Intrathecal Morphine Versus Morphine-dexmedetomidine Combination for Postoperative Pain Control After Total Knee Replacement

Total knee replacement is one of the most painful orthopedic surgical procedures. Patients who undergo total knee replacement are usually older and have limited cardiac and pulmonary reserves. The increased sensitivity of elderly patients to drugs makes it necessary to choose postoperative analgesia agents and methods that have minimal side effects. Intrathecal injection of morphine to provide postoperative analgesia during the initial 24-h after the operation is a widely used technique, however, opioid therapy is limited because of the side effects (hypotension, pruritus, nausea, urinary retention, respiratory depression) and intolerance. Alpha-2 adrenergic agonists (clonidine and dexmedetomidine) have pharmacologic characteristics (sedation, hypnosis, anxiolysis, sympatholytic, and analgesia) that make them suitable as adjuvants to multimodal analgesia. Their anti-nociceptive effect is attributed to the stimulation of a2- adrenoceptors located in the central nervous system. The rationale for combining analgesics that produce similar therapeutic effects or synergistic interactions is to accentuate the analgesic efficacy and decrease the side effects by permitting dose reduction of each agent. Human studies on the antinociceptive effects of co-administrated intrathecal morphine (ITM) and dexmedetomidine in postoperative pain are still few. On the other hand, Abdel-Ghaffar et al., results do not support improved analgesia with the combination of intrathecal morphine and dexmedetomidine, despite the absence of significant adverse effects. We hypothesized that the addition of dexmedetomidine to ITM would improve the quality of perioperative pain control and decrease the side effects of postoperative systemic opioid use.