Method for measuring and correcting the fit of fixed dental prostheses

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BACKGROUND

The long-term success of prosthodontic treatment largely depends on the quality of dental prosthesis fabrication. One of the key quality indicators for fixed dental prostheses is the accuracy of their fit to the prepared tooth. There is a direct correlation between the service life of fixed dental prostheses and the degree of their fit to the prepared tooth [1]. Inadequate fit results in pronounced marginal gaps, microleakage, and reduced retention of the prosthesis, necessitating an increased thickness of the luting cement layer [2, 3]. Consequently, the risk of such complications as secondary caries in the cervical area of the tooth, periodontal tissue pathology, formation of periodontal pockets and tooth mobility, premature loss of retention increases [4].

Errors that inevitably occur during both the clinical and laboratory stages of dental prosthesis fabrication lead to variations in the degree of fit of the completed prostheses. Therefore, after fabrication in the dental laboratory, each prosthesis must be clinically evaluated before definitive cementation. The degree of adaptation of the prosthesis is determined by measuring the gap between the internal surface of the prosthesis and the prepared tooth surface. Measurement from the internal surface of the prosthesis to the walls of the abutment tooth, perpendicular to their surface, is defined as the internal gap, whereas measurement along the external margin is defined as the marginal gap [5]. Numerous methods have been described for evaluating the marginal adaptation of fixed dental prostheses [6, 7]; however, most of them are of primary scientific interest and have not gained widespread clinical acceptance because of technical complexity and impracticality.

Currently, the most common clinical method for evaluating prosthesis fit intraorally is the use of a fine dental probe, which enables the clinician to assess the marginal gap of an indirect restoration and, based on this finding, to draw conclusions about the degree of prosthesis fit to the tooth surface [8, 9]. The limitation of this method is the lack of clear and reliable information about the internal gap. As a result, the clinician is unable to identify and eliminate areas that interfere with complete seating of the prosthesis.

Another method for evaluating the adaptation of dental prostheses to the prepared tooth surface involves the use of a low-viscosity silicone material [9–11]. In this technique, the clinician applies a small amount of low-viscosity silicone to the internal surface of the prosthesis and, under pressure, positions it onto the prepared tooth. After polymerization, the prosthesis is removed, and the set silicone layer provides a three-dimensional assessment of the degree and uniformity of prosthesis adaptation. The clinician evaluates the transparency of the silicone layer: the more translucent the material appears, the tighter the fit of the prosthesis at that site. The limitation of this method is that it provides only approximate information. The clinician cannot measure the film thickness or obtain quantitative data (e.g., in micrometers) regarding the space between the prosthesis and the prepared tooth surface. Such measurements are necessary to determine whether the fit of the prosthesis falls within clinically acceptable limits and whether correction is required. In the latter case, the prosthesis must be adjusted to improve its seating.

In light of these considerations, there is a need for a method that allows for quantitative evaluation of the fit of fixed dental prostheses to the prepared tooth surface, as well as identification and elimination of areas that interfere with complete seating.

This work aimed to improve the effectiveness of prosthetic treatment by enhancing the fit of fixed dental prostheses.

Study objectives:

1. To develop a clinical method for quantitative assessment of prosthesis fit after fabrication in the dental laboratory.
2. To improve the method of prosthesis adjustment to enhance the fit of fixed dental prostheses.
3. To perform a comparative analysis of prosthesis fit before and after clinical adjustment.

METHODS

For the quantitative evaluation of prosthesis fit, a specialized device—the calibrator [12]—was developed. During the silicone test, the calibrator enables the clinician to obtain quantitative measurements of the gap between the prosthesis and the tooth surface at different sites of the prepared tooth (see Fig. 1).

 

Fig. 1. Calibrator for measuring silicone film thickness.

 

The study was conducted in a group of patients who required full-coverage single crowns. A one-step, two-phase impression technique was used with addition silicone impression materials: Affinis putty soft and Affinis light body (Coltene, Switzerland). The impression was poured with type IV high-strength dental stone FujiRock (GC, Japan). Both the working cast and the separate working die were scanned with a laboratory scanner Medit T510 (Medit, Republic of Korea). During digital crown design, the operator specified a uniform internal gap of 40 µm. Crowns were milled from zirconium dioxide blocks (Everest, UNC, Republic of Korea) using a K5 milling machine (VHF, Germany).

After delivery from the dental laboratory, the crowns’ internal surfacewas cleaned with an alcohol solution. The provisional crown was removed, and the abutment tooth was thoroughly cleaned of temporary luting cement.

To determine the gap between the crown and the hard dental tissues, a low-viscosity silicone material, Speedex light body (Coltene, Switzerland), together with the calibrator, was used. The silicone was mixed strictly according to the manufacturer’s instructions. Immediately after mixing, part of the material was applied to the internal surface of the crown, and the remainder was placed into the calibrator, which was then closed with its upper plate. The crown was seated on the prepared tooth under firm finger pressure. After polymerization, the silicone strip was removed from the calibrator. The strip exhibited color changes depending on material thickness, with visible markings at 20, 40, 60, 80, 100, 200, 300, and 400 µm. This silicone strip served as a scale for measurement (see Fig. 2).

 

Fig. 2. Silicone strip removed from the color calibrator.

 

After removing the crown from the abutment tooth, it was taken out of the patient’s mouth. The internal surface of the crown, covered with a thin layer of silicone, was examined. Areas of the inner crown walls that were in direct contact with the abutment tooth—where the silicone was completely displaced—were marked with a fine pencil Fig. 3). The silicone film was then carefully removed from the crown and placed against the measurement scale (Fig. 4). The film thickness was determined by matching the color with the corresponding reference on the scale. The control measurement site was the film in the region of the occlusal surface of posterior teeth or the incisal edge of anterior teeth (see Fig. 4). If the thickness at this site exceeded the designed gap of 40 µm, this indicated incomplete seating of the crown on the abutment tooth.

 

Fig. 3. Marking of displaced areas of the silicone film with a fine pencil.

 

Fig. 4. Silicone film removed from the crown before adjustment.

 

If the film thickness at this site measured 100 µm or more, crown adjustment was performed. Using a diamond bur at low speed and with mandatory water cooling, the operator selectively reduced the areas of the crown that had been marked with a pencil (Fig. 5).

 

Fig. 5. Adjustment of the internal surface of the crown.

 

After adjustment, a repeat measurement of film thickness was carried out using the method described above (Fig. 6).

 

Fig. 6. Silicone film removed from the crown after adjustment.

 

The obtained results were recorded in study protocols. In cases requiring adjustment, film thickness was measured twice: before and after clinical correction of the prosthesis.

RESULTS AND DISCUSSION

A total of 46 zirconia (ZrO₂) crowns fabricated in the dental laboratory were analyzed. The mean fit value for all crowns was 105 µm. In this study, the internal gap was assessed by measuring the silicone film thickness. Previous reports have noted that during seating, a crown may shift or deviate from the intended path of insertion, resulting in uneven adaptation to the abutment tooth along the axial walls [1]. Therefore, the control measurement site was chosen at the occlusal tip of the abutment tooth (see Figs. 4, 6), which, in our opinion, best reflects the accuracy of vertical crown positioning. It was found that all prostheses delivered from the dental laboratory exhibited varying degrees of fit. The range of values was 20 to 200 µm. This variation is attributable to errors at multiple stages of crown fabrication, each of which contributes cumulatively to the final result. Adaptation of completed crowns may also be compromised by residual particles of temporary cement on the abutment tooth that were not completely removed during cleaning and thus remained unnoticed by the dentist.

Currently, there is no consensus on the threshold of clinically acceptable crown fit. Some authors consider the optimal range to be 25 to 50 µm, although such values are difficult to achieve under clinical conditions; therefore, other researchers accept marginal gaps of up to 120 µm [4, 13–15]. In this study, a gap of less than 100 µm was defined as clinically acceptable. When the silicone film thickness exceeded this value, crown adjustment was performed; when it was below this threshold, adjustment was not required.

In total, 50% of the crowns demonstrated a gap of 100 µm or more. The mean fit value for these crowns was 167 µm. All of these crowns underwent adjustment, which reduced the silicone film thickness. The degree of reduction varied from 20 to 140 µm. After adjustment, the mean fit value for these crowns decreased to 68 µm. On average, crown seating improved by 59.28%. Similar results were reported by Davis et al. [11], who evaluated the adaptation of 18 cast crowns fabricated on resin dies simulating abutment teeth. Their in vitro study demonstrated that adjustment of cast crowns using low-viscosity silicone improved adaptation twofold. In addition, the authors recommended the use of C-silicones for the silicone test because their viscosity and flow characteristics resemble those of freshly mixed luting cements.

The proposed silicone test allows measurement of the silicone film thickness across the entire internal surface of the crown, providing dentists with additional information necessary to determine the appropriate amount of luting cement during crown cementation. This is clinically important because the volume of cement placed inside the crown may influence the final marginal gap after cementation [16].

In addition, film thickness measurement can aid in the selection of the luting cement itself. Different cements exhibit different minimal film thickness values; therefore, the available internal space of the crown is a critical parameter for each material. Wu et al. [17] emphasized the importance of internal crown space for optimal seating during definitive cementation. Dentists should account for this parameter to ensure accurate seating of restorations during the cementation stage.

Based on the findings of this study, we propose the following practical recommendations:

1. Fixed dental prostheses delivered from the dental laboratory should always be evaluated using the silicone test before definitive cementation.
2. Adjustment of the internal surface of the prosthesis should be performed selectively and only at sites that interfere with complete seating.

CONCLUSION

Fixed dental prostheses placed intraorally may exhibit varying initial degrees of fit to the prepared tooth surface. Frequently, this fit is considerably below clinically acceptable limits. The proposed clinical method for assessing and correcting the marginal fit of fixed dental prostheses enables the dentist to monitor this parameter and achieve clinically acceptable results.

ADDITIONAL INFORMATION

Funding sources: No funding.

Disclosure of interests: The authors have no relationships, activities, or interests (personal, professional, or financial) with third parties (for-profit, not-for-profit, or private entities) whose interests may be affected by the content of this article. The authors also report no other relevant relationships, activities, or interests within the past three years.

Author contributions: M.A. Muradov: investigation, resources, writing—original draft, writing—review & editing; V.A. Erokhin: methodology, formal analysis; A.N. Ryakhovsky: supervision, writing—original draft, writing—review & editing; D.A. Trunin: supervision, writing—review & editing. All authors approved the version of the manuscript to be published and agreed to be accountable for all aspects of the work, ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

About the authors

Murad A. Muradov

Central Research Institute of Dentistry and Maxillofacial Surgery

Email: kemine160@mail.ru
ORCID iD: 0000-0003-1960-5715
SPIN-code: 3448-4866

MD, Cand. Sci. (Medicine)

Russian Federation, Moscow

Vladislav A. Erokhin

Samara State Medical University

Author for correspondence.
Email: vladalex.171097@mail.ru
ORCID iD: 0000-0003-1096-7568
SPIN-code: 4724-5883
Russian Federation, Samara

Alexander N. Ryakhovsky

Central Research Institute of Dentistry and Maxillofacial Surgery

Email: avantis2006@mail.ru
ORCID iD: 0000-0002-0308-126X
SPIN-code: 5774-4493

MD, Dr. Sci. (Medicine), Professor

Russian Federation, Moscow

Dmitry A. Trunin

Samara State Medical University

Email: trunin-027933@yandex.ru
ORCID iD: 0000-0002-7221-7976
SPIN-code: 5951-4659

MD, Dr. Sci. (Medicine), Professor

Russian Federation, Samara

References

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