Dual Poly Liner Mobility Optimizes Wear and Stability in THA - Healio

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■ Emergent Hip Technologies? A dual-mobility acetabular component consists of a large, fixed, porous-coated acetabular component and a bipolar femoral component (Figure 1). These components are often called tripolar components. This configuration provides a stable, well-fixed implant platform against bone and 2 articular interfaces, a large polyethylene surface directly apposed to a highly polished metal implant, and a standard-sized (28- or 32-mm) femoral head captured within polyethylene (Figure 2). This concept was conceived, developed, and described by the French orthopedic surgeon, Gilles Bousquet, in 1976. 1 The dual-mobility implant was developed to reduce the incidence of dislocation in patients at risk of instability who required total hip arthroplasty (THA). Patients at increased at risk may include those: (1) older than 75 years; (2) who have had prior hip surgery; (3) with neuromuscular diseases; (4) with cognitive dysfunction; and (5) with American Society of Anesthesiologists scores 3. Hip instability remains one of the most common causes of THA failure, especially following revision THA. The dual-mobility acetabular component potentially provides substantially greater range of motion than conventional articulations with large femoral heads (Figure 3). This theoretical protection in stability has been confirmed in a number of studies (Table 1). 1 The dual-mobility cup has also been used successfully in revision THA, where the dislocation rate using conventional, nonconstrained implants may be 10% to 20% (Table 2). This high rate of stability achieved with the dualmobility cup in patients at increased risk of dislocation has been accompanied by a survival rate that compares favorably with THA using conventional, unconstrained articular surfaces (Table 3). 2 Recent modifications of the original dual-mobility implant have sought to increase the durability of the device, reduce the potential for soft tissue impingement that might be associated with the original prosthesis, and increase the flexibility of the acetabular shell. The dual-mobility design using firstgeneration cross-linked polyethylene has been shown to reduce polyethylene wear to rates below those reported for conventional articulations using conventional polyethylene. Moreover, when the dualmobility concept is paired with the current generation of highly cross-linked polyethylene, the wear rates are 97% lower than those obtained with the first-generation cross-linked polyethylene in dual-mobility cups (Figure 4). The original dual-mobility acetabular shell was hemispherical. This design, especially in larger sizes, had the potential to be associated with iliopsoas impingement. An anatomic shell has been developed that seeks to avoid this complication. A surgical technique with dedicated instrumentation has been developed to optimize the insertion of this shell. A major drawback of the original monoblock dual-mobility and monoblock anatomic variation of the acetabular shell has been the inability to augment the press-fit fixation with cancellous screws. Moreover, the monoblock versions require surgeons to use a unique implant that is not routinely a part of an operating room acetabular cell inventory. In the past 6 months, a modular dual-mobility shell has become available (Figure 5). Standard hemispherical shells can be converted to a dual-mobility implant with the insertion of a highly polished, thin metal insert that articulates with the bipolar polyethylene femoral head. Thus, a standard acetabular design that is routinely stocked by hospitals, is familiar to orthopedic surgeons, and is inserted using conventional instrumentation can be converted intraoperatively to a dual-mobility cup if additional stability is needed. This modular 1 2 3 Figure 1: Fixed porous shell and bipolar femoral component. Figure 2: The 3 components of a dual mobility cup: fi xed porous shell, polyethylene bipolar component, and femoral head. Figure 3: Potential range of motion of a dual-mobility cup (ADM) vs conventional articulations with large femoral heads. e446 ORTHOPEDICS | ORTHOSuperSite.com
■ Emergent Hip Technologies? Table 1 Dislocations in Primary THA Using the Dualmobility Cup 2 Study No. of Cases No. of Dislocations a Philippot, 2004 106 0 Aubriot, 1993 110 1 Vanel, 2003 127 1 Béjui-Hughes, 167 0 2006 Philippot, 2006 70 0 Abbreviation: THA, total hip arthroplasty. a Overall dislocation rate is 0.4%. Table 2 Dislocation Rate in Revision THA Using the Dual-mobility Cup 2 No. of Study Revision THAs No. of Dislocations a Aubriot, 1995 13 0 Beguin, 2002 42 0 SFHG, 2006 403 8 Guyen, 2009 54 3 Abbreviation: THA, total hip arthroplasty. a Overall dislocation rate is 2.1%. Table 3 Dual-mobility Survival Rates 2 Study No. of Hips Survivorship Aubriot, 1993 100 97% at 5 y Farizon, 1998 135 95.4% at 10 y Leclerc, 1999 153 96% at 10 y, 94% at 15 y Philippot, 106 94.6% at 10 y 2004 Philippot, 2006 100 95% at 10 y 6A 6B Figure 6: Radiographs of a dysplastic hip with severe degenerative arthritis (A) treated with a modular dual-mobility cup (B). Figure 4: The impact of dual-mobility design on polyethylene wear (Trident and ADM; Stryker, Mahwah, New Jersey). 5 Figure 5: The modular dual-mobility cup consists of a conventional press-fi t shell, a polished modular metal or standard X3 poly inserts (Stryker, Mahwah, New Jersey), a large X3 poly head (Stryker), and a 22- or 28-mm cobalt chrome or ceramic head. Abbreviations: CoCr, cobalt chrome; MOM, metal-on-metal. 4 design has been used at our institution in various situations, including: (1) dysplasia (Figure 6); (2) osteoarthritis with a large horizontal offset; (3) avascular necrosis in patients requiring small cups; (4) failed metal-on-metal implants; (5) failed bipolar implants; (6) infected bipolar implants; (7) acute dislocations after revision THA; and (8) recurrent early and late dislocations after primary THA. The dual-mobility cup appears to offer a safe, effective, durable solution to hip instability. The concept has extensive laboratory and clinical support. Although the long-term durability of these implants is unknown, the tested wear rates of a dual- SEPTEMBER 2011 | Volume 34 • Number 9 e447
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