Loosening of the porous coating of bicompartmental prostheses in patients with rheumatoid arthritis.

Thirty-four porous-coated anatomical (PCA) total knee arthroplasties in twenty-eight patients with rheumatoid arthritis were followed for an average of seventeen months. In these thirty-four knees, twenty-three tibial and fifteen femoral components were cemented. The rest were uncemented. At follow-up, more than half of the knees (nineteen of thirty-four) showed radiographic evidence of displacement of some of the beads from the porous coating. In fourteen knees the beads loosened more than three months postoperatively, and all of these knees had a radiolucent zone. Six knees had beads within the joint. Eight of nine non-cemented tibial components (excluding one tibial component) and five of the nineteen femoral components that were not cemented showed late loosening of beads (after three months). This study indicates that the metallurgical technique for porous coating with heat-sintered cobalt-chromium beads does not produce a coating that is sufficiently strong to prevent loosening in the case of the porous-coated anatomical prosthesis. The long-term clinical relevance of our findings is unclear. The mechanical strength of the porous coating on the prosthetic surface should be improved and the prosthesis should be tested in limited clinical studies before cementless fixation can be recommended.