Von Frey's method of measuring pressure sensibility in the hand: An engineering analysis of the Weinstein-Semmes pressure aesthesiometer

The development of microsurgical techniques has generated a resurgence of interest in estimating local pressure sensibility as a measure of sensory improvement. Because our experience with Weinstein's modification of Von Frey's probes yielded variable and poorly understood results, we measured two sets of probes and examined them in the light of the engineering principles on which their behavior is based. The mechanical behavior of the nylon monofilaments can be described as buckling with one end built in and the other end pinned. The probes are relatively uniform and consistent. However, no loss in sensitivity would accompany division of the set into two or three equivalent sets. Variations in the buckling stress as high as a factor of eight are difficult to avoid. Gross errors arise from careless application, variations in the elastic modulus due to changes in temperature and humidity, and variations in the attachment of fibers to handles and differences in the ends of the filaments. Interpreting results for this instrument requires an understanding of the factors which can influence those results. The rpobes are simple to use but easy to misinterpret.