Bedding-in and true wear in two different generations cementless porous-coated acetabular cups

Radiographic measurement of polyethylene wear cannot distinguish between so-called bedding-in and true wear in porous-coated cups. In this study, sequential wear performance was compared in two different generations of two cementless porous-coated cups. Group 1 consisted of 42 Harris-Galante I cups (HG I; 32-mm femoral head) and 37 Harris-Galante II cups (HG II; 28 mm); group 2 comprised 30 ACS liners using Profile or Trilock cups (32 mm) and 53 Duraloc cups (28 mm). All liners were made of conventional polyethylene sterilised by gamma irradiation in air. All cups showed radiographic bone ingrowth. Follow-up ranged from 5 to 17 years. Anteroposterior pelvic radiographs were scanned digitally and sequential femoral head penetration was estimated using a software package based on Kim's method. Bedding-in was 0.17 ± 0.09 mm for HG I and 0.09 ± 0.02 for HG II cups (p=0.035) in group 1, and 0.60 ± 0.50 for the ACS liners and 0.11 ± 0.41 for the Duraloc cups (p<0.01) in group 2. Femoral head penetration at the end of follow-up was 1.50 ± 0.28 mm for HG I (mean 14 years) and 0.85 ± 0.01 for HG II (8 years) in group 1, and 2.86 ± 0.68 for ACS (12 years) and 0.90 ± 0.41 for Duraloc (8 years) in group 2. Mean wear was 0.09 mm/year for HG I and 0.10 for HG II in group 1, and 0.20 for the ACS liners and 0.09 for the Duraloc cups in group 2. In conclusion, sequential femoral head penetration using Kim's digitised method allows us to distinguish between bedding-in and true wear.