Finally! 4-6 implants have integrated, the patient has paid in advance for an implant supported full arch prosthesis, and its time to take an impression. Your goal is to record the position of the implants as accurately as possible, in order to achieve a passively fitting framework. Why is this so important? Ill-fitting frameworks have been implicated in biological and mechanical complications including screw loosening/fracture, implant failure, bone loss and even framework fracture. However, it has been shown that with even the most stringent and meticulous impression techniques, some degree of misfit will always exist between the implant platform and the prosthesis. These posts are meant to review the current literature on complications resulting from misfit, mention how different impression techniques contribute to misfit and outline some of my preferred techniques for full arch implant impressions.
In this, the first of three instalments on passive fit and implant impressions, I’ll try to define passive fit, and discuss the complications which might ensue if its not obtained.
Define: Pass·ive fit /pæsɪv fɪt/ noun --
In my prosthodontic residency when an instructor would grill me on definitions or classifications I would sneak a look at my phone to look it up in the ever holy Glossary of Prosthodontic Terms (8th edition). Unfortunately for us, there’s not a single mention of passive fit or misfit. Since there is no formal definition of passive fit, here are a few of the definitions offered over the years.
Over the last several decades we have seen the evolution of both the term and the importance of passive fit. There is never perfectly passive fit. Not once in any study was there a case where an abutment or framework fit perfectly onto the implant platform. We also know that the majority of implants, implant crowns and implant prosthesis survive without complications. Therefore, there must be a certain biologic and mechanical tolerance for misfit. In other words, passive fit isn’t as much of a concern as quantifying the acceptable misfit in a given situation. In an excellent review by Taylor, took this dilemma and raised two important questions: ,
Complications resulting from misfit:
Many publications state misfit as a cause of biologic and mechanical complications. These publications cite numerous articles to support their position. If you dive deeper, you’ll find that the articles cited are often just more opinion papers. In fact, no study to date has found a statistical correlation between misfit and any biologic complication. Therefore, these statements remain primarily hypothetical, but for good reason -- Anyone with a knowledge of implant components and a basic understanding of physics can understand how misfit can propagate stresses throughout the system. Stresses are the boogeyman of dentistry. They have been implicated in bone/implant loss, screw loosening/fracture, framework fracture, porcelain chipping, tooth wear/fracture and a slew of other biological complications. It seems intuitive that since misfit leads to stress and stress leads to complications, then misfit… causes complications, and I’m sure to some degree it does, but without evidence based research and guidelines, misfit will remain the monster under bed. Here is some of the evidence we have so far:
Achieving an implant supported framework that exhibits absolute passive fit has not yet been accomplished. However, there are many successful implant restorations in service so level of misfit must be tolerated. There is no consensus on what constitutes an acceptable amount of misfit but there is general agreement that it should be as close to passive as possible despite not yet being able to provide a statistical correlation to any negative mechanical or biological outcomes. Laboratories and clinicians struggle to do anything and everything in their power to both evaluate and minimize misfit. In the next installment I will discuss methods to evaluate component fit as well as laboratory and clinical methods to minimize the amount of misfit present.
 Branemark PI, Zarb GA, Albrektsson T (1985) Tissue –integrated prostheses. Quintessence, Chicago, p 253
 Klinberged IJ, Murray GM (1985) Design of superstructures for osseointegrated fixtures. Swed Dent J 28:63–69
 Jemt T (1991) Failures and complications in 391 consecutively inserted fixed prostheses supported by Branemark implants in
edentulous jaws: a study of treatment from the time of prosthesis placement to the first annual checkup. Int J Oral Maxillofac Implants 6:270–276
 Sahin S, Cehreli MC. The significance of passive framework fit in implant prosthodontics: current status. Implant Dent 2001; 10:85–92.
 Taylor TD, Agar JR. Implant prosthodontics: Current perspective and future directions. Int J Oral Maxillofac Implants. 2000;15:66–75.
 Carlsson L. Built-in strain and untoward forces are the inevitable companions of prosthetic misfit. Nobelpharma News. 1994;8:5.
 Kallus T, Bessing C. Loose gold screws frequently occur in full-arch prostheses supported by osseointegrated implants after 5 years. Int J Oral Maxillofac Implants. 1994;9:169–178.
 Jemt T, Book K. Prosthesis misfit and marginal bone loss in edentulous implant patients. Int J Oral Maxillofac Implants. 1996;11:620–625.