Precision of full-arch digital impressions: a major clinical challenge

Passive fit of large-span implant prostheses is a sine qua non condition to avoid mechanical stress and biological complications. While digital workflow has become the standard, the accuracy of intra-oral scanners (IOS) on a full arch remains a subject of debate compared to emerging technologies. This in vitro study directly addresses this issue by comparing the fidelity of three digital capture protocols for the realization of maxillary rehabilitations.

Study objective and methodology

The specific objective of this study was to evaluate the fit accuracy and marginal adaptation of machined titanium jigs following three types of impressions. The study used a reference edentulous maxillary model, incorporating four parallel implant analogs, to generate 30 digital impressions (n=10 per group):

• Group I: Conventional intra-oral scanner with vertical scan bodies (VS).
• Group II: Grammetry via the MEDIT TruScan ioConnect system and horizontal scan bodies (GR).
• Group III: Extra-oral photogrammetry using the T-Marker system coupled with an iPad Pro (PG).

The authors tested the hypothesis that these specific capture technologies would significantly influence the root mean square (RMS) and the marginal gap, measured here at 240 points by optical microscopy (50×), to determine which protocol guarantees the best prosthetic passivity.

Study methodology

This in vitro study was conducted on an edentulous maxillary model designed by 3D printing, featuring four implant analogs positioned in a parallel manner. The objective was to compare the accuracy of three digital acquisition protocols (n = 10 impressions per group):

• Group I (VS): Conventional intraoral scanner using vertical scan bodies.
• Group II (GR): Grammetry using the MEDIT TruScan ioConnect system with horizontal scan bodies.
• Group III (PG): Extraoral photogrammetry via the T-Marker system coupled with an iPad Pro.

For each acquisition, titanium transfer jigs were digitally designed and machined. The accuracy assessment was based on two main criteria:

• Fit precision: measured by calculating the root mean square (RMS) error via surface comparison software.
• Marginal adaptation: analyzed at 240 points per group using a digital optical microscope (50× magnification).

Statistical analysis was performed using a one-way ANOVA supplemented by a Tukey post-hoc test and a Pearson correlation, with a significance threshold set at α = 0.05.

Digital precision and marginal adaptation: the results

Data analysis highlights a clear hierarchy between the three digital impression protocols tested. Photogrammetry (Group III) consistently outperforms the other methods, both in terms of digital fidelity (RMS) and clinical fit (marginal gap).

Statistical significance and comparisons

Analysis of variance (ANOVA) reveals significant differences between groups (p < 0.05). Post-hoc tests provide major insights for the clinician:

• Superiority of photogrammetry: Group III shows significantly higher precision than the conventional intraoral scanner (p < 0.05). Regarding marginal adaptation, photogrammetry displays significantly smaller gaps than grammetry and IOS (p < 0.001).
• Grammetry vs IOS: No statistically significant difference was observed between the grammetry protocol and the conventional intraoral scanner, whether for RMS deviation or marginal fit (p > 0.05).

Qualitative observations and correlations

The evaluation of marginal adaptation, performed by digital optical microscopy at 50× magnification on 240 points per group, confirms the digital measurements. The study establishes a moderate positive correlation (r = 0.468; p = 0.009) between the RMS deviation (software metric) and the actual marginal gap measured on the machined titanium copings. This data is crucial: it confirms that the accuracy of the digital impression is a direct predictor of the quality of the final prosthetic fit in the dental practice.

Clinical analysis: the supremacy of photogrammetry

The results of this in vitro study are conclusive: the photogrammetry protocol via the T-Marker system significantly outperforms conventional intraoral scanning (IOS) and grammetry in terms of precision and marginal fit. With a marginal gap of only 22.75 ± 1.92 µm, photogrammetry guarantees superior prosthetic passivity compared to the 33.28 µm observed with standard IOS. For the implantologist, this 10 µm difference is critical for reducing mechanical stress on the framework of a screw-retained full-arch bridge.

A predictive correlation for the digital workflow

A major contribution of this work lies in the moderate positive correlation (r = 0.468, p = 0.009) established between the RMS deviation (measured digitally) and the actual marginal gap observed under an optical microscope. This confirms that software accuracy is a reliable predictor of the final clinical fit. While grammetry (30.83 µm gap) proved to be statistically comparable to conventional IOS, it remains clinically behind the precision of photogrammetry.

Limitation and experimental framework

These findings should be tempered by the in vitro nature of the study. The printed model featured four parallel implants, an ideal situation rarely encountered in clinical practice where angulations, saliva, and limited mouth opening complicate data capture. Although comparative literature remains limited on these emerging technologies, the authors confirm here that the T-Marker system significantly optimizes passive fit compared to conventional vertical scan bodies.

Implications for practice

For the practitioner performing full-arch restorations on implants, the choice of the impression protocol is decisive. These data suggest that the integration of photogrammetry offers superior precision security to ensure the long-term stability of the implant-prosthesis interface, minimizing the risk of mechanical complications related to a lack of passivity.

Summary of results

Photogrammetry (T-Marker system) surpasses conventional optical impression and grammetry with a digital precision (RMS) of 21.6 ± 5.2 µm and a marginal fit of 22.75 ± 1.92 µm. Conventional intraoral impression and grammetry show inferior and statistically similar results, with marginal gaps exceeding 30 µm (p > 0.05), proving that photogrammetry is the most accurate method for the full arch.

In concrete terms, for the practitioner:

• Prioritize photogrammetry: For your full-arch rehabilitations, this protocol guarantees prosthetic passivity and marginal adaptation superior to standard digital methods.
• Be vigilant regarding grammetry: The use of horizontal scanbodies (ioConnect type) does not offer a significant statistical advantage in final accuracy compared to conventional vertical scanbodies.
• Reliability of indicators: As the root mean square (RMS) deviation is positively correlated with marginal fit (r = 0.468), the quality of your initial scan is a direct predictor of the clinical success of the prosthesis.

Technical lexicon of the study

Passive fit: Clinical condition of an implant prosthesis that fits without inducing mechanical stress on the implants or the surrounding bone, ensuring long-term stability.

Root Mean Square (RMS deviation): Statistical measure used in surface comparison software to quantify the overall accuracy of the fit by calculating the deviation between the scanned data and the reference model.

Grammetry: Digital scanning protocol using specific scan bodies (here MEDIT TruScan ioConnect horizontal) to capture implant positions on a full arch.

Photogrammetry: Extra-oral capture method (here T-Marker system with iPad Pro) using markers and imaging algorithms to accurately locate the spatial position of implants.

Marginal gap: Microscopic distance measured between the cervical margin of a prosthetic part (titanium jig) and the implant abutment, evaluating the quality of marginal adaptation.

Scan bodies: Transfer devices fixed to implant abutments to enable their digital identification; the study compares vertical (classic IOS) and horizontal (grammetry) geometries.

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