A novel noncontact optical coherence tomography based air-jet indentation system was developed for characterizing the biomechanical properties of soft tissue in a noncontact way. This study aimed to measure the stiffness of diabetic foot ulcer tissues by using this air-jet indentation system, and examining the test/retest reliability. Eight subjects with diabetes (seven males, one female), with a total of 10 foot ulcers between them, participated in the study. A total of 20 measuring sites located at the central wound bed (n=10) or peri-ulcer areas (n=10), respectively, were evaluated with the air-jet indentation system. Four cycles of loading and unloading, each with a duration of approximately 36 seconds at an indentation rate of 0.08 mm/s, were carried out for each indentation trial. The test/retest reliability was examined at all measuring points. The average stiffness coefficient of the peri-ulcer area (mean±SD: 0.47±0.15N/mm) was significantly larger than that of the central wound bed area (mean±SD: 0.35±0.23N/mm; p=0.042). A high value for test/retest reliability was shown (intraclass correlation coefficient: 0.986; Pearson's correlation: r=0.972, p < 0.001). Our preliminary findings showed that the peri-ulcer area had greater stiffness than the central wound bed. This greater magnitude of hardness and inelasticity at the peri-ulcer region may scatter part of the contractile forces for wound contraction during the healing process. We found the novel air-jet system to be a reliable tool for characterizing the stiffness of soft tissues around the wound in a noncontact way.
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