Ergonomic Risk and Musculoskeletal Disorders Related to the Upper Limbs in Dental Medicine Students

Background: In light of the high prevalence of musculoskeletal disorders in dentists, an extended knowledge of the phenomena, including the onset time and treatments presenting higher risk, is needed. Thus, this study aimed to 1) assess exposure to ergonomic risk factors, musculoskeletal symptoms and associated functional limitation in dental students, 2) estimate the prevalence of musculoskeletal disorders in dental students and 3) assess the exposure to vibrations associated with the use of an ultrasonic scaler.

Methods: For the ergonomic evaluation, two questionnaires were applied: Assessment of Repetitive Tasks (ART-Tool) and the Strain-Index Score Sheet. The ultrasonic scaler's assessment of exposure to vibratory forces was carried out by a certified engineer using an accelerometer Cubic Triaxial Deltatron attached to a mounting clip attached with cyanoacrylate to the handpiece of the scaler, according to the 3 axes of the hand-arm system (x, y e z). Standardized questionnaires were used to identify symptoms and associated disability.

Results: All the recorded values exceeded the maximum limit of reference. This difference was more accentuated in the Strain-Index. The maximum exposure value is much lower than the limit imposed. An exposure time equivalent to 8 hours of work in a day would still not exceed the maximum action value. As a result of the clinical evaluation, different musculoskeletal disorders were identified.

Conclusion: Ergonomic risk exposure among dental students is a reality. Preventive strategies aimed at reducing the incidence of musculoskeletal injuries in dentists should be initiated at an early stage in their lives, particularly during their academic training.

The prevalence of musculoskeletal disorders of the upper limb in dentists is higher than that found in the general population [1,2]. Several studies demonstrate the exposure of this professional class to numerous ergonomic risk factors, such as emitting sources of mechanical vibrations, vicious postures, sustained and repetitive clamping movements [3,4]. The exposure may start at very early stages of life, even before the beginning of professional activity, namely during academic training and thus increasing the risk of developing occupational disease in the future [5].

In dental students, exposure to the listed risk factors is enhanced by the technical inexperience inherent in their lack of clinical practice, which exposes them to ergonomic risks, namely due to the incorrect handling of some work equipment, such as the ultrasonic scaler [6,7]. In this specific case, the exposure to vibrations in the hand-arm system is influenced by the clamping force exerted when handling the handpiece handle [8], which is often increased in inexperienced populations [9,10]. As a result, dental students are a population more likely to develop musculoskeletal disorders in the future [3-5].

This study aimed to 1) assess exposure to ergonomic risk factors, musculoskeletal symptoms and associated functional limitation in dental students, 2) estimate the prevalence of musculoskeletal disorders in dental students and 3) assess the exposure to vibrations associated with the use of an ultrasonic scaler.

Using a convenience sampling method, students from the 5th year of the integrated master’s in dental medicine at the University of Porto were evaluated.

For the ergonomic evaluation, two questionnaires were applied: Assessment of Repetitive Tasks (ART-Tool) and the Strain Index Score Sheet.

The ultrasonic scaler's assessment of exposure to vibratory forces was carried out by a certified engineer using an accelerometer Cubic Triaxial Deltatron attached to a mounting clip attached with cyanoacrylate to the handpiece (KAVO Sonosoft Lux) of the ultrasonic scaler, according to the 3 axes of the hand-arm system (x, y and z).

Data were collected during the performance of an ultrasonic scaling procedure in full acrylic prostheses, with a duration of 6 minutes, by vibration analyzer Brüel & Kjær 4447 and analyzed with the software Vibration Explorer Version 2.0.1.

The total value of the vibration (a_hv) was calculated through the quadratic sum of the components of the 3 axes (a_hwx, a_hwy, a_hwz). Daily exposure to vibrations (a_hv) was expressed in a 8 hours of use period, represented by A (8), where T is the total daily duration of exposure to vibrations and T0 is the reference duration of 8 hours, following Portuguese legal requirements (Law Decree No. 46 of February 24, 2006). Similarly, it was possible to calculate the amount of exposure to weighted vibration for an estimate of 4 hours per week, that translates the vibration exposure to which the elements of the population under analysis are effectively subjected, using the formula A (8) = a_hv √T/T₀.

The clamping force was estimated using a dynamometer Jamar®. The participants’ forearm was supported on a table, adjusting the chair height, so that it made a 90º angle with the arm. Subsequently, the maximum clamping force was applied to the dynamometer handle. For each arm / hand, three measurements were made before using the ultrasonic scaler and another three immediately after the end of its handling. The values determined for everyone were averaged at each of the two moments. The average values, determined in pounds / force, were then converted to the SI unit, Newton.

Standardized questionnaires were used to identify symptoms and associated disability, Nordic and DASH (Disabilities of the Arm, Shoulder and Hand), and a clinical evaluation consultation was carried out with an occupational physician.

This study was approved by the Ethics Committee of Faculty of Dental Medicine of the University of Porto (2015/01) and complies with the Ethical Principles expressed in the Helsinki Declaration. Informed consent was obtained from all subjects involved in the study.

We evaluated 55 individuals, mostly women (n = 43; frequency = 78%), aged between 22 and 32 years (mean = 22.94 years; standard deviation = 1.82). In the ergonomic evaluation, all the recorded values exceeded the maximum reference limit (Table 1). This difference was much more accentuated in the Strain-Index.

With regard to vibration exposure (Table 2), the maximum exposure value (0.67m/s2) is much lower than the limit imposed by law (Law Decree No. 46 of February 24, 2006) (2.5m/s2) [11].

The clamping force increased after performing the procedure with the ultrasonic scaler (Table 3).

Shoulder symptoms were the most prevalent with about 30% of individuals reporting pain. The intensity of symptoms for the regions assessed varied between 0.09 and 2.76, with the minimum values in the elbow region and maximum values in the shoulder region (Table 4).

The DASH questionnaire revealed a general module with an average value of 6.32, as well as a module related to work and another related to the practice of sports / music with means of 13.55 and 10.57, respectively. As a result of the clinical evaluation, 3 cases of musculoskeletal disease were identified, including carpal tunnel syndrome and epicondylitis.

Exposure assessment of ergonomic risk among dental students exceeded the maximum reference limits, which is in line with the high proportion of musculoskeletal symptoms found in this sample. A possible explanation for the results of ergonomic evaluation was the degree of subjectivity in the interpretation of the analyzed parameters, such as the definition of “breaks” and “cycles” of work. The work positions adopted by dental students put them at a considerable level of ergonomic risk. This conclusion was already achieved in other studies [12,13].

Even if the exposure time was equivalent to 8 hours a day, which makes the 40 hours a week of the normal working period legally stipulated for full-time employment, the maximum value, although approximate, would still not exceed the maximum action value defined by law [11,14]. From the above, the use of the ultrasonic scaler does not appear to be a significant ergonomic risk factor.

The increase in the clamping force was higher after performing the procedure with the scale, although not statistically significant. This can be explained by the reduction in tactile sensitivity, triggered by the vibration of the device and by the establishment of a temporary tonic muscle reflex. Inexperienced workers tend to handle instruments with more force than necessary [8,9,15,16].

The limitations of this study include the way the ultrasonic scaler was used (not in patients, but in prosthetic devices) which may account for some bias. Furthermore, the total cumulative exposure period to the vibratory forces is potentially smaller than the one on typical dentist populations.

Exposure to ergonomic risk among dental students is a reality. Musculoskeletal symptoms are present in a high proportion of this population, with associated pathology diagnosed in 5.4% of cases. Consequently, preventive strategies aimed at reducing the incidence of musculoskeletal injuries in dentists should be initiated at an early stage in their lives, particularly during their academic training. These results must be considered when recognizing future occupational diseases. To reduce the chance of injury, work tasks should be designed to limit exposure to ergonomic risk factors

This research received no external funding

The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the Ethics Committee of Faculty of Dental Medicine of the University of Porto (2015/01).

Informed consent was obtained from all subjects involved in the study.

The authors declare no conflict of interest.

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