Threshold Detection and Semmes-Weinstein Monofilaments

Semmes-Weinstein monofilaments provide a repeatable instrument stimulus with a small standard deviation in contrast to other handheld test instruments, making them an optimum choice for objective sensory testing in a variety of clinics. Normal sensory detection thresholds for the entire body, and the stimulus force for each filament, were determined by Weinstein. He found a nylon filament of 0.005 in wide and 38 mm long (mean force, 68 mg) to be a good predictor of "normal" light touch-deep pressure threshold for the hands and most of the body. However, manufacturers of the nylon used in making the filaments allow an 8-10% tolerance in diameter. This small change in diameter can result in small variations in mean force among filaments of a given size. It has not been previously determined what effect this small variance in force has on the accuracy of the 2.83 (marking number) 0.005-in wide filament most often used for normal threshold detection. This study compared the 2.83 filaments available at the Gillis W. Long Hansen's Disease Center, which have a mean force of 62 mg, with those from North Coast Medical, Inc., which have a mean force of 95 mg. The filaments were used by 6 examiners in a standard testing protocol for the hands, arms, faces, legs, and feet of 130 subjects. Heavier and lighter filaments of measured force were also included. Results showed a high correlation in responses for two values for the 2.83 filaments in the range specified. On detailed analysis between kits there were some differences for site and age. The 2.83 filament was a good predictor of normal in both kits for the hand, the arm, and the leg, as was expected. It was suprathreshold for the face, making use of a lighter filament possible for facial testing, and subthreshold for the plantar surface of the foot, which required a slightly heavier filament (3.61 marking number, mean force, 279 mg) for normal threshold detection as measured in this study.