Experiences with the Plasmacup--early stability, wear, remodelling, and outcome.

Whilst advances in cementing technique have led to improvement in the survival of cemented femoral stems in total hip arthroplasty (THA), cup failure due to aseptic loosening remains a major clinical problem. These observations have led to a move away from cemented cup designs, particularly in young patients, towards uncemented implants. The Plasmacup is a hemispherical, press-fit, cementless, titanium-shelled, acetabular component with a polyethylene liner. In this article we review our experience of its pattern of early migration, wear, bone remodelling, and mid-term survival. In 18 cups followed for 2 years in subjects with a mean age at operation of 58 years, the mean total vectorial cup migration was 0.75 mm, and cup orientation remained stable (EBRA method). The mean polyethylene linear wear rate over this period was 0.21 mm/year. In 27 cups followed for 6 months using dual-energy X-ray absorptiometry (DXA), average bone loss was -5%, and the pelvic bone-remodelling pattern was consistent with the rim-loading principle of the cup design. In a clinical review of the outcome of 128 cups in 104 patients with a mean age at operation of 51 years and follow-up of 59 months, we found that 82% of patients had a good or very good Merle D'Aubigne score, and cup survival rate was 98% (Kaplan-Meier). Four cups had small radiographic areas of focal osteolysis and three had been revised (two for recurrent dislocation and one for deep sepsis; none were revised for aseptic loosening). The mean linear wear rate in this series was 0.14 mm/year. In conclusion, the Plasmacup shows satisfactory early stability, a wear rate similar to other uncemented cups, and favourable mid-term clinical function and survival rates.