Pain inhibitory mechanisms and response to weak analgesics in patients with knee osteoarthritis

BACKGROUND: Conditioned pain modulation (CPM) and offset analgesia are different features of descending pain inhibition. This study investigated CPM, offset analgesia and clinical pain measures in patients with knee osteoarthritis (KOA) before and after treatment with the combination of a non-steroidal anti-inflammatory drug (NSAIDs) plus acetaminophen.

METHODS: Forty-two patients with KOA received Ibuprofen 1.2 g/daily and acetaminophen 3.0 g/daily for three weeks. Before administration, CPM magnitude was assessed as the difference between cuff pain detection (cPDT) with and without a conditioning stimulus (evoked by tourniquet pain). Offset analgesia was assessed as the pain intensities evoked by a constant 46°C for 30-seconds stimulus compared to an offset analgesia paradigm of 46°C for 5-seconds, 47°C for 5-seconds, and 46°C for 20-seconds. The worst pain within the last 24-hours and pain during activity were assessed before and after treatment.

RESULTS: Clinical pain significantly decreased after treatment (P<0.001) and less efficient CPM before treatment was associated with weaker analgesic effect (R=0.354, P=0.043). No significant modulation of CPM or offset analgesia were found for the treatment.

CONCLUSION: This study found that less efficient CPM is associated with reduced analgesic effect of NSAIDs plus acetaminophen in patients with KOA whereas the treatment did not modulate CPM nor offset analgesia magnitude. This article is protected by copyright. All rights reserved.