Biomechanical characterization of earlobe keloid by ring suction test

Abstract

Introduction Keloid is a fibro-proliferative disease characterized by an overgrowth of scar tissue beyond wound margins. Keloids appear on some specific anatomical sites often believed to be permanently under mechanical load. Besides biological and anatomical factors in keloid growth, the importance of the tissue mechanical environment is well witnessed (Ogawa 2008). Thus, biomechanical studies are still needed to understand the keloid expansion mechanism in order to treat patients or prevent keloid recurrency. External mechanical forces can be used to prevent keloid development or recurrency. In this way, an innovative pressotherapy device (silicon forceps, equipped with magnets) has recently been developed at the University Hospital of Besançon in collaboration with biomedical engineer school of ISIFC (Institut supérieur d'ingénieurs De Franche-Comté). Since 2018, this compression device is under clinical evaluation (Scar Wars study, NCT03312166) at 'Centre d'Investigation Clinique' CIC INSERM 1431 (Lihoreau et al. 2020). The device is applied on earlobe after earlobe keloid surgery in order to prevent from keloid recurrency. A wide mechanical characterization of earlobe keloid would provide us information to improve the efficiency of such pressotherapy, i.e., compression level and load duration. As the ear lobe surface is small, few mechanical tests can be performed within a restricted area, such as indentation and suction tests. Developed by Courage+Khazaka Electronic, the Cutiscan®is a promising tool to quantify the elastic and viscoelastic properties, as well as anisotropy and the tension lines of the human skin in vivo (Rosado et al. 2017). Tough, before investigating the mechanical behavior of earlobe keloid with the latter tool, we apply the ring suction test on a more wide and flat site, for instance, forearm. In this pre-study, we present the validation of Cutiscan®device on healthy flat skin and its relevant sensitivity to environmental conditions. A primary application on the earlobe is presented as well. 2. Methods In a preliminary study, 24 series of ring suction tests with the Cutiscan®probe have been performed on a healthy soft tissue. In each series, 21 negative pressure levels p 2 [100, 500] mbar have been applied within an annular section with inner diameter dinner = 5 mm and outer diameter douter =14 mm. The suction test is conducted over two phases: creep phase, where the pressure is maintained for 3 s, and the unloading phase which lasts 3 s also (Figure 1). During the creep-unloading process, the in-plane displacement of the skin within the central zone is recorded with a uEye®camera and converted onto spatial coordinates through the Digital Image Correlation technique (DIC). In the provided commercial software, in each of 360 directions, only the maximum displacement along that direction is gathered. Hence, the inplane displacement field is not recorded. To overcome this issue, we have adapted an alternative DIC algorithm based on OpenCV (Computer Vision), a graphic opensource Python library. After validation of the experimental process on the healthy soft tissue, the ring suction test has been applied on a keloid scar situated on a young male African-American earlobe skin. In a second time, the ring suction test has been conducted on healthy contralateral earlobe. 3. Results and discussion In healthy soft tissue cases, we calculated mean displacements and their relevant standard deviations over 24 test series along the direction 0°: The resultsexhibit a nonlinearity of the mechanical response if we assume a zero-displacement for zero-load. Also, the measurement uncertainty increase for large deformation. By comparing mutually between the 24 data series, we have identified many factors occurring in varying roughly the measurements: humidity, probe stability, DIC process uncertainty and optical artifacts. As an example, we have observed that wet skin is stiffer because of stored water. Consequently, the better option to reduce measurement sensitivity is to set a standardized stable protocol with well-controlled tools and devices, by taking into account the mentioned factors earlier. For that purpose, we have developed a stable probe holder. Concerning the earlobe case, an additional issue has been noticed: its diameter is sometimes smaller than the ring suction diameter, which makes the aspiration non-operational. The use of a smaller probe is mandatory in this situation. In Figure 2, the primary application of Cutiscan on earlobe highlights several characteristics of the mechanical behavior of human skin and keloid. Whatever the materials, the displacement curves exhibit an anisotropic and viscoelastic response. For each direction, the elastic displacement in healthy skin is higher than in keloid. Besides, an increasing displacement for a constant load, from 0:5 s to 3 s, is witnessing the dissipation phenomenon. 4. Conclusions In this work, a biomechanical characterization process based on a ring suction test has been introduced. It has been applied on healthy soft tissue as a validation phase, and on a young male African-American earlobe skin, with and without keloid, as a primary study. The next steps consist in standardizing the clinical protocol and conducting an experiment of ring suction test to characterize earlobe mechanical properties.