Total knee arthroplasty (TKA) is one of the most common surgeries performed nowadays. However, severe postoperative pain after TKA made the patient suffering and limit postoperative recovery. The periarticular cocktail injection was widely used in TKA to effectively reduce pain[14, 15], although there is no consensus yet of the ingredients. The current study was designed to compare three cocktail regimens for postoperative analgesia and assess the potential side effects of added morphine and dexamethasone.
The patients in Group I demonstrated the lowest VAS scores in the first 4 days after TKA, which confirmed the regimen effectively reduce the damaging stimulus and the release of inflammatory mediators, prevent pain sensitization and alienation, and achieve balanced analgesia. It was due to the main components in the regimen work synergistically on different targets. Bupivacaine is a long-acting amide local anesthetic. It mainly inhibits the sodium ion channel of the nerve cell membrane to block nerve excitement and conduction.[16] Its anesthesia intensity is 16 times that of procaine and 2-3 times that of lidocaine, and the duration of action is 8 times that of procaine. [16, 17] The surface of periosteum was covered by a layer of dense fibrous membrane. The internal surface of the periosteum is rich in unmyelinated nerve fibers and small vessels. The small unmyelinated nerve fibers in the periosteum are sensitive to local anesthetic. Thus, the regimen could provide a basic pain-controlled effect due to bupivacaine. Flurbiprofen axetil is a non-steroidal analgesic with lipid microspheres as the drug carrier.[18, 19] After the drug was injected into the body, flurbiprofen axetil is released from the lipid microspheres, and is rapidly hydrolyzed under the action of carboxylesterase to produce flurbiprofen[20], which inhibits the synthesis of prostaglandins and reduce pain.[21, 22] Morphine is a complete agonist of opioid receptors. It can mimic endogenous opioid active substances by binding to opioid receptors in different brain regions to exert pharmacological effects, such as with dorsal thalamus, ventricles, peri-aqueduct gray matter, and spinal cord glue.[23, 24] Compared to intermittent sharp pain and visceral colic, morphine has a better effect on persistent dull pain than[25], which is ideal for reducing the postoperative pain after TKA. Opioids usually exert an analgesic effect 10-20 minutes after one dose, the strongest effect is 1 to 2 hours, and it can be maintained for 4-6 hours.[26] Morphine also has a significant sedative effect, which is related to the binding of opioid receptors in the limbic system to eliminate emotional changes caused by pain and can eliminate anxiety caused by pain.[27] Emotional reactions such as tension and fear, thus significantly improving the tolerance to pain.[28, 29] Steroids have anti-inflammatory effects and may reduce these effects by inhibiting the cyclooxygenase pathway, stabilizing neuronal cell membranes, and reducing the level of bradykinin in the tissue.[30] Dexamethasone has also been shown to inhibit the release of neuropeptides from nerve endings after tissue injury, such as calcitonin gene-related peptide and substance P, which were both for reducing pain.[31, 32]
The patients in Group I also demonstrated the biggest active and passive VOM in the first 4 days. It was partially due to the pain was reduced to the lowest level, so the range of passive knee movement could achieve the highest level. The reduced pain also encouraged the patients to conduct the active exercise. The good ROM could also be partially reasoned with the use of corticosteroids. More than directly suppressing inflammation, corticosteroids also increase vasoconstriction, reduce vascular permeability, antagonize the expansion of blood vessels by inflammatory mediators such as histamine, reduce local congestion, and inhibit the exudation of white blood cells and body fluids. Thus, corticosteroids used local efficiently reduce exudation and edema, which was beneficial for achieving a higher ROM in the beginning.
No side effects were observed during the hospital stay, it was mainly because the morphine and dexamethasone were used in a small amount. In addition, the applied dexamethasone decreased capillary permeability, so the morphine entered the blood circulation at a slow rate, so the risk of side effects related to morphine was minimized. The well-controlled pain and limb swelling are beneficial for early rehabilitation after TKA, which could help to restore the general condition and immune system. Therefore, the infection was not observed by monitoring serum CRP and ESR continuously.
This current study possessed several limitations though. First, the conducted study was a single-center study, and the sample size was relatively small. Future studies with a larger sample size conducted in multicenter might potentially help find a clearer intergroup difference. Second, although all the surgical processes were conducted by the same surgical team, the different requirements of osteotomy and soft tissue release of specific individuals might have a certain but undefined influence on pain and movement evaluation. Finally, in the present study, the outcomes were analyzed largely based on VAS evaluation, which was a subjective rating scale. As different patients possess a different sensitivity and accuracy of expression to pain, so the VAS was inevitably biased. An objective rating scale was needed in the future to precisely record and compare the outcome.
In conclusion, for patients undergoing primary unilateral TKA, the morphine and dexamethasone incorporated cocktail regimen significantly reduced postoperative pain and promoted ROM which was beneficial for early postoperative enhanced recovery. The combined cocktail did not raise any side effects.