Occlusal precision factors of crowns on implants with analogue and digital workflow

The precision of crowns at the occlusal level is an extremely important aspect of prosthetic rehabilitation (1).

From a clinical point of view, excessive contact on a tooth/implant can lead to pre-contact or interference during eccentric movements of the mandible, while loose contact would be inconsistent with the occlusion (2).

In both cases, the risk of mechanical complications involving the prosthetic element and/or biological complications can increase significantly (3,4).

In compliance with the occlusal schemes with which implant patients are rehabilitated, there are often occlusal clearances to be respected between the antagonistic surfaces; consequently, accuracy in the processes of clinical data acquisition and prosthesis manufacture is fundamental (5–8).

Occlusal precision factors

Depending on the workflow (analogue/digital), and the materials used to create the prosthesis, there are several factors capable of influencing the occlusal precision of crowns on implants.

These include:

1. The accuracy of the master model;
2. The detection of the analogue/digital occlusion and the subsequent positioning of the models in the articulator;
3. The movement of the analogues in the plaster model originating from the impression. When working to produce a digital model, the placement of analogues in the 3D printed model and the model manufacturing tolerances relative to the 3D printer or resin used;
4. The tolerances between the different implant-prosthetic components (9,10);
5. The prosthesis manufacturing procedures and the presence of aesthetic coatings;
6. The finalization procedures, such as glazing and polishing.

The accuracy of the master model

As regards the first point, i.e. the accuracy of the master model, the analogue impression technique (pick-up or tear-off), the materials used (polyvinyl siloxanes or polyethers), the splinting or not of the transfers and the manufacturing technique of the plaster model can certainly influence the accuracy of the master model (11,12).

Scanning strategies and techniques

If, on the other hand, the workflow is digital, intraoral scanning with all the components and techniques (scan body, scanning strategy, arch length, etc.) or scanning of the master modelwith lab scan bodies if moving from a physical to a digital workflow are decisivefor the realization of thevirtual model (13,14).

Model movements and jaw positioning

To date, the creation of type IV plaster models from PVS impressions appears to be more accurate than intra-oral scans followed by 3D printing of the model (15).

The positioning of the analogue in the 3D printed model and the management of the manufacturing tolerances of the model also have an impact here (16).

When moving from an analogue to a digital workflow and thus scanning a plaster model with a laboratory scanner, other errors resulting from the scanning process can accumulate (17), depending on the type of desktop scanner used (18,19).

The spatial positioning of the patient’s jaws relative to the hinge axis and consequently also the maxillomandibular relationship can determine discrepancies and inaccuracies in the manufacturing of the prostheses as they directly influence the modelling of the cusps and pits, and the inclination of the dental elements in the prosthesis (20–22).

Tolerances between the different implant-prosthetic components

The manufacturing tolerances between the different implant-prosthetic components, i.e. between the implant and pick-up transfer, between transfer and analogues, between analogues and abutments or Ti-base, and between Ti-base and implants at the connection level are also decisive in the precision of construction of the prosthesis (9).

Even in the digital workflow, many of these tolerances are present if the workflow involves printing the model in 3D (23); if the workflow is full-digital; however, discrepancies can still result from an incorrect alignment between the scan body mesh and the library files used by the dental technician to identify the 3D position of the implant-prosthetic connection (24,25).

Manufacturing and finalization procedures

Instead, as regards the manufacturing mechanisms of the prosthesis, these can alter the final result also in relation to the materials with which they are made.

For example, a zirconia crown will show errors coming from the milling tolerances of the pre-sintered block, from the dimensional contractions due to sintering, from the glazing, polishing and cementation procedures on Ti-base (5,26,27).

Within this category it must be remembered that different thicknesses and shapes of zirconia restorations could cause different types of contractions with consequent errors in the occlusal precision of the crowns (26).

Polishing procedures can increase occlusal clearance with the antagonist, given the removal of a thin layer of glass ceramic of approximately 25 μm (28).

On the other hand, the cementation on Ti-base could reduce occlusal clearance, resulting in a crown overhang of approximately 20 μm (27).

Conclusions

In conclusion, the occlusal precision of crowns on implants depends on a variety of factors linked to both the workflow used (analogue or digital) and the materials and manufacturing techniques used.

Understanding and controlling these factors is essential to reduce occlusal discrepancies and ensure optimal prosthetic results.

Careful management of each phase of the process can significantly improve the quality and predictability of prosthetic rehabilitations.

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