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Femoral Nerve Block

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Femoral Nerve Block

Take home messages

  • Use ultrasound - it makes onset faster and needs less local anaesthetic
  • Remember that bones take innervation from all the surrounding nerves
  • You'll need sciatic blockade of some description for knee surgery

Why block the femoral nerve?

Aside from 'for fun' and 'ARCP', patient-centred reasons for blocking the femoral nerve include:

  • Analgesia for femoral neck and shaft fractures
  • Post operative analgesia (knee replacements, patellar surgery, skin grafts from anterior thigh)
  • Post-amputation phantom limb pain
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Bones are innervated by branches from all of the surrounding nerves

Single shot vs catheter

If you're doing a day case procedure, or the patient is hoping to depart home within 24-48 hours, then a single shot should be adequate (otherwise the procedure is probably not suitable for day case surgery).

If you're anticipating the patient is going to be in for a while, that it's going to be very painful, or there are other issues to think about such as:

  • Chronic pain syndromes
  • Previous opioid dependance
  • Likely to need revision or relook in the same admission
  • Palliation for pathological fracture with very short prognosis

Then a catheter is probably a wise choice.

Just remember this will need a dedicated Tuohy needle and catheter, which usually comes a specific regional nerve block set.

Briefly summarise the anatomy of the femoral nerve

  • Originates from L2 - L4 via lumbar plexus
  • Emerges lateral to psoas majour
  • Passes inferolateral to midpoint of inguinal ligament
  • Divides into motor and sensory branches
  • Motor supply to quadriceps, sartorius and pectineus
  • Sensory supply to anterior and medial thigh and knee
  • Sensory supply via saphenous nerve to medial leg, ankle and foot

How to block the femoral nerve

This being anaesthetics, there are multiple ways to skin a nerve and it doesn't really matter how you bathe the nerve in something ending in 'caine' as long as you do it safely.

That being said, the following have historically been the most commonly used approaches.

Femoral Nerve Block

  • Targets femoral nerve
  • Usually done under ultrasound
  • Great for anterior thigh and knee surgery
  • Doesn't cover lateral femoral cutaneous nerve or obturator nerve
  • Profound motor block

Three-in-One block

  • Targets femoral, obturator and lateral femoral cutaneous nerves
  • Injection is essentially at exactly the same site as femoral nerve block
  • Higher volume than femoral nerve block
  • Pressure applied to femoral nerve distal to injection to encourage proximal spread to cover obturator nerve as well
  • Inconsistent cover of obturator, often called '2-in-1' block

Largely superseded by ultrasound guided femoral nerve blocks.

The obturator nerve is primarily motor however it does have some sensory branches to the hip, making it useful to block in hip fracture surgery.

Fascia Iliaca block

The pop pop classic.

  • Targets femoral and lateral femoral cutaneous nerves
  • Can hit the obturator too if you're lucky
  • Large volume of relatively dilute mix injected lateral to femoral artery deep to the fascia iliaca
  • Less of a motor block than direct femoral nerve blockade
  • Generally considered to be safer as further from important neurovascular structures

Let's not beat around the bush here - they're all basically the same, they just vary ever so slightly with injection location, and they have different volumes of injectate.

As a general rule, the closer you can get the needle tip to the target nerve, the smaller the volume of local anaesthetic required.

The opposite correlation doesn't seem to be true - a bucket of 0.5% placed next to the patient does very little.

There is also the lumbar plexus block and newer PENG block, which we'll discuss later on.

Use Ultrasound

As if you need another excuse to drag the old reliable scanner out - any excuse to play with the toys.

Blocks have historically been performed using a combination of landmarks and nerve stimulators, however the vast majority are now taught and performed under ultrasound guidance, and for good reason.

What are the benefits of using ultrasound?

  • Less likely to inject intravascularly
  • Smaller volume of drug needed
  • Faster onset
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Tilting the probe can help visualise the nerve more clearly.

An OSCE station

This is an old block that's not used a whole lot these days, but as always, remains highly examinable.

What are the landmarks for performing a 3-in-1 block?

  • Anterior superior iliac spine
  • Inguinal ligament
  • Pubic tubercle

Palpate the ASIS and pubic tubercle. The inguinal ligament that joins these two points.

Mark the midpoint (above the femoral artery). The femoral nerve should be 1cm lateral to this. This is the needle insertion point, and a short blunt needle is advanced to around 3cm depth until paraesthesia felt.

Which nerves are blocked by a 3-in-1 block?

  • Femoral nerve
  • Lateral cutaneous nerve of the thigh
  • Obturator nerve

This is ideally. Much of the time, the obturator isn't covered very effectively, making it more of a '2-in-1 block'

Which nerve roots supply these nerves?

  • Anterior primary rami of 2nd, 3rd and 4th lumbar roots
  • These supply the lumbar plexus

What volume of local anaesthetic is used?

  • 20 - 30 ml

What precautions should be taken during injection?

  • Standard AoA monitoring
  • Monitor for intravascular injection and ensure intralipid available
  • Apply pressure over femoral nerve distal to the injection point, to encourage proximal spread of local anaesthetic

What current is supplied by a nerve stimulator?

  • 30 mA at a voltage of 6 - 8 V
  • Impulse of <0.3 msec
  • Frequency of either 2Hz or 50Hz

Higher frequencies produce pain before lower frequencies.

Which electrode of the stimulator should be applied to the nerve, and which should be placed on the skin?

  • Negative applied to nerve as it has a higher current density
  • This means 30% less current needed to produce a stimulus
  • This helps identify the nerve more accurately

Which nerves are stimulated first, motor or sensory?

  • Motor

The GOAT

Here's NYSORA's video, shared with permission.

Video credit

Here's a useful podcast episode on blocks for knee surgery

Useful Tweets

References and Further Reading

Ultrasound-Guided Femoral Nerve Block - NYSORA
The ultrasound (US)-guided technique of femoral nerve block, used for femur, patella, quadriceps tendon, and knee sugery; analgesia for hip fracture.
NYSORAOperater
Ultrasound-Guided Fascia Iliaca Nerve Block - NYSORA
Ultrasound-Guided Fascia Iliaca Block for anterior thigh and knee surgery, analgesia following hip and knee procedures
NYSORAOperater
The Hip (PENG) Block - NYSORA
Several alternative analgesic techniques have been recently suggested for hip pain (e.g., surgery, fractures, chronic pain). They all consist of an injection of a local anesthetic in proximity to the acetabulum, and in the plane between the iliopsoas muscle and the proximal insertion of the anterior hip capsule. The techniques vary in the transducer orientation, needle insertion and recommended volumes of local anesthetic. This new addition to NYSORA’s web-app is the approach used in our clinical practice for patients who qualify for fast-track after a primary total hip replacement (anterior approach). Functional Anatomy Pain after total hip replacement comes primarily from the anterior hip capsule which is innervated by the terminal nerves of the lumbar plexus: Femoral nerve: The articular branches from the femoral nerve reach the plane between the iliopsoas muscle and the iliofemoral ligament (iliopsoas plane) and innervate the anterior and lateral aspects of the hip capsule. Obturator nerve: The articular branches innervate the anterior and medial aspects of the hip capsule. Accessory obturator nerve: Contributes to the innervation of the hip in 10%-30% of the patients, and supplies the inferomedial aspect of the hip capsule. The cutaneous innervation of the anterolateral thigh is conferred by the lateral femoral cutaneous nerve (LFCN), which travels underneath the inguinal ligament, medially to the anterior superior iliac spine (ASIS) and courses distally, superficial to the sartorius muscle. The posterior aspect of the hip joint is innervated by the sciatic nerve and branches of the sacral plexus (superior and inferior gluteal nerves, and an articular branch from the quadratus femoris nerve).
NYSORANYSORA

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