement, without compromising clinical results. The purpose of the current study was to review clinical outcomes in three groups of primary THA patients receiving a dual mobility acetabular shell. Methods. In two US based, post-market, multicenter studies, 450 patients received a primary cementless dual mobility THA. Patients were split into three groups based on cup size: ≤ 50mm, 52mm–56mm, and ≥ 58mm. Harris Hip Scores (HHS), Short Form 12 Physical Components (SF12 PCS), Lower Extremity Activity Scores (LEAS), and Euroqol 5D Score (EQ-5Ds) were collected preoperatively and through 2 years postoperative. Results. The current study displays gender differences among the three groups, with 90% female patients in the ≤ 50mm group, 66% male patients in the 52mm–56mm group and 100% males in the largest cup size group. A posterior/posterolateral approach was used in 94% of cases. The mean age range among the 3 groups was 60.5–61.7 and the two most common concurrent medical conditions were cardiovascular and musculoskeletal. There were no differences observed in clinical outcomes among any of the groups, all of which displayed significant increasing trends through 2 years postoperative (Figure 1). The HHS increased significantly from an average preoperative score of 54.5 to 92.9 and 93.7 at 1 and 2 years. Clinically significant improvements were seen at 2 years in SF12 PCS (+16.5) and the LEAS (+2.4) (Figures 1 and 3). The EQ-5D TTO increased from 0.62 preoperative to 0.91 at 2 years postoperative (Figure 2). There have been no failures due to dislocation reported in the current study population. Conclusion. Positive clinical outcomes for primary THA patients receiving a dual mobility system were seen in the current study, supporting their effectiveness. Regardless of the relative head size, all patients showed significant improvements postoperative with continued stability. As the primary risk factors for instability can include gender, age and increased comorbidities, the contemporary dual mobility system used in this study can address each patient's anatomic differences, improving quality of life and reducing the risk for dislocation, as well as the significant cost implications

Although the incidence of total hip dislocation has decreased, it still remains a major problem particularly if recurrent. The actual incidence is around 1–2% but it has been documented as the leading cause for hip revision in the United States. In patients with recurrent hip dislocation, technical issues of leg length inequality, incorrect offset, and poor implant position should be addressed surgically and the abnormality corrected. In patients with recurrent hip dislocation, the articulation is preferably converted to a more stable articulation, with constrained sockets and dual mobility being the choices. In my experience, dual mobility articulations remain an excellent option for recurrent hip dislocation and its use is increasing significantly. It provides improved hip stability and data have demonstrated good success with recurrent hip dislocation. However, with use of the modular variety of dual mobility which is needed for acetabular cup fixation with screw augmentation, dissimilar metals are placed in contact (titanium socket and cobalt chrome liner insert) which potentially can pose a fretting or corrosion problem in longer term outcomes. Constrained sockets of the tripolar configuration provide another option which is useful in those patients with severe abductor dysfunction or insufficiency. Constrained sockets can also be cemented into the existing shell in cases where there is a well-fixed cup and cup removal may lead to significant bone loss and a need for complex acetabular reconstruction. It is important to remember that there are two types of constrained sockets, tripolar and focal constraint. Results with the tripolar constrained socket have been significantly better than the focal constraint variety which adds a polyethylene rim piece to the liner. In a mid-term follow up (2–9 years) of 116 constrained tripolar sockets, recurrent dislocation was only 3.3%. In papers reporting on focal constrained sockets, recurrent dislocation was in the 9–29% range. There continues to be a role for constrained sockets and selection of implant type has made a difference in ultimate outcome

Introduction. The rapidly expanding aging population in the UK are living longer than ever before, which is reflected by the rising number of relatively fit and healthy people that sustain fracture NOF (neck of femur). According to current NICE (National institute for health clinical excellence) guidelines a large proportion of fracture NOF patients meet the requirements to have a total hip arthroplasty (THA) for this injury. Dislocation rate of THA can be as high as 20% for patients with fracture NOF, which is a disastrous complication in these vulnerable patients. Numerous techniques have been adopted to minimise the risk of dislocation. The use of dual mobility (tripolar) acetabular components is one such strategy with a proven track record in the literature that is employed by surgeons at our institute. Objectives. To assess the dislocation rate in patients with fracture NOF treated with dual mobility (tripolar) THAs in our unit. Method. Retrospective study analyzing clinical notes and radiographs for dislocation rate, cup inclination, and limb length discrepancy. Results. A total of 17 patients with fracture NOF were treated with biarticular THAs during a 3½ year study period, with an average follow up of 22 months. Mean cup inclination was 42°, with mean limb length discrepancy of 3.4 mm. All patients mobilized comfortably without the use of walking aids. There have been no dislocations in our study group to date. Conclusions. This small series has excellent results, with a 0% dislocation rate, in treating fracture NOF patients with dual mobility (tripolar) acetabular cups. This is comparable to larger studies in the literature. Dual mobility cups provide a valuable option to decrease dislocation risk without increasing polyethylene wear rate. This is a safe, effective technique with a proven advantage to reduce dislocation risk in patients undergoing THA for fracture NOF

Studies have indicated that the shallow Ultra High Molecular Weight Polyethylene (UHMWPE) acetabular socket or the socket with no head center inset can significantly increase the risk of hip joint dislocation. A previous study suggested the rim loading model in UHMWPE socket and metal femoral head can generate an intrinsic dislocating force component pushing head out of socket. Recently there has been renewed interest in dual mobility articulations due to the excellent stability. The outer bearing couple of the dual mobility articulations are comprised of the UHMWPE femoral head and metal acetabular socket while inner bearing is the locked conventional metal-poly construct. The acetabular socket is also featured by an anatomically shaped head inset wall. The purpose of this study was to theoretically compare the intrinsic dislocating force between conventional metal head on UHMWPE socket articulations and the poly head on metal socket articulations used in the dual mobility cup under direct loading. The 3-D finite element analysis (FEA) models were same as previous study but with different material combinations. Sixty FEA model assemblies were consisted of CoCr or UHMWPE femoral heads and their corresponding 10mm thick generic UHMWPE or CoCr acetabular sockets. There were five different head center insets of 0, 0.5, 1, 1.5 and 2mm for each of six bearing diameters of 22, 28, 32, 36, 40 and 44mm for either sockets. The joint load of 2,446N was applied through the femoral head center as the same fashion as previous study. The dislocating force generated by the joint loading force intrinsically pushed femoral head out of socket. FEA results were verified with two data points of physical testing of actual UHMWPE 28mm ID liners with 0 and 1.5mm head center insets. The highest dislocating force was 1,269N per 2,446N of rim loading force for the 0mm head center inset in poly cup with 22mm CoCr femoral head or the case of easiest to dislocate. The lowest dislocating force was 17.7N per 2,446N force for the 2mm inset in CoCr socket with 44mm poly head which therefore was the least likely to dislocate. The average dislocating force decreased by 78% from metal head- poly cup couple to poly head - metal cup couple. The dislocating force decreased as the head center inset and head size increased in all material cases. The study suggests that not only the head center inset and head size but also the bearing material combinations can affect the intrinsic dislocating force component. The dual mobility poly head and metal socket couple generates less intrinsic dislocating force in all comparable conditions for conventional metal head and poly socket couple. During the hip separation and vertical placement of the cup, all variables found in this study may play the important rules to maintain joint stability. The stiffened cup rim reduces the deformation and thus reduces the potential cup wedge effect to generate dislocating force. The result of this study should provide the guidance to improve acetabular cup design for better joint stability