Ultrasound guidance of needle tip position for femoral nerve blockade: an observational study.

Background: The femoral nerve lies in the ilio-fascial space in a groove formed by the iliac and psoas muscles (GIPM) posteriorly, and overlaid by the iliac fascia. Recommendations for needle insertion for femoral blockade using ultrasound imaging are to insert the needle tip behind the iliac fascia at the lateral side of the femoral nerve, but this part of the nerve is poorly visualised in some patients. A more accurate location of the lateral part of the femoral nerve might be achieved by identifying the GIPM and its lateral segment.

Objectives: The objectives of this study are to determine the frequency of ultrasound visibility of the lateral part of the femoral nerve and GIPM, and to note the motor response to electrostimulation of the nerve and the spread of local anaesthetic when positioning the needle tip at the lateral segment of the GIPM.

Design: A prospective observational (case series) study.

Setting: Department of Anaesthesiology of a University Hospital.

Patients: Inpatients undergoing hip or knee surgery scheduled to have femoral nerve blockade were eligible to participate.

Interventions: The ultrasound probe was positioned in the inguinal region, and direct ultrasound identification of the femoral nerve, lying on the GIPM behind the iliac fascia, was obtained. A stimulating needle, inserted in-plane and advanced lateral to medial was directed towards the femoral nerve until it made contact with the target structure defined as the lateral segment of the GIPM.

Main Outcome Measure: Ultrasound identification of the lateral part of the femoral nerve and GIPM.

Results: An image compatible with the lateral part of the femoral nerve was observed in 91 out of 100 patients. In the remaining nine patients, when the lateral part of the femoral nerve was not seen, GIPM could be visualised in five (55%) patients. The iliac fascia and GIPM were clearly visualised in 68 and 85 patients respectively. In 85 cases when the needle tip was placed at the lateral segment of GIPM, a quadriceps femoris muscle motor response was obtained, and the distribution of the anaesthetic solution was observed behind the iliac fascia in all patients. In two patients, only the iliac fascia was identified, and in the two patients, none of these structures was correctly visualised.

Conclusion: The GIPM was seen in the majority undergoing ultrasound-guided femoral nerve blockade, even when the lateral part of the femoral nerve was not visualised. Using the lateral segment of GIPM as a target for needle tip location in an in-plane lateral to medial approach of the femoral nerve deserves further investigation.