ADAPTATION CORRECTION OF FIXED DENTAL PROSTHESES TO THE PROS-THETIC BED

Danish Scientific Journal No101, 2025 47 MEDICAL SCIENCES ADAPTATION CORRECTION OF FIXED DENTAL PROSTHESES TO THE PROSTHETIC BED Rustamov E., Doctor of Philosophy in Medicine,Assistent Zeynalova N., Doctor of Philosophy in Medicine Chief laboratory Assistant Ashrafov D., Department of Orthopedic Department Assistant Karimova A., Department of Orthopedic Department Assistant Azerbaijan Medical University, Baku, Azerbaijan https://doi.org/10.5281/zenodo.17493469 Abstract One of the key indicators of the quality of fixed dental prostheses is their close adaptation to the prosthetic bed. Errors that occur during the clinical and laboratory stages of prosthesis fabrication can result in varying degrees of fit in the finished fixed prostheses. Most methods for assessing the degree of adaptation are primarily of scientific interest and have not found wide clinical application due to their complexity and inconvenience in use. Keywords: adaptation of fixed prostheses; silicone test; test calibrator; silicone film thickness; marginal gap. Introduction The long-term success of prosthodontic treatment largely depends on the quality of dental prosthesis fabrication. One of the key indicators of the quality of fixed prostheses is their close adaptation to the prosthetic bed. There is a direct correlation between the service life of fixed prostheses and the degree of their adaptation to the prosthetic surface [1]. Poor adaptation results in a pronounced marginal gap, microleakage, and reduced retention of the prosthesis, which in turn requires an increased thickness of the cement layer [2, 3]. As a result, the likelihood of complications such as secondary caries in the cervical area, periodontal tissue pathologies, periodontal pocket formation and tooth mobility, as well as premature decementation of the prosthesis, increases [4]. Errors that inevitably occur during both the clinical and laboratory stages of prosthesis fabrication lead to variability in the degree of adaptation of the final prosthesis. Therefore, fixed prostheses must always be checked intraorally before final cementation. The degree of adaptation is determined by measuring the gap between the prosthesis and the prosthetic bed. The measurement from the internal surface of the prosthesis to the walls of the abutment, taken perpendicular to their surface, is referred to as the internal gap, while the measurement along the outer margin is known as the marginal gap [5]. A sufficient number of methods have been described for evaluating the marginal fit of fixed prostheses [6, 7], most of which are of scientific rather than clinical interest due to their complexity and practical inconvenience. Currently, the most common clinical method for evaluating the fit of prostheses intraorally is the use of a fine dental probe, which allows the clinician to assess the marginal gap of an indirect restoration and draw conclusions about the adaptation of the prosthesis to the tooth surface [8, 9]. However, this method provides no clear or reliable information about the magnitude of the internal gap. Consequently, the clinician is unable to identify or correct areas that prevent the prosthesis from fully seating. Another method for evaluating prosthesis adaptation to the prosthetic bed involves the use of a low-viscosity silicone material [9–11]. The clinician applies a small amount of low-viscosity silicone to the internal surface of the prosthesis and seats it under pressure onto the prosthetic bed. After polymerization, the prosthesis is removed, and the cured silicone layer allows a threedimensional visual assessment of the degree and uniformity of adaptation. The translucency of the silicone layer serves as an indicator: the more transparent the material appears, the closer the fit of the prosthesis in that area. However, this technique provides only approximate information. The clinician cannot measure the film thickness or obtain quantitative data (e.g., in micrometers) on the space between the prosthesis and the prosthetic bed. Such measurements are necessary to determine whether the prosthesis adaptation is within clinically acceptable limits or requires correction. In the latter case, the clinician must adjust the prosthesis to ensure optimal seating. In this regard, the development of a method that allows for the quantitative assessment of the adaptation of fixed dental prostheses to the prosthetic bed, as well as the identification and correction of areas preventing proper seating, is of significant clinical relevance. Purpose of the Study To improve the effectiveness of prosthodontic treatment by enhancing the adaptation of fixed dental prostheses. 48 Danish Scientific Journal No101, 2025 Objectives of the Study 1. To develop a clinical method for quantitative evaluation of prosthesis adaptation after fabrication in the dental laboratory. 2. To refine the method of adjusting fixed prostheses to improve their adaptation. 3. To perform a comparative analysis of prosthesis adaptation before and after clinical correction. MATERIALS AND METHODS To obtain a quantitative assessment of the adaptation of fabricated prostheses, a special device—a calibrator—was developed [12]. This tool allows the clinician, during the silicone test, to quantitatively determine the size of the gap between the prosthesis and the tooth surface in different areas of the prosthetic bed. The study was conducted on a group of patients receiving full single crowns. The working impression was taken using a one-step, two-viscosity technique with A-silicone impression materials Affinis putty soft / Affinis light body (Coltene, Switzerland). The impressions were poured using Type IV high-strength dental stone FujiRock (GC, Japan). Both the working model and a separate die were scanned using a laboratory scanner Medit T510 (Medit, Korea). During digital design, an even internal gap of 40 µm was set. The crowns were milled from zirconium dioxide (Everest, UNC, Korea) using a K5 milling unit (VHF, Germany). Upon receipt from the laboratory, the internal surfaces of the crowns were cleaned with an alcohol solution. The temporary crowns were removed, and the prepared tooth surfaces were carefully cleaned of temporary cement residues. To determine the gap between the crown and the tooth hard tissues, a low-viscosity silicone material (Speedex light body, Coltene, Switzerland) and the calibrator were 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 another portion was placed into the calibrator, which was then closed with its top plate. The crown was seated on the prosthetic bed under firm finger pressure. After polymerization, the silicone was removed from the calibrator, forming a strip that served as a reference scale. The color of this strip gradually changed according to the thickness of the material, with markings corresponding to 20, 40, 60, 80, 100, 200, 300, and 400 µm. This silicone strip was used as a color scale for thickness measurement. The crown was then removed from the abutment and taken out of the patient’s mouth. The internal surface of the crown, covered with a thin silicone film, was examined. Areas where the crown walls were in direct contact with the abutment (where the silicone had been completely displaced) were marked with a fine pencil. The silicone film was then carefully removed from the crown and compared with the measurement scale (Fig. 4). The film thickness was determined by matching the color intensity with the calibrator scale. The control measurement point was located on the occlusal surface for posterior teeth or on the incisal edge for anterior teeth. If the film thickness in this area exceeded the 40 µm internal gap specified during digital design, this indicated incomplete seating of the crown on the tooth. If the film thickness in this region was 100 µm or greater, crown correction was performed. Using a diamond bur at low rotational speed (with mandatory water cooling), the marked area of the crown was carefully adjusted. After correction, the silicone test was repeated using the same protocol described above. The obtained results were recorded in research logs. When correction was performed, the silicone film thickness was measured twice—before and after clinical adjustment of the prosthesis. RESULTS AND DISCUSSION A total of 46 zirconium dioxide (ZrO₂) crowns received from the dental laboratory were analyzed. The mean adaptation of all crowns was 105 µm. The internal gap was evaluated by measuring the thickness of a silicone film. Previous studies have noted that during seating, a crown can shift or deviate from the intended path of insertion, resulting in uneven adaptation along the lateral walls of the abutment [1]. Therefore, the control measurement point was selected at the apex of the abutment, as this location most accurately reflects the vertical positioning accuracy of the crown. All crowns received from the laboratory exhibited variable adaptation. The range of measured gaps was 20–200 µm. This variability is due to cumulative errors occurring at multiple stages of crown fabrication. Additionally, residual particles of temporary cement left on the abutment can further compromise the fit if not thoroughly removed by the clinician. Currently, there is no consensus on the clinically acceptable adaptation of crowns. Optimal values are thought to range from 25 to 50 µm, but these are rarely achievable in routine clinical practice. Some authors allow adaptation values up to 120 µm [4, 13–15]. In this study, a clinically acceptable internal gap was defined as less than 100 µm. Crowns exceeding this threshold underwent clinical adjustment, while those below this value did not require modification. Fifty percent of the crowns had a gap of 100 µm or greater, with a mean adaptation of 167 µm. All of these crowns underwent correction, resulting in a reduction of silicone film thickness. The degree of reduction varied between 20 and 140 µm, and the mean adaptation of these crowns decreased to 68 µm. On average, crown seating improved by 59.28%. Similar results were reported by S.H. Davis et al. [11], who studied 18 cast crowns on plastic dies simulating abutments and demonstrated in vitro that adjustments using low-viscosity silicone improved crown fit twofold. They also recommended K-silicone materials for the silicone test due to their viscosity and flow characteristics, which closely resemble freshly mixed cements. The proposed silicone test allows measurement of film thickness across the entire internal surface of the crown, providing the clinician with critical information to determine the appropriate amount of cement during final cementation. This is important because the volume of cement affects the final marginal gap after cementation [16]. Moreover, film thickness measurements assist in selecting the most suitable cement, as Danish Scientific Journal No101, 2025 49 different cements have varying minimum film thickness requirements. Thus, the internal space within the crown is a key factor for optimal seating during permanent fixation [17]. Based on these findings, the following practical recommendations can be made: 1. Fixed prostheses received from the dental laboratory should always be evaluated using a silicone test before permanent cementation. 2. Adjustment of the internal surface of the prosthesis should be performed selectively and only in areas that prevent full seating of the crown. Conclusion Fixed dental prostheses may exhibit varying initial adaptation to the prosthetic bed in the patient’s mouth. In many cases, the degree of adaptation is significantly lower than clinically acceptable values. 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