Efficacy of Magnesium vs Dexamethasone as an Adjuvant to Ropivacaine in Ultrasound Guided Femoral and Sciatic Nerve Block for Postoperative Analgesia: A Prospective, Double Blinded Randomized Controlled Study

In patients undergoing lower limb orthopaedic surgery, unrelieved post-operative pain not only results in discomfort to the patients but also predispose to the development of chronic pain syndromes. The dawn of ultrasonographic-guided techniques has led to increased interest in femoro-sciatic nerve block (FSNB) for lower limb orthopaedic surgeries. Efficacy of various adjuvants have been studied to prolong the block and analgesia. In recent years, there is growing interest in magnesium sulphate (MgSO4) and dexamethasone as adjuvants to local anaesthetics in nerve blocks. Duration of post-operative analgesia was the primary outcome of our study, whereas the rescue analgesics requirement, VAS scores and haemodynamic parameters and time required for toe movement were the secondary outcomes.

Sixty-patients scheduled to undergo below knee orthopaedic surgeries under subarachnoid block were divided into 2 groups: Group RM(n=30) patients received 38 mL of 0.375% Ropivacaine with MgSO4 150 mg in 2 mL NS and Group RD(n=30) patients received 38 mL of 0.375% Ropivacaine with Dexamethasone 8 mg(2mL) to make total drug volume of 40 mL. In all patients, 20 mL of LA solution was injected around femoral nerve and 20 mL around sciatic nerve. The primary outcome was duration of post-op analgesia and secondary being requirement of rescue analgesia and time for toe movement. Mean variables were analysed and compared with unpaired t-test. Proportions were compared with Chi-square test and Fischer-exact test.

Duration of analgesia was prolonged with Dexamethasone (18.8 ± 7.8) as compared to MgSO4 (8.8 ± 4.2). In regards to early ambulation, MgSO4(6.78 ± 2.25) was a cut above Dexamethasone (16.43 ± 4.56).

Both MgSO4 or Dexamethasone added to Ropivacaine prolonged the duration of analgesia, decreased requirement of rescue analgesia. Dexamethasone delays requirement of rescue analgesics with better pain scores as compared to MgSO4.