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Femoral Nerve Block: Pain Control for Knee Surgery

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I am a board-certified anesthesiologist in Lake Tahoe, California. I write from the perspective of both a doctor and a patient.

In a femoral nerve block, numbing medicine is injected around the femoral nerve at the top of the leg. The front of the leg down to the knee is numbed since this area is innervated by the femoral nerve.

In a femoral nerve block, numbing medicine is injected around the femoral nerve at the top of the leg. The front of the leg down to the knee is numbed since this area is innervated by the femoral nerve.

What Is a Femoral Nerve Block?

A femoral nerve block (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.

  1. Risks and benefits discussed with the patient; questions answered
  2. Full monitoring (EKG, blood pressure, oxygen monitor) and oxygen mask applied. Site (correct patient, site and side of surgery confirmed with staff and patient)
  3. Light sedation is given for patient comfort, but still able to communicate effectively
  4. Patient positioned flat in bed, leg to be numbed straight and out a bit from the body, relaxed
  5. Area where the front of the hip joins the top of the leg is cleaned with sterile soap
  6. Femoral artery found and marked (either with pen or finger)
  7. Needle inserted about 1cm lateral (away from the midline of the body) to the femoral artery
  8. Nerve stimulator on and set a 1 mAmp
  9. 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 twitch
  10. Aspiration (pulling back) on syringe attached to needle confirms no blood- not in femoral artery (or vein)
  11. 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.
  12. 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).
  13. 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 stimulator 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.

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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.

When Are They 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 a continuous flow of pain medicine if the block would be needed for longer than a few hours or a day or so.

Location of the femoral nerve with an ultrasound probe

Location of the femoral nerve with an ultrasound probe

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 in 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).

This content is for informational purposes only and does not substitute for formal and individualized diagnosis, prognosis, treatment, prescription, and/or dietary advice from a licensed medical professional. Do not stop or alter your current course of treatment. If pregnant or nursing, consult with a qualified provider on an individual basis. Seek immediate help if you are experiencing a medical emergency.


Bill on September 05, 2017:

I had a block and after surgery my blood pressure drop and my heart rate drop and I take blood pressure meds dr told me to stop my blood pressure. Meds for to weeks , it's been two months now and my blood pressure and heart rate is still not right , how often dose this happen

Heidi on October 31, 2015:

I am 5 days out from the. I had a nerve block done. Since it wore off I have had lots of groin pain. They have me on muscle relaxers bc they think its spasms.. I'm not too sure of that??? It is slowly getting beyter. Any ideas and anything I can do to help the pain

Zen on January 19, 2015:

I have femoral neuropathy I am currently seeing an orthopedic doctor but I want a second opinion. Should I see an orthopedic surgeon or neurosurgeon? I was wondering which type of doctor would be best to see. Thanks

Beau on May 19, 2014:

The nerve block is fantastic!! I have has 1 TKA and a revision and tomorrow I am having another TKA to fix revision..if that makes sense!! Anyway, the block WORKSzz

TahoeDoc (author) from Lake Tahoe, California on December 10, 2012:

Hi Steelernation, Sorry it took so long to answer you. I'm from Pittsburgh area originally, btw and went to med school at Pitt.

It sounds like you could have some residual nerve irritation or nerve injury. If it's possible, see if you can contact the anesthesia department where you had surgery. They might be able to have the anesthesiologist who took care of you get in touch with you. Some larger anesthesia departments, such as at academic centers, may have a clinic where you can be seen.

I understand you not wanting to take narcotics for this, but it may benefit you to take some anti-inflammatory medicines. There is also a med called Neurontin (gabapentin) that was originally an anti-seizure med, but is now used for nerve-type irritation or pain. It causes some sleepiness at first, but has relatively few side effects and is non-narcotic.

Of course without seeing you, I can't give you a specific diagnosis or say for sure if any of the things mentioned here would be appropriate for you as there are many variables that have to do with your history, exam and other meds.

So, please try to contact the anesthesiology department. If you cannot, then start with your surgeon and ask to be referred to someone who can help (pain management, neurologist, etc).

Good luck, and if possible, please try to let me know what happens. These things DO almost always resolve within a period of time (days, weeks, or sometimes months), but your discomfort should be treated in the meant time.

Steelernation on December 06, 2012:

Had an ankle surgery one year ago with nerve block & good results, no problems. However, had a knee resurface arthroplasty (opposite leg) 5 weeks ago, with complications, I think from the femoral nerve block. The skin of my thigh (mostly inner) and knee is extremely sensitive, often feels like a huge piece of tape is being pulled off the skin. Loose clothing bothers it to no end. It's a large area that's affected. Also feel pain going down my thigh in mostly a narrow straight line. Not bad pain, but again, bothersome. I don't want/won't take narcotic pain medicine, but wish there was more info available on how to treat or alleviate. Physical therapist says to rub it frequently with varying textures to try to desensitize it? Any other ideas?

TahoeDoc (author) from Lake Tahoe, California on June 17, 2012:

Thank you both! Marcy- you are right, you may have had a different type of nerve block. Same principle, different location. Nerve blocks are useful in many situations. Most frequently, I use femoral nerve blocks for knee surgery (replacements and ACL surgery) and interscalene blocks in the neck for shoulder surgery. They are very satisfying for the patient and doctors. For me, they are enjoyable to perform, as well.

Marcy Goodfleisch from Planet Earth on June 13, 2012:

I think I had a nerve block for foot surgery (it was given lower in my leg, as I recall) and it was a much easier way to go through it than the anesthetic I'd had for similar surgery on the other foot.

I had no pain during the surgery, was alert within a short time after the surgery, and they made me sleepy enough that I didn't hear much (thankfully) and was able to talk a bit but wasn't totally out. I would absolutely opt for this choice again.

Great hub - you always write such amazing and helpful hubs, and I learn so much from you!

Nettlemere from Burnley, Lancashire, UK on June 11, 2012:

Very thorough and useful hub, should be required reading for anyone about to undergo this procedure

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