• Peter Skrejborg, Kristian K Petersen, Jacob Beck, Michael Ulrich, Ole Simonsen, Poul T Nielsen, Lars Arendt-Nielsen, and Mogens Laursen.
• Department of Health Science and Technology, Center for Sensory-Motor Interaction (SMI).
• Clin J Pain. 2020 May 1; 36 (5): 352-358.

Background And AimsSevere preoperative and acute postoperative pain have been associated with the development of chronic postoperative pain. Chlorzoxazone (a muscle relaxant) has been suggested to enhance acute postoperative pain recovery, but the lack of larger randomized controlled trials has, however, questioned the continued use. Despite this, chlorzoxazone is still used for acute postoperative pain management following total knee replacement (TKR) or total hip replacement (THR). The current randomized, double-blinded, placebo-controlled, parallel-group, clinical trial aimed to assess the effect of chlorzoxazone for postoperative pain management following TKR or THR.MethodsA total of 393 patients scheduled for TKR or THR were included in the trial. Patients were assigned to 250 mg chlorzoxazone 3 times daily for the first 7 days postoperatively or to placebo. The primary outcome was pain after 5 m walk assessed 24 hours postoperatively. Secondary outcomes included changes in preoperative pain at rest, worst pain in the last 24 hours, and Oxford Knee or Hip Score compared with 12 months' follow-up. In addition, adverse events were assessed in the perioperative period.ResultsNo significant differences were found for any of the outcome parameters after TKR or THR. As regards TKR or THR, no effects were demonstrated for pain after 5 m walk 24 hours after surgery (P>0.313), or for any of the secondary outcomes (P>0.288) or adverse events (P>0.112) in the group receiving chlorzoxazone compared with placebo.ConclusionThe current study demonstrated no analgesic effects of postoperative chlorzoxazone administration compared with placebo on acute or chronic postoperative pain 12 months following TKR and THR.

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