Introduction: Uncemented press fit cups function well in the medium term, but often seem to develop backside osteolysis in the long term. This study was done to compare migration and osteolysis for cups with different fixation to find out if augmentation is needed or more a risk factor.
Method: Eighty seven hips in 81 patients planned for a THA due to OA were randomised and operated with a Reflection cup inserted with under reaming and press fit. Group 1 had a porous coated cup without holes; group 2 had the same cup with additional HA coating. Group 3 had a porous coated cup with six holes and fixed with three screws; and group 4 an identical cup fixed with three pegs. Early cup stability, migration, osteolysis and function were followed over five years with RSA, x- rays and Harris hip score and the groups compared statistically.
Results: At five years the mean (SD) translation for all cups was 0.13 (0.28) mm proximally, 0.002 (0.41) mm medially, and 0.13 (0.42) mm anteriorly. The inclination increased 0.13 degrees (0.84 degrees), anteversion 0.13 degrees (0.84 degrees), and anterior tilt 0.15 degrees (0.92 degrees). There were no differences in migration between the four modes of fixation (p=0.053–0.9). An inducible displacement test of stability one week after operation showed only minor movements, below the detection limit for RSA and without group differences. Thirty-five cups had a central gap post-operatively, all disappeared within two years and did not imply either inferior fixation or radiolucencies. The HA-coated cups displayed less radiolucent lines (p=0.003) than the other groups when measured as percentage of the total interface. Most lucent lines were seen in zones 2 and 3 and developed during the initial two years. At five years, minor focal osteolytic lesions were found in 13 hips, most in cups with screws and all progressing slowly. The proximal wear was 1.02mm and the 3D wear 1.05 mm after five years, equal for the four modes of fixation and higher for young and male patients. Harris hip score was 96 at 5 and 93 at five years and equal for the groups.
Conclusion: In this study, screws and pegs did not add any stability to the already excellent fixation of cups but lead to less satisfactory interfaces. Since osteolysis is the main clinical problem and not stability, both facts suggest more cups should be used without holes, to minimise particle penetration and osteolysis.
