Evaluation of digital and skinfold caliper measurements of the Achilles tendon width

Background: A critical review of the literature shows that very few objective methods for clinically evaluating Achilles tendinopathy have been described. Aim: To evaluate the validity and reliability of common digital and skinfold calipers in measuring the Achilles tendon width. Methods: Reliability study of ten healthy adult volunteers (five males and five females) was performed at the Rheumatology Unit of Barzilai Medical Center, Ashkelon, Israel. Achilles tendon width was measured by ultrasound at two points of both legs (the tendon attachment and 5 cm proximally). Using regular and skinfold calipers, two further measurements were made, three hours apart, at the same sites. Results: Test-retest reliability was high for the skinfold caliper at both the upper (intraclass correlation coefficient (ICC)=0.863, p<0.001) and lower (ICC=0.931, p<0.001) points, and moderate for the regular caliper at upper (ICC=0.730, p<0.001) and lower (ICC=0.641, p<0.001) points. Moderate association was found between ultrasound and caliper measurements at the lower point (Spearman’s rank correlation coefficient (rho)=0.721, p=0.019 for regular calipers; rho=0.646, p=0.043 for skinfold calipers). At the upper point, the associations were high (rho=0.778, p=0.008 for regular calipers; rho=0.960, p<0.001 for skinfold calipers). Conclusions: The skinfold caliper showed a higher correlation with ultrasound measurements and higher intraobserver reliability than the regular caliper. Therefore, the skinfold caliper can be recommended in clinics and trials when repeated measurements need to be performed.