Intraoperative femoral nerve stimulation in evaluation of patellar tracking: tourniquet effects and catheter placement.

BACKGROUND

Dynamic intraoperative assessment of patella tracking utilizes femoral nerve stimulation to contract the quadriceps muscles in assessing the proper distance to transfer the tibial tubercle during distal realignment procedures for patellofemoral instability.

PURPOSE

We describe the effects of tourniquet inflation and catheter placement on intraoperative femoral nerve stimulation for assessment of patellar tracking.

METHODS

Percutaneous electromyographic (EMG) needles were placed into the quadriceps and sartorius muscles to monitor muscle activity and changes in amplitude threshold (mA) required for femoral nerve stimulation with increasing tourniquet inflation times. Eleven patients used ultrasound for catheter placement and ten were manually placed based upon body landmarks.

RESULTS

Tourniquet application time correlated positively with the change in amplitude threshold required to generate muscle contraction. Patients had an average four-fold increase in required stimulus amplitude from the baseline thresholds (pre-tourniquet inflation) to final thresholds (tourniquet inflated) with a two-hour tourniquet inflation time. The use of ultrasound for catheter placement significantly decreased the baseline amplitude required in comparison with catheters placed without ultrasound, (p = 0.0330).

CONCLUSIONS

Increased tourniquet inflation times require greater stimulus amplitude to generate quadriceps muscle contraction. Ultrasound guidance for catheter placement can provide femoral nerve stimulation at low amplitudes.