Impression material accuracy and precision for the fabrication of full-arch All-on-four implant prosthetics

The Journal of Prosthetic Dentistry recently published an interesting study evaluating the accuracy and precision of different elastomer materials for impression-taking on implants (1). More specifically, using a master model simulating an edentulous patient requiring All-on-four rehabilitation, the authors evaluated the extent to which certain impression materials were accurate and precise in recording the three-dimensional spatial position of the implants.

To do so, they created a Plexiglas master model in which 4 implants were inserted, two anterior implants with a forward tilt of 5° and 10° and two posterior implants perpendicular to the model base. After attaching the transfers to the implants and splinting the transfers together with carbon-resin rods, pick-up impressions were taken using the following materials:

• Hydrorise Implant Monophase
• Hydrorise Implant Heavy + Light
• DMG Honigum Monophase
• DMG Honigum Heavy + Light
• Impregum Monophase
• Permadyne Heavy + Light

For the Hydrorise Implant groups (Heavy + Light and Monophase) impressions were also taken with unsplinted transfers.

Ten impressions of the master model were taken for each of the 8 groups; each impression was dried and analysed within 48 hours. In order to reduce polymerization shrinkage effects and standardise the amount of material, a rigid plastic impression tray was used for all the impressions, which restricted the thickness of the impression material to approximately 5 mm. The spatial position of the transfers on both the master model and in each individual impression was acquired with the OGP 300 (OCMM) metrology system, which is used for three-dimensional measurements in the industrial field.

The 3D positional error between the position of the transfers of each impression and the position of the transfers in the master model was then calculated using a dedicated software programme (Rhinoceros). Accuracy and precision were measured for each group. Accuracy was quantified by the differences between the three-dimensional position of the transfer in the impression and the three-dimensional position of the transfer in the master model. Precision, on the other hand, was detected by calculating the differences within each group of impressions.

Following the statistical data analysis, the results of the study showed that:

• the impressions taken using Hydrorise Implant Heavy + Light and Hydrorise Implant Monophase with transfer splinting afforded the best accuracy and precision;
• of all the groups of materials, polyethers showed the worst accuracy and precision;
• there was a statistically significant difference between the accuracy and precision of the Hydrorise Implant impressions with transfer splinting and splinted polyether impressions;
• The non-splinted Hydrorise Implant groups were seen to be equivalent to the splinted polyether groups in terms of accuracy and precision.

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There could be a number of reasons for these results. Firstly, the results may have been affected by the polymerization of the polyethers, which continues for several minutes and hours after the material’s clinical setting time (5 minutes and 30 seconds) (2). The different polymerization kinetics (Snap-set behaviour) and lower elastic recovery associated with greater rigidity may also have had a negative influence on the performance of the polyethers (3). As has been highlighted by a number of published studies (4–6), an impression taken using a very rigid and inelastic material is more susceptible to distortions during removal from the patient’s mouth, especially when the implants are not parallel to one another.  Furthermore, the force applied when removing the impression from the patient’s mouth also depends on the hydrophilicity of the material, which results in a higher friction coefficient with the oral tissues due to the greater surface free energy. Indeed, it has been demonstrated that polyether impressions require a greater force than polyvinylsiloxanes, despite the difference in rigidity and hardness (7).

Another interesting result regards splinting, which according to other scientific studies (8-12), significantly improved both the accuracy and precision of impressions taken using polyvinylsiloxane. However, of the non-splinted groups, Hydrorise Implant Medium showed greater accuracy and precision than Hydrorise Implant Heavy + Light. The reasons for this behaviour could lie in the fact that the Medium material has a lower viscosity than the Heavy material, which is thought to adapt better to transfers, and in the greater flexibility that results in a reduction in the force required to remove the impression from the oral cavity. This result suggests that Hydrorise Implant Medium could be successfully used even without transfer splinting for All-on-4 impressions, consequently simplifying the clinical process.

Nevertheless, this was an in vitro study devoid of certain potentially decisive variables, such as the oral tissues, saliva, blood contamination and patient movements. Furthermore, the transfer fabrication tolerances were not known and consequently any dimensional errors in their production were not taken into consideration. In vivo studies are therefore required in order to ascertain the validity of these results also in conventional clinical practice.

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