Objective  To investigate the value of high-frequency ultrasound (HFUS) for the quantitative assessment of the nail unit and periungual tissues in healthy adults and to analyze the influencing factors of HFUS measurements.

Methods  A total of 210 healthy volunteers were recruited. HFUS was used to measure the thickness of the dorsal and ventral nail plates, nail bed, nail matrix, skin over the distal interphalangeal (DIP) joint, and terminal extensor tendon, as well as to assess nail bed vascularity using Doppler grading, across all fingers of both hands. A linear mixed-effects model was applied to analyze the effect of these factors on the thickness measurements of the nail unit and periungual tissues. Correlations of age and blood flow rating with the thickness measurements were further analyzed.

Results  Except for the skin over the DIP joint, thickness measurements of all structures were higher in males than in females, with the greatest difference observed in the nail matrix at approximately 0.18 mm (95% CI: 0.10-0.26 mm, P < 0.001). The thickness of the dorsal nail plate, nail bed, nail matrix, and terminal extensor tendon increased with age (P < 0.05). Except for the ventral nail plate, all structural thickness measurements were greater in the right hand than those in the left hand, with the most pronounced difference observed in the nail matrix at approximately 0.07 mm (95% CI: 0.06-0.09 mm, P < 0.001). The thicknesses of all measured structures showed a decreasing trend from the thumb to the little finger, with the thumb showing the highest thickness measurements, and the differences between the thumb and each of the other fingers were statistically significant (P < 0.05). Age and sex had the most pronounced effects on the thickness of the nail bed, nail matrix, and terminal extensor tendon, with the difference between males and females aged 41 to 60 years reaching 0.30 mm (95% CI: 0.19-0.40 mm, P < 0.001) for the nail matrix. The median thickness ranges of the dorsal nail plate, ventral nail plate, nail bed, nail matrix, skin in the region of the DIP joint, and terminal extensor tendon were 0.20-0.30 mm, 0.10 mm, 0.90-1.40 mm, 1.80-2.50 mm, 0.60-0.70 mm, and 0.60-1.40 mm, respectively. A positive correlation was found between nail bed thickness and nail matrix thickness (r = 0.61 95% CI: 0.51-0.69, P < 0.001). No significant differences in nail bed vascularity grading were observed regarding sex or age (P > 0.05).

Conclusion  HFUS enables systematic assessment of the thickness of the nail unit and surrounding tissues in healthy adults. In this study, comprehensive and stratified normative reference ranges for these measurements were established. The reference ranges are significantly influenced by anatomical and demographic factors, including sex, age, left or right hand, and the specific finger, and may provide a reliable basis for the objective diagnosis of nail diseases and the monitoring of treatment response.