Measuring Joint Pain Through Tibiofemoral Flexion: A Novel Approach to Assess Pain Behavior and Analgesic Profiles in Rats.

Assessing knee joint pain in experimental osteoarthritis (OA) models remains a significant challenge. Our study demonstrates that the use of an adapted electronic von Frey (aVF) device, featuring a modified tip, surpasses the standard von Frey (sVF) in detecting knee joint pain behavior and evaluating the efficacy of analgesic treatments in an OA model induced by monoiodine acetate (MIA) in male rats. The sVF induced a behavior profile in naïve animals characterized by a hind paw flinching and withdrawal reflex. This behavioral response was affected by intraplantar lidocaine, which increased mechanical thresholds related to sVF and validated it as pain related behavior. In the aVF method, rats displayed no alterations in their mechanical thresholds in absence or presence of lidocaine, suggesting minimal stimulation of hind paw by the modified methodology. In MIA-OA rats, the aVF was able to detect a significant reduction on joint mechanical thresholds. The behavior linked to aVF was significantly affected by systemic delivery of morphine, confirming a nociceptive-like phenotype and suggesting a pain behavior predominantly triggered by joint flexion. The aVF was also able to detect an analgesic profile in MIA-OA rats treated with dexamethasone, LPS-RS, and fucoidan, indicating effectiveness in measuring differential responses triggered by analgesic drugs. Our results suggest aVF as a more appropriate method to evaluate joint pain in rats.