Does intraoperative magnesium lessen pain after knee replacement surgery?

“The disease is painless; it’s the cure that hurts.” – Katharine Whitehorn

Magnesium sulphate attenuates acute postoperative pain and increased pain intensity after surgical injury in staged bilateral total knee arthroplasty: a randomized, double-blinded, placebo-controlled trial. Shin et al. British Journal of Anaesthesia. doi: 10.1093/bja/aew227

This study looked at the effect of intra-operative Magnesium Sulphate on pain after knee arthroplasty, which is an operation associated with a high rate of chronic pain.  The authors acknowledged that Magnesium has previously been shown to be an effective analgesic, and postulated that it may also help minimise the hyperalgesia seen following staged bilateral surgeries (where the second operated knee is usually significantly more painful than the first).

They were based in Seoul, South Korea and it took place in March 2015.

44 patients were divided into 2 groups:

  • The intervention group received MgSO4 50mg/kg intra-operatively + 15mg/kg/hr for the duration of surgery
  • The control group received saline of same volume

Anaesthetists, patients, and assessors were blinded to the intervention. Drugs were prepared by nurses not involved in the study. Patients were almost exclusively female, aged between 50-80 years old, and ASA 1 or 2.  There were quite extensive exclusion criteria, including any pre-existing pain syndrome, chronic opioid use, BMI >35 and cardiovascular disease.

The primary endpoint of the study was the difference in pain scores at rest, before and after each operated knee (surgeries were performed only one week apart). Secondary endpoints were rescue analgesic use (prn intravenous ketoprofen) and cumulative fentanyl PCA use at 24h and 48h post-operatively. Pain was assessed using a visual analogue scale.

All patients received a complex but protocolled anaesthetic, including midazolam premedication, and multi-modal analgesia prior to induction as well as post-operatively.  This was supplemented by a femoral nerve catheter, a spinal anaesthetic with fentanyl and an intra-articular injection. Post-operatively patients received a fentanyl PCA and prn ketoprofen in addition to their regular analgesics.

The results of the study were impressive, with the Magnesium group demonstrating significantly lower pain scores compared to control after both surgeries, as well as significantly lower analgesia use compared to control at 48h after the second operated knee. The control group demonstrated an increase in pain scores and analgesia use after the second surgery compared to the first – something not seen in the Magnesium group.

The authors concluded that Magnesium was a safe and effective adjunct to intra-operative analgesia and may help attenuate post-operative pain and increased pain intensity in staged surgeries. The mechanism remains unknown, but may involve NMDA inhibition.

Some drawbacks of the study include the fact that it was not clear how or where patients were treated in the time between their two operations, and that the study group was small with a specific demographic of healthy elderly females. It would have also been helpful to know whether long-term pain outcomes were assessed and to what degree patient satisfaction was affected.

Overall, this was a well-planned study with convincing results, which could easily be applied to our everyday practise.

Summary by Dr Victoria Buswell

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