A Proposal for Determining a Diagnostic Cutoff Value of a New Ultrasonographic Index of Retronychia Inversely Correlating with Disease Severity

Background Ultrasonography has demonstrated a shortening of the distance between the origin of the nail plate and the base of the distal phalanx in retronychia. The aim of this study was to analyze the clinical and ultrasonographic features of retronychia. Methods We evaluated the clinical findings in 18 patients with retronychia, along with the ratio of ultrasonographic distance a, extending between the nail plate origin and the base of the distal phalanx, to distance b, perpendicular to distance a, extending between the nail plate origin and the upper margin of the distal phalanx. Results Retronychia was present in 26 nails. The mean ± SD distance a was 7.66 ± 1.64 mm and distance b was 3.56 ± 1.95 mm. The mean ± SD a/b ratio was 2.59 ± 1.11. There was a significant inverse correlation between a/b ratio and clinical severity (Pearson correlation = –0.668; P < .001). The cutoff value of this ratio was 3.319, with specificity of 90% and sensitivity of 69%. Conclusions The ratio of distance a /distance b and the cutoff value of this ratio may help in making the diagnosis, in objectively determining the disease severity, and in selecting a patient-specific treatment approach.