Primary stability of two uncementedacetabular components of different geometry: hemispherical or peripherallyenhanced?

Abstract

Objective: This study compared the primary stability of two commercially available acetabular components from the same manufacturer, which differ only in geometry; a hemispherical and a peripherally enhanced design (peripheral self-locking (PSL)). The objective was to determine whether altered geometry resulted in better primary stability.

Methods: Acetabular components were seated with 0.8 mm to 2 mm interference fits in reamed polyethylene bone substrate of two different densities (0.22 g/cm(3) and 0.45 g/cm(3)). The primary stability of each component design was investigated by measuring the peak failure load during uniaxial pull-out and tangential lever-out tests.

Results: There was no statistically significant difference in seating force (p = 0.104) or primary stability (pull-out p = 0.171, lever-out p = 0.087) of the two components in the low-density substrate. Similarly, in the high-density substrate, there was no statistically significant difference in the peak pull-out force (p = 0.154) or lever-out moment (p = 0.574) between the designs. However, the PSL component required a significantly higher seating force than the hemispherical cup in the high-density bone analogue (p = 0.006).

Conclusions: Higher seating forces associated with the PSL design may result in inadequate seating and increased risk of component malpositioning or acetabular fracture in the intra-operative setting in high-density bone stock. Our results, if translated clinically, suggest that a purely hemispherical geometry may have an advantage over a peripherally enhanced geometry in high density bone stock. Cite this article: Bone Joint Res 2013;2:264-9.

Keywords: Acetabular cup; Geometry; Peripherally enhanced; Primary stability; THR; Uncemented.

Fig. 1

Photograph showing the peripheral self-locking (PSL) Trident acetabular component (Stryker, Newbury, United Kingdom) mounted on the brass rod connecting assembly used to secure the component to the load cell.

Fig. 2

Diagrammatic illustration of the moment arm determined for ‘lever-out’ testing. A steel cable, low friction pulley and universal connection are used to apply a lever-out moment to the introducing assembly. The moment arm was defined by the distance of the universal joint from the surface of the synthetic block.

Fig. 3

Seating force for hemispherical and peripheral self-locking (PSL) implant designs in low- (0.22 g/cm³) and high-density (0.45 g/cm³) substrate. The error bars denote the range of values (* p = 0.006).