Thumb Pronation Angle in Thumb Opposition Evaluated by the Nail Tip Angle of Thumb-Ring Finger Opposition

Background: This study aimed to devise the method for the evaluation of the thumb pronation angle and calculate its reference value.Methods: The study was conducted from October 2021 to March 2022, and 600 hands of 300 healthy volunteers were included in this study. The participants were divided into 6 age groups with 50 participants each. In study 1, 12 participants from group 1 were randomly selected, which included 6 male and 6 female participants. We measured the thumb pronation angle using FASTRAK system and the nail tip angle of thumb-ring finger opposition (T-R angle) and also evaluated the relation between them. In study 2, we measured the T-R angle of 600 hands of 300 healthy volunteers. The T-R angle mean (SD) and 95% confidence interval (CI) of each age group were calculated as reference values.Results: The thumb pronation angle was correlated with the T-R angle. The T-R angle mean (SD) and 95% CI of the 6 groups were as follows: group 1, 142.5 degrees (12.4) and 117.7 to 167.3; group 2, 139.9 degrees (10.1) and 119.7 to 160.1; group 3, 142.3 degrees (9.7) and 122.9 to 160.5; group 4, 140.2 degrees (12.6) and 115.0 to 165.4; group 5, 138.3 degrees (11.9) and 114.5 to 162.1; and group 6, 135.4 degrees (13.0) and 109.4 to 161.4. The 95% CI tended to decrease with an increasing age.Conclusions: The T-R angle was a suitable evaluation method for the thumb pronation angle, and it should be evaluated by age groups.