Reduced articular surface of one-piece cups: a cause of runaway wear and early failure

Abstract

Background: Despite the clinical success of modern metal-on-metal articulations, concerns with wear-related release of metal ions persist. Evidence suggests metal ion release is related to the effective coverage of the head in the metal shell (the cup's functional articular arc). A recent study suggests a reduced functional articular arc is associated with increased ion release and the arc is a function of component design, size, and the abduction angle.

Questions/purposes: The purposes of this study were to (1) measure the functional articular arc in different sizes of currently available one-piece metal shells from several different manufacturers; and (2) compare the functional articular arc of these one-piece metal shells with the 180 masculine arc of conventional hip arthroplasty acetabular components.

Methods: We calculated the available articular surface arc for 33 one-piece metal cups using measurements of cup depth and internal cup radius.

Results: The arc of the articular surface varied among manufacturers and generally decreased with decreasing shell diameter. The mean functional articular arc was 160.5 degrees +/- 3.6 degrees (range, 151.8 degrees -165.8 degrees), which was less than the 180 degrees arc of a conventional acetabular component.

Conclusions: Our data show certain cup designs are at higher risk for failure as a result of the decreased articular surface arc. This, along with analysis of abduction angles, supports the recent findings of bearing failure with vertically placed implants. 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.

Fig. 1

The functional articular arc (a) is a function of radius (r) and depth of the cup (d). The articular arc angle is a measure that is design-specific. The amount of coverage laterally over the head is a function of the abduction angle of the cup and the functional articular surface. Reproduced (in amended form) with permission and copyright © of the British Editorial Society of Bone and Joint Surgery (De Haan R, Pattyn C, Gill HS, Murray DW, Campbell PA, De Smet K. Correlation between inclination of the acetabular component and metal ion levels in metal-on-metal hip resurfacing replacement. J Bone Joint Surg Br. 2008;90:1291–1297, Fig. 2).

Fig. 2

Head size (mm) versus functional articular arc (degrees) is graphed for different manufacturer shell designs assessed in this study.