Background: Theoretically, the ideal volume of local anaesthetic for adductor canal block (ACB) would ensure sufficient filling ofthe canal and avoid proximal spread to the femoral triangle. In this dose-finding study, we aimed to investigate the minimaleffective volume for an ACB needed to fill the adductor canal distally in at least 95% of patients (ED95).
Methods: We performed a blinded trial, enrolling 40 healthy men. All subjects received an ACB with lidocaine 1%. Volumes wereassigned sequentially to the subjects using the continual reassessment method followed by Bayesian analysis to determine theED95. Distal filling of the adductor canal was assessed by magnetic resonance imaging (primary outcome). Secondary outcomeswere the effect of volume on proximal spread to the femoral triangle (also assessed by magnetic resonance imaging), quadricepsmuscle weakness (decrease by ≥25% from baseline) and sensory block.
Results: The ED95 was 20 ml, with an estimated probability of sufficiently filling the canal of 95.1% (95% credibility interval: 0.91–0.98). Proximal spread to the femoral triangle was seen in 0/4 (0%), 7/12 (58%), 4/8 (50%), and 8/16 (50%) subjects with the 5, 10, 15,and 20 ml doses, respectively (P=0.25). Seven subjects had a reduction in muscle strength, but there was no difference betweengroups (P=0.85).
Conclusions: For an ACB, the dose closest to the ED95 needed to fill the adductor canal distally was 20 ml. There was nosignificant correlation between volume and proximal spread or muscle strength.Clinical trial registration: NCT02033356.