The Behind-the-Knee test: an efficient model for evaluating mechanical and chemical irritation

The 'Behind-the-Knee' method (BTK test), using the popliteal fossa as a test site, evaluates both the inherent chemical irritation, and the potential for mechanical irritation of substrates and products. This approach eliminates some of the difficulties of in-use clinical test systems while still providing reliable results. In this publication, examples of the results of BTK tests on several materials are presented with direct comparisons, where possible, with results of in-use clinical testing conducted on the same materials.

In in-use clinical tests, volunteer panelists were provided with catamenial products to use in place of their normal product. In the BTK test, samples were applied daily to the popliteal fossa using an elastic athletic band. In both studies, irritation reactions were scored visually.

Levels of irritation in the BTK test are consistently higher than those of standard patch tests, illustrating the contribution of mechanical irritation to the overall irritant potential of materials and products. Repeated tests on identical test materials demonstrated that the BTK test results are reproducible. Side-by-side comparisons of the BTK test and in-use clinical tests demonstrated that the BTK test produces results of similar quality to the in-use clinical. By using several concurrent panels with a common test material, it is possible to compare the irritant properties of several materials at once.

We have tested over 25 different materials in over 35 BTK studies. The test method has proven reliable and versatile in testing a wide variety of materials, including menstrual pads, topsheets, interlabial pads, pantiliners, tampons and lotion coatings on products. Unlike in-use clinicals, the BTK test allows the direct comparison of two products at one time on the same individual, and is easily adapted to investigative programs. It is subject to fewer confounding factors, is much easier to implement, has a shorter turnaround time, and is less expensive than in-use clinical testing. Importantly, unlike standard patch tests, the BTK test evaluates both the inherent chemical irritation associated with materials and the mechanical irritation owing to friction. Although the BTK test was developed using catamenial products, the test system provides a valuable alternative for evaluating any material where mechanical irritation may play a role, including textiles, facial tissues, baby and adult diapers, and laundry products that may leave residues on fabrics.