We read with interest the recent article by Kissin et al (1) reporting on the validity of a durometer to objectively measure skin hardness in systemic sclerosis (SSc). The authors demonstrated that durometer-measured skin hardness correlates well with the modified Rodnan skin score (MRSS) and is more reliable than the MRSS. The durometer is a small, easy-to-use, noninvasive instrument that appears promising for everyday clinical use and therapeutic trials. Measurement with a durometer of the hardness of a specimen is based on the extent of the depression produced by the indenter when the resilient force of the specimen becomes equal to the pressure load produced by an inner spring. The durometer was originally designed for use with various industrial materials such as rubber, plastic polymers, and elastomers, so there are many kinds of durometers that have indenters of different shapes and sizes to suit the material. It has not yet been determined which of these is best for assessment of SSc.
We therefore used various durometers to measure skin hardness and examined correlations of durometer hardness values with MRSS. The durometers featured 12 different shapes and sizes of indenters consisting of 6 columns and spheres, each with a diameter of 0.5, 1, 2, 4, 6, and 8 mm. A hand-held deep-hole type (GS-719H; Teclock, Nagano, Japan) designed for the measurement of uneven surfaces and relatively soft materials was used. Skin hardness was measured at 17 sites of the body corresponding to those of MRSS in 17 SSc patients (7 diffuse and 10 limited cutaneous types) and 9 healthy control subjects. The mean ± SD MRSS of the patients was 18.5 ± 11.0.
As shown in Figure 1, correlation of durometer hardness with MRSS varied according to the shape and size of the indenter. The most significant correlations were attained with a column-shaped indenter with 2-mm diameter at both dorsal hands and forearms (r = 0.76 and 0.73, respectively). The durometer with this indenter demonstrated similar results at other sites, whereas those with other indenters did not always show statistically significant correlations between durometer hardness and MRSS. We discontinued measurements using indenters with a diameter of 0.5 mm and 1 mm because of pain associated with the touch of the instrument. Our findings indicate that the correct selection of a durometer is important. Of the 12 different indenters used in our study, the most suitable is one with a column-shaped indenter with a 2-mm diameter.
Positive correlation of durometer hardness with MRSS has also been reported by other investigators (2, 3). Durometry, which is objective and easily performed, effectively provides information about skin involvement in SSc and should be considered for use in clinical trials, although it will be necessary to determine the shape and size of the indenter most suitable for the evaluation of skin hardness.