Int. J. Environ. Res. Public Health2021, 18, 9171 9of 15 4. Discussion We chose to survey a population of actively employed surgical device mechanics and compared them with a group of employees believed not to be exposed to repetitive hand and arm movements to such a large extent. The overall prevalence of subjective upper extremity complaints (i.e., symptoms) was 47% (33/70). Eight out of 20 (40%) grinders, 14/24 (58%) packers, and 11/26 (42%) people in the control group reported such symptoms. One or more upper extremity WMSDs (i.e., diagnoses) at the elbow, forearm, and/or wrist were found in 56% (39/70), of which 12/20 (70%) were grinders, 14/24 (54%) were packers, and 13/26 (42%) were control persons. These results are consistent with previous studies showing a prevalence rate of symptoms between 21% and 71% in the study group and between 6.0% and 50% in the control group [5,39–50]. With regard to diagnoses, existing studies report a prevalence between 21% and 56% in the study group compared to 5.0% to 22% in the control group [5,39,41–43,45]. Within the framework of the standardized clinical examination, we applied rather “softer” criteria, as recommended by Vikari-Juntura [51]. This might explain the higher detection of WMSDs via examination as compared to the lower number of subjective complaints reported in the questionnaire. An above average co-occurrence of medial epicondylitis (golfer’s elbow) and nerve entrapment at the medial elbow (cubital tunnel syndrome) was found in 30% of our test persons and 54% of the individuals with upper extremity complaints in this cohort [52–54]. Study designs and research methods used to collect and process data via WMSDs are extremely heterogeneous. Our literature search revealed that, out of 262 original research articles, only about 4.0% of the studies contained a clinical examination of the individuals with or without structured questionnaires/interviews including a control group [17]. Most studies were based on self-administered questionnaires and results from health insurance databases without a control group. The clinical endpoint values of our study population were largely consistent with the reference values of the general population, but in some cases (grip strength, PPB tests) also showed below average values. This is surprising and contradicts the study situation, as our cohort tends to have an above-average physical load [55]. The reasons for this could be mechanical support, shorter working hours, and a historical shift in populations’ reference values without the first two points mentioned. A closer look into the scatter plot revealed an obvious floor effect in the DASH distribution (Figure 1). DASH values of probands with WMSDs concentrated more in the ten-point range, whereas the “no WMSDs” group very often had a DASH score of 0. This caused the slight tilt of the ellipse, correlating with the Pearson correlation coefficient, which, in the first approximation, visualized an interrelation between the DASH score and WMSDs (Figure 2). However, the non-normality contradicted the application of the parametric Pearson regression and caused a switch to the non-parametric Spearman correlation. Its coefficient was indeed much more sensitive to this effect, due to its robustness towards non-normal distribution of the DASH values. For the same reason, the normality assumption was not given for ROM and VAS distributions, so the Spearman method had to be preferred for evaluation of interactions between WMSDs and these endpoints. This particular non-parametric correlation analysis with one dichotomous variable is known as rank-biserial correlation. In the bivariate analysis, we found a correlation between the DASH score and WMSDs as well as VAS under strain and WMSDs. A simple clinical explanation for this could be that pathology (WMSDs) manifests itself through pain, especially when the hand is used forcefully. This aspect is a common feature of the DASH questionnaire and also manifests itself with VAS under strain. VAS at rest, ROM, grip strength, the PPB Test, and the indication of subjective complaints by the study participants were not suitable to detect WMSDs in our study. However, the question of subjective complaints is a central component of many studies and the basis for their interpretation.
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