Position and anatomical landmarks
The patient is placed in the supine position with a pillow under his knees to increase comfort, especially if extension of the legs is painful. The lateral femoral cutaneous nerve (LFCN) can sometimes be identified by palpation. A point 2 cm medial and 2 cm caudal to the ASIS is selected as the injection site.
The needle (22-25-G) is inserted past the deep fascia of the thigh (fascia lata) where a 'pop' is heard when perforating the fascia. Thereafter, paresthesia can usually be quickly generated in the area of the lateral side of the thigh. The point is sought where there is maximum paresthesia.
The ASIS can be identified by means of ultrasound. The tissue caudal to the ASIS is scanned with the ultrasound probe (6 -13 Hz) in a transverse position in order to identify the sartorius muscle. The LFCN lies at this level above the sartorius muscle in the area under the fascia lata and above the iliac fascia. Running caudally, the LFCN enters a lenticular compartment between the sartorius muscle and the tensor fasciae lata muscle, formed by a double layer of the fascia lata and filled with fat. A 22-G needle is inserted in line with the ultrasound probe. The thermocouple is connected to the needle. The location of the LFCN is confirmed when the patient experiences reproducible paresthesia by means of stimulation (1 Hz; 1 mA). Thereafter, a 45-V pulsed radio-frequency current lasting for 120 seconds is administered with no preceding local anaesthetic. The maximum temperature should not exceed 42°C, but if this occurs the output should be reduced. If necessary, a second treatment period of 120 seconds can be administered if the impedance is above 500 Ω, prior to injection of 1 ml NaCl 0.9% solution.
- The most serious complication after the injection is infection.
- Small subcutaneous bleedings may result in a temporary increase in pain after the injection.