A quantitative in vitro assessment of fit and screw fixation on the stability of a cementless hemispherical acetabular component

Abstract

This investigation quantifies in vitro the effect of component fit, as well as the effect of adjuvant screw fixation, on the initial stability of cementless hemispherical titanium acetabular total hip arthroplasty components and assesses apposition of the acetabular components to bone. Six, fresh human hemipelvi (3 matched pairs) were harvested at autopsy. Titanium alloy acetabular components with a porous surface of commercially pure titanium fiber mesh (Harris Galante Porous acetabular components, Zimmer, Warsaw, IN) were used for implantation. Initially, each acetabulum was underreamed to achieve a 2 mm press-fit with the acetabular component. Pressuresensitive film had been placed along the dome and medial wall at the bone-implant interface to assess the completeness of seating. After the implant was impacted into the acetabular cavity, relative motion between the implant and bone was measured during simulated single leg stance. Adjuvant fixation of the implant was then obtained with the insertion of four 6.5 mm cancellous screws. High-contrast roentgenograms of the specimens in multiple views were obtained after initial cup insertion and again after screw insertion. The stability of each implant under load was measured with four, three, two, one, and no screws in place. Further reaming of the bone was done to create a 1 mm press-fit. The sequence was then repeated. Further reaming was done to create an exact-fit and the sequence was repeated again. Under these conditions, 1 mm press-fit with or without screws provided the optimum combination of fit and stability. With the 2 mm press-fit, incomplete seating of the implant was found in every case, resulting in gaps between the dome of the implant and the bone. This reduced the area of the porous surface in contact with bone, and resulted in an increase in micromotion with the dome of the implant. The addition of supplemental screws did not significantly enhance stability of the implant under press-fit conditions. Exact-fit reaming with the use of two, three, or four screws yielded acetabular component stability at the dome equal to 1 mm press-fit, but at the rim the acetabular component was less stable than 1 mm press-fit. The optimum combination might be a press-fit introduction of the acetabular component, and the advantage of screws must be weighed against possible particulate debris and vascular damage.