Is a patella motor response necessary for continuous femoral nerve blockade performed in conjunction with ultrasound guidance?

Background: Successful continuous femoral nerve blockade (CFNB) has been associated with the elicitation of a patella motor response during needle and catheter insertion. We evaluated whether a patella motor response is necessary when CFNB is performed in conjunction with ultrasound (US) guidance.

Methods: Ninety-eight patients undergoing CFNB (along with sciatic nerve block and spinal anesthetic) for total knee arthroplasty participated in this cohort observational study. Using out-of-plane US guidance alone, the tip of an insulated Tuohy needle was positioned superficial to the midpoint of the femoral nerve visualized in short axis. A nerve stimulator was turned on and the type of motor response (patella versus medial muscle) and minimum stimulating current from the needle were recorded. A stimulating catheter was then inserted and the type of motor response and minimum current from the catheter were recorded. Ten milliliters mepivacaine 2% was injected through the catheter. The primary outcome was sensory block defined as loss of sensation to pinprick on the anterior surface of the distal thigh measured 20 minutes after mepivacaine injection.

Results: Forty-three patients demonstrated a patella motor response, 43 demonstrated a medial motor response, and 12 demonstrated no motor response from the catheter. The proportion of patients with sensory block differed according to motor response from the catheter (patella [98%], medial [91%], and no motor response [75%]; P = 0.02), but there was no significant difference between a patella (98%) and medial (91%) motor response from the catheter (P = 0.58). The proportion of patients with motor block 20 minutes after local anesthetic injection also differed according to motor response from the catheter (patella [95%], medial [77%], and no motor response [67%]; P = 0.03). In addition, there was a significant difference between a patella (95%) and medial (77%) motor response from the catheter (P = 0.01). The mean minimum stimulating currents did not differ between patella and medial motor responses elicited from the catheter (P = 0.06). Postoperative pain and analgesic consumption were similar regardless of the type of motor response from the catheter.

Conclusions: Based on observational data, a patella or medial motor response from the catheter similarly results in sensory block of the anterior thigh when CFNB is performed in conjunction with out-of-plane US guidance.