Fixation failures of dual mobility cups: a mid-term study of 2601 hip replacements.

Background: The use of dual-mobility cups has increased because of a low rate of dislocations combined with a 96% 15-year survival rate. However, late cup migrations have been attributed to their fixation (tripod - exact fit with two pegs and one extraacetabular screw) and the absence of porous coating. In a second-generation device, the designs were modified to achieve press-fit fixation and a layer of titanium beads was sintered on stainless steel cups.

Questions/purposes: We therefore (1) determined the midterm survival of press-fit, grit-blasted, second-generation cups with or without additional screws, compared with original tripod and (2) compared survival of grit-blasted dual-mobility cups with bimetallic porous-coated cups.

Methods: From a multiinstitutional trial, we reviewed 2408 patients with osteoarthritis implanted with 2601 prostheses of seven designs of a second-generation dual-mobility cup. The criteria for failure were migration, widening radiolucencies in any zone of the interface, or revision for cup loosening. The minimum followup was 5 years (mean, 7.7 years; range, 5-11 years).

Results: The 8-year survival rate of press-fit, grit-blasted cups was lower than that for press-fit, grit-blasted cups fixed with screws (91% versus 100%) and for tripod fixation (98%). The 8-year survival rate of press-fit, grit-blasted cups was less than that for press-fit, porous-coated cups made of the same alloy (91% versus 95%).

Conclusions: The data suggested primary fixation of grit-blasted dual-mobility cups should be secured with screws. Porous coating sintered on the convex side improved midterm survivorship. No deleterious effect of metallosis resulted from sintered titanium beads on stainless steel. Long-term followup is required to confirm these findings.