[Mechanical changes in the femur induced by a joint prosthesis. Comparative extensometric studies].

This study using electrical extensometry is aimed at comparing the relative deformations of the same fresh femur before and after the successive implantation of the following titanium prostheses: a cement-free prosthesis with metaphyseal endocortical support, a cemented prosthesis with, then without collar, and finally a screwed titanium prosthesis. The 56 deformation measurement paths produce an overall exploration of the behavior of the femur. The results applied to the same straight section of the femur are different from those of earlier works. The density of the measurement areas has particularly been increased in the upper metaphysis by the use of bidirectional gages. The loading device ensures the absence of variations of the external stress torsion transmitted to the femur between the individual prostheses. The results recorded with the cement-free prosthesis with metaphyseal endocortical support and with the collar-free cemented prosthesis are very similar. The incidence of the "collar effect" is very low in spite of optimal in vitro implantation conditions ensuring good contact between the collar and the neck of the femur. The screwed prosthesis is the implant that most stiffens the upper end of the femur. The testing procedure chosen has allowed recording the deformations caused by the mere insertion of the implants and the bone. The importance of the upper metaphysea endocortical support of the cement-free, screwless implant contributes in ensuring a better primary anchorage, producing stresses that will then foster the secondary anchorage through endosteal osteogenesis.