Femoral Nerve Block: Pain Control for Knee Surgery
Femoral Nerve at the Top of the Leg
What is a Femoral Nerve Block?
A femoral nerve blocks (FNB) is a commonly used type of peripheral nerve block. This means that a specific nerve, or group of them, is numbed in order to anesthetize a particular, relatively large, area of the body.
Anesthesiologists are the doctors who most often place peripheral nerve blocks such as FNBs. The anesthesiologists' training includes intensive training in performing and managing these nerve blocks.
FNBs are most frequently used for knee surgery, particularly total and partial knee replacements and ACL — anterior cruciate ligament — surgery.
They are often placed to help with pain relief after surgery using a long-acting local anesthetic rather than as the primary anesthetic for surgery.
Note: In the photo, the femoral nerve can be seen at the top of the thigh in an area called the 'inguinal crease'. The femoral artery is just medial (toward the midline of the body) to the femoral nerve.
Femoral Nerve Block Technique (with nerve stimulator)
There are variations in this technique, but this is a basic outline of the procedure.
Risks and benefits discussed with the patient- questions answered.
Full monitoring (EKG, blood pressure, oxygen monitor) and oxygen mask applied. Site (correct patient, site and side of surgery confirmed with staff and patient).
Light sedation given for patient comfort, but still able to communicate effectively.
Patient positioned flat in bed, leg to be numbed straight and out a bit from the body, relaxed.
Area where the front of the hip joins the top of the leg is cleaned with sterile soap.
Femoral artery found and marked (either with pen or finger).
Needle inserted about 1cm lateral (away from the midline of the body) to femoral artery.
Nerve stimulator on and set a 1 mAmp.
Needle carefully advanced until muscles in front of leg down to the knee twitch, nerve stim turned down to .3 to .5 mAmp to confirm disappearance of twich.
Aspiration (pulling back) on syringe attached to needle confirms no blood- not in femoral artery (or vein).
Local anesthetic injected slowly. Incremental injection punctuated by repeated aspiration to confirm needle has not advanced into a blood vessel. I do this every 5mL.
During the entire procedure, communication is maintained with the patient. Confirmation that there is no pain, electric shock type sensation, no signs that the medicine is going into a blood vessel (ringing in the ears, numb or tingling tongue, metallic taste in the mouth, etc).
Vital signs carefully monitored. Block checked by testing sensation to either cold or light pinprick or 'heavy' feeling when trying to lift the leg.
How is an FNB Done?
After discussing the risks and benefits of an FNB, the anesthesiologist will likely sedate you a bit for the procedure. You may or may not remember the procedure being done since these medicines can cause amnesia after they are given. Whether or not you remember it, you will be conscious and alert enough to answer the anesthesiologist's questions while they are placing the block. This helps him or her make sure they find the right spot and avoid complications.
Currently, the most common techniques for placing the FNB are by using either a nerve stimlator or an ultrasound image for guidance.
Ultrasound allows visualization of the nerve and surrounding structures as well as observing the needle as the block is being done. Learning to use the ultrasound and understanding the transmitted images takes some instruction and practice.
The nerve stimulator technique uses a special, insulated needle connected to a device that emits a light electrical discharge. When the correct spot is located, the nerve stimulator causes the nerve to fire and the muscles on the front of the thigh to twitch. When this twitch is found at an appropriate milliamp setting on the monitor, the numbing medicine is injected.
The particular local anesthetic used is chosen to be long-acting enough to provide pain relief after surgery, but with as little side effect or complication risk as possible.
The video below shows the anatomy relevant to the femoral nerve block. Then, at about the 3:20 mark, a demonstration of the actual block occurs, showing the nerve stimulator twitches, if you wish to skip the anatomy lesson.
Personal note: Although it looks uncomfortable, over the hundreds to thousands of these blocks that I've done, I've not had anyone say that they 'hurt'. People who remember the block (a minority), might say it felt 'weird' when their leg twitched without effort on their part, but pain is not usually a complaint. Also, despite what the video says, many of us are using the FNB alone for post-op pain relief after knee replacement. It does not take away all of the pain, but it reduces the amount of intravenous narcotic pain medicine needed for the first day after surgery.
Teaching Video on Femoral Nerve Block
TahoeDoc is a board-certified, practicing physician anesthesiologist. By writing these articles, she hopes to bridge the gap between what physicians think they explained to patients and what patients actually heard or understood.
When is an FNB Used?
As mentioned, and shown in the video, the FNB is most useful for surgery on the knee and front of the thigh. It can be used alone to take the edge off of post-op pain, or combined with a sciatic and/or other peripheral nerve blocks for anesthesia during knee surgery.
(In my practice, probably 90 percent of total knee replacements and 70 percent of ACL surgeries get a femoral nerve block. Almost everyone, it seems, who has one will elect to have another for other surgeries when it is possible.)
In addition to the 'single-shot' injection of local anesthesia described and shown, it is possible to place a catheter into the space around the nerve to provide continuous flow of pain medicine if the block would be needed for longer than a few hours or a day or so.
Risks, Problems, and Complications of FNB
All peripheral nerve blocks have some risks in common. Because a needle is used, despite its small size, there is always a risk of bleeding of nearby blood vessels. Bleeding can cause a hematoma — collection of blood — under the skin. If this hematoma is large enough, it can cause damage to nerves and other adjacent structures. Also, any entry into the body introduces the risk of infection. These are rare, but possible complications.
The biggest issue encountered with the femoral nerve block is incomplete pain relief. Realizing that the FNB will not cover the pain on the back of the knee is important. If this isn't discussed ahead of time patients, often and understandably, believe the block has failed or will be less than satisfied. Unfortunately, this fact seems to be omitted a lot in preop discussions.
Injury to the nerve can occur, but is less likely with the nerve stimulator or ultrasound techniques than with older processes that directly contacted the nerve with the needle.
Actual block failure is another risk. Although it happens less often than with other blocks, it is still possible to not be able to get a good block, with or without good procedural technique.
More rare complications can occur from the injected local anesthetic themselves. A most feared complication, fortunately rare, is injection or absorption of the numbing medicine into a blood vessel. This can cause seizures and cardiovascular collapse (cardiac arrest).