ZEdu Article: The implant-prosthetic misfit: analogue and digital impression and other factors involved

According to the Glossary of Prosthodontics Terms (1), “fit” is defined as the “adaptation of any dental restoration to its site, in the mouth”.

In implantology and implant prosthetics, the correct adaptation of a prosthesis to dental implants is very important.

In this case the, passive fitting search turns out to be a fundamental step without which it is impossible to proceed with the final processing of implant prosthetic products (2).

In fact, this fitting at implant/prosthetic connection level needs to avoid generating stress or other situations that could cause biological or mechanical complications over the long term. (3).

In fact, a mismatch between the prosthesis and the implants, or the secondary abutments (in the case of a multi-unit abutment), generates stress and tension which, depending on the type of misfit, can be exerted on the prosthetic superstructures (framework, connection screws, coating materials), on the implants or on the peri-implant bone (4,5).

The misfit can be vertical, horizontal, angular or rotational, depending on the spatial axis in which unequal interactions between the implant/prosthetic coupling surfaces occur. (6,7).

The main factors that influence the fitting of an implant-prosthetic structure

Among the factors that influence the fitting of an implant-prosthetic structure we include the type of impression (8,9).

#1 The type of impression

The process, the technique, the impression materials or the scanners, with their different scanning strategies used to take the impression on implants, are the initial factors responsible for inaccuracies in producing the cast from which the prosthesis will then be made (10).

Nowadays, digital technologies make it possible even to think of working without a cast in a fully digital workflow (11).

However, it is always the clinical case that determines whether a digital process can be fully trusted or not.

In single-implant situations or in cases where multiple implants are rehabilitated, but which are parallel to each other and in a hemi-arch, it is possible to carry out a full-digital protocol (8,9), whereas, for the complete arch, the analogue workflow with a rigid material impression and production of a plaster cast still seems to be the more reliable solution (8,9).

In the full-digital workflow, when working without any physical cast, any misfit issues that may exist in the frameworks cannot be verified before production of the prosthetic framework and cannot be corrected without taking another intraoral scan (11).

In cases where these types of difficulties can be identified, it is therefore always useful to produce at least one 3D printed cast, which can be used both to verify the impression made and as a working model even in the event that the prosthetic framework is modified (12).

Obviously, production of the 3D printed model entails discrepancies related to the printer setting parameters, type of resin, and so on, which can in turn influence the position of the implant analogues in the cast (13).

However, the possibility of building a verification jig and modifying the model by repositioning the analogues, as is usually done with plaster casts, is of considerable importance in complex cases in order to avoid misfit problems. (14)

#2 The manufacturing technology of prosthetic frameworks

Another very important factor is the manufacturing technology of the prosthetic frameworks (15,16).

Thanks to the advent of digital technology, there are new methods and new materials available nowadays, which are used to manufacture implant-prosthetic frameworks, such as milling or 3D printing.

Milled zirconium, for example, is a material that can be used nowadays for this purpose; in addition to its numerous aesthetic advantages, it may not require coating material, as was the case with metal (17,18).

However, zirconium always requires a Ti base at implant/prosthetic connection level; in fact, being a polycrystalline ceramic, it is not a material suitable for threading, which would inevitably lead to microfractures.

Furthermore, a prosthetic screw produced from titanium or other metals compatible with the internal threads of the implants or MUAs would present problems in terms of adaptation and mechanical stability with the zirconium structure, given the materials’ different resistance and ductility properties (19).

The advantage of Ti bases, then, is to reduce any tension in the prosthetic structure resulting from manufacturing inaccuracies, thanks to the small gap necessary for cementation with the framework (20).

#3 Other factors implicated in misfit

However, there are also other factors implicated in misfit, such as:

• The framework design (21);
• The application of coating material (22);
• The type of implant/prosthetic connection (23);

and others that we can mention but which require further study.

Conclusions

The correct fit of a prosthesis on dental implants is crucial to ensure the longevity and clinical success of implant rehabilitations.

The search for passive fitting is an essential step to avoid long-term biological and mechanical complications.

The choice of materials, manufacturing technologies and workflow always requires specific considerations for each clinical case aimed at maximising outcomes.

References:

1. The Glossary of Prosthodontic Terms. The Journal of Prosthetic Dentistry. 2017 May;117(5):C1-e105.
2. Lops, D., Bruna, E., & Fabianelli, A. (2014). La protesi implantare. Dental Cadmos, 6(82), 386.
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23. Bédouin Y, Lefrançois E, Salomon JP, Auroy P. Abutment rotational freedom on five implant systems with different internal connections. J Prosthet Dent. 2023 Mar;129(3):433–9.

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