Comparison of porous-threaded and smooth-threaded acetabular components of identical design. Two- to four-year results.

Between April 1985 and April 1987, 82 smooth-threaded and 62 porous-threaded acetabular components of identical design were implanted without structural bone grafting as primary acetabular replacements. At a minimum two year follow-up examination, statistically significant differences were found in the roentgenographic and clinical results between the two matched groups. Twenty-nine percent of the smooth-threaded cups and none of the porous-threaded acetabular components were classified as roentgenographically unstable. Clinically, groin or buttock pain was present in 25% of patients with smooth-threaded cups and in only 7.5% of the patients with the porous-threaded components. Six smooth-threaded components and none of the porous-threaded components have been revised to date. Because of the unacceptable high incidence of instability at short-term follow-up, the smooth-threaded acetabular component is no longer being used at this institution. The addition of the porous coating was associated with the superior results noted in this group.