The use of dual mobility cups has been extensively evaluated in primary and revision THA and is a cost-effective and reliable option to prevent and treat dislocation in high-risk patients [1, 2, 5, 6, 9, 14, 15, 17, 20, 29, 31, 33]. The principle of dual mobility cups relies on three prosthetic articulations: the small articulation between the femoral head and the mobile component, the large articulation between the mobile component and the metal shell, and the third articulation between the mobile component chamfer and the femoral neck. Despite 40 years of continuous improvement in implant design, cementless fixation and, more recently, highly cross-linked polyethylene for mobile components, intraprosthetic dislocation is a specific long-term wear-related complication of dual mobility cups and remains a concern [4, 7, 9, 10-12, 14, 16, 18, 19, 21, 22, 26-28, 30, 32]. Intraprosthetic dislocation occurs because of a loss of retentive power of the mobile component for the femoral head that is related to wear of the chamfer and retentive area as a result of chronic impingement at the third articulation [4, 8, 12, 21, 23, 24, 27, 28, 30, 32]. After the first clinical description of intraprosthetic dislocation by Lecuire et al.  with the original Bousquet’s dual mobility cup, Philippot et al.  proposed a three-type classification system of different mechanisms leading to this complication, with Type 1 being pure wear-related intraprosthetic dislocation, Type 2 being intraprosthetic dislocation related to restricted motion of the mobile component at the large articulation because of arthrofibrosis or impingement of the iliopsoas tendon, and Type 3 being associated with dual mobility cup loosening [4, 12, 27]. Importantly, intraprosthetic dislocation Types 1 and 2 are both characterized by a well-fixed dual mobility cup metal shell [22, 30].
Dual mobility cups are increasingly being used worldwide in primary and revision THA, even in younger and active patients, and therefore the management of intraprosthetic dislocation should be defined, particularly when a dual mobility cup is not loose. To our knowledge, no large studies, except for case reports, have described the strategy to manage long-term wear-related intraprosthetic dislocation occurring with a well-fixed dual mobility cup metal shell [21, 22, 30].
Therefore, this study, which retrospectively analyzed an institutional total joint registry, aimed to (1) determine the prevalence of intraprosthetic dislocation in this patient population and the macroscopic findings at the time of surgical revision and (2) evaluate whether isolated mobile component exchange could be an option to manage intraprosthetic dislocation when the dual mobility cup metal shell is not loose.
Patients and Methods
From January 1991 to December 2009, a continuous series of 5874 THAs systematically performed with an intermediate-generation 316L stainless-steel cementless threaded hemispherical dual mobility cup (Bousquet-Cittiefe-Pasquali cup, Cittiefe, Caldera di Reno, Bologna, Italy), a titanium alloy cementless threaded femoral stem (Bousquet-Cittiefe-Pasquali stem, Cittiefe, Caldera di Reno, Bologna, Italy) with an 11/13-mm Morse taper cylindrical and polished modular neck, and a 316L stainless-steel 22.2-mm-diameter femoral head were prospectively enrolled in the senior author’s (JBH) institutional total joint registry. Six hundred implants (10%; 600 of 5874 THAs) in patients who were lost to follow-up at the time of evaluation or died before 2 years of follow-up were excluded from this study. Therefore, a series of 5274 THAs (4546 patients; 2773 women; mean [range] age at THA 58 years [22-87]; 728 bilateral) were included in this study and retrospectively analyzed at a mean (range) follow-up interval of 14 years (3-26). The indications were primary osteoarthritis in 62% (3269 of 5274), revision for aseptic loosening in 10% (529 of 5274), femoral neck fracture in 8% (424 of 5274), developmental dysplasia of the hip in 8% (422 of 5274), avascular necrosis of the femoral head in 8% (412 of 5274), and inflammatory arthritis in 4% (218 of 5274). This study was performed in accordance with the ethical standards of the 1964 Declaration of Helsinki. Owing to the local regulation at the time of inclusion, institutional review board approval was not required for this study. However, informed consent was obtained from all patients before surgery for data collection, analysis, and publication.
All THAs were performed through a standard posterolateral approach by or under the direct supervision of two fellowship-trained total hip surgeons (GA, JBH) with more than 15 years of dedicated arthroplasty experience and performing more than 300 THAs per year combined. The acetabulum was reamed and the proximal femur was broached to the size of the components to be implanted. The dual mobility cup’s position was in relation to neighboring anatomic landmarks, aiming for anteversion of 15° to 20° and cup abduction of 40° to 45°. Then, the femoral stem was threaded and the modular neck was positioned, aiming for anteversion of 10° to 30° relative to the posterior femoral condylar axis. A 22.2-mm-diameter femoral head was captured in force using a snap-fit technique into a conventional ultra-high molecular-weight polyethylene mobile component that was sterilized with γ-radiation under vacuum pressure. Then, the implant was intraoperatively tested for ROM, stability, absence of intraprosthetic impingement between the femoral neck and the dual mobility cup metal shell, and leg length. The hip closure method was standard, without capsular repair. Patients were admitted to the hospital after surgery. All patients received routine antibiotic prophylaxis and appropriate anticoagulation for deep venous thrombosis prophylaxis. Structured physical therapy was initiated the day after surgery and continued during the in-hospital stay. Patients were then mobilized fully weightbearing with the help of two crutches and were educated to prevent dislocation. Discharge was allowed when patients could ambulate 30 meters, could ascend and descend 10 steps, and had pain that was well-controlled with oral medications. Then, patients returned for postoperative follow-up visits at 3 months, 6 months, 1 year, and annually thereafter. Patients underwent a clinical examination, and plain AP and lateral radiographs of the pelvis and the operated-on hip were obtained. Radiographs taken at 3 months were considered baseline radiographs for comparison at follow-up. If patients were unable to attend the annual visit, they were contacted by telephone, and radiography was performed.
The occurrence of intraprosthetic dislocation was systematically reported in the total joint registry. At the latest follow-up examination, each patient with intraprosthetic dislocation was retrospectively analyzed regarding the demographics, reason for THA, radiographs, intraoperative findings, management of intraprosthetic dislocation (dual mobility cup reoperation or revision), and outcome. Reoperation was defined as the isolated exchange of the mobile component (polyethylene mobile component and femoral head), leaving the well-fixed dual mobility cup metal shell and femoral stem in place. During reoperation, débridement of the hip with synovectomy was performed, and the existence of impingement between the iliopsoas tendon and mobile component was assessed [16, 27]. Revision was defined as removal of the acetabular and/or femoral component and acetabular and/or femoral reconstruction.
Before THA reoperation or revision, a radiographic analysis was performed, and particular attention was paid to evidence of osteolysis and aseptic loosening of the implant according to the criteria of Massin et al.  and Engh et al. . Intraoperatively, implant stability, periarticular metallosis, and intraprosthetic impingement lesions on the dual mobility cup metal shell and/or femoral neck were assessed. Each polyethylene mobile component was macroscopically analyzed for the location of wear and damage lesions, and particular attention was paid to the chamfer and retentive area. Loving et al.  defined intraprosthetic dislocation as: (1) polyethylene wear and damage lesions circumferentially spread throughout the chamfer and retentive area that corresponded to a mobile impingement of a well-functioning dual mobility cup (intraprosthetic dislocation Type 1) or (2) focalized, patched polyethylene wear and damage lesions related to a fixed impingement between the polyethylene mobile component and the femoral neck that occurred in a dual mobility cup with restricted motion of the large articulation (intraprosthetic dislocation Type 2).
Descriptive data are presented as the mean and range. An intergroup comparison of two continuous and quantitative variables was performed using two-sample t-tests. Qualitative variables were compared using chi-square tests. Statistical analyses were performed using IBM SPSS Statistics, version 22.0 (IBM Corp, Armonk, NY, USA), with the level of significance set at p < 0.05.
At a mean (range) follow-up duration of 14 years (3-26), 3% of intraprosthetic dislocations (169 of 5274) were reported, with a mean (range) time from THA of 18 years (13-22). Intraprosthetic dislocation occurred predominantly in younger male patients (p < 0.001), but was not influenced by the indication for THA (p = 0.9) (Table 1).
In all, 9% of intraprosthetic dislocations (16 of 169) were Type 3 with aseptic loosening of the dual mobility cup and were managed with acetabular revision, without recurrence at a mean (range) follow-up of 7.5 years (5-11). Ninety-one percent of intraprosthetic dislocations (153 of 169) were related to wear of the mobile component chamfer and retentive area without aseptic loosening of the dual mobility cup , and managed with isolated mobile component exchange. Periarticular metallosis with subsequent osteolysis of the greater trochanter was reported in 69% of these patients (105 of 153) (Fig. 1). Intraprosthetic impingement lesions on the dual mobility cup metal shell and/or femoral neck were reported in 65% of these patients (99 of 153) (Fig. 2A-B). In all of these patients, a macroscopic analysis of the explanted mobile component revealed circumferential polyethylene wear and damage lesions of the chamfer and retentive area, with subsequent loss of retaining power for the femoral head (Fig. 3). There was no obvious blockage of the large articulation because of impingement of the iliopsoas tendon and the mobile component was observed during the revision procedure. In addition, all femoral heads showed macroscopic damage lesions of pitting and scratching on their bearing surface that was related to metal-on-metal friction after intraprosthetic dislocation (Fig. 4).
At a mean (range) follow-up of 7.5 years (5-11), 82% of the patients (125 of 153) who underwent isolated mobile component exchange did not sustain further surgery because they were without evidence of intraprosthetic dislocation recurrence or aseptic loosening of the dual mobility cup. Early recurrence of intraprosthetic dislocation was reported in 6% of the patients (nine of 153) at a mean (range) time of 3 years (2-4.5) after reoperation. These nine patients underwent conventional acetabular revision, without intraprosthetic dislocation recurrence at a mean (range) follow-up interval of 5 years (3-8). In addition, 12% of the patients (19 of 153) later underwent acetabular revision at a mean time of 1.5 years (0.5-3 years) after reoperation for severe premature wear of the polyethylene mobile component at the small articulation, which was associated with aseptic loosening of the dual mobility cup (Fig. 5 A-B). No intraprosthetic dislocation recurrence or aseptic loosening of the dual mobility cup was reported at a mean (range) follow-up duration of 6.5 years (5-9) after revision. Importantly, in 18% of the patients in whom reoperation failed (28 of 153 patients undergoing isolated mobile component exchange), severe periarticular metallosis and intraprosthetic impingement lesions on the implants were observed (Figs. 6 A-B).
This study retrospectively analyzed an institutional total joint registry with prospectively collected data to evaluate whether isolated exchange of a mobile component could be an option to manage intraprosthetic dislocation when a dual mobility cup is not loose. If conventional acetabular revision is indicated to manage intraprosthetic dislocation associated with dual mobility cup loosening at diagnosis (intraprosthetic dislocation Type 3), the strategy to manage long-term wear-related intraprosthetic dislocation occurring with a well-fixed dual mobility cup metal shell (intraprosthetic dislocation Types 1 or 2) was not clearly defined in previous studies [4, 18, 21, 22, 28, 30]. The most important finding of this study was that a failure rate of 18% was observed within 5 years after isolated exchange of the mobile component to manage intraprosthetic dislocation when a dual mobility is not loose.
This study had four major limitations. First, the evaluated construct is no longer commercially available as dual mobility cup design and technology evolves with time, especially with the introduction of highly cross-linked polyethylene and a better understanding of the relationship between the mobile component and femoral neck at the third articulation [4, 8, 12, 14, 23, 24, 30, 32]. Therefore, our results might not be generalizable to modern constructs. However, with the increased worldwide use of dual mobility cup in younger and more demanding patients who undergo primary and revision THA, clinical studies with longer follow-up durations to evaluate modern designs might later report intraprosthetic dislocation [6, 14, 17, 33]. Consequently, management of intraprosthetic dislocation must be defined according to the available historical data. Second, 10% of the implants (600 of 5874) were not evaluated in this study because patients were lost of follow-up at the time of evaluation or deceased before two years of follow-up. This might represent a bias in this study, as intraprosthetic dislocation was a relatively low event in this patient population. Third, intraprosthetic dislocation Types 1 and 2 were difficult to differentiate in this study, as the classification system of Philippot et al.  has been proposed recently. Therefore, it was not possible to precisely establish in this study whether failures of the isolated mobile component exchange could be attributable to intraprosthetic dislocation Type 2 with restricted motion of the dual mobility cup’s large articulation [12, 16, 27]. Four, this study was uncontrolled regarding an outcome comparison of isolated mobile component exchange and conventional acetabular revision to manage intraprosthetic dislocation Type 1 or 2.
The intraprosthetic dislocation rate of 3.2% reported with this intermediate-generation dual mobility cup was similar to the rates seen with the use of other constructs evaluated during the same period, although intraprosthetic dislocation is less common in the latest generations of implants [4, 18, 20, 21, 22, 30]. In a systematic review including nearly 18,000 THAs with dual mobility cups, intraprosthetic dislocation decreased to a mean rate of 0.7% in primary THA and 1.3% in revision THA, with near-disappearance in the most recent studies evaluating modern dual mobility cup [4, 6, 9, 28, 33]. However, most of those studies were limited to a 10-year follow-up duration, with dual mobility cups implanted in selected patients with a high risk of dislocation and, frequently, with low functional demand [9, 14, 28, 31]. Retrieval and biomechanical studies confirmed that dual mobility cup motion and wear physiologically predominate at the small and third articulations [3, 4, 8, 12, 23, 27, 32]. Boyer et al.  reported that in a well-functioning dual mobility cup, the in vivo evaluation of wear requires three-dimensional measurements. Because the mobile component is intended to move freely along three axes, the conventional linear rate of penetration measured on conventional two-dimensional radiographs was shown to be poorly related to volumetric wear . Three-dimensionally, a shift in the centers of rotation of the mobile component’s convexity and concavity with time was correlated with volumetric wear, particularly at the small articulation . Although the cut-off threshold of femoral head penetration to predict intraprosthetic dislocation is still unknown when examining dual mobility cup wear in routine clinical practice, we advise surgeons to closely evaluate patients when the femoral head appears to have penetrated several millimeters into the dual mobility cup and even propose that a preventive isolated exchange of the mobile component should be performed before intraprosthetic dislocation occurs (Fig. 7A-D). In addition, Loving et al.  and Nebergall et al.  demonstrated that in a well-functioning dual mobility cup, polyethylene deformation and wear lesions were circumferentially spread throughout the chamfer and retentive area and were not contained in one particular area, suggesting that the mobile component was not impeded in a fixed position. Conversely, if there is restricted motion at the large articulation and for a given amount of load transferred to the third articulation, the pattern and extent of polyethylene deformation and wear lesions would likely be affected and concentrated in areas where impingement occurs between the femoral neck and fixed mobile component [23, 27]. Therefore, along with in vivo polyethylene wear, oxidation, and even crack propagation, there is the potential for failure of the retentive area and subsequent intraprosthetic dislocation [8, 23, 24, 27]. Moreover, D’Apuzzo et al.  demonstrated that, when comparing damage scores between the small and large articulations, scratching, pitting, and embedded metallic debris were greater at the small articulation. The propensity of the small articulation to trap metallic debris could be of concern if the large articulation is stationary with restricted motion, particularly if periarticular metallosis is present . Therefore, extensive synovectomy should be performed during reoperation or revision of a dual mobility cup to reduce the amount of metal debris and subsequent third-body wear of the small articulation that could account for the modes of failure reported in the current study.
In the current study, intraprosthetic dislocation Type 3 was successfully managed with conventional acetabular revision, without mechanical failure at mid-term follow-up. In contrast, after isolated mobile component exchange to manage Type 1 or 2 intraprosthetic, the two reasons for failure were early intraprosthetic dislocation recurrence or severe premature polyethylene wear of the mobile component leading to dual mobility cup loosening. Importantly, in all patients with failure, severe periarticular metallosis with damage to the femoral head and intraprosthetic impingement lesions on the metal shell and femoral neck were observed [19, 21, 26]. Although such failures could be explained by third-body wear at the small and third articulations because of metallic debris, the timing when periarticular metallosis occurs is difficult to establish (Fig. 6A-B) [8, 16, 19, 26]. Indeed, intraprosthetic impingement lesions could have occurred before intraprosthetic dislocation because of implant malposition or femoral-head penetration into the mobile component that was related to polyethylene wear with time and created conditions leading to femoral-neck impingement on the rim of the metal shell (Fig. 5) [3, 4, 12, 22, 26, 30]. Inversely, those lesions could have occurred after intraprosthetic dislocation as the femoral head emerged from the mobile component and lodged itself into the dual mobility cup metal shell with direct articulation. Lastly, a 22.2-mm-diameter femoral head with a 11/13-mm Morse taper was used in each primary THA and reoperation in this series, resulting in a unfavorable head-to-neck ratio. In the first description of intraprosthetic dislocation by Lecuire et al. , four principles were advocated to reduce the risk of intraprosthetic dislocation that total hip surgeons must consider: (1) achievement of the most favorable head-to-neck ratio that is actually allowed with the use of 28-mm and 32-mm femoral heads snapped into a cross-linked polyethylene mobile component, reserving 22.2-mm femoral heads only for the smallest dual mobility cup size; (2) use of a femoral stem with the narrowest cylinder and smoothest femoral neck, avoiding a skirted femoral head with a long offset, which causes the longest tapered trunnion to impinge on the third articulation; (3) positioning of the dual mobility cup in the bony acetabulum without excessive anteversion, to avoid impingement with the femoral neck that could induce metal debris of the third body, particularly with a non-hemispherical dual mobility cup metal shell; and (4) removal of all fibrotic tissue and osteophytes that could restrict motion of the large articulation, with intraoperative verification of the perfect mobility of the mobile component at the large articulation [4, 9, 16, 20, 22, 27].
In conclusion, isolated exchange of a mobile component demonstrated a high failure rate at short-term follow-up and is therefore not recommended to manage long-term wear-related intraprosthetic dislocation occurring with a well-fixed dual mobility cup metal shell. The exception is intraprosthetic dislocation occurring in elderly or frail patients, for whom a conventional acetabular revision procedure would be associated with an unjustified surgical or anesthetic risk. At least, isolated mobile component exchange could be used to manage a pseudo-intraprosthetic dislocation with well-positioned, well-fixed, and non-damaged implants occurring after external maneuvers to reduce an acute dual mobility cup dislocation [7, 9, 10, 11, 16, 18, 19, 26]. Conventional acetabular revision with synovectomy should remain the standard procedure to manage intraprosthetic dislocation, particularly if periarticular metallosis is present. Further studies should be dedicated to defining methods for mobile component wear evaluation usable in routine follow-up examination to propose isolated mobile component exchange before intraprosthetic dislocation and metallosis occur.
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