This clinical trial was approved by the Clinical Trial Ethics Committee of Kyungpook National University Dental Hospital (IRB No. KNUDH-2021-04-04-00). The study was conducted in compliance with the 1964 Helsinki Declaration and its subsequent amendments. Before inclusion, all participants provided informed written consent.

The sample size was determined using power software (G*Power version 3.1.9.2; Heinrich-Heine-Universität Düsseldorf), with 14 individuals selected for each IOS group based on a previous pilot study with 3 participants who utilized the same method (actual power = 99.8%; power = 99%; α = 0.05). Recruitment for the clinical trial commenced in June 2021, targeting clinicians who graduated from the School of Dentistry and had experience in creating dental prosthetics using digital dental workflows. The clinical trial began in October 2021, with all evaluations being carried out within the School of Dentistry. The trial was successfully completed in November 2022.

The participants recruited were right-handed male and female dentists with experience in digital dentistry, specifically in producing dental restorations using IOS and digital dental workflows. The study included eight men and six women with a mean age of 29.7 ± 4.1 years, height of 169.1 ± 5.5 cm, weight of 67.2 ± 10.1 kg, and clinical dental experience of 3.0 ± 1.5 years. The detailed physiological data are presented in Table 1. The experimental order was randomly assigned to each participant. Participants were randomly allocated using the program (Random Allocation; Isfahan University of Medical Sciences, Isfahan, Iran).

All participants had no history of musculoskeletal disorders related to the musculoskeletal system. Comprehensive interviews focusing on musculoskeletal disorders were conducted with the participants. The questions included: Have you experienced any musculoskeletal pain or fatigue during recent dental clinical activities? If there was fatigue, did it persist beyond nocturnal rest? Participants who reported persistent musculoskeletal discomfort even after rest were excluded from the study. The questionnaire was meticulously designed to identify symptoms related to musculoskeletal issues commonly exacerbated by intraoral scanner use, focusing on the neck, shoulders, arms, and back. This exclusion criterion aimed to minimize confounding variables and ensure that observed effects on muscle activation and fatigue were directly attributable to IOS usage.

To minimize the potential impact of muscle thickness, which may vary by gender, participants were selected to ensure similarity in height and age. Although the present study involved the same participants using two different types of IOS, the relative effect of muscle thickness was deemed less significant. To counter any potential influence of muscle morphology or thickness on cumulative fatigue, each session included a 10-min rest period, and sessions using different scanners were conducted after a full day's rest. Direct fatigue levels were monitored through interviews, with experiments proceeding only if participants reported no cumulative muscle fatigue.

To monitor muscle activity during dental procedures, the placement of electrodes used in previous studies was referenced [6,7,8,9,10, 13,14,15,16]. Surface EMG measurements were performed on various muscles, including the arm (EDC and FDS), neck (SCM and SC), and shoulder muscles (T) (Fig. 1). Each muscle was measured using a pair of 20-mm diameter electrodes attached to the surface of the skin using a pre-gelled adhesive (Covidien, Mansfield, USA). Before the electrodes were applied, the skin was cleaned using a 70% isopropyl alcohol swab to ensure proper adhesion. The electrodes were placed over the muscle fibers according to the Surface Electromyography for the Non-Invasive Assessment of Muscles (SENIAM) protocol, with a 20-mm distance between two electrodes [6,7,8,9,10]. The grounding electrode was attached to the coracoid process of the left scapula (Fig. 1) [6,7,8,9,10].

Schematic representation of electrode attachment positions. EDC, extensor digitorum communis; FDS, flexor digitorum superficialis; SCM, sternocleidomastoid muscle; SC, splenius capitis; T, trapezius descendens

In this study, four specific muscles were selected based on previous research investigating the ergonomics of dentists during intraoral dental work [16,17,18,19]. The EDC and FDS muscles in the arm, responsible for finger and hand extension, respectively, were chosen due to their frequent use during intraoral scanning tasks. The SCM and SC muscles in the neck were selected because they are commonly employed to maintain head and neck posture during dental procedures. The T is utilized to elevate the shoulder, supporting heavy medical devices carried over from the arm. The EDC electrode was positioned between the lateral epicondyle of the humerus and the styloid process of the ulna, while the FDS electrode was placed at the midpoint of the medial border of the coronoid process of the ulna and the medial epicondyle of the humerus (Fig. 1) [11, 12]. The SCM electrode was attached to the sternal portion of the muscle, at one-third of the distance from the mastoid process to the sternal notch (Fig. 1) [11]. The SC electrode was placed at the midpoint between the mastoid process and the seventh cervical vertebra, and the T muscle electrode was positioned at the midpoint between the acromion and the seventh cervical vertebra (Fig. 1) [11]. These specific muscle groups and electrode placements were chosen to accurately assess the muscle activation and fatigue levels during intraoral scanning procedures, providing valuable insights into the ergonomic risks associated with the use of wired and wireless IOSs.

The electrodes were connected to an EMG measurement system (WEMG-8; LAXTHA). The signal from each channel was amplified to 244 μV through the EMG preamplifier in the measurement system and digitized at a sampling rate of 1000 Hz using an AD converter.

A common approach to normalize EMG data is to use MVC measurements. MVC measurements were conducted according to the guidelines established by the SENIAM Protocol [12, 15] and were performed with participants seated on a chair that provided back support. For measurements involving the arm muscles, the participant’s forearm was placed on a desk, and the elbow was flexed at a 90° angle. To measure the EDC muscle in the arm, maximal resistance was applied during hand and finger extension, whereas the FDS muscle was measured using a grip strength meter with maximal force applied to the fingers and palms. The SCM muscle in the neck was measured by providing maximal resistance during head rotation to the left and right with both arms lowered, whereas the SC muscle was measured by providing maximal resistance when tilting the head down and then lifting it up. The T muscle in the shoulder was measured by providing maximal resistance while lifting the shoulder. Three trials were conducted for each muscle, with a 5-s interval between each trial. The MVC value was then determined by selecting the highest value, which was used to normalize the EMG activity.

After the MVC measurement, participants performed intraoral scanning simulations on a dental manikin and typodont (Simple Manikin III, NISSIN) installed in a dental unit chair system (MEGAGEN). Muscle activation was recorded using eight EMG channels. Wireless and wired IOS (i700; MEDIT) were utilized for intraoral scanning simulations (Fig. 2). According to the manufacturer, the wireless type is an IOS with a module added for wireless data transmission to the same optical system as the wired type, and there is no difference in appearance, such as the size of the scan tip, other than the weight difference (328 g for wireless IOS and 280 g for wired IOS). A computer system with specifications superior to those recommended by the manufacturer was employed, as computer specifications can significantly impact scanning speed. All experiments with wireless and wired IOS (i700; MEDIT) were conducted using the same software version (MEDIT) on the same computer. Participants adjusted the patient chair and dental stool to ensure comfort during the intraoral scanning process. The wireless IOS was fully charged prior to use, with the scanning procedure displayed on a chairside monitor of dental unit chair system connected to a laptop (Fig. 3). In contrast, the wired IOS, due to its cord constraints, projected the scanning process directly onto the laptop (Fig. 2).

Intraoral scanners (IOS) used in this study. A Wired IOS. B Wireless IOS

Scanning simulation using a wireless intraoral scanner

A total of 14 participants received approximately 30 min of education on the operation of the two types of IOSs and performed one practice session per IOS (Fig. 2). The experimental order was randomly assigned to each participant, and the next type of IOS experiment was conducted after a rest day. Furthermore, the health status of each participant was assessed, and through interviews, experiments were carried out when participants were in their optimal condition. Recognizing the potential impact of intraoral scanning proficiency on muscle fatigue, this study selectively recruited participants with approximately 3 years of general dental clinical experience and a minimum of one year of specialized experience in intraoral scanning. Additionally, the duration of work was strictly limited to no more than 10 min per session. This constraint was rigorously monitored and enforced by the investigators to ensure compliance and participant awareness.

Wireless and wired IOSs were performed for four repeated tasks per participant (Fig. 3). The decision to limit the number of operations to four per IOS was based on findings from a pilot study aimed at determining adequate sample sizes. It was observed that conducting more than four tasks in a single day resulted in fatigue that could not be alleviated even with breaks longer than 10 min. Consequently, it was established that more than four tasks per session would compromise the validity of the results due to excessive fatigue accumulation. To prevent cumulative muscle fatigue in each session, a break of at least 10 min was taken before moving on to the next session, which was conducted only when the participant did not feel fatigued. The scanning process began by scanning the maxilla of the typodont model prior to scanning the mandible. The scan strategy was to scan from the left maxillary second molar to the right maxillary second molar in the occlusal and cross-sectional directions, then sequentially scan in the contralateral direction and complete the maxillary scan in the ipsilateral direction. The complete mandibular arch was scanned using the same scan strategy as that used for the maxillary arch. The participants consecutively checked for defects in the scanned area during the scanning process and completed the complete arch scan to ensure that there were no holes in the tooth area. An investigator monitored the participants’ scanning process in real-time and observed the inappropriate use of the IOSs. Additionally, one investigator (K.S.) recorded muscle activation in real-time only when the participant performed an action for simulation and did not record muscle activation otherwise. Working time for both wired and wireless IOSs was meticulously recorded, focusing exclusively on the periods when participants actively engaged with the IOS, in alignment with muscle activation recording. The working time was noted as the duration required to complete the scan in each of the four repetitions.

Muscle activation and fatigue were evaluated using an EMG measurement software (EMG-Works 4.0; Delsys Inc) during the simulated dental tasks. EMG data were normalized and expressed as a percentage of each muscle’s MVC using the following formula [6,7,8,9,10, 15]:

$$RMS\, EMG(\%MVC) = \frac\right)}\times 100.$$

The root mean square (RMS) EMG (%MVC) represents the level of muscle activation during tasks relative to MVC. A higher RMS EMG (%MVC) value may indicate an increased risk of musculoskeletal disorders. Previous research classified ergonomic risk level based on the level of activation of each muscle: 0–10% MVC indicates “low risk”, 11–20% indicates “medium risk”, and 21% or higher indicates “high risk” [6,7,8, 10, 15].

Muscle fatigue was evaluated by analyzing the median frequency (MF) of the EMG signal, with a decrease in MF indicating an increase in muscle fatigue [10,11,12]. The MF was obtained by applying a fast Fourier transform to the EMG signal and calculating it within a frequency range of 20–500 Hz. Muscle fatigue was calculated by comparing the MF in the second half of the total work time to the MF in the first 60 s using the following formula [6,7,8,9,10, 15]:

$$Muscle\, fatigue\left(\text\right) = \frac\times 100.$$

If the MF in the second half of 60 s is lower than that in the first 60 s, resulting in a negative value, muscle fatigue is considered to have increased [6,7,8,9,10,11]. In this study, to analyze EMG activity across consecutive tasks, the mean graph of the median frequency of EMG signals was derived for a 60-s interval at the midpoint of each task's inception and conclusion.

All data were analyzed using statistical analysis software (Statistical Package for the Social Sciences ver. 25.0; IBM) with a significance level of α = 0.05. First, the normality of the data was examined using the Shapiro–Wilk test, which indicated that the data did not follow a normal distribution. The Wilcoxon rank-sum test was used to compare the wired and wireless IOSs in terms of EMG and muscle fatigue. To compare the differences in EMG and muscle fatigue during repeated intraoral scanning simulations in the fourth session, the Friedman test was used with a significance level of α = 0.05. The Bonferroni correction was applied for multiple comparisons, with a significance level of α = 0.05.

To determine the influence of consecutive usage and working duration on muscle activation and fatigue, a correlation analysis was conducted (α = 0.05). Spearman’s rank correlation coefficient was used for this analysis. Correlation strength was categorized as follows: a coefficient of ± 0.3 or lower signified a slight correlation, ± 0.3 to 0.5 indicated a low correlation, ± 0.5 to 0.7 suggested a moderate correlation, ± 0.7 to 0.9 represented a high correlation, and a coefficient of ± 0.9 or higher denoted a very high correlation [41]. Multivariate variance tests with partial eta-squared from the Kruskal–Wallis and Mann–Whitney U tests were performed to determine the effects of independent variables and interactions.