The clinical significance of distortion in implant prosthodontics: is there such a thing as passive fit?

Passive fit of the implant prosthesis onto osseointegrated and functional fixtures intraorally is the "Holy Grail" of the discerning implant prosthodontist. There is growing recognition that the old and "tried" clinical procedures and laboratory fabrication methods transferred from traditional fixed and removable partial denture prosthodontics, fall short of achieving true passivity of fit. The stages of impression making, master cast, resin verification jig, waxing, investing, casting, veneer addition and finishing can all add to the final distortion of the prosthesis. Studies are emerging that report the possible distortion associated with each stage. Alternative strategies have also been suggested to by-pass these traditional stages, for example laser welding or the elimination of the impression using optical digitising and CAD-CAM. In the final analysis, totally "passive" fit is probably unachievable. The challenge is to determine a minimum level of distortion and associated stress that is acceptable and that confers adequate clinical longevity to the prosthesis. Developing clinical tests that measure stress levels intraorally as the prosthesis is fitted would be the ideal gold standard by which to judge the accuracy of fabrication procedures. This paper summarises and rationalises the growing body of literature dealing with accuracy of implant prosthodontic procedures.