Impact of Magnifying Loupes on the Finish Lines of Fixed Prosthesis Preparations

Abstract

Background: The use of magnification appears to offer advantages in dental preparation for fixed prosthetics and is widely employed in clinical practice, although it has not yet been thoroughly documented. Therefore, the primary objective is to determine the impact of magnification on the quality of finish lines during the performance of preparations for fixed prostheses. Methods: Sixty-four natural teeth were randomized into two groups: Group O (preparation without additional magnification) and Group L (preparation with Kitus^® 2.5× magnifying glasses). The teeth were prepared for full crowns, and the finish lines were evaluated under the OPMI^® PicoZeiss dental microscope at 10× magnification, based on the criteria of Continuity, Roughness, and Thickness. Results: There were no significant differences between the groups in any of the parameters evaluated. In Thickness, Group O had a median (IQR) of 600 µm (500; 800 µm) and Group L, 600 µm (400; 800 µm). Group L was Continuous in 64.8% of the cases, Slightly Continuous in 26.1% of the evaluations, and Not Continuous in 9.1% of the cases, thus having a slight advantage over Group O, whose values were 58.0%, 35.2%, and 6.8%, respectively. Group L was Polished in 71.0% of the cases and Rough in 29.0% of the evaluations, against 69.3% and 30.7% of Group O, respectively. These results were obtained using IBM SPSS ^® software, version 29.0. Conclusions: The 2.5× magnification magnifiers demonstrated a slight positive impact on improving the quality of dental preparations for fixed prostheses. Nevertheless, since the results are not statistically significant, it is difficult to extrapolate them to the broader population.

1. Introduction

Fixed prostheses are one of the most important forms of prosthetic rehabilitation, as they restore the function and aesthetics of missing teeth, as well as correct damaged or less harmonious dental elements. This form of rehabilitation has become increasingly popular, not only because of the growing aesthetic demands but also due to the current decrease in edentulism [1,2].

Dental preparation is an essential stage of fixed prostheses, and there are therefore clinical guidelines aimed at guaranteeing their quality [3]. However, we are far from finding an ideal protocol in the literature; what is agreed upon is that the technique followed must respect the biomechanical principles of the preparations as well as the biological and aesthetic principles of the teeth [2,3]. From all the features of dental preparations for fixed prostheses, the finish line is considered a critical stage of fixed prostheses since it defines their fit [4].

Additionally, irregular finish lines can increase the risk of microleakage and secondary cavities, leading to rehabilitation failure [5,6]. As long as these finish lines are well defined and free of unsupported enamel, they are able to contribute to a good impression quality and adaptation of the restorations, guaranteeing good aesthetics and functional, long-lasting results. This phase of preparation is extremely important and rigorous, requiring tremendous precision in execution [7].

Monolithic zirconia restorations, manufactured exclusively by CAD/CAM technology, require more conservative dental preparation.

Professional knowledge and mastery of the instruments used are demanding requirements for obtaining excellent tooth preparation for fixed prostheses. Dentists report difficulties in preparing the finish line on the entire surface of the tooth, especially in conservative preparations, which is why various instruments have been studied to improve this characteristic of preparations by reducing speed and changing rotation and oscillation. Consequently, ultrasonic instruments and specific diamond drills have emerged, guaranteeing that the tooth structure has a more precise cut [6,7,8]. Furthermore, the finish line is considered the most challenging part of the dental preparation, seeing that 92.5% of the preparations performed are not deemed adequate [9]. Coordination between vision and manual dexterity, which is highly demanding in the dental field, as well as tactile perception, requires a high level of skill. By using magnification aids, a dentist not only achieves ergonomic results but can also possibly define better diagnosis and treatment [10,11].

Magnifying instruments are therefore increasingly used by dentists to improve their visual acuity and compensate for certain deficits that arise with advancing age. Most magnifying glasses used by these professionals provide magnification in the range of 2.5× to 3.5×. Microscopes, on the other hand, guarantee magnifications of 4.0× to 25.0× [12]. Several branches of dentistry have already studied the impact of magnification on the quality of their work, such as in endodontics [13,14] and Restorative Dentistry [15,16]. Despite the seeming advantages of using magnification in dental preparation for fixed prosthetics and its widespread use in clinical practice, it has not been well documented.

Dentistry involves many clinical skills that are acquired primarily in a preclinical environment, by combining various forms of learning, such as artificial models of patients and demonstrations by trainers [17].

Assessing the quality of dental preparations can be carried out in a variety of ways: by individually evaluating each feature of the finish line, by assessing the entire preparation, or even by taking into account their future performance in vivo. It also depends on the experience of the researchers [8]. Instruments such as a probe and mirror, scanners, silicone replicas, computerized tomography scans, and/or microscopes can be used for this purpose [18].

The aim of this study was to determine the impact of magnifying loupes on the quality of the finish lines when making preparations for fixed prostheses.

2. Materials and Methods

2.1. Research Hypothesis

The null hypothesis of this study is that there is no significant difference in the Continuity, Roughness, or Thickness of the finish line of dental preparations for fixed prostheses between the use or non-use of magnifying loupes.

2.2. Research

The sample size was determined bearing in mind the aim of this study and based on the assumption that there is a significant difference of 500 µm in the Thickness of the tooth preparation carried out with or without magnifying loupes, a standard deviation of 259.6 µm for the group, observed with magnifying loupes in another study [18], and a 95% confidence level; a minimum of 5 teeth was obtained for each group, using Open Epi^® software version 3.01 (San Diego, CA, USA). To obtain as representative a sample as possible, a total of 64 natural teeth were used.

The teeth in this study were extracted between 2019 and 2022 for orthodontic, prosthetic, periodontal, or surgical reasons. Teeth that were decayed, fractured, previously restored, endodontized, or with excessive occlusal/incisal wear were excluded. This resulted in 16 incisors (I), 16 canines (C), 16 premolars (PM), and 16 molars (M).

This study was approved by the Fernando Pessoa University Board, the Fernando Pessoa University (UFP) Ethics Committee (FCS/MED-340/22-3), and the Technical Department of the Faculty of Health Sciences (FCS) of UFP.

In the group of teeth with magnifying loupes, the same Kitus^® (Porto, Portugal) 2.5× magnifying lenses were always used, calibrated for the principal investigator. In order to minimize bias, the dental preparations were always carried out by the same researcher [19], using the same Gnatus Artus^® dental chair (Ribeirão Preto, Brazil) with a single-phase induction motor power of ¼ HP, as well as the same turbine with an air pressure of 30LBF/POL2 and a water pressure of 30 Ilbf/pol². The lamp lighting used was OSRAM^® (Berlin, Germany), with a voltage of 12 V and a power of 55 W. The drills used were measured with an Iwanson^® Caliper (Munich, Germany) every 5 complete uses (complete preparation) to make sure that the diameter of the cutter used for the preparation was that recommended by the brand. When this diameter was smaller than suggested, the drill was replaced with a new one with the same reference.

Since the researcher who carried out the preparations had no previous experience of using loupes, he carried out the entire fixed prosthesis preparation protocol on 15 teeth, making it possible to better adapt and become used to them. All the teeth prepared during this training period were not counted to assess the impact of magnifying loupes on dental preparations for fixed prostheses, but rather to assess the learning curve of using loupes. In other words, the point was to understand whether there was a significant difference in the quality of the preparations before and after the training period. This group was called LT.

The natural tooth pieces were hydrated in distilled water for 24 h and dried naturally, without being exposed to heat sources or direct sunlight. Once the teeth were dry, they were randomly and equally distributed into two different groups: the first, called Group O, was prepared using only medically prescribed glasses, without any additional magnification, and the second one was labeled Group L, whose teeth were prepared using 2.5× magnification loupes. After separating the pieces into groups, they were given a sticker code with the initials O or L, in accordance with their study group, so that they could be mixed and stored in one place and assessed impartially by the researchers.

Subsequently, a base was made with Reus Pink Wax that completely concealed the roots, exposing only the dental crown, to simulate its insertion into the oral cavity and facilitate support for the preparation. Wax was placed on top of the sticker (O/L), making it impossible to identify the group at the time of analysis, limiting the possibility of bias. They were given another sticker code on top of the wax, consisting of a letter according to the type of tooth (I, C, PM, M), followed by a random number from 1 to 16, which would allow the various assessments carried out on each tooth to be recorded (example: C16—Figure 1).

The material needed to carry out the tooth preparations was an NSK^® turbine (Tokyo, Japan), with Edenta^® (Au, Zürich, Switzerland) turbine diamond drills: 6368.314.023, 8368.314.023, 6856.314.018, 8856.314.018, 6856.314.014, 8856.314.014. To polish the preparations, the NSK^® contra-angle was used in conjunction with the 3M^® (St Paul-Minneapolis, MN, USA) soft lex medium-grit, super-fine polishing discs with a diameter of 12.7 mm fitted to the mandrel.

All grinding respected the tooth’s anatomy. Due to the absence of pre-existing cavities and restorations, a standardized preparation technique was ensured [19]. Returning to the turbine, tooth preparation began with incisal/occlusal (I/O) wear using the 018 coarse-grained diamond drill for a reduction of 1.5–2 mm. Vestibular (V), Palatine/Lingual (L), Mesial (M), and Distal (D) wear was carried out with the 014 coarse-grained diamond drill for a reduction of 1.2–1.5 mm, followed by a finish line with the 014 deep beveled coarse-grained diamond drill, using half the drill thickness so that the final measurement of the finish line was 0.8 mm, as described in [20]. This protocol is illustrated in Figure 2. On the palatal surface (P), on the I and C, before grinding with the 014 drill, the 023 candle flame diamond tip was used for grinding on the mid-incisal ⅓. A double inclination was also carried out with the 018 diamond drill in V and L/P. Moving on to finishing, all the drills with the same diameter as those previously used were used, this time with a finer grain size. The final polishing used polishing discs fitted to the mandrel and mounted at a contra-angle (Figure 1). Once all the preparation was complete, the dental pieces were rehydrated until the assessment. On average, the ambient sunlight was 500,000 K, the room temperature 17 °C, and the relative humidity 72%.

The data were collected at the VitalPlace Dental Clinic (evaluation of the quality of dental preparations for fixed prostheses) at two different times: in the first phase, by two researchers, and in the second phase, 2 weeks apart, by just one researcher. All groups of teeth (O, L, and LT) were assessed during this period, following the same protocol described below. An OPMI^® Pico Zeiss dental microscope (Jena, Germany) was used, with manual apochromatic magnification, 10× field magnification eyepieces, a straight tube, focal length f = 170 mm, focus objective lens f = 250 mm, and 100 W halogen illumination, supported on the ground. The ambient sunlight was 550,000 K, the ambient temperature was 20 °C, and the relative humidity was 68%.

All parameters were assessed at 4 points on the anterior teeth, 6 points on the premolars, and 8 points on the molars, according to another study [9]. Due to the lack of specific guidelines for assessing the quality of the finish line, a bibliographic search was carried out in which information was gathered on the criteria for assessing dental preparations for fixed prostheses, from which the criteria corresponding only to the finish line were selected and, finally, those co-existing in the various studies were chosen. Thus, Roughness, Continuity, and Thickness, present in [3,21,22,23], were the criteria used to assess the quality of the finish lines. Of the characteristics assessed, the first two criteria are ordinal qualitative assessments and the third is a continuous quantitative assessment.

Continuity was classified on an increasing scale from 1 to 3, with the following designations: (1) Non-Continuous (Continuity present in less than 50 per cent of the length); (2) Slightly Continuous (Continuity present in more than 50 per cent of the length); or (3) Continuous (Continuity perfectly visible throughout the length); this parameter was evaluated visually, depending on whether the finish line was uninterrupted or not. Roughness was classified as 1 or 2, i.e., (1) Rough (Roughness detected at 10× magnification) or (2) Polished (Roughness not detected at 10× magnification); this parameter was evaluated visually, depending on whether the finish line was uninterrupted or not. Thickness was quantified in micrometers (µm), with a range of 500–1000 µm considered favorable. To measure it, a k8 endodontic file was used, on which the Thickness of the finish line was marked with a stop. To find out the exact measurement, a Mitutoyo^® caliper (Kawasaki, Japan) was used. The Thickness was measured 1000 µm above the actual finish line, as described in [22].

In the qualitative assessment (Roughness and Continuity), the limitation of [8] was the lack of criteria and calibration of the examiners. To overcome these limitations, the examiners in this study were calibrated and specific criteria were used to increase the objectivity of the conclusions presented.

The group of each dental piece was revealed here after the assessment was completed by removing the wax base, allowing it to be identified solely and exclusively at this point (Figure 3).

To assess intra- and inter-investigator agreement, about 1/3 of the sample was used, i.e., 24 teeth, equally divided by study group and tooth class. The assessment for intra-investigator agreement was carried out 2 weeks before the main assessment.

2.3. Statistical Data Processing

Data analysis was performed using the IBM SPSS^® (Chicago, IL, USA) vs. 29.0 software with a significance level of 0.05 for all comparisons and R version 4.2.2. to calculate Cohen’s Weighted Kappa.

Qualitative results (Continuity and Roughness) were described using counts and percentages, while Thickness was described using the median and the respective 1st and 3rd quartiles, i.e., the interquartile range (IQR). For comparison with data from the literature, it was also decided to include the mean and its standard deviation, but subsequent comparisons do not use these statistics. Differences in Continuity and Roughness between Groups O and L (and LT) were assessed using Pearson’s Chi-square test.

For the quantitative parameter (Thickness), a comparison was made between the median and the AIQ using the Mann–Whitney U-test, since Thickness did not show a normal distribution, as assessed by the Kolmogorov–Smirnov test.

The evaluation of the variance (variability) of the Thickness measure was assessed by Levene’s test (between L and LT).

Intra- and inter-rater reliability was also assessed. Cohen’s Weighted Kappa test was used for the qualitative criteria and the intraclass correlation coefficient (ICC) for the quantitative parameter.

3. Results

Inter-rater reliability was carried out before the full assessment of the 64 teeth to ensure that it had a good degree of reliability. The less experienced researcher was calibrated by the more experienced one and had a training period with one-third of the teeth. After this training, the same number of teeth were assessed by each of the researchers to check the reliability of the results. The ICC for Thickness was 0.825, considered “very good” reliability, Roughness assessed by the Weighted Kappa test was 0.660, interpreted as “good agreement”, and Continuity, assessed by the same test, was 0.471, understood as “fair agreement”. Intra-researcher reliability in this study varies in the same way as inter-researcher reliability, with 0.844 for Thickness, considered “very good” reliability, 0.686 for Roughness, assessed as “good agreement”, and 0.599 for Continuity, understood as “regular agreement”. The significance for both cases was less than 0.01 (

Table 1. Statistical analysis of inter-rater reliability by Cohen’s Weighted Kappa and Interclass Correlation Coefficient.

	Cohen’s Weighted Kappa	ICC
Inter-rater reliability
Continuity	0.471	-
Roughness	0.660	-
Thickness	-	0.825
Intra-rater reliability
Continuity	0.599	-
Roughness	0.686	-
Thickness	-	0.844

). Intra-rater reliability (Table 1) was slightly higher.

The statistical analysis of the Thickness assessment criterion can be seen in

Table 2. Thickness (µm) comparison between Groups O and L, (%), for all dental pieces and according to dental piece type (I, C, PM, and M).

Dental Pieces	Statistics	Group O	Group L	p *
ALL	Median (P25; P75)	600 (500; 800)	600 (400; 800)	0.482
	Mean ± StDev	628.4 ± 213.5	619.3 ± 205.9
Incisor	Median (P25; P75)	700 (500; 800)	500 (400; 800)	0.040
	Mean ± StDev	671.9 ± 211.3	562.5 ± 205.960
Canine	Median (P25; P75)	600 (500; 675)	500 (400; 800)	0.553
	Mean ± StDev	581.3 ± 183.9	556.3 ± 228.512
Premolar	Median (P25; P75)	500 (400; 700)	500 (300; 600)	0.231
	Mean ± StDev	533.3 ± 176.7	495.8 ± 182.1
Molar	Median (P25; P75)	700 (500; 875)	800 (525; 900)	0.073
	Mean ± StDev	701.6 ± 224.3	771.9 ± 268.1

* according to the Mann–Whitney test.

. When descriptive statistics were analyzed between the two study groups, Group O had a median (AIQ) of 600 µm (500; 800 µm) and Group L 600 µm (400; 800 µm), but no significant differences were detected (p = 0.482). Individually, in the incisors, there was a significant difference in the Thickness values between Groups O and L. The median (AIQ) for Group O’s incisor class was 700 µm (500; 800 µm) and Group L’s was 500 µm (400; 800 µm). It is also important to emphasize that although there is a significant difference, both values are within the ideal Thickness parameters defined earlier, between 500 and 1000 µm. However, no significant difference was detected in the canines, premolars, and molars, as the p-values were 0.553, 0.231, and 0.073, respectively. The median (AIQ) of the canines in Group O was 600 µm (500; 675 µm) and in Group L, it was 500 µm (400; 800 µm); the median (AIQ) of the premolars in Group O was 500 µm (400; 700 µm) and in Group L, it was 500 µm (300; 600 µm); and the median (AIQ) of the molars in Group O was 700 µm (500; 875 µm) and in Group L, it was 800 µm (525; 900 µm).

Table 2 provides a statistical analysis of the Thickness comparison statistics for all dental pieces, as well as per tooth type.

Regarding Continuity (

Table 3. Continuity comparison statistics for Groups O and L.

Continuity	NC n (%)	PC n (%)	C n (%)	p *
Group O	12 (6.8)	62 (35.2)	102 (58.0)	0.165
Group L	16 (9.1)	46 (26.1)	114 (64.8)

* Chi2 test.

), Group L was found to be Continuous in 64.8 per cent of cases, Slightly Continuous in 26.1 per cent, and Not Continuous in 9.1 per cent of cases, thus having a slight advantage over Group O, whose values were 58.0 per cent, 35.2 per cent, and 6.8 per cent, respectively. The p-value was 0.165, indicating that the distribution of Continuity in Group O or L was equal. When classes I, C, PM and M were compared individually, in the incisors, it was found that Group O had a slight advantage, being Continuous 54.9 per cent of the time, Slightly Continuous in 31.3 per cent of the cases, and 9.4 per cent Non-Continuous, while Group L, despite sharing the percentage of Non-Continuous cases, only showed a difference in one assessment in the Slightly Continuous and Continuous parameters, translating into 34.4 per cent and 56.3 per cent in that order. The canines in Group L showed better Continuity than those in Group O, being Continuous in 65.6% of cases, Slightly Continuous in 28.1% of cases, and Not Continuous 6.3% of the time; Group L was Continuous in 50% of cases, Slightly Continuous 40.6% of the time, and Not Continuous in 9.4% of assessments. Premolars showed the same favorable trend as the Loupe Group, with the latter being Continuous 62.5% of the time, Slightly Continuous in 33.3% of cases, and Non-Continuous only 4.2% of the time. Group O had Continuous evaluations in 56.8% of cases, Slightly Continuous in 37.5% of cases, and 6.3% of evaluations were Non-Continuous. The molars maintained their lead in the highest Continuity classification in Group L, with 70.3% of cases compared to 62.5% in Group O. On the other hand, Group L had 15.6% of evaluations that were Slightly Continuous and 14.1% that were Non-Continuous, compared to 32.8% and 4%, respectively, in Group O. The p-values were 0.963, 0.449, and 0.778, respectively, for incisors, canines, and premolars, i.e., there was no significant difference. On the other hand, molars showed a p-value of 0.027, which means that this class of teeth showed a significant difference in the distribution of Continuity between Groups O and L, indicating the rejection of the null hypothesis.

The loupes favored the canines, with 68.8% of the evaluations being Polished and 31.3% Rough, while Group O showed results of 59.4% and 40.6% in the same order (

Table 4. Roughness comparison statistics for Groups O and L [n (%)], for all dental pieces and according to dental piece type (I, C, PM, and M).

Dental Pieces	Result	Group O	Group L	p *
ALL	R	54 (30.7)	51 (29.0)	0.727
	P	122 (69.3)	125 (71.0)
Incisor	R	10 (31.3)	12 (37.5)	0.599
	P	22 (68.8)	20 (62.5)
Canine	R	13 (40.6)	19 (59.4)	0.434
	P	10 (31.3)	22 (68.8)
Premolar	R	15 (31.3)	33 (68.8)	0.519
	P	18 (37.5)	30 (62.5)
Molar	R	16 (25.0)	48 (75.0)	0.279
	P	11 (17.2)	53 (82.8)

* Chi2 test.

). In the premolars, Group L had slightly lower ratings, and this class had similar results to the incisors, with 62.5% of cases being Polished and 37.5% Rough, and Group O had 68.8% and 31.3% in the same order. The molar class obtained the highest classifications, 82.8% and 75.0% Polished for Group L and O, respectively, and 17.2% and 25% Rough in the same order, with Group L showing a slight advantage when using the loupes. Despite the high percentages, as they were shared by both groups, the distribution of Roughness in Groups O and L was the same, i.e., there was no significant difference in any of the dental classes (incisors p = 0.559; canines p = 0.432; premolars p = 0.519; molars p = 0.279).

The Thickness of Group LT showed a median (AIQ) of 300 µm (200; 500 µm) (

Table 5. Results of Group LT for Continuity (n (%)), Roughness (n (%)), and Thickness (µm) parameters.

Continuity			Roughness		Thickness
NC n (%)	PC n (%)	C n (%)	R n (%)	P n (%)	Median (P25; P75)	Mean ± St Dev.
52 (57.8)	32 (35.6)	6 (6.7)	81 (90)	9 (10)	300 (200; 500)	405.6 ± 301.1
p < 0.001			p < 0.001		p < 0.001

). The dental pieces were Continuous in 6.7% of cases, Slightly Continuous in 35.6 per cent of evaluations, and Not Continuous in 57.8 per cent of cases. As for Roughness, the teeth in Group LT were Rough in 90% of the evaluations and Polished in 10% of the cases. Thus, there is no significant difference in the variance of the Thickness observed between Groups L and LT (Levene’s test, p = 0.635), but in Group LT, there are very strange observed values (outliers) with a value higher than the ideal (Figure 4).

4. Discussion

This study is relevant as it aims to address a gap in the literature. It is important to understand the impact of magnifying loupes on daily clinical performance, given their increasing use in dental practice. The existing literature, although limited, presents a significant controversy regarding the association between magnifying loupes and dental preparation.

After analyzing the results, it was evident that 2.5× magnification loupes had no significant clinical impact on the quality of dental preparations for fixed prostheses. Generally, the qualitative assessment parameters (Continuity and Roughness) showed a slight positive impact with the use of loupes, with differences of 6.8% (12 more maximum assessments for Continuity) and 1.7% (3 more maximum results for Roughness) out of a total of 176 assessments for each group. The quantitative criterion, Thickness, remained unaffected by the loupes, as both groups exhibited equal medians within the ideal range (500 to 1000 µm).

Individually, there was a significant difference in the distribution of Thickness values for incisors in both groups. However, this difference, although statistically relevant, lacked significant clinical relevance, as the 200 µm difference did not take either group out of the ideal range. Molars also demonstrated a significant difference in Continuity, favoring Group L, with half the intermediate rating compared to Group O. However, the loupes led to nine more Non-Continuous evaluations, thus negating the clinical significance of this statistically significant difference.

The null hypothesis, suggesting no significant difference in Continuity, Roughness, or Thickness of the finish line of dental preparations for fixed prostheses with or without magnifying loupes, was retained.

Generally, loupes can provide fixed magnifications between 2.5× and 6×. In prosthodontics, the vast majority of magnifying loupes used by professionals provide an amplification of around 2.5× to 3.5×.

The results of this study align with some publications in the field of dentistry, such as [15,24]. The former assumed that higher magnifications imply better resolution and the ability to identify small structures, which could be useful in various areas. However, when they evaluated the effect of magnification on cavity diagnosis, they found no differences with 2.5× magnification, and higher magnifications resulted in false positives at certain cavity detection levels. In the second study, when testing the effect of magnification on damage to adjacent teeth during class II preparation, it was found that neither 2.5× nor 6.4× magnification significantly decreased iatrogenic errors compared to preparations without magnification, suggesting the use of materials to protect adjacent teeth.

In contrast to the results found in this study, ref. [16] reported a significant improvement in the precision of tooth preparation with either 2.5× magnifying glasses or a 6.4× microscope compared to preparations without magnification. This improvement was noted with increasing magnification.

In endodontics, ref. [13] reached conclusions both identical and contrary to the aforementioned study. They demonstrated that magnification improved the motor skills necessary in endodontics, with the increase being progressive according to the degree of magnification used (naked eye, 2.5× magnifying glasses, and 8.0× microscope). Setzer et al. [14], in a systematic review, found a favorable possibility of greater success in endodontic surgery with greater magnification, but they noted that there might be an adaptation phase to the use of magnification. Taschieri et al. [25] concluded that magnification had a positive impact on endodontic surgery; however, they found no significant difference in various degrees of magnification, such as magnifying glasses or microscopes.

In prosthodontics, only one study [18] compared the impact of 3× and 10× magnifications and demonstrated that the preparation of finished crowns with greater magnification resulted in more precise marginal adaptation, contrasting with preparations without magnification. This last study was based on the assumption that tooth preparation is the most relevant factor for marginal adaptation, referencing [26].

The evolution of the quality of dental preparations between Groups LT and L was notable, with an inversion of the assessments; Group LT presented the majority of its classifications in the worst possible criteria (Rough and Non-Continuous), while most of the teeth in Group L obtained the best classification. Thickness also had more dispersed evaluations in Group LT, the majority falling below the ideal range, with several aberrant values (outliers). Therefore, it is important to consider that there is a learning curve in using these magnification instruments, as mentioned by [14].

Some authors argue that the assumption that “the use of magnification instruments increases the precision of clinical work” lacks scientific support due to scarce and ambiguous evidence [11]. Despite the lack of consensus on the effect of magnifying loupes in all areas of dentistry, they continue to be widely used. The same team stated that magnifying loupes can not only compensate for the decline in dentists’ visual capacity with advancing age but can also bring ergonomic advantages in small working areas. On the other hand, as they limit movement due to the restricted position of the head, they can hinder the execution of full-arch preparations. The authors also noted that magnifying loupes are typically used in conjunction with an intense LED light source, which significantly influences visibility, especially in the oral cavity. LED light was not used in this study since it was not the focus, ensuring the same working conditions in both groups, albeit differing from in vivo clinical practices.

This in vitro study simulates the use of magnifying glasses in tooth preparation for fixed prostheses in natural dental pieces and at a working distance; however, it does not simulate the oral cavity environment. Factors such as the dark environment, access difficulty, interference of the tongue and cheek, patient movements, limited mouth opening, exact tooth location, mirror position, and its reflection of light were not evaluated, nor was direct or indirect vision [11]. Therefore, the results cannot be extrapolated to an in vivo situation without understanding and identifying the interferences present in the oral cavity, which were not considered here.

Atlas et al. and Al Fouzan [18,27] stated that different periods of tooth extraction can cause varying degrees of dehydration in the teeth, which is why they were hydrated for 24 h in distilled water, as indicated by Atlas et al. [18].

For statistical purposes, the comparison between groups and the evaluation of the learning curve should have been made using the same tooth (unfeasible, as dental preparations involve irreversible changes) or by re-running standard teeth (frasaco). However, the researchers chose to simulate the impact of magnifying loupes on dental tissue, eliminating other variables such as material differences and changes in the effectiveness of plastic drills. Above all, the researchers prioritized the variability inherent in nature.

Subjectivity was identified as a flaw in several studies, such as [8,9]. The latter evaluated the quality of the 1–4 finish line based solely on the researchers’ experience without any calibration. To overcome this inaccuracy, several evaluation criteria were applied instead of a single general classification; the opinions of two researchers, calibrated by the most experienced, were used, and a 10x microscope was employed to enhance visual capacity and allow the detection of as many details as possible.

Evaluating the quality of dental preparation is a subjective exercise, yet it is this subjectivity that dentists encounter in their daily clinical practice. They depend on knowledge and experience to assess whether a certain characteristic of the preparation is adequate, as referred to by [8].

5. Conclusions

The 2.5× magnification loupes demonstrated a slight positive impact on improving the quality of the finish line of fixed prosthesis preparations; however, as the results are not statistically significant for the sample, it is impossible to extrapolate that slight positive impact to the population.

Limitations

Although the sample was considerably larger and more representative than the minimum suggested by Open Epi ^® version 3.01 software (San Diego, CA, USA), it did not prove to be sufficient, suggesting the need for further studies with a more in-depth analysis or an alternative methodological approach to better understand the relationships between the variables investigated.

As there are magnifying glasses of different magnifications on the market, the lowest magnification was used in this study. This magnification level of 2.5× may not have been enough to detect differences when compared to the naked eye.

As it was not possible to use the same lighting with and without magnifiers, LED lighting was not used, which can greatly enhance the ability to see and identify details.

The subjectivity inherent in quality assessment can and should always be considered. Although efforts have been made to minimize this influence by applying standardized procedures, using independent assessors, and adopting pre-established criteria for data analysis, it is important to recognize that researchers’ perceptions and assessments can be influenced by their individual experience and perceptions, and there is still the possibility that subjectivity has influenced the results obtained.