Histomorphometric evaluation and pathogenesis of prosthetic loosening.

On the basis of a vast experience deriving from the histological study of explants of hip joint prostheses, the authors emphasize the role of histomorphometric testing in allowing for a quantitative evaluation of periprosthetic cellular reaction, of the bone structure, and of the wear phenomena of the implant, in order to gain a better understanding of the pathogenetic mechanisms responsible for the phenomenon of loosening. In agreement with what has been reported by other authors, they describe what they were able to observe by image analysis, microradiography, x-ray diffraction, and microdurimetry in various types of explanted prostheses. In particular, they describe data on the features of the wear particles, the periprosthetic cellular component, the thickness of the interfacial membrane and of the joint neocapsule, the volume of bone-ingrowth, the contact surface, and the structure of the bone-implant interface. Based on the changes observed using histomorphometry, a pathogenetic hypothesis was formulated according to which physical phenomena closely related to the type of implant used, such as wear and metal ion release, and non-physiological stress exerted on the bone, cause a series of cellular chain reactions. Within this context, macrophages, lymphocytes, osteoblasts and osteoclasts are in a continuous state of activation and, both directly and by means of the production of cytokines and other substances, they are responsible for bone resorption and thus loss of continuity between the bone and the implant, resulting in prosthetic loosening.