Accuracy Comparison of Impression by Alginate Materials between Conventional and New Methods (Injection)—an In-vitro Study

Purpose: In this research, we used a new technique (injection) to improve the accuracy of recording the surface details of alginate impression material. This research aims to compare the new method (injection) accuracy with the conventional method using alginate impression material.

Material and Method: In this in vitro study, the Typodont Tooth model of the Maxillary Arch was used. The second molar tooth was prepared from the second quadrant of the maxillary arch model, and ten points were identified for examination with an extra-oral laboratory 3D scanner. Poly Works, SPSS, and EXO CAD software were used for analysis and comparison.

Results: Among the ten reference points used on the trimmed teeth after impression, casting, and scanning, the casts made by the new method contain the most recorded points, with the highest accuracy in recording details. By using Poly-Works software, the degree of matching of the scanned plaster casts with the original model was compared, and the highest match was related to casts number 1, 2, and 3 (injection) and cast No. 4, 5, and 6 (conventional) had the minor match.

Conclusion: Based on the results obtained from this study, the injection technique was more accurate than the traditional technique.

Introduction

Impression is a negative form, the initial form obtained by placing soft and semi-liquid materials in the mouth after it hardens [1]. Depending on the type of material used, the impression may be rigid or elastic. The materials used for cast restorations must often be elastic when they leave the mouth [2]. The reconstruction of the positive form or its cast is made from the negative form obtained from the teeth and the buildings around them [3]. There are comprehensive types of impression materials:

1. Rigid impression materials include Plaster, Zinc Oxide, Eugenol, and compound

2. Elastic impression materials include hydrocolloids, divided into reversible and irreversible parts

3. Elastomeric impression materials have three main groups: 1-Polysulfides, 2-Polyethers, and 3-Silicones [4].

To make an accurate restoration, the following should be considered:

1. One critical factor in achieving a precise restoration is the ideal tooth preparation. This includes a smooth finish line, appropriate taper, and a preparation that conforms to the tooth’s anatomy.

2. Accurate impression taking also includes high-quality impression material, appropriate tray selection, use of adhesive for the tray, utilization of retraction cord, and proper impression technique.

3. Laboratory fabrication and procedural steps are carried out according to principles.

4. Pre-installation assessment of the restoration, including passive fit evaluation, examination of restoration margins, and assessment of proximal and occlusal contacts.

5. Lastly, the appropriate and precise cement must be selected [5].

Failure to adhere to each of these points can lead to the fabrication of an inappropriate restoration. The primary reasons for the inadequacy of many restorations produced in city-level laboratories in Herat, Afghanistan, are related to the clinical section handled by the dentist or the laboratory section and the stages of restoration fabrication.

One of the significant clinical errors (related to the dentist) is the need for more attention to impression-taking principles. Dimensional stability, accurate margin registration, and the absence of bubbles are characteristics of a suitable impression [6].

The important issues that improper impression-taking can create for both the dentist and the patient are the incorrect fit of the restoration to the prepared tooth and the need for precise adaptation to the finish line [7]. Improper impression-taking can lead to several complications for the dentist and the patient. It can result in microleakage, secondary caries, decreased crown fit and retention stability, and incorrect force distribution on the tooth under the crown. Microleakage in the prepared crown can cause secondary caries, which in turn can weaken the tooth and even lead to tooth loss. Additionally, microleakage increases the risk of inflammation around the tooth (gingivitis) [2].

The continued occurrence of these issues in impression-taking can lead to problems such as poor adaptation, increased periodontal issues and inflammation, bad breath, and a higher risk of tooth loss. Therefore, carefully selecting impression material and technique is crucial for the dentist and the patient [8].

Alginate material is widely available and cost-effective for dentists; hence, researchers have been investigating its properties to enhance surface detail registration [9]. One notable property studied is the impact of viscosity on alginate impression accuracy. Studies have shown that increasing alginate viscosity enhances the registration of undercuts and surface details [10]. Considering this characteristic of alginate, more precise crowns and bridges can be fabricated [11].

Artificial elastomers offer good detail registration, dimensional stability, and overall quality. These materials include polysulfides, polyethers, and various silicones, which are adaptable through different techniques in the oral cavity [12]. The monophase technique is used for medium-viscosity polyethers and certain addition silicones. This method, also known as the single-phase or single-viscosity technique, is similar to the multiple-mix technique but involves a single mix with a consistent viscosity. Part of this mix is placed in the tray. At the same time, the remainder is used to infiltrate around the prepared tooth, ensuring adaptation [13]. The success of this method depends on shear-thinning properties (reduction in viscosity upon application), which allow the material to penetrate effectively around the prepared tooth while maintaining medium viscosity in the tray to ensure precise contact with sensitive areas of the prepared tooth [14].

Shear thinning is one of the properties of non-Newtonian fluids. In these fluids, viscosity is not constant and decreases under pressure. These fluids include ketchup, blood, nail polish, silicone oil, and others [15].

In this study, alginate materials were prepared and examined using the Monophase Technique. During this investigation, the traditional method, which involves using an impression tray, was compared to the new method, which utilizes a disposable syringe to inject alginate around the prepared tooth (Monophase Technique) and then adapts the tray into the mouth cavity.

Materials and Methods

This research was conducted as an in vitro study using the Typodont Tooth Model (Beijing AMC Co. Limited), specifically focusing on the maxillary arch. The second molar in the second quadrant was selected as the reference tooth and was meticulously prepared for a more detailed examination. On this prepared tooth, ten reference points were identified: two on the occlusal surface, two on the buccal surface, two on the lingual surface, two on the mesial surface, and two on the distal surface, using a round bur number 41 under loop vision. Four points were placed on the prepared margins as a shoulder (Fig. 1).

Fig. 1. Reference model with prepared teeth.

The mentioned model was impressed with the alginate brand YONGXING MEDICAL, Ultra-High Precision Type. The impression was performed six times, three times using the new (injection) method and three times using the conventional method. For all impressions, tray number 3, 25 grams of alginate, and 40 cc of water at room temperature were used. The same person performed all impressions. The mixing time for all impressions was 33 seconds, and the setting time was 6 minutes (Fig. 2).

Fig. 2. Injected alginate material around the prepared tooth.

The primary model and the molds were numbered and cast using the Chinese brand Dental Super Hard Gypsum from Yong Xing Yiliao, with 12 grams of gypsum mixed with ten ccs of water. The cast models were numbered and sent to the laboratory. They were scanned using the Qscan Pro Dental Lab Scanner made in China for digital analysis and comparison. The scanned models from each numbered gypsum cast were compared with the original model using PolyWorks software. SPSS, PolyWorks, and ExoCad software were used for data analysis (Figs. 3 and 4).

Fig. 3. Prepared cast models.

Fig. 4. Poly-works, matching scanned cast with original model.

Results

Among the ten reference points used on the prepared tooth, after impression, casting, and scanning, the first cast registered all 10 points, the second cast registered 9 points, the third cast registered 10 points, the fourth cast registered 5 points, the fifth cast register 6 points, and the sixth cast registered 4 points (Fig. 5). When we compared both techniques, in total, including 10 points, 5 points were recorded in the conventional method and 9.66 points were recorded in the new (injection) method (Fig. 6). PolyWorks software compared the degree of match between the scanned gypsum casts and the original model, and the mean deviation of all models from the original model was evaluated. The results indicated that the highest conformity was related to casts 1, 2, and 3, while the lowest was found in casts 4, 5, and 6 (Fig. 7).

Fig. 5. Comparison of registered points in six alginate-prepared models.

Fig. 6. Comparison of the number of registered points in injection and conventional methods.

Fig. 7. Mead deviation of each model.

Discussion

Due to the novelty of this study, related articles are currently unavailable, and no prior research has been conducted in this area. This investigation aimed to enhance the accuracy of surface detail registration using alginate impression material through the Monophase Technique. The results of this study indicated that using the injection method yielded better and more precise registration of finish lines and reference points compared to the conventional method.

After reviewing the latest articles on the accuracy of impression materials, it was found that alginate has the lowest surface detail accuracy and dimensional stability compared to elastomeric impression materials [16]. The most recent study on the comparison of impression materials was conducted by Eugen S. Bud and colleagues in Romania in 2022. This study compared three types of impression materials, condensation silicone, addition silicone, and alginate, regarding surface registration accuracy. The results showed that condensation and addition silicones had the highest accuracy, while alginate had the lowest. This comparison was done using a three-dimensional method, where the impressions made from each material were scanned with an extra-oral scanner and digitally compared to the reference model. The current study followed the exact mechanism and method [16], [17].

Another study in 2022 by Hua Xi Kou and colleagues in China aimed to address the inaccuracy of alginate registration by developing a medium-consistency, putty-like alginate that could be injected and adapted into the impression tray, similar to some addition silicones. The results showed that the impressions made with the new putty-like injectable alginate were more accurate and better at registering details without bubbles than those made with conventional alginate (powder and liquid) [17].

Conclusion

Based on the results of this study, the injection method for impression-taking significantly enhances the accuracy of the surface detail of alginate impression material. According to the data obtained, out of the ten reference points created on the model, almost all points were registered using the injection method. In comparison, only about 5 points were registered using the conventional method. This indicates approximately a 50% increase in point registration accuracy with the injection method.

When the reference model and the gypsum casts were compared three-dimensionally using PolyWorks software, the mean deviation revealed that the casts made using the injection method had the slightest deviation. Therefore, this method can produce more accurate impressions to fabricate crowns and bridges.

Recommendations

This study introduced a new alginate impression technique and compared it only with the conventional method, demonstrating higher accuracy. It would be beneficial to introduce this new technique to dental practitioners. Additionally, it is recommended that this new technique be compared with impression-taking using additional silicone and condensation silicones to determine the accuracy of the new injection technique. This comparison would help evaluate the accuracy of alginate impression-taking using the injection method against that of elastomeric impression materials such as condensation and additional silicones.