EvF… Continuity with your filament data

Why does EvF give different thresholds?
  • The probe on an EvF is the same diameter, irrespective of the force it applies, whereas each von Frey filament has a different tip diameter. As the threshold force increases (approximately) linearly with tip diameter (Pressure = Force/Area doesn’t apply in this case), filaments are actually applying a rather confusing set of stimulii. The variation in tip diameter “stretches” the force scale, lowering treatment thresholds with hyperalgesia compared to EvF. It’s not a bad thing, it’s a plus for filaments: it’s a built-in “amplifier” that can make the treatment effect clearer. But EvF, with its constant probe area, doesn’t do it, so you need to know if you move from one method to the other. Please click here for a 1 page pdf that explains the effect (without too much maths).
  • This area effect is further complicated by the fact that the effective area of each filament changes drastically as it buckles: it’s then presenting something between a semicircle and an edge to the plantar surface. So if the mouse reacts just as it buckles, is that a response to that filament or not? In contrast, the EvF probe maintains a constant area.
In Summary:

MouseMet (or RatMet) may not give you the same baseline threshold as your von Frey filaments and it may indicate a reduced treatment effect compared to your previous studies. But the measurements of mechanical threshold force will be more accurate and just as reproducible (we know what the spread of your data should be and we’ll coach you to achieve a good standard deviation with the early baselines). They will also be quicker (and therefore cheaper) to generate, and less dependent on the operator.