Document Type : Original Article
Authors
1 Department of Prosthodontics, School of Dentistry, Mashhad University of Medical Sciences, Mashhad, Iran.
2 Department of Prosthodontics, School of Dentistry, Mashhad University of Medical Sciences, Mashhad, Iran
3 Dental Research Center, Mashhad University of Medical Sciences, Mashhad, Iran.
Abstract
Keywords
Main Subjects
Implant-supported prostheses have become the preferred option for masticatory rehabilitation because they provide predictable functional and aesthetic outcomes. Successful rehabilitation of edentulous patients with implant-supported prostheses depends on achieving a passive fit between the prosthetic framework and the underlying implants. A misfit can induce biomechanical stresses, leading to complications such as screw loosening, peri-implant bone loss, and implant fracture (1).
The initial step in achieving a passive fit between the prosthetic framework and the underlying implants is the accurate transfer of the three-dimensional (3D) implant positions from the patient's mouth to the working casts (1, 2). Several variables can affect transfer accuracy, including implant characteristics (number, angulation, and depth), the impression technique, and the use of coping splinting methods (3).
Among the available impression techniques, the open-tray method using polyvinyl siloxane or polyether is widely accepted for multiple implants (4-6). However, the inherent dimensional instability of impression materials has led clinicians to recommend rigid splinting to maintain the spatial relationships between coping (7). As a result, various rigid splinting materials have been investigated, with autopolymerizing acrylic resin being the most commonly used material (3, 8, 9).
The effect of splinting on impression accuracy remains controversial. In a systematic review by Baig (3), 24 studies were evaluated; ten reported no significant difference between splinted and non-splinted techniques, seven supported the superior accuracy of the splinted technique, and six showed mixed results. Another study found that splinting provided a better
|
Figure 1. Acrylic resin cast with surgical guide |
passive fit in the maxilla but showed no statistically superior fit in the mandible (10). Furthermore, Ma et al. (11) suggested that splinted copings produced
|
C |
|
A |
significantly larger marginal gaps than non-splinted
|
E |
ones, suggesting the superiority of the non-splinting technique. In contrast, Richi et al. (1) reported that splinting the impression copings enhanced the accuracy of the impression technique.
In clinical practice, parallel implant placement is frequently restricted by anatomical constraints and aesthetic requirements (1). In edentulous cases, implants are often placed at varying angulations to optimize bone support, avoid anatomical structures such as the mental foramen, and achieve favorable prosthetic emergence profiles. Although some studies have examined the accuracy of splinted and non-splinted open tray impression techniques, there is limited data regarding their application in cases involving angled implants (12).
This study aimed to compare the accuracy of splinted and non-splinted open-tray impression techniques for angulated implants in an edentulous mandibular model. The study tested the following null hypotheses: first, there would be no statistically significant difference in the 3D accuracy of implant analog positions between the splinted open-tray and non-splinted open-tray techniques; and second, implant angulation (0°, 15°, and 25°) would have no significant effect on the accuracy of each impression technique.
Materials and methods
This in vitro study was approved by the ethics committee of Mashhad University of Medical Sciences (IR.MUMS.DENTISTRY.REC123456).
Model preparation
Three acrylic edentulous mandibular models with identical dimensions were fabricated. Using a custom surgical guide, implant sites were marked at three positions: one in the middle (C) and two distal to the middle point (A & E; Figure 1) (18). A milling machine (Nouvac AG, Goldach, Switzerland) was used to prepare osteotomies in three angulation configurations:
Model 1: All implants were placed parallel (0°).
Model 2: A 15° buccolingual divergence was created between implants A and C, and a 15° mesiodistal divergence between implants A and E.
Model 3: A 25° buccolingual divergence was created between implants A and C, and a 25° mesiodistal divergence between implants A and E.
The angulations were controlled using the machine’s tilting table, which was calibrated with a smartphone gyroscope application.
Three B2 Ø4.1/14 mm internal-connection implants (Biodenta Swiss, Berneck, Switzerland) were secured in each model using autopolymerizing acrylic resin (Figure 2). To ensure reproducible tray positioning, a peripheral resin seat was fabricated around each model. Custom impression trays with three 5 × 5 mm openings were prepared.
Impression techniques
Five impressions were obtained from each model using two techniques (n=15 per technique, total n=30):
|
|
Group A (Non-splinted impression technique): Square open-tray copings (Biodenta Swiss) were finger-tightened onto the implants. A custom impression tray was loaded with high-viscosity polyvinyl siloxane (Putty Fast Set; Panasil, Kettenbach, Germany). Light-body material (Initial Contact X-Light; Panasil) was then injected around the copings on the model. The assembly was placed in an incubator at 35°C for 8 minutes (13).
The set impression was removed, and implant analogs were connected. After a 4-hour setting period at room temperature, the impressions were poured with type IV dental stone (Elite rock; Badia Polesine, Zhermack Rovigo, Italy). The casts were stored at room temperature for one week before analysis.
Group B (Splinted impression technique): A cast was made according to the protocol explained in group 1 to form the splinting framework. The copings were connected using dental floss (Mina, Iran) as a scaffold. Duralay resin (Reliance Dental Mfg Co, Wort, IL, USA) was applied around the dental floss (Figure 3). After 20
|
Figure 2. Implant placement. 0-degree model (right) and 25-degree model (left)
|
|
Figure 3. Splinting the impression copings (left) and separating the resin bar using a cutting disk (right)
|
minutes, the framework was sectioned using a disk and allowed to set for 24 hours.
For the final impression, the sectioned splint was carefully positioned on the master model, and the impression copings were secured within it. The two-sectioned parts of the splint were then reconnected using fresh Duralay resin. After 5 minutes, the impression was taken using the same procedure as in Group A.
Reference scans
After attaching the scan bodies (Figure 4), the casts and master models were scanned using a high-precision laboratory scanner (Smart optics Activity 885, Smart optics, Bochum, Germany) with 6 μm accuracy (14). These reference scans served as the gold standard. The STL files of the casts were compared with their corresponding master models using Geomagic Control X (3D Systems, Rock Hill, SC, USA). To reduce potential dimensional distortion errors, only the geometries of the three scan bodies were used for superimposition, while the other regions of the mandible were excluded. For superimposition of the scan bodies, the best-fit algorithm was applied (13). Subsequently, by superimposing the files obtained from the casts and the corresponding master model, the average spatial deviation (Average deviation) of the scan bodies in all three spatial dimensions was calculated (15).
Statistical analysis
Data analysis was performed using IBM SPSS Statistics for Windows, version 22.0 (IBM Corp., Armonk, NY, USA). The normality of the data was confirmed using the Shapiro-Wilk test (P>0.05). A two-way analysis of variance (ANOVA) was used to assess the effects of the impression technique and implant angulation (0°, 15°, 25°) on impression accuracy. A P-value less than 0.05 was considered statistically significant.
Results
The descriptive data for the 3D deviations across the study groups are summarized in Table 1. The non-splinted technique had a lower mean deviation (57.90 ± 12.65 µm), compared to the splinted technique (61.84 ± 15.84 µm). The mean deviation was lowest at 15° (56.38 ± 14.83 µm) and highest at 25° (64.44 ± 14.62 µm).
Two-way ANOVA indicated that implant angulation did not have any significant effect on the accuracy of the impression technique (P=0.401). Moreover, no significant difference was observed among different impression techniques regarding their accuracy (P=0.535). There was no significant interaction between impression technique and implant angulation (P=0.383).
In the non-splinted group, the mean deviation increased with increasing angulation. The splinted group exhibited an initial decrease in deviation from 0° to 15°, followed by an increase at 25°.
|
Table 1. Mean ± standard deviation (SD) of 3D deviations (µm) between test and reference datasets across angulations and impression techniques
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||
|
Figure 4. Attached scan bodies on the resin cast surrounded by a peripheral resin seat
|
Discussion
This in vitro study evaluated the accuracy of splinted open-tray and non-splinted open-tray impression techniques for recording angulated implants in edentulous mandibular models. "The results showed no significant difference in accuracy between the splinted and non-splinted impression techniques, nor between different implant angulations. Therefore, the null hypotheses were accepted.
The findings of this study align with several investigations that reported comparable accuracy between splinted and non-splinted open-tray techniques (14, 16, 17). A systematic review by Pesce et al. (18) suggested that the pick-up technique is adequate for up to three implants without splinting, while splinting is recommended for more than three implants to ensure accurate transfer of spatial relationships to stone casts. Lee et al. (19) suggested that acrylic resin splinting often presents clinical difficulties in obtaining an accurate impression due to its time-consuming nature and the risk of material distortion. In contrast to the outcomes of this study, Akca and Çehreli (20) reported that the splinting technique for implant impressions yields greater accuracy, which contributes to improved long-term stability of the implants. Richi et al. (1) reported that the splinted technique had lower deviations compared to the non-splinted one.
The lack of a significant difference in accuracy between the splinted and non-splinted techniques may be attributed to the limited number of implants (three per model). Furthermore, using rigid acrylic models eliminated soft tissue resilience and movement, which are factors that could contribute to impression distortion during tray removal in the clinical setting. Additionally, the positioning of implants at sites A, C, and E may have minimized the cumulative dimensional changes caused by polymerization shrinkage of the impression material. Site C was positioned centrally on the mandibular model, while Sites A and E were symmetrically placed on the distal sides, forming a balanced triangular pattern with short inter-implant distances. By keeping the implants closer together, the study minimized the total distance affected by shrinkage.
Implant angulation (0°, 15°, 25°) showed no significant effect on impression accuracy for either splinted or non-splinted techniques. This finding aligns with previous research by Choi et al. (21), who reported that implant angulation up to 25° does not compromise the accuracy of conventional impression methods. The outcomes of this study are in contrast to those of other studies that demonstrated the splinting technique and implant angulation would affect the accuracy of impressions (5, 10). These studies used higher angulations or more implants and attributed the increased accuracy of the splinted technique to reduced movement of the copings during impression removal and analog attachment (5, 10). Richi et al. (1) examined the accuracy of open-tray impressions in edentulous maxillary models with six implants, comparing parallel placement (0°) to angulations of 10° and 20°. Their results showed that implant angulation significantly affected impression accuracy, with deviations increasing as the angulation rose from 0° to 20°. This contrasting result is likely due to the use of greater implants (six compared to three in this study), which may increase cumulative distortion.
The absence of angulation-related errors in the present study may also be attributed to the design of the square impression copings, which provide rotational stability and prevent misalignment during impression removal. Additionally, the open-tray technique ensures that impression copings remain rigidly connected to the implant analogs after tray removal, preserving their spatial orientation regardless of the initial implant angulation.
The present findings suggest that clinicians can place implants at angulations up to 25° without compromisimg impression accuracy when using conventional open-tray techniques. This study has some limitations. The in vitro design does not fully replicate clinical conditions such as soft tissue resilience, patient movement, and limited intraoral access. Additionally, testing only one splinting material (Duralay resin) limits the generalizability of the results to other splinting methods. Future studies should include clinical validations with a variety of splinting materials and full-arch implant configurations.
Conclusions
Within the limitations of this in vitro study, the following conclusions can be drawn:
Acknowledgements
Not applicable.
Conflicts of interest
The authors declare no conflict of interest.
Ethical considerations
This in vitro study was approved by the ethics committee of Mashhad University of Medical Sciences (IR.MUMS.DENTISTRY.REC123456).
Author contributions
M.R.N. and H.R.H. contributed to the design and supervision of the research; S.S. assisted with the conceptualization of the study, data collection, and data analysis; and M.S. contributed to the research implementation and writing of the manuscript. All authors read and approved the final manuscript.
Funding
The present study was funded by the vice-chancellor for research at Mashhad University of Medical Sciences (Thesis number: 625).
)