Basic Methodology Skin Surface Impressions

The site to be sampled should be delineated by affixing adhesive paper rings with orientation tabs such as those manufactured by CuDerm (Dallas, TX). Because the crow's feet furrows taper and become less pronounced as you move away from the periorbital area, it is extremely important that the site be located precisely. To facilitate relocating this site for subsequent serial samples, close-up photographs can be taken of the region with the adhesive rings properly placed for each panelist, as shown in Figure 1 for the crow's foot region. Of the dental-impression materials that have been used, Silflo from Flexico Develop

Ring Impression Skin

Figure 1 Placement of CuDerm Replica Locator Ring for obtaining silicone rubber impression from the crow's feet region.

Figure 1 Placement of CuDerm Replica Locator Ring for obtaining silicone rubber impression from the crow's feet region.

ments Ltd. (Potters Bar, UK) is the best choice for skin-surface replicas. It not only offers a very high degree of fidelity, but the white opaque surface is ideal for viewing under reflected light used in image analysis. Moreover, these types of samples could be stored for at least 2 years without any fear of alteration in microtopography.

To make a impression, a thin layer of freshly prepared Silflo is gently spread over the bounded area of the ring and allowed to flow into the various furrows, creases, and fine lines that mark the surface. Within a few minutes, the material will polymerize and the replica is removed by gently lifting away from the skin using the orientation tab of the paper ring. It is important that the panelist remains calm with eyes closed and face relaxed during the polymerization phase. Because of the hydrophobic properties of silicon rubber, holes will form if the panelist is sweating from being too warm or emotionally stressed. Other artifacts such as bubbles can be created if the mixture is stirred too vigorously, causing it to froth. Alternatively, if the resin is not adequately mixed with the hardener or the mixture is allowed to partially polymerize before application, the specimens will lack detail.

Image Analysis

The general principles of image analysis for measuring the microtopography of the skin surface as captured in replica specimens have been previously described [6,7]. Briefly, these instruments consist of a high-resolution, black-and-white digital camera that is interfaced into a computer that contains specially designed image-processing hardware and software. The resulting image consists of a 640 X 480 pixel matrix with 256 gray levels. By selecting proper thresholds based on gray-level values, the image can be segmented into features of interest, such as wrinkles, and subsequently analyzed. One of the advantages of using replicas over photographs is that only topographic features are captured in the white replicas, which can be studied in all three dimensions by taking lighting angles into consideration. In striking contrast, not only are thephotographs limited to two dimensions, but color variations attributable to mottled pigmentation greatly complicate the analysis.

In this application, the replica specimen is sidelighted using a fiberoptic illuminator set at a precisely defined angle to bring out the surface details of interest. In general, the lower the light source the greater the detail will be. In the case of a child, an angle of 15° to 20° will enable the observer to see a large number of fine lines, whereas for deeper furrows and creases such as crow's feet in an adult, an angle of 38° to 45° is optimal. In both cases, lines and wrinkles will cast shadows that are contrasted against the white background of the replica. Figure 2 illustrates that differences in the degree of wrinkling in the crow's foot region can be readily appreciated in this manner.

Because of the extreme anisotrophy of the skin, it is extremely important to take note of the position of the light source relative to the orientation of the specimen. Indeed, the major furrows and lines that are recognized as crow's feet are highly directional with 180° symmetry. For technical reasons, it is far simpler to rotate the sample than to have the lighting system revolve to simulate the movement of the sun. This is accomplished by a using a lazy Susan as a revolving sample holder, and great care is taken to ensure that the replica is held perfectly flat and centered with regard to both the light source and video camera during this movement. In the automated system of Corcuff and Leveque [10], the specimen is rotated at 9° steps through 360°, giving a series of 40 values. When plotted as polar coordinates according to the angle of rotation, one obtains a ''rose of direction.'' A min-max at 180° is readily apparent, and taking measurements in both orien-

Silflo Crow Foot Replica
Figure 2 Representative silicone rubber impressions cast from the crow's foot region showing different degrees of wrinkling.

tations is sufficient for most applications. In our convention, the north-south axis is when the orientation of the lighting is perpendicular to the major furrows, whereas with the east-west it is parallel.

In addition to the angle of lighting, the uniformity of the incident light is critical. Interfering lights and changes in ambient lighting will influence the reproducibility of the measurements; it is best to work in total darkness. Fluctuations in the electronic circuits of the digitizing camera must also be minimized. As a rule, all these types of errors can be controlled by routinely using a series replicas as reference standards to ensure the analysis is being properly conducted.

The digitized image can be mathematically represented as a three-dimensional matrix of numbers. The x and y values are polar coordinates that provide the location of the pixel whereas the z value represents the brightness of the pixel in terms of its gray level. In one analytical approach, the digitized image is segmented into a binary image consisting of shadows and background by choosing an appropriate gray-level threshold. The percentage of the surface area occupied by shadows in a standard field of view is directly related to its topography. Obviously, if the surface is rather smooth and flat, there will be few shadows and this value will be small. On the other hand, if the skin is wrinkled and rough, the shadowed areas will be correspondingly larger. Moreover, because the angle of illumination is known, the horizontal projection of these shadows can be used to estimate their mean depth. Indeed, more sophisticated analyses such as the coefficient of developed skin surface (CDSS), which is a mathematical expression of true-versus-appar-ent surface area as pioneered by Corcuff's group [7-10], are possible.

In optical profilometry, a profile that represents the surface features at that specific location is created by plotting the gray-level values across a horizontal segment of this digitized image. This graphic display (Fig. 3) is similar to that achieved through mechanical profilometry with stylus devices, and we can extract numeric information that describes the microtopographic attributes in much the same way. Of the many parameters available for assessing skin-surface topography, both Rz and Ra have proved to be the most useful. To compute Rz, the profile is first divided into five equal segments along the x-axis. The minimum-maximum differences within each of the five segments are then determined,

Figure 3 Basic set-up and profile of skin surface topography generated by image analysis of side-illuminated silicone rubber impressions.

and Rz is calculated as the average of these five local values. To compute Ra, an average line is generated to run through the center of the profile, and the area that the profile describes above and below this reference line is determined.

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  • Rahel
    How to take silicone replicas crow's feet?
    6 years ago

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