Largely through the pioneering advances made by John Charnley,' total joint replacements with man-made metal and plastic materials have revolutionized the surgical treatment of millions of patients with end-stage hip and knee arthritis for the past 30 years. Approximately 500,000 total hip and knee replacements are done each year worldwide costing billions of dollars, benefitting patients far in excess of the cost.
OSTEOLYSIS AND ASEPTIC LOOSENING-THE PROBLEM
Traditionally, total joint replacement surgery involved anchoring the prosthetic components made-of superalloy metals to the skeleton with methylmethacrylate bone cement. Ultrahigh molecular weight polyethylene provided the articulation with a very low coefficient of friction. The clinical results of primary total hip replacements are good to excellent for over 95% of the patients.2 Even though they are for the most part successful, aseptic loosening of the prosthetic components and pathologic bone resorption (periprosthetic osteolysis) associated with prosthetic loosening become the major problems with total hip arthroplasty 5 to 10 years after surgery. The total hip replacements done in the 1970s had a femoral component loosening rate between 30 and 50% and acetabular components loosening rate between 10 and 15% by 10 year^.^,^ Interestingly, the acetabular loosening rate continues to increase with time, 50% being loose radiographically at 15 years.4 Virtually all of the loose components develop progressive periprosthetic oste~lysis.~ However, aggressive periprosthetic osteolysis appearing as focal cystic lesions in bone occurs prior to component loosening in 7% of patients at a mean follow-up of 11 years6
Dissatisfaction with the high loosening rates led to the popularity of cementless prosthetic components in the early 1980s. Improved metallurgical techniques have led to the fabrication of implants with porous surfaces into which bone can grow, thereby mechanically anchoring the prosthesis without cement. As follow-up studies became Views expressed in this column are those of the author, and do not necessarily reflect those of the Society for Biomaterials, the Journal of Applied Biomarerinls or its editors, or Dr. Frederick J. Schoen, Editor, Clinical Reviews Column.