With the increase in the number of resurfacing and THAs performed with large, nonhemispheric one-piece metal cups, new complications associated with wear have been noted. These complications include runaway wear, pseudotumor formation, and acute lymphocytic vascular-associated lesions [8
]. Several reports have recently been published regarding acetabular component position as it relates to serum metal ions [5
] and cup loosening [4
]. Cups implanted with inclinations ranging from 45° to 60° showed signs of increased edge loading, loosening, and serum metal ion levels. Additionally, De Haan et al. have demonstrated the importance of functional articular arc as it relates to wear complications with large metal-on-metal articulation [5
]. De Haan et al. showed elevated metal ion release correlates with a reduced functional articular arc, in which the arc is a function of component design, component size, and the abduction angle of the cup.
The purpose of the current study, therefore, was to build on the work of De Haan et al. [5
] and (1) to measure the functional articular arc in different sizes of currently available one-piece metal shells from several different manufacturers; and (2) to compare the functional articular arc of these one-piece metal shells with the 180º arc of conventional hip arthroplasty acetabular components.
We acknowledge several limitations of the current study. The primary limitation is that articular surface wear is a multifactorial process and there are many variables that affect it which we do not address, including bearing diameter and cup anteversion. The focus of this study, however, is functional articular arc. All else being equal, functional articular arc matters [5
] and surgeons should be more aware of it. Quantification of functional articular arcs for cups presently on the market may help surgeons improve their outcomes by enabling them to understand which shell designs are at increased risk for edge loading, specifically if they are positioned suboptimally. Second, we did not evaluate the full range of implants for each manufacturer. Although we would have preferred to evaluate many more cups, we were only able to study those cups donated by the different manufacturers. However, all cups donated were evaluated and included in the present analysis. Future analyses should include more cups from different manufacturers with a complete range of cup sizes.
We observed a wide variation of functional arcs among currently available manufacturers and cup sizes in which functional arc generally decreased as shell diameter decreased. In general, the Conserve cup (Wright Medical) had the largest functional arcs, whereas the BHR (Smith and Nephew) and the Cormet (Stryker) had the smallest functional arcs across the range of cup sizes. Of all the cups studied, the 44-mm ASR (DePuy) had the smallest functional articular arc. The reason for the variation among manufacturers is related to design. Design decisions that compromise hemisphericity are based on many factors, including retention of bone stock and reducing the risk of impingement in a resurfacing situation. Some cups are designed to be less than a hemisphere to prevent impingement of the native femoral neck against the shell edge. Some designs reduce hemisphericity by incorporating a thickened dome to stiffen the shell and lessen the deformation that can occur during press-fit implantation. Still other designs incorporate a tapered radius at their rim to eliminate sharp edges and maintain fluid ingress into the articulation. Overall, these design differences result in variability in functional articular arcs. This difference between designs is clearly demonstrated by a recent clinical report. In a single center study of 660 metal-on-metal resurfacings, Langton et al. [12
] reported 17 failures from adverse reactions to metal debris in patients implanted with ASR implants (3.5%) and no failures of this nature in patients implanted with BHR (Smith and Nephew) implants. Langton et al. suggest the increased failure of the ASR cup (DePuy) secondary to the increased generation of metal debris was the result of its reduced functional articular arc as compared with that of the BHR (DePuy) component [12
The articular surface arcs of cups in the current study averaged 160.6°, which was substantially less than the 180° arcs associated with standard THA components. The smaller bearing surface arc demonstrated with these and other one-piece metal cups results in a smaller amount of coverage laterally over the head, which could lead to edge loading at much lower abduction angles, resulting in elimination of fluid film lubrication and increased metal wear. This is consistent with the reports of edge loading and component loosening as well as increased metal ions noted in subjects with inclination angles averaging 45° or more [5
]. As previously noted, the amount of arc available for coverage is a function of the abduction angle of the cup, the functional arc of the particular cup design, and the size of the implanted cup. Cups with a smaller functional articular surface are at higher risk of edge loading and high wear rates. As an example, if a cup with a functional articular surface of 151° is implanted at 55° abduction, it will behave like a 180° cup implanted at 69.5° of abduction and be at risk for edge loading. A traditionally acceptable 45° inclination angle leaves no room for error in these nonhemispheric cups, particularly in smaller sizes. This supports the findings of Ollivere et al. who studied the rate and mode of early failure in 463 Birmingham hip resurfacings [16
]. They reported a 3.1% rate of metallosis-related revision at 5 years with risk factors for revision including female gender and a high abduction angle. When combined with excessive anteversion, these one-piece cups provide even less coverage of the weightbearing portion of the femoral head. Given this possibility, and the increasing number of reports of runaway wear and loosening as well as pseudotumor formation and devastating soft tissue necrosis and nerve palsies, it may be advisable to implant nonhemispheric resurfacing cups between 40º to 45º of abduction and 15º of anteversion and check for impingement intraoperatively. Because cup position error is not well tolerated with these designs, an intraoperative radiograph may be advisable to ensure accurate position.
Our data and analysis of abduction angles are consistent with recent findings of bearing failure with vertically placed implants. It is evident certain designs are at higher risk for failure as a result of the decreased articular surface arc. Care must be taken when implanting these shells to ensure they are placed in less abduction to avoid edge loading and the potential for early bearing failure.