An In Vitro Study on the Accuracy of a Splint for the Transfer of Dental Implants

Abstract

The purpose of this study is to measure the positional accuracy of transfers during the splinting printing process using four biomaterials (two acrylic resins and two bis acrylic resins). Materials and Methods: A master model was constructed for the acrylic simulation of edentulous mandibles with four multiunit analogs (Bioinnovation, Sao Paulo, Brazil) positioned between the lateral incisor and premolar areas. Eighty samples (n = 80) were created in total. Four different dental materials, Duralay (Reliance, Chicago, IL, USA); Pattern Resin LS (GC, Tokyo, Japan) with a ratio of one part liquid and two parts powder; Structur (Vocco, Colonia, Germany); and Protemp (3 M ESPE, Neus, Germany), were used as splints with five samples each. Measurements were obtained for distances between points A, B, C, and D in sequential order through an INSIZE digital calibrator (Inzise, Taiwan, Seng, China). Results: The results were as follows: Distance A: better performance was observed for Pattern Resin (mean = 38.59 mm) and Duralay (mean = 38.58 mm) compared to the base (mean = 38.59 mm); distance B: Pattern Resin (mean = 19.08 mm) and Duralay (mean = 19.07 mm) were compared to the base (mean = 19.08 mm); distance C: no significant differences in relation to the base (prom = 18.85 mm) were shown for the three materials of Pattern Resin (prom = 18.86 mm), Structur (prom = 18.85 mm), and Duralay (mean = 18.85 mm); and distance D: Pattern Resin (mean = 20.46 mm) and Duralay (mean = 20.46 mm) provided lower performance compared to the base (mean = 20.47 mm), ANOVA (Prob = 0.333 > 0.05). Conclusions: Pattern resins and Duralay acrylic resins exhibit better dimensional stability when used as splints for the transfer of multiple dental implants.

1. Introduction

Nowadays, the use of conventional complete total dentures has been improved by the use of overdentures or complete fixed prostheses attached to implants [1]. These types of implant-retained or implant-supported prostheses provide several benefits, such as bone preservation, better masticatory performance, and greater patient satisfaction, improving patient’s quality of life [2]. The long-term osseointegration of implants is substantially impacted by the precise and passive adjustment of prostheses. Thus, it is necessary to accurately record the positions and distances of the implants by taking a precise and stable impression [3]. When exact parallelism is not achieved, several biomechanical problems can occur, including non-passivity in the entire structure. This could lead to the loosening of the abutments or implant screws, contributing to a higher risk for the screws to fracture [3]. A gap can occur between the abutment and the prosthesis, causing plaque accumulation and undesirable reactions in the peri-implant tissues [4].

There are some characteristics that impression materials need to fulfill in order to reduce shrinkage and improve dimensional stability, For example, Duralay (Reliance, Chicago, IL, USA) and Pattern Resin LS (GC, Tokyo, Japan), are considered self-curing monomers (Type II) and methyl methacrylate (methyl methacrylate CAS NUMBER: 80-62-6: 100 PEL (OSHA): 100 ppm, 410 mg/m³, 8 Hr. TWA TLV (ACGIH): 100 ppm 410 mg/m³, 8 Hr. TWA INEOS recommended: 50 ppm, 205 mg/m³, 8 Hr. TWA; 100 ppm, 410 mg/m³ 15 min) is an excellent acrylic with low dimensional change, generating heat moldability point without residues or by-products. This biomolecular characteristic allows the degree of shrinkage to be reduced to 1%, limiting any tensile force that could occur. [5,6]. Structur Resin (Vocco, Colonia, Germany) and Protemp TM 4 resin (3 M ESPE, Neus, Germany) are bisacrylic resins: composite materials made with resinous matrix, filler material (BIS-GMA and UDMA) bonding agent, z-curing, adhesives and pigments. These materials were selected because they are the most used analog impression techniques for dental implants. Passivity is essential in the impression, as well as in the elaboration of the respective prosthetic device [7].

Diverse techniques have been proposed to achieve a more accurate impression, such as direct and indirect techniques. In the direct technique, the splint can be used for transfers. The indirect technique involves transfers for a closed bucket and basket strategy (Snap/One). Even though the closed bucket technique is more frequently recommended, for the purpose of this study, we did not use it as the screws can be difficult to loosen when the analogs are accurately placed on the printing material [8]. The different materials and techniques used to splint print transfers that ensure the most accurate shape have been the subject of much research. There is conflicting evidence regarding splinting and print transfers [9]. Elshenawy stated that compared to other techniques, the printing technique with the splinting of transferors results in prostheses that fit more precisely on the implants. The clinical situation of the implants can also be altered with dimensional changes due to the emptying material and volumetric contraction of the metal casting, among other factors [10]. The objective of this study was to measure the positional accuracy of the transfers during the splinting printing process using four biomaterials (two acrylic resins and two bis acrylic resins).

2. Materials and Methods

For the null hypothesis, there was no significant difference between the degree of positional accuracy of transfers while using four splinting materials to take impressions on dental implants.

In this quantitative, experimental, cross-sectional study, a master model was made using clear self-curing acrylic resin thermal plasticization. In a glass socket, the base was isolated with liquid glycerin. For the sample calculation, the procedure based on the power of the test was used. This test consisted of contrasting the hypothesis of equal means or treatments, ANOVA. Assuming a significance level of 0.05 with a sample standard deviation of 0.00577 (

Table 1. Finite population parameters.

Maximum Difference	Sample Size	Target Power	Real Power
0.009	11	0.8	0.844
0.009	13	0.9	0.909
0.009	16	0.95	0.961

).

Then, the first layer of acrylic was placed. Subsequently, dental implants were positioned while respecting the angulation and distance established according to the digital model (lateral incisors/seconds premolars zone). Once the first layer was set, the glass mold was placed, completing the matrix with the acrylic resin, and carefully engaging the implants (plumber’s PTFE pipe sealant tape flex and waterproof seal tape were placed for the protection of implant entrances), simulating edentulous mandibles (Figure 1 and Figure 2). Four multiunit analogs for dental implant cone morse connection were positioned (Biomorse 3.5 × 10, Bioinnovation, Sao Paulo, Brazil). These were located in the lateral incisor and second premolar areas. The initial procedure consisted of measuring the base with the installation of the transfer caps without any material, and the final measurements involved the bases with the splints. The platform of the multiunit was at the same height with a difference of ±1 mm. In the front view from left to right, the analogs of the multiunit implants were named M1, M2, M3, and M4. A total of 20 splints were created following the power of the test to obtain the sample size. The inclusion criteria for rotational multiunit transfers, which are analogous to those of the standard model, included a torque of 10 Ncm inside the acrylic resin matrix, splints 6 mm wide/3 mm high, and the same splinting material. Of the 80 samples in total, Group 1 (n = 20), Group 2 (n = 20), Group 3 (n = 20), and Group 4 (n = 20) were divided into an additional four groups by splinting materials: Duralay (Reliance, Chicago, IL, USA), Pattern Resin LS (GC, Tokyo, Japan), Structur resin (Voco), and Protemp TM 4 resin (3 M ESPE/). (

Table 2. Testing materials.

Material/Acrylic/Bisacryl Resin	Manufactory	Country	Presentation	Composition
Duralay GC (A)	Reliance Dental Mfg Co. Chicago/Ilinois	USA	Powder/liquid	Powder: Polymethylmetacrylate Phthalate of dialkyl peroxide benzoyl with red pigments Liquid: Methylmethacrylate Ethylene Glycol Dimethacrylate Amin-type chemical initiator Self-polymerizable
Pattern Resin (A)	GC Americ/Chicago/Ilinois	USA	Powder/liquid	Powder: Polymethylmetacrylate Phthalate of dialkyl peroxide benzoyl with red pigments Liquid: Methylmethacrylate Ethylene Glycol Dimethacrylate Amin-type chemical initiator Self-polymerizable
Structur (Voco) (B)	Voco/Anton Fletner	Germany	Catalyst base system	Nalkyl dimetacrylate, bis-GMA, UDMA, silica, benzoyl peroxide, amines, terpenes, BHT
ProTemp TM4 (B)	3 M ESPE/Minnessota	USA	Catalyst base system	Bismethacrylate polymers of Z acetate silanized strontium glass silica

).

Four distances were established for the initial and final measurements: Distance A: the distal wall from M1 to the distal wall of M4; distance B: the distal wall of M1 to the mesial wall of M2; distance C: the distal wall of M2 to the distal wall of M3; and distance D: the mesial wall M3 to the distal wall of M4 (Figure 2).

The selection of materials for the study was based on their physical and chemical properties, as well as if they were well-known and used by the dental community, which is considered in the final selection. Methyl metalcrylate, for example, has excellent fluidity [11]. This allows the clinician to create accurate models with direct or indirect techniques. Thixotropic properties prevent displacement while the material is positioned. Also, it has excellent manipulation, controlled construction, and is easy to model and carve, allowing it to work in large extensions or spans without problems. Positive material characteristics were considered for the selection, such as minimal shrinkage, favorable fluidity, great wettability, perfect fit, high hardness, high resistance, a lack of residues generated, a fast setting, perfect adhesion, unlimited dimensional stability, great stability, and no risk of deformations, even when applied in thin layers. The physical properties like the amount of residual ash after combustion, Vickers hardness, flexural strength, and the volumetric polymerization shrinkage of each material were considered. Bisacrylic resins have chemical properties such as the distribution of methacrylate particles, the polarity of the monomers, the stability of the pigments, the capacity of the initiator system of the provisional materials, key data sheets to produce the different alterations in the polymerization, the absorption of liquids, and consequently, less color stability.

Duralay (Reliance, Chicago, IL, USA), Pattern Resin LS (GC, Tokyo, Japan) acrylic resin splints were fabricated [12] when the acrylic resin reached the plastic stage, and small amounts of acrylic resin were incrementally added with the help of a brush toward the center of the dental floss ligation (Figure 3 and Figure 4).

The acrylic resin (1 g–0.5 mL) continued to be placed incrementally until a full thickness of 6 mm wide/3 mm high was obtained, and the dental floss was covered. After 7 min (the exact time of total setting at room temperature at 20 °C with an altitude of 2830 m above sea level), the acrylic was polymerized (Figure 5).

Cuts were made in the splint at a central point between the transfers, using a multi-cut diamond disc (Edenta AG, Bauru, Brazil) with a thickness of 0.30 mm, a diameter of 19 mm, and a speed of 15,000 rpm to relieve the stresses caused by polymerization shrinkage. (Figure 6 and Figure 7). After waiting 15 min for the material to set, the print transfers were unscrewed, and A, B, C, and D were removed from the master model sequentially.

For the bisacryl resin samples, the first group used Structur resin (Vocco, Colonia, Germany), and Protemp TM 4 resin (3 M ESPE, Neus, Germany) crowns (as a bridge for provisionals) were used for the second one. Throughout, using an injection gun, a small amount of bisacryl resin was added to the silicone matrix in incremental layers. With the help of an injection tip, the material was placed toward the center of the dental floss ligation [13]. This process continued until a thickness of 6 mm wide/3 mm high was obtained, covering all dental floss. After 7 min, the bisacrylic resin was allowed to completely polymerize. The screws of the print transfers were unscrewed and sequentially removed from master models A, B, C, and D.

When the 80 splint samples were finished, they were stored at room temperature for subsequent measurement within 24 h. The preparation of all the samples for the final measurement was carried out in the same order, obtaining the measurement reference with a digital caliper (INSIZE/Inzize Corp., Taiwan/Sheing, China). The scanning of all the samples was carried out with PrismaScan (Sirona, Erlangen, Germany), obtaining the image in an STL file. The images were opened in EXOCAD DentalCAD 3.2 Elefcina (Darmstadt, Germany), a software that allowed the measurement of the reference points (Figure 2) to be compared with the measurements obtained by the digital calibrator. The values were obtained and recorded in the corresponding table (Table 2) (Figure 8).

3. Results

The totality of the samples (n = 80) was compared using measurements obtained by the digital caliper and the software. No significant differences were noted.

For the replicates, the significance level for distance A was 0.05 (5%); this provided statistical evidence that the mean distances of A for each group of replicates were equal (Prob = 0.333 > 0.05). The mean distances of A for each splinting material were different (Prob = 0.000 < 0.05). (

Table 3. Groups replicas of A, B, C, and D.

G †	R ‡	M § mm	V ¶	P	IC%	G †	R ‡	M § mm	V ¶	P	IC%
A	1	38.60	1.25	0.33	95	B	5	19.08	87.7	0	95
	2	38.59					4	19.09
	5	38.59					3	19.08
	4	38.59					2	19.08
	3	38.59					1	19.08
A	6	38.59	1.25	0.33	95	B	6	19.08	87.7	0	95
	7	38.59					7	19.08
	10	38.59					8	19.08
	9	38.59					10	19.08
	8	38.60					9	19.09
A	11	38.59	1.25	0.33	95	B	11	19.08	87.7	0	95
	12	38.59					12	19.08
	15	38.60					13	19.09
	14	38.59					15	19.08
	13	38.59					14	19.08
A	16	38.59	1.25	0.33	95	B	20	19.09	87.7	0	95
	17	38.59					19	19.08
	20	38.59					18	19.08
	19	38.60					17	19.08
	18	38.59					16	19.08
G†	R‡	M § mm	V¶	P	IC%	G†	R‡	M § mm	V¶	P	IC%
C	1	18.86	1.85	0.173	95	D	4	18.86	30.48	0	95
	3	18.85					5	18.85
	4	18.85					3	18.85
	2	18.85					1	18.85
	5	18.85					2	18.85
C	6	18.85	1.85	0.173	95	D	9	18.85	30.48	0	95
	8	18.85					10	18.86
	9	18.86					7	18.85
	7	18.85					6	18.85
	10	18.85					8	18.85
C	11	18.85	1.85	0.173	95	D	14	18.86	30.48	0	95
	13	18.85					15	18.85
	14	18.85					13	18.85
	12	18.85					11	18.85
	15	18.86					12	18.86
C	16	18.85	1.85	0.173	95	D	19	18.81	30.48	0	95
	18	18.85					20	18.85
	19	18.85					18	18.86
	17	18.85					16	18.85
	20	18.86					17	18.85

^† G = group; ^‡ R = replicas; ^§ M = media in mm; ^¶ V = variance; P = prob; IC = confidence interval.

).

The average distances of B for each group of replicates were equal (probability = 0.173 > 0.05). The comparison between the two procedures led to the deduction that the measurement of distance B in the initial procedure was more homogeneous than that in the final procedure. The mean of distance B for each splinting material differed (Prob = 0.000 < 0.05). The splinting material was different from the mean distance from point B. The average distances of C for each group of aftershocks were different (Prob = 0.001 < 0.05) (

Table 4. Group materials.

G	MT n = 80/20 *	M mm	G	MT n = 80/20 *	M mm	G	MT n = 80/20 *	M mm	G	MT n = 80/20 *	M mm
A	Base	38.60	B	Base	19.08	C	Pattern Resin	18.86	C	Pattern Resin	18.86
A	Pattern Resin	38.59	B	Pattern Resin	19.08	C	Structur	18.85	C	Structur	18.85
A	Duralay	38.58	B	Duralay	19.07	C	Base	18.85	C	Base	18.85
A	Structur	38.56	B	Structur	19.06	C	Duralay	18.85	C	Duralay	18.85
A	Protemp	38.54	B	Protemp	19.03	C	Protemp	18.85	C	Protemp	18.85

* n = 80 for the total sample * n = 20 samples for each group. MT = material; M = media in mm.

). Based on the comparison between the two procedures, the measurement of distance C in the initial procedure was statistically identical to that in the final procedure. The mean distances of C for each splinting material were different (Prob = 0.005 < 0.05). The average distances of D for each group of replicates were equal (Prob = 0.712 > 0.05). The average distances of D for each splinting material were different (Prob = 0.000 < 0.05). The ferulization material was different from the average distance of points A, B, C, and D because of its physical properties; moreover, the materials exhibited different polymerization contractions, which were related to the positional accuracy of the transfers. Although performing the measurements was difficult, the points marked as a reference for the measurement were on the proximal walls of the transfer copings of M1, M2, M3, and M4. The comparison between the two procedures led to the conclusion that the measurements of distances A, B, C, and D in the initial procedure were more homogeneous than those in the final procedure. (

Table 5. Statistical analysis.

G	V	P	IC%
A	9.25	0.001	95
B	4.44	0.005
C	0.54	0.712
D	29.85	0

G = group; V = variance; P = prob; IC = confidence interval.

).

ANOVA statistical analysis was performed to determine the probability of each material having the optimum degree of resistance; this is important when choosing to work in an analog way.

4. Discussion

Impression techniques in oral rehabilitation have always been a challenge. In implantology, it has not been the exception. The search for passivation in impression attachments helps the success of treatment. Several studies have evaluated the accuracy of such impressions, considering variables such as the material with which it is splinted, the materials for printing, and the printing techniques [14]. Incorporating transfer techniques compared to direct techniques without splinting has the objective of stabilizing the transfers and impression screws in complete neutrality so that the implants do not have tractions and micro-tractions that endanger their permanence over time [15]. This procedure involves rigid limits of the material, with a reduction in the dimensional changes generated by the impression material. The material selected for splinting is decisive (prostheses on implants), influencing the preservation of the health of the peri-implant tissue [16]. The appearance of digital impressions has exponentially reduced the analog handling of additaments. However, there are always clinical cases where such management does not work, leading to the use of conventional analog forms. Compared the accuracy of models obtained from direct impression techniques without a splint and with a splint that incorporated a digital component; splints with transfer caps with a bite registration (polyether) were the closest to the reference model, followed by Pattern Resin, caps without splints [17] and fin-shaped caps with the addition of bite registration silicone. Compared to splints with acrylic resins, digital handling with Scan Bodys was even more accurate. The evidence found and obtained indicated that different protocols for the handling of acrylic resins, bisacrylics, and other resins differ according to ambient temperature, geographic location, humidity, and product [18]. Pattern Resin and Duralay acrylic resins in the printing sockets achieved the passive and optimal fit of the prosthetic components used for production. The self-curing resin groups used in the compensation procedure generated less distortion than other methods [19]. In 2018, Saini et al. studied the different techniques of impression, the transfer of splinted attachments, and the dimensional accuracy of multiple implant impressions, determining that the impressions obtained with photopolymerizable hybrid resin were more accurate than the impressions made to splint the implants with self-curing acrylics [20]. The photopolymerizable resin splinting technique and the non-splinting technique were significantly less accurate than the self-curing acrylic resin technique [21,22]. Acrylic resins have been shown to exhibit better efficacy than bisacrylic resins. Studied the accuracy of models obtained from different impression techniques with different implant angulations (0, 15° and 30°) using indirect techniques, direct techniques without splints, and direct techniques with acrylic resin splints; the results coincide with the study for the use of a self-curing acrylic resin, and the protocols for the preparation of the splint were similar to those used [23]. In the preparation protocol for splinting with self-curing acrylic (Duralay–Pattern Resin) [24], a few minutes after the start of polymerization, there was an 80% reduction in the effects of polymerization shrinkage [25,26], which is an important consideration that allows a perfect impression to be obtained [27]. Additionally, in the future, it would be interesting to test the accuracy of a splint for the transfer of dental implants in combination with recently introduced technology such as smartphone applications. [28]

Analog printing has been handled adequately for the last 10 years; however, the incorporation of digital technology has changed the landscape of the profession. AI in these types of processes has been incorporated into all design software, and this has facilitated the handling of the meshes to obtain the final product [29].

The importance of this study lies in the provision of guidelines for knowledge of the effectiveness and fidelity of the materials used to perform splinting in the taking of implant impressions; the scientific community is also invited to carry out in vitro research comparing the techniques.

5. Conclusions

Pattern Resin and Duralay acrylic resins exhibit better dimensional stability when used as a splint for the transfer of multiple dental implants.