Analgesic efficacy of local anaesthetic wound administration in knee arthroplasty: volume vs concentration.

Wound administration of local anaesthetic may be effective for postoperative pain management in knee arthroplasty, but the analgesic efficacy of local anaesthetic in relation to volume vs concentration has not been determined. In a double-blinded trial, 48 patients scheduled for total knee arthroplasty were randomly assigned to receive either a high volume/low concentration solution of ropivacaine (20 ml, 0.5%) or a low volume/high concentration solution of ropivacaine (10 ml, 1%), 6 and 24 h postoperatively through an intracapsular catheter. Pain was assessed for 2 h after administration. Pain was reduced in both groups with ropivacaine administration 24 h postoperatively (p < 0.02), but with no difference in analgesia between groups at all time intervals. No reduction in pain scores was observed with ropivacaine injection 6 h postoperatively. The median (IQR [range]) dose of oxycodone administered was 12.5 (10-19 [0-35]) mg in the high volume/low concentration group, and 20 mg (16-40 [0-65]) mg in the low volume/high concentration group (p = 0.005). In conclusion, intracapsular administration of local anaesthetic may have limited analgesic efficacy with no volume vs concentration relationship after total knee arthroplasty.