Magnesium Sulfate Route Trial After Total Knee Arthroplasty
Key Takeaways
- In adults undergoing primary unilateral total knee arthroplasty for knee osteoarthritis, combined intravenous plus topical magnesium sulfate did not improve the prespecified 24-hour resting VAS primary endpoint versus either route alone.
- The combined regimen was associated with significantly lower resting pain at 6 and 12 hours and lower movement pain at 12 hours after surgery.
- Those early differences remained below the prespecified 1-point minimal clinically important difference, limiting their clinical importance.
- No statistically significant differences were observed in rescue morphine use, early functional recovery, stress measures, length of stay, complications, or measured outcomes between the intravenous-only and topical-only groups.
At West China Hospital, investigators conducted a prospective, randomized, blinded controlled trial in patients undergoing primary unilateral total knee arthroplasty (TKA) for knee osteoarthritis, as reported in the Wang et al randomized TKA trial of intravenous, topical, and combined magnesium sulfate. Among 247 screened patients, 150 were randomized 1:1:1, and 50 in each group were analyzed by intention to treat with no losses to follow-up. Eligible participants had American Society of Anesthesiologists (ASA) physical status I to III. Regimens were intravenous magnesium sulfate alone, topical magnesium sulfate added to the local infiltration analgesia (LIA) cocktail alone, or both together; intravenous dosing was 40 mg/kg over 15 minutes followed by 10 mg/kg/h until surgery end, and topical dosing was 2.5 mg/mL in a 100 mL LIA cocktail. All groups also received celecoxib, pregabalin, an adductor canal block, standardized general anesthesia, and rescue morphine for prespecified Visual Analogue Scale (VAS) pain thresholds. The primary endpoint was resting VAS pain at 24 hours, with secondary outcomes covering other pain time points, rescue morphine use, function, stress measures, length of stay, and complications.
No significant between-group difference emerged for resting VAS pain at 24 hours after surgery. The combined regimen was associated with lower resting pain at 6 and 12 hours and lower movement pain at 12 hours, but absolute between-group differences remained under 1 VAS point. In repeated-measures analyses, the treatment-group effect was significant for resting pain (p<0.001) and movement pain (p=0.003). Intravenous-only and topical-only administration did not differ significantly, and rescue morphine use, knee range of motion, ambulation capacity, length of stay, stress markers, and complication rates were similar across groups.
Those early differences did not meet the prespecified minimal clinically important difference, so the same-day statistical signal did not establish a clinically meaningful advantage for combined treatment. Follow-up lasted only through hospitalization, and the sample size was powered for the 24-hour resting pain endpoint rather than uncommon adverse events or all secondary outcomes. Because the trial was conducted at a single center in China within a specific multimodal protocol, direct applicability to other perioperative pathways, including North American practice, may be limited. Some personnel involved in treatment preparation were unblinded, and the comparison was not designed to address convenience, drug exposure, cost-effectiveness, or route-specific adverse effects.
The investigators concluded that combined intravenous plus topical magnesium sulfate did not confer a clinically significant advantage over either route alone for postoperative pain management, stress response, or early functional recovery after TKA in this trial. They called for further studies to clarify the optimal route, dose, and regimen for perioperative magnesium sulfate.
Clinician Questions
Which total knee arthroplasty patients were included in the magnesium sulfate route comparison?
The comparison enrolled consecutive patients undergoing primary unilateral total knee arthroplasty for knee osteoarthritis at a single center, aged 18 to 80 years, with ASA physical status I to III. The trial excluded several higher-risk or nonstandard cases, including major cardiopulmonary insufficiency, significant hepatic or renal dysfunction, prior open knee surgery, neuromuscular dysfunction, severe deformity, active infection, opioid addiction, and inability to complete VAS assessment.
What perioperative analgesia did patients receive alongside magnesium sulfate in this TKA trial?
All groups in this TKA magnesium sulfate trial received multimodal background analgesia with celecoxib and pregabalin starting the day before surgery and continuing postoperatively, an adductor canal block before induction, standardized general anesthesia, prophylactic sufentanil plus antiemetics near the end of surgery, and rescue subcutaneous morphine for prespecified pain escalation.
Why did the early pain reductions with combined magnesium sulfate not translate into a clinically meaningful advantage after TKA?
The authors' interpretation was that the combined regimen separated from the single-route groups at some early postoperative time points, but the magnitude of benefit remained below the prespecified minimal clinically important difference for VAS pain and the primary 24-hour resting pain endpoint was unchanged. Rescue morphine use, functional recovery, stress measures, and complications were also similar, which limited the practical impact of the early pain signal.
Recommended Reading
- For more on postoperative opioid-sparing analgesia: Liposomal Bupivacaine Cut Opioid Use After Lumbar Fusion