Building, testing and validating a set of home-made von Frey filaments: A precise, accurate and cost effective alternative for nociception assessment

Background: A von Frey filament (vFF) is a type of aesthesiometer usually made of nylon perpendicularly held in a base. It can be used in paw withdrawal pain threshold assessment, one of the most popular tests for pain evaluation using animal models. For this test, a set of filaments, each able to exert a different force, is applied to the animal paw, from the weakest to the strongest, until the paw is withdrawn. New method: We made 20 low cost vFF using nylon filaments of different lengths and constant diameter glued perpendicularly to the ends of popsicle sticks. They were calibrated using a laboratory balance scale. Building and calibrating took around 4. h and confirmed the theoretical prediction that the force exerted is inversely proportional to the length and directly proportional to the width of the filament. Results: The calibration showed that they were precise and accurate. We analyzed the paw withdrawal threshold assessed with the set of home-made vFF and with a high quality commercial set of 5 monofilaments vFF (Stoelting, Wood Dale, USA) in two groups (n= 5) of healthy mice. Comparison with existing methods: The home-made vFF precisely and accurately measured the hind paw withdrawal threshold (20.3. ± 0.9. g). The commercial vFF have different diameters while our set has the same diameter avoiding the problem of lower sensitivity to larger diameter filaments. Conclusion: Building a set of vFF is easy, cost effective, and depending on the kind of tests, can increase precision and accuracy of animal nociception evaluation.