The pathology of total joint arthroplasty. I. Mechanisms of implant fixation.

The clinical results of total joint arthroplasty are usually excellent, but surgeons, radiologists, and pathologists are often called upon to evaluate, in one way or another, the stability of the implants. These evaluations are aided by an understanding of the basic pathophysiology of total joint arthroplasty. The first part of this two-part review, will summarize the mechanisms whereby total joint implants achieve fixation. The second part will describe and illustrate the most important mechanisms of implant loosening. The "gold standard" for hip and knee arthroplasty is to use polymethylmethacrylate bone cement to anchor the implant to bone, but the optimal surface texture of cemented implants is controversial. Some surgeons advocate a rough implant texture to facilitate bonding between implant and cement; other surgeons prefer a smooth, polished implant to minimize abrasion of cement. Implant loosening can be initiated by particles of cement generated at either the implant/cement, or cement/bone interface. Uncemented implants with porous metal surfaces achieve a variable amount of bone ingrowth, but some designs have excellent clinical results. Maximal bone ingrowth usually occurs along surfaces that are relatively close to cortical bone. Implants with bioactive coatings, such as hydroxyapatite achieve rapid bone apposition. The amount of bone that persists on uncemented implants long-term is determined by many variables, inlcuding the quality of the coating, the overall implant design, and factors that influence local bone remodeling.