Use of the Semmes-Weinstein 5.07/10 gram monofilament: the long and the short of it.

The Semmes-Weinstein monofilament has been developed for the detection of patients at risk of neuropathic ulceration. In this study we evaluated how the physical characteristics of the monofilaments can impact on their performance. Commercially purchased monofilaments from the Hansen's Disease Center (HDC) and a batch produced 'in-house' (RPAH) were calibrated using a Mettler Balance. To assess the effects of varying lengths on buckling force, the monofilaments were tested repeatedly while the length of the filament was reduced stepwise from 4.1 to 3.1 cm. The correct length of the monofilament to generate a buckling force of 10 g was also determined theoretically by applying the Euler's Buckling Equation. Results showed neither batch of monofilaments buckled at a strength of 10 g (HDC 6.8 g, CI 5.7-7.9, and RPAH 7.2 g, CI 7.1-7.3). In addition HDC showed a wide interfilament variation range, 4.1-10.3 g with CV 29% versus corresponding figures of range 7.1-7.9 g, CV 4.9% for the RPAH monofilaments. As predicted by Euler's Buckling Theory, buckling force can be increased by reducing the length of the filament. These results demonstrate that the physical characteristics of the monofilament are important determinants of buckling force and are not necessarily uniform in commercial filaments. The clinical relevance of variance in buckling force remains to be determined.