Passive fit of metal frameworks in implant restorations: It’s clinical significance

Abstract

Passive fit of metal frameworks has been a major concern for the researchers because of its possible contribution in the development of complications that could endanger the prognosis of implant-supported prosthesis. The transmission of forces from a non passive metal framework, through the abutment, to the underlying bone has raised concerns about the possible complications. The extend of the problem is enhanced by the fact that achieving passive fit with current know-how seems not feasible. The uneven distribution of forces combined with their increased values, related to the lack of passive fit, have been related to both biological and mechanical problems concerning bone-abutment-superstructure system. In literature, loss of osseointegration, pain and inflammation, loosening or fracture of screw joints, abutments, metal framework or even implants, comprise the possible complications attributed to the misfit. At the same time, the term misfit is not completely clarified and the variety of clinical methods that are used to identify it, display many restrictions. Apart from lack of determining with specific criteria the fit of the framework clinically, the confusion concerning the degree of misfit that is biologically accepted merely perplexes the problem. Current bibliography has not yet achieved to document a clear correlation between those complications and misfit of metal framework. On the other hand, the presence of unfavorable forces cannot be underestimated. Therefore in order to prevent this phenomenon in the laboratory a great number of techniques and different alloys have been developed so as to achieve passive fit of superstructures if possible. However, this has not been accomplished for the time being.