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The femoral nerve is formed from the lumbar plexus, consisting of branches from L2 to L4.
It supplies motor innervation to the extensors of the knee (Quadriceps Femoris
and Sartorius), sensory innervation to the anterior thigh,
anteromedial aspect of the knee, medial lower leg and the medial aspect of the
ankle and foot (via it's terminal sensory branch, the saphenous nerve) and supplies the periosteum of the femur.
It emerges below the inguinal ligament lateral to the vascular bundle (femoral vein and artery), and quickly divides into sensory and motor groups. It is covered by, and separated from the vascular bundle by the fascia iliaca encloses it laterally and flattens the nerve between itself and iliopsoas on which the nerve lies. Overlying fascia iliaca, there is fascia lata which runs together with the fascia iliaca laterally, whilst separating from it and overlying the femoral vessels medially. The fascia iliaca compartment within the pelvic brim also contains the lateral cutaneous nerve of the thigh laterally. This nerve supplies sensory supply to the lateral thigh.
Clinically relevant information
The femoral nerve is commonly blocked for surgeries involving the hip, knee or ankle as it supplies a varying proportion of sensory innervation to both cutaneous sensation, and joint innervation. The lateral cutaneous nerve of the thigh is useful for cutaneous analgesia of the incision site for hip procedures. Prolonged knee extensor weakness is a common worry of surgeons following blockade of the femoral nerve, but this can be minimized by the use of lower concentrations/volumes of local anaesthetics.
Choice between a femoral nerve block, or a FICB is usually down to anticipated use. A FICB is a volume block (as described below), and so although blockade of the LCNT is anticipated, the volumes needed (and hence concentrations required) mean that surgical anaesthesia is unlikely. It is extremely useful for analgesia of a fractured neck of femur, and can be safely performed in A&E with adequate monitoring. A femoral nerve block is more useful where LCNT blockade is not required, and potentially can be used in conjuction with blocks of other nerves due to lower doses of local anaesthetic required (e.g in combination with sciatic nerve block for total knee replacement). The major advantage of a FICB is that the nerve and artery can be avoided, therefore reducing potential complications of needle trauma.
Use a high frequency, linear probe for this block. Orientate the probe transversely perpendicular to the inguinal ligament. Patient should be supine, with (ideally) the leg slightly externally rotated. The femoral nerve can be visualized below the inguinal ligament as a flattened hyperechoic, fasiculated structure on the medial surface of iliopsoas, with the femoral artery immediately medial to it (see figure 1). Watch for the lateral femoral circumflex artery, which occasionally branches directly from the femoral artery, and can traverse the image. Use of colour flow doppler can often help.
The fascia iliaca (figure 2) should be identifiable as a bright, hyperechoic linear structure superficial to the iliopsoas muscle which passes over the femoral nerve medially to form the lateral wall of the structure surrounding the femoral vessels (the lacuna vasorum). It can be difficult to visualize, as it may closely resemble the fascial structures around it. Careful angulation of the transducer caudally will often reveal the nerve.
If the femoral artery has split into 2, then probe position is too low, and should be repositioned more cephalad.Needle position for a femoral nerve block is usually to approach in plane of the transducer, from lateral to medial, aiming to deposit local anaesthetic above the femoral nerve laterally, just below the fascia iliaca.Needle position for a FICB is more laterally, aiming to penetrate first the fascia lata, then the fascia iliaca (second "pop") 1-2 cm lateral to the nerve.Local anaesthetic spread should be visible above the femoral nerve in both blocks, which should appear to "float" in a pool of hypoechoic local anaesthetic (figure 3).
Conduct of the block
Femoral Nerve Block
We recommend you prepare a block tray with the following; a short bevel block needle (with or without nerve stimulation), syringes, drawing up needle, 25g needle for local infiltration, skin preparation, a small sterile drape (preferably with a hole), sterile ultrasound gel, sterile ultrasound probe cover (sheath or tegaderm), 1% lidocaine for infiltration and 10ml of 0.25% Bupivicaine (or similar), sterile gloves.
Apply standard monitoring.
Give the patient appropriate sedation, such that they are comfortable and able to tolerate the procedure.
After a pre scan to determine optimal probe position and normal anatomy, prepare the skin with your antiseptic, place drape and cover ultrasound probe ensuring no air gap. Scan as described above.
Insert the local infiltration adjacent to intended point of injection, and wait sufficient time to achieve cutaneous analgesia.
Flush the block needle completely with local anaesthetic, then insert in plane (parallel to the long axis of the ultrasound transducer, in the centre of the beam), attempt to visualise the needle at the lateral edge of the picture (it should become visible after it has been inserted 1-2cm into the skin, if not, reposition the probe until it becomes visible, do not insert any further). Angle the needle so that the tip pierces fascia iliaca on the superior surface of the femoral nerve at a safe distance from the femoral artery.
Aspirate to ensure the needle has not penetrated a vessel, then cautiously inject 1-2ml of local anaesthetic and ensure that spread is visible from your needle tip. If you are unable to see hypoechoic local anaesthetic emerging, then you have not adequately visualised the needle tip and should reposition the transducer. If the spread is visible below the fascia iliaca, then continue to slowly inject the whole 10ml observing for adequate spread around the femoral nerve, and watching carefully for enlargement of the nerve (which could represent intraneural placement of the needle tip). Injection pressures should be low, and the patient should be observed for paraesthesias or pain during injection.
If a catheter is to be placed, we use a 19g Tuohy tip set, using the same approach. Full aseptic precautions are necessary when placing catheters. Remember to hydrodissect using either local anaesthetic or saline to create "room" for the catheter to enter. Fixation can be achieved with a "Lockit" along with Opsite adhesive spray.
Fascia Iliaca Compartment Block
Conduct as for Femoral Nerve block, with the following changes.
Can be safely performed asleep as long as the target is truly lateral to the femoral nerve.
Use 30-40ml of local anaesthesia, calculate the concentration based on body weight. Remember this block is for cutaneous analgesia, not surgical anaesthesia.
Target for injection should be 1-2cm lateral to the lateral border of the femoral nerve.
Spread should be visible under the fascia iliaca, and medially to the femoral nerve. Much of the spread will be cephalad, and therefore not directly visible.