Inadequate polyethylene thickness and osteolysis in cementless hip arthroplasty.

A matched series Porous Coated Anatomic (PCA), Howmedica, Rutherford, NJ) cementless hip arthroplasties comparing a 26-mm with a 32-mm head were reviewed. The 26-mm head was used in 63 hips of 56 patients (group 1), and the 32-mm head was inserted into 97 hips of 90 patients (group 2). The two groups were matched for age, sex, and primary diagnosis. The postoperative follow-up period ranged from 3 to 6.9 years (mean, 4.2 years) in group 1, compared with 3 to 7.9 years (mean, 4.5 years) for group 2. The mean clinical scores for pain, movement, and function were similar for the two groups. Erosive osteolytic lesions were encountered in 25.7% of the group 2 hips but in none of the group 1 cases. These lesions were not seen before 3 years after surgery and were present in Gruen zone 7 of all involved hips, although in two patients they were also noted in zone 1. Osteolysis was encountered only with the 32-mm head where the outer diameter of the metal backed cup was 52 mm or less. Therefore, inadequate polyethylene thickness, rather than head size, is implicated as the cause of plastic-particle mediated osteolysis.