Systemic sclerosis (SSc) is a complex multisystem autoimmune connective tissue disease. SSc has been classified into two subtypes: (1) limited cutaneous systemic sclerosis (lcSSc) in which skin thickness is limited, presenting distal to the elbows and knees, with or without face involvement, and (2) diffuse cutaneous systemic sclerosis (dcSSc) in which the extent of skin involvement presents above the elbows and knees, with or without face involvement . The most common symptom and cause for concern among SSc patients is skin thickening .
The assessment of severity and extent of skin thickness is crucial as it is a surrogate marker of disease activity, severity, and prognosis as well as treatment responsiveness. The methods for skin thickness assessment thus need to be valid, reliable, precise, and practicable [3,4,5,6].
The skin biopsy validated, gold standard for skin thickness assessment for SSc is the modified Rodnan skin score (mRSS) [7,8,9,10,11,12]. The mRSS assesses skin thickness from 17 body sites: the face, chest, abdomen, arms, forearms, hands, fingers, thighs, legs, and feet. A score of 0 indicates normal skin thickness, 1 mild skin thickness, 2 moderate skin thickness, and 3 severe skin thickness with an inability to make skin folds between two fingers. The score is calculated by summing the rating from all 17 areas (range 0–51) [13, 14].
Although the mRSS has been validated at many centers, it has some limitations such as significant inter- and intra-rater variability due to (a) its subjective methodology, (b) physician inexperience, (c) significant differences between ethnic groups, (d) inaccuracies during the edematous and atrophic skin phase, and (e) lack of sensitivity in measuring minimal changes [7, 15,16,17].
To overcome these limitations, researchers have tried to develop new objective and quantitative methods for skin assessment. Mechanical devices and new imaging techniques include the durometer—a handheld device that measures skin hardness [18,19,20]; the plicometer—a medical device that measures skin folding ; the cutometer—a device that measures skin elasticity [22, 23]; the vesmeter—a computer-linked device that measures skin hardness, elasticity, and viscosity ; the twistometer—a device that measures skin rotation ; high-frequency ultrasound—an objective and quantitative tool that measures skin thickness [26, 27]; elastosonography—a tool that measures skin elasticity [28, 29]; shear wave elastography—a tool that measures skin thickness ; magnetic resonance imaging—a tool for demonstrating abnormalities of the skin and subcutaneous tissues [31, 32]; and, optical coherence tomography—a tool that identifies the microscopic features of the skin [33,34,35]. None of these techniques has matched the OMERACT standard of the mRSS for assessing the validity of outcomes; usually, because the techniques are not feasible in clinical practice due to time constraints, accessibility, dependence on trained experts, or lack of clarity defining what aspect of the skin to assess [8, 36].
Since mRSS is a validated outcome in scleroderma, specialist rater training was needed to improve accuracy and reduce variability. Limitations of the mRSS skin assessment include; (a) training of the mRSS skin assessment needs experienced rheumatologist as a trainer, the training process might be affected if there are limited numbers of experienced rheumatologist; (b) the mRSS assessment method takes time and the trainee needs a learning curve, and; (c) subjective skin assessment according to the mRSS method needs recall memory, so it might affect the accuracy of data and causes a recall bias. The idea of using a skin model arose as a way to address these limitations. Trainees can use this skin model, which has been validated by experienced rheumatologists, as a reference for skin thickness severity assessment without needing to take specialist skin assessment training from a rheumatologist and using recall memory after training. Moreover, nurses and/or healthcare workers can also perform skin assessments using the skin model as a reference. If the skin model can be validated, it would be help ensure initial early disease severity assessments, early management, and early referral to specialists. In addition, using the skin model might save resources and provide better care for SSc patients.
The skin model has four grades of skin thickness, just like the mRSS (viz., 0, 1, 2, and 3 as validated by experienced rheumatologists) [14, 37]. The model is used as a nonviable trainer for inexperienced physicians who palpate each site of the patient’s skin, compare it to the model, and score according to the mRSS assessment method. The study's objectives were to determine the inter- and intra-rater variability of the mRSS assessment after using the skin model. If the model achieved good agreement vis-à-vis both inter- and intra-rater variabilities for inexperienced or non-expert assessors, the model could be used as a reference of skin thickness assessment as per the mRSS assessment method for health education, routine clinical practice, and/or clinical trials.