**1. Introduction**

In dental clinical practice, the use of a dental unit chair system (UCS) is essential for patient diagnosis and treatment [1,2]. Dentists spend most of their work time in the dental UCS for patient care [3]. The dental UCS consists of an operating light and a patient seat, foot controller, water fountain and cuspidor, monitor, bracket table, and dentist's chair [4]. In addition, the dental UCS has been developed to facilitate the use of various dental medical devices and treatment tools [5,6].

Musculoskeletal disorders (MSDs) frequently occur among dental practitioners [7]. It is very difficult for a dentist to adopt an optimal working position because of the limited working space and long duration of treatment [8]. In addition, a high degree of concentration is required by the dentist during treatment resulting in a static posture being maintained for a long time [9]. In the process of maintaining a static posture, the parts of the dentist's body most affected are the back, shoulders, and neck [10,11].

**Citation:** Son, K.; Lee, J.-M.; Son, Y.-T.; Kim, J.-W.; Jin, M.-U.; Lee, K.-B. How Does the Use of an Intraoral Scanner Affect Muscle Fatigue? A Preliminary In Vivo Study. *Bioengineering* **2022**, *9*, 358. https://doi.org/10.3390/ bioengineering9080358

Academic Editors: Christina Zong-Hao Ma, Zhengrong Li and Chen He

Received: 8 July 2022 Accepted: 29 July 2022 Published: 1 August 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Electromyography (EMG) is a method for measuring electrical signals generated in the skeletal muscles to quantitatively evaluate the magnitude of muscle fatigue or exerted strength [9–12]. Since EMG evaluation can diagnose the functional abnormalities of muscles, it is widely used in various fields, such as medical research, rehabilitation medicine, sports science, and design engineering [9–12]. Muscle fatigue refers to a temporary decrease in the ability of a muscle or muscle group to generate force or perform physical activity and is an essential factor affecting working efficiency [10–12]. Therefore, muscle fatigue is highly correlated with muscle EMG activity and the root mean square (RMS) of EMG [10,11].

Several muscle groups, including the arms, neck, shoulders, and back, are activated during dental work. The arm muscles, flexor digitorum superficialis (FDS), and extensor digitorum communis (EDC) are activated during bending of the wrist and application of force for gripping dental instruments [13]. The sternocleidomastoid muscle (SCM) is involved when turning the head, and the splenius capitis (SC) is involved when bending the head to observe the patient's mouth [9,10,12–15]. The trapezius descendens (T), which is used to raise the shoulder, has also been frequently used for assessment of EMG in dentists [9,10,12–15]. Therefore, it is important to reduce or prevent MSDs in the aforementioned muscles. There are several examples of application of ergonomics in dentistry, including in the patient chair, operator chair, operating light, hand instrumentation, and cabinetry.

Recently, as the application of dental computer-aided design and computer-aided manufacturing (CAD/CAM) has rapidly increased. The use of intraoral scanners has also increased [16]. Although manufacturers have reduced the weight and size of intraoral scanners for usability, these scanners are still one of the heaviest medical devices used directly in the oral cavity [17]. The weight of the intraoral scanner suggested by the manufacturer generally ranges from 113 g to 585 g; the scan time is more than five minutes per complete arch and the device is used repeatedly [17,18]. Although studies have reported the evaluation of EMG when a dentist performs tooth preparation using a high-speed handpiece [9,11], there have been no reports on the effect of intraoral scanner use on the dentist's MSDs.

There is a need for further research on muscle activity and fatigue considering MSDs in various dental practices. Therefore, the purpose of this study was to evaluate muscle activation and fatigue in the operator during tooth preparation and intraoral scanning by simulating these two tasks in the two types of dental UCS. The null hypothesis of this study was that there is no significant difference in muscle activity and fatigue between the two types of dental UCS and the two types of tasks (tooth preparation and intraoral scanning). Additionally, we hypothesized that there is no difference in muscle activity and fatigue caused by repeated use of the intraoral scanner.

#### **2. Materials and Methods**

#### *2.1. Participants*

This clinical trial was approved by the Clinical Trial Ethics Committee of Kyungpook National University Dental Hospital (IRB No. KNUDH-2021-04-04-00). Right-handed participants with no history of MSDs were recruited. The study inclusion criteria specified that individuals with right-handedness or who presented with musculoskeletal disorders were excluded. The study exclusion criteria specified that individuals with musculoskeletal disorders, sensory or mental abnormalities, debilitating medical conditions, and/or who were pregnant, or lactating were not eligible for assessment in this study. For blinding, all participants did not know the purpose of the present study, and the experiment was performed only according to the instructions of one investigator. The sample size was calculated as at least four participants per group based on the results of a previous study [10] (G\*Power version 3.1.9.2; Heinrich-Heine-Universität Düsseldorf, Düsseldorf, Germany) (actual power = 99.11%; power = 95%; α = 0.05); the present study included six participants per group. The mean age of the participants was 31.5 ± 3.9 years. The participants had a

mean height of 170 ± 6.2 cm, mean weight of 66.3 ± 10 kg, and a dental clinical experience of 3.6 ± 1.1 years. The six participants consisted of two women and four men.

#### *2.2. Data Collection: Ag/AgCl Electrode Placement on Sampled Muscles*

The present study refers to the location for evaluation of MSDs that develop during dental treatment in the dental UCS as observed in previous studies [6–11]. The muscles to be assessed for surface EMG, EDC, and FDS were the arm muscles; neck muscles (SCM and SC); and shoulder muscle (T) (Figure 1). For the arm muscles, a pair of 20 mm diameter silver or silver chloride solid adhesive pre-gelled electrodes (Covidien, Mansfield, MA, USA) were attached only to the right hand to perform the task (Figure 1). For the other muscles, the electrodes were symmetrically attached to the left and right sides (Figure 1). Before attaching the electrode, the attachment site was made free of excess hair and thoroughly washed with a 70% isopropyl alcohol swab. According to the guidelines of the surface electromyography for the non-invasive assessment of muscles (SENIAM) protocol for each muscle location, two electrodes were attached to the movement point of each muscle in the direction of the muscle fiber [19]. The center distance between the two electrodes was 20 mm, and the ground electrode was attached to the sphenoid process of the left ulna (Figure 1) [19].

**Figure 1.** Schematic of the electrode attachment position for electromyography. EDC, extensor digitorum communis; FDS, flexor digitorum superficialis; SCM, sternocleidomastoid muscle; SC, splenius capitis; T, trapezius descendens.

For the EDC, the electrodes were attached to the quarter point between the lateral epicondyle of the humerus and the styloid process of the ulna (Figure 1) [20,21]. For the FDS, the electrodes were attached to the quarter of the medial border of the medial epicondyle of the humerus and the coronoid process of the ulna (Figure 1) [20]. For the SCM, the electrodes were attached at the third point between the mastoid process and the sternal notch toward the sternal portion of the muscle [20]. For the SC, the electrodes were attached to the midpoint between the mastoid process and vertebra C7. For the T, the electrodes were attached to the midpoint between the acromion and vertebra C7 (Figure 1) [20].

After electrode placement, the electrode was connected to an EMG measuring system (WEMG-8; LAXTHA, Daejeon, Korea). In the measurement system, each channel was

amplified to 244 µV through the EMG preamplifier, and the analog and digital signals were converted to a 10-bit resolution through the AD converter. The sample was collected at a sampling rate of 1024 Hz. Real-time EMG measurement software (TeleScan ver 3.29; LAXTHA, Daejeon, Korea) was used to collect real-time EMG data.
