Femoral Block

Anatomy



The femoral nerve is a branch of lumbar plexus.  It arises from L2 to L4 lumbar nerves.  It descends through the fibers of the psoas major muscle and then is positioned between it and the iliacus muscle.  Here it is covered by the iliac fascia.  It then passes under the inguinal ligament.  It then divides into anterior and posterior division.  It lies on the lateral side of the femoral artery separated from it by iliac fascia. The anterior division gives rise to muscular branch (Sartorius and pectineus muscle) and cutaneous branches (medial and intermediate cutaneous nerves of thigh). Posterior division gives rise to muscular branches to quadriceps muscles and articular branches to the hip and knee joint. Articular branches are given by nerves to the quadriceps muscle.  

Anatomy relevant to the block:




The red line shows the level of the inguinal ligament.    However the level of inguinal crease is much lower than that. You will note that in the picture at the level of inguinal ligament a single femoral nerve lies lateral to the single femoral artery. 


However both femoral nerve and artery start to divide into multiple branches as soon as they pass under the inguinal ligament.  Femoral artery will divide into superficial and deep femoral artery.  Lateral femoral circumflex artery (LFCA) arises from the deep femoral artery.  LFCA may lie between the branches of femoral nerve.  At the level of inguinal crease more than one femoral artery may be seen.  




Femoral nerve also divides extensively below the inguinal ligament. We have discussed the branches before.  In an interesting paper published in 2010, Kenneth J et al looked at the fascicular anatomy of the femoral nerve.  They dissected eight femoral nerves   in four female cadavers.  They found the distance from the inguinal ligament to the first branching point of the femoral nerve was 1.50 0.47 cm (range 12cm).  Most branching occurred within a small length of the nerve. Branches to the pectineus muscle, sartorius muscle, and sensory nerves (saphenous and medial cutaneous) branched first and medially in seven out of eight specimens. In all but one specimen, the branch to the sartorius originated on the ventral surface. The branches innervating  vastus medialis, vastus intermedius, and vastus lateralis muscles were centrally located and the branch to rectus femoris muscle was lateral in the branching pattern of all specimens. The femoral nerve was an elongated ellipse  rather than circles. Femoral nerve width was almost five times the height thus was a more flattened structure.   This paper is available on the internet:
http://www.rehab.research.va.gov/jour/09/46/7/pdf/gustafson.pdf


So when ultrasound probe is placed below the inguinal crease you may see more than one artery or nerve.  Moving the probe cephalad as shown will result in visualization of single artery and nerve.




Femoral nerve lies underneath the iliac fascia.

The following video shows the ultrasound anatomy of the femoral nerve.


It is the main nerve above the knee. Below the knee the main nerve is sciatic nerve. Obturator nerve plays a small part on the medial side of the knee. br

Indications:
1.Providing postoperative analgesia for surgery on knee and hip joint (e.g. knee replacement, hip replacement).
2.Femoral fracture surgery: Very good for providing pain relief.  Beneficial opioid sparing effect in elderly.
3.Above knee amputation for pain relief.  Femoral nerve is the major component above the knee.  It plays a smaller role in providing pain relief following below knee amputation.  Sciatic nerve plays a greater part below the knee.

Contraindications: Absolute contraindications are patient refusal and infection at the site of femoral nerve block.

Equipment
1.Ultrasound machine with high frequency linear probe
2.Stimuplex or equivalent 100 mm needle
3.Chlorhexidine 2%  for asepsis
4.Local anaesthetic: For providing analgesia I use 0.25% levobupivacaine 20 ml.  If I do the block for providing anaesthesia then I use a higher concentration of local anaesthetic.  

Technique:


Patient lies supine.  Anaesthetist stands on the side of the patient the block is going to be done. The machine is positioned as shown in the picture so that everything is easily visible and accessible to the anaesthetist picture Ultrasound probe is placed below the inguinal crease and parallel to it so that a cross sectional view of a single femoral artery is obtained.  If more than one artery is seen the probe is moved cephalad till a single artery is seen.  At this point femoral nerve is located lateral to it.  Femoral nerve may be seen as a single structure or may have already divided into two divisions or multiple branches.  In that case an attempt is made to see a single nerve or to see the two divisions very close.  Sometimes if the patient is obese then it is not possible to move the probe cephalad  in that case best possible view of the nerve or its divisions is obtained. Needle is introduced in-plane so that the needle is seen approaching the nerve from lateral side.  The aim is to surround the nerve (or the divisions) with local anaesthetic. If nerve stimulator is used   I have observed that  when the needle is close to the posterior division a patellar twitch is seen and when the needle is closer to anterior division then a twitch of the Sartorius muscle is seen.



Following video is from RA-UK.  It shows in-plane ultrasound guided femoral nerve block



Video.