*2.6. Statistical Analysis*

IBM SPSS statistical Statistics for Windows, version 25 (IBM Corp., Armonk, NY, USA) was used to analyze all data (α = 0.05). First, the distribution of the data was investigated using the Shapiro–Wilk test; the data were not normally distributed. Therefore, Mann– Whitney U test was performed to compare the two types of dental UCS in EMG and muscle fatigue and to compare dental tasks (intraoral scanning and tooth preparation simulation). A Kruskal–Wallis H test was performed to compare the differences in EMG and muscle fatigue according to the muscles. The Bonferroni adjustment was performed for multiple comparisons.

#### **3. Results**

The mean working time was 444.7 ± 195.2 s for the tooth preparation task and 509.6 ± 142.6 s for the intraoral scanning task (1st: 571.5 ± 169.0 s, second: 496.3 ± 145.2 s, third: 461.0 ± 113.6 s). The time for the intraoral scanning task showed a significant decrease during the three repetitions (*p* < 0.001).

In both types of dental UCS, the *RMS EMG* of the tooth preparation task was higher than that of intraoral scanning, but there was no statistically significant difference (*p* = 0.147; Table 1). In addition, there was no significant difference between the muscle fatigue for the two types of simulations measured in the two types of dental UCS (*p* = 0.435; Table 2).


**Table 1.** Comparison of mean *RMS EMG* (%*MVC*) according to muscle type and dental unit chair system.

Significance was determined by the Mann–Whitney U test (\*, comparison according to unit chair system in each muscle; \*\*\*, comparison of unit chair systems in overall mean; and \*\*\*\*, comparison of two simulations), *p* < 0.05. \*\*, Significance determined by Kruskal–Wallis H test (comparison of each muscle), *p* < 0.05. RMS, root mean square; EMG, Electromyography.

**Table 2.** Comparison of mean muscle fatigue (%) according to muscle type and dental unit chair system.


Significance was determined by the Mann–Whitney U test (\*, comparison according to unit chair system in each muscle; \*\*\*, comparison of unit chair systems in overall mean; and \*\*\*\*, comparison of two simulations), *p* < 0.05. \*\*, Significance determined by Kruskal–Wallis H test (comparison of each muscle), *p* < 0.05.

The intraoral scanning task and tooth preparation task showed a low risk level only in the SCM and a moderate risk level in other muscles (Table 1). During the intraoral scanning task, the digital integrated dental UCS showed significantly higher *RMS EMG* in the EDC and T (*p* < 0.001), while the conventional dental UCS showed significantly higher *RMS EMG* in the right T (*p* < 0.001; Table 1). During the tooth preparation task, both types of dental UCS showed significantly higher *RMS EMG* in the EDC, FDS, left SC, and T (*p* < 0.05; Table 1). There was also no significant difference between the *RMS EMG* with the two dental UCS (intraoral scanning task, *p* = 0.237; tooth preparation task, *p* = 0.543; Table 1).

In digital integrated dental UCS, there was no significant difference in muscle fatigue according to muscle in the intraoral scanning task (*p* = 0.138) and tooth preparation task (*p* = 0.219; Table 2). Similarly, in conventional dental UCS, there was no significant difference in muscle fatigue according to the muscle in the intraoral scanning task (*p* = 0.417) and tooth preparation task (*p* = 0.141; Table 2).

When comparing the two tasks (intraoral scanning and tooth preparation), there was a significant difference in the *RMS EMG* of EDC (*p* = 0.033), and there was no significant difference in the *RMS EMG* and muscle fatigue between the two tasks in other muscles (*p* > 0.05). Both tasks showed moderate risk levels of *RMS EMG* in the T and EDC (Figure 3), and high muscle fatigue in the EDC and FDS (Figure 4).

**Figure 3.** Comparison of *RMS EMG* (%*MVC*) according to the experimental task. (**A**) extensor digitorum communis; (**B**) flexor digitorum superficialis; (**C**) sternocleidomastoid muscle; (**D**) splenius capitis; (**E**) trapezius descendens.

**Figure 4.** Comparison of muscle fatigue (%) according to the experimental task. (**A**) extensor digitorum communis; (**B**) flexor digitorum superficialis; (**C**) sternocleidomastoid muscle; (**D**) splenius capitis; (**E**) trapezius descendens.

Repeated use of the intraoral scanner three times did not show a significant change in *RMS EMG* (*p* = 0.639; Table 3) but showed a significant difference in muscle fatigue (*p* < 0.001; Table 4). In the FDS and SCM, using the intraoral scanner three times increased the muscle fatigue significantly (FDS, *p* = 0.043; SCM, *p* = 0.027; Table 4).

**Table 3.** Comparison of mean *RMS EMG* (%*MVC*) in the first, second, and third repetitions of the intraoral scanning task.


The same superscript lowercase letters (column) are not significantly different according to the Mann–Whitney U-test and Bonferroni correction method. Significance was determined by the Kruskal–Wallis H test (\*, comparison of each muscle; \*\*, comparison of task repetitions in each muscle; and \*\*\*, comparison of task repetitions overall); *p* < 0.05. RMS, root mean square; EMG, Electromyography.

**Table 4.** Comparison of intraoral scanning task mean muscle fatigue (%) in the first, second, and third repetitions.


The same superscript lowercase letters (column) and same superscript uppercase letters (row) are not significantly different according to the Mann–Whitney U-test and Bonferroni correction method. Significance was determined by the Kruskal–Wallis H test (\*, comparison of each muscle; \*\*, comparison of task repetitions in each muscle; and \*\*\*, comparison of task repetitions overall); *p* < 0.05.

#### **4. Discussion**

The purpose of the present preliminary in vivo study was to evaluate muscle activation and fatigue in dentists during tooth preparation and intraoral scanning by performing simulations of the same with two types of dental UCS. The null hypothesis of our study was partially rejected (*p* > 0.05). There was no significant difference between muscle activity and fatigue with the two types of dental UCS (*RMS EMG*: *p* = 0.237 and *p* = 0.543; muscle fatigue: *p* = 0.228 and *p* = 0.287; Tables 1 and 2), and there was no significant difference between muscle activity and fatigue with the two types of simulations (*RMS EMG*: *p* = 0.147; muscle fatigue: *p* = 0.435; Tables 1 and 2). Repetitive learning of the intraoral scanner had no effect on muscle activity (*p* = 0.639; Table 3) but had a significant effect on muscle fatigue (*p* < 0.001; Table 4).

The learning effect (reduction in working time) according to repeated learning with the intraoral scanner has been confirmed in previous studies [26–28]. Similarly, in the present study, a significant decrease in the working time was observed with repetition of the intraoral scanning task (*p* < 0.001). In the previous study, the mean time of full-arch scanning using the intraoral scanner was reported to be 1255 s [29], but in the present

study, the mean time was 509.6 s. This difference in scan time is due to rapid advances in intraoral scanners and the shift toward digital workflows. Although the task time was shortened, muscle activation was confirmed to be the same during the three repetitions due to the quantitative amount of the same task (*p* = 0.639; Table 3). However, contrary to the results of muscle activation, muscle fatigue showed significant accumulation after three repetitions (*p* < 0.001; Table 4); in particular, significant accumulation of muscle fatigue was confirmed in the arm (FDS: *p* = 0.043) and neck muscles (SCM: *p* = 0.027) after three repetitions (Table 4).

The weight of the intraoral scanner has been found to range from 113 g to 585 g [17]. In addition, because the manufacturing process of dental prostheses is being digitalized, the use of intraoral scanners is increasing. Therefore, considering the weight and increasing use of the intraoral scanner, it becomes necessary to evaluate muscle activation and fatigue. To the best of our knowledge, the present study is the first to evaluate this. The weight of an intraoral scanner used in the present study was 280 g. Our results suggest that continuous and repetitive intraoral scanning tasks should be avoided, and sufficient rest is important after an intraoral scanning task. In a previous study, a difference in muscle activation was observed with the type of muscle involved in performing the task [8–10]. Contrary to these results, a previous study reported that there were no significant differences in elbow or shoulder pain in 110 participants using either a light wide-handle curette or a narrow-handled heavy curette for scaling in 16 weeks [30].

In the present study, the intraoral scanning task and the tooth preparation task both showed a low risk level only in the SCM and a moderate risk level in the other muscles (Table 1). In the present study, high muscle activation was observed in the shoulder muscle (T) during the intraoral scanning task and in the two arm muscles (EDC and FDS) and in the shoulder muscle (T) in the tooth preparation task (Table 1). A previous study reported that a force of 0.9 N or more is applied to the teeth during tooth preparation for a desired shape [28]. Therefore, it can be inferred that the high activation of the arm muscles (EDC and FDS) during the tooth preparation task in the present study was because of gripping the dental ultra-fast handpiece and pressing it against the teeth (Figure 3). In addition, because the intraoral scanner is heavier than the high-speed dental handpiece [17], it can be inferred that the shoulder muscle (T) showed relatively high muscle activation during the intraoral scanning task compared to that during the tooth preparation task (Figure 3).

A previous study reported a difference in the neck muscle activation depending on the posture of the dentist when observing the oral cavity [8]. The posture for observing the oral cavity was corrected through the use of magnification lenses, and this lowered the activation of the neck muscles [8]. A previous study reported that the use of an ergonomic saddle and a dental magnifying glass improved working posture [31]. In a previous study, it was reported that the vision of an operator may accompany changes in the head and neck posture, which may affect the EMG [32]. In the present study, it was observed that activation of the neck muscle (SCM) increased during the intraoral scanning task compared with that during the tooth preparation task (Figure 3). This is because the intraoral scanning task is performed while observing a separate monitor while the scan is in progress, and the tooth preparation task is performed by bending the neck to observe the oral cavity (Figure 2). Muscle fatigue occurred regardless of the muscle type in both the intraoral scanning and tooth preparation tasks (Table 2). Therefore, it is important to note that activation of the neck muscles can be increased during the tooth preparation task [8], and sufficient rest is required after the task.

According to previous studies, various designs for dental UCS have been considered to help dentists provide treatment in the dental clinical environment [1–3]. In the present study, the design of the dental UCS had no effect on muscle activation and fatigue (*p* > 0.05; Tables 1 and 2). Therefore, before performing each task, the participants adjusted the dentist's chair and the patient's chair according to their needs. Since both types of dental UCS used in the present study were adjusted for body type and convenience, it is presumed that the difference in dental UCS did not affect muscle activation and fatigue.

The present preliminary in vivo study has several limitations. First, the following variables were not considered during the simulation: postures, other than sitting, for treatment; various types of teeth involved in tooth preparation tasks; and types of highspeed dental handpieces and intraoral scanners. The mannequin used in the present study was difficult to reflect the patient's oral environment. In actual clinical practice, the oral cavity does not remain fixed even if the patient cooperates. Moreover, the muscle tone associated with the presence of temporomandibular joint disorder can affect the degree of opening of the mouth, which can affect the dentist's posture. This is a preliminary in vivo study, which has limitations in experimental configuration, and the findings should be further verified through additional studies. Second, although the sample size was determined by referring to a previous study [10], the present study included a small number of participants (six participants). In the present study, various factors were controlled for, and only participants who had a high willingness to participate, were very cooperative, and had a high understanding of its purpose were included. In addition, it is difficult for participants recruited in the present study to represent the results of various age and sex groups [33]. With increasing age, musculoskeletal disorders may increase, which may affect muscle fatigue and activation during certain activities. Finally, factors that may affect fatigue and muscle activation during work activities were not considered: subjective working positions, vision, practitioner parafunctions and bad habits, type of services performed, daily working hours, individual physical activity, degree of experience in the use of specific dental equipment. Conversely, a long-term clinical trial should be conducted by increasing the number of participants.

#### **5. Conclusions**

The difference between the two types of dental UCS did not affect muscle activation or fatigue. In addition, similar muscle activation and fatigue were observed during intraoral scanning and tooth preparation. However, in the present in vivo study, a moderate risk level of muscle activation was confirmed in the arm muscle (EDC) and shoulder muscle (T), and successive and repeated use of the intraoral scanner may have caused an increase in the muscle fatigue. Therefore, to reduce the occurrence of MSDs in dentists, it is recommended to take appropriate rest after performing continuous intraoral scanning and tooth preparation tasks. In addition, further studies are needed considering the number of participants and factors affecting fatigue and muscle activation during work activities.

**Author Contributions:** K.S. contributed to the conception and design, analysis, and writing of the original draft; Y.-T.S. contributed to data acquisition and interpretation; J.-M.L. contributed to data acquisition and interpretation; J.-W.K., M.-U.J. and K.-B.L. contributed to supervision and project administration. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was financially supported by the Ministry of Trade, Industry, and Energy (MOTIE) and the Korea Institute for the Advancement of Technology (KIAT) through the National Innovation Cluster R&D program (P0016241\_User-friendly chair unit development for digital information provision).

**Institutional Review Board Statement:** The study protocol was approved by the Kyungpook National University Dental Hospital Institutional Review Board (approval number: KNUDH-2021-04- 04-00). All methods were carried out in accordance with relevant guidelines and regulations. The informed consent was obtained from all subjects.

**Informed Consent Statement:** Informed consent was obtained from all subjects involved in the study. Written informed consent has been obtained from the patients to publish this paper.

**Data Availability Statement:** The datasets used and/or analyzed during the current study are available from the corresponding author on reasonable request.

**Acknowledgments:** The authors thank the researchers at the Advanced Dental Device Development Institute, Kyungpook National University, for their time and contribution to the study. This research was financially supported by the Ministry of Trade, Industry, and Energy (MOTIE) and the Korea Institute for the Advancement of Technology (KIAT) through the National Innovation Cluster R&D program (P0016241\_User-friendly chair unit development for digital information provision).

**Conflicts of Interest:** The authors declare no conflict of interest.
