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Regional Anaesthesia for the Knee

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Regional Anaesthesia for the Knee
Photo by CHUTTERSNAP / Unsplash

The sun is shining, the coffee's hot and you've been assigned to the trauma list with your favourite consultant.

You just know it's going to be a great day.

There are going to be some blocks, some haemodynamic wobbles to sort out and a bit of perioperative geriatric medicine thrown in for good measure.

You get changed, grab yourself a list and look up the first patient's details before going to do your assessment.

  • 67 year old man
  • Tibial plateau fracture after falling off a horse (amazing)
  • Edoxaban for atrial fibrillation, last dose yesterday
  • History of spinal stenosis and previous spinal surgery with metalwork from L2 to S1

What's your plan?

Hopefully the phrase 'GA and a block, obviously' sprung to mind.

But what blocks to choose?

Read on.

Some anatomy

If you're asked in an OSCE or SOE question about the innervation of the knee, use the following introduction to set off to a blinding start:

"The knee is supplied by components from the lumbar plexus, with the femoral and obturator nerves, and the sacral plexus, with the sciatic nerve."

Or something similar.

What is Hilton's law?

  • Hilton's Law states that a joint is supplied by the nerves supplying the muscles moving that joint, and the skin overlying their insertions.

Which nerves supply the anterior knee?

This is almost entirely by branches of the femoral nerve:

  • Saphenous nerve
  • Nerve to vastus medialis
  • Intermediate femoral cutaneous nerve
  • Medial femoral cutaneous nerve

So a proximal femoral block will provide excellent coverage of the anterior knee, however it will also knock out the motor components, dramatically hampering physio and recovery.

Which nerves supply the medial knee?

  • Saphenous nerve
  • Articular branches of the obturator nerves

Which nerves supply the lateral knee?

  • Branches of the common peroneal nerve
  • Lateral retinacular branches

Which nerves supply the posterior knee?

  • Branches of the posterior popliteal plexus
  • Articular branches of the tibial nerve
  • Genicular nerves

What is the popliteal plexus?

This network of nerves is formed of contributions from:

  • Tibial nerve
  • Posterior branch of the obturator nerve
  • Common peroneal nerve

It's the main target of the IPACK block (see below).

Primary nerve supply by area of interest

  • Anteromedial knee - saphenous nerve
  • Anterior capsule - femoral branches
  • Medial capsule - obturator
  • Posterior capsule - tibial nerve articular branches and the popliteal plexus
  • Skin over patella - femoral cutaneous branches
  • Medial leg - saphenous nerve

What are the components of the adductor canal?

  • Femoral artery
  • Femoral vein
  • Saphenous nerve

It may also contain the nerve to vastus medialis, as well as contributions from the medial femoral cutaneous and obturator nerves.

Which nerves supplying the knee arise from the lumbar plexus?

The lumbar plexus contributions are:

  • Femoral nerve (L2-L4)
  • Saphenous nerve (terminal sensory branch of femoral nerve)
  • Nerve to vastus medialis
  • Medial femoral cutaneous nerve
  • Intermediate femoral cutaneous nerve
  • Obturator nerve articular branches

These mainly supply the anterior and medial aspects of the knee.

An adductor canal block primarily targets the saphenous nerve, although other sensory branches may also be blocked in the process.

Why would an adductor canal block not cover posterior knee pain?

  • The posterior capsule is mainly supplied by articular branches from the tibial component of the sciatic nerve and popliteal plexus
  • The adductor canal block covers the saphenous nerve, and sometimes the nerve to vastus medialis, which contributes to innervation of the anterior knee
  • This means you need some sciatic cover too

Which nerves supplying the knee arise from the sacral plexus?

  • This is basically just the sciatic nerve and its branches
  • The tibial component produces articular branches to the posterior capsule and contributes to the popliteal plexus
  • The common peroneal also fires off articular branches to the lateral joint

Why might a patient have persistent incision pain after a seemingly successful block?

  • This might be due to failure to capture the medial femoral cutaneous and anterior femoral cutaneous nerves
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Hopefully now it's obvious that you're usually going to need two blocks - one for the lumbar plexus components, and one for the sacral.

What are we trying to achieve?

The question we're trying to answer here is:

"How do we block sensory innervation of the entire knee, while maintaining as much motor function as possible, to facilitate early mobilisation and recovery?"

Why is knee analgesia challenging?

  • Complex joint with overlapping innervation from two major nerves
  • Contributions from both lumbar and sacral plexi
  • Distinct anterior and posterior sensory territories
  • Motor and sensory branches in close proximity

This means that no single peripheral block will reliably cover the whole knee joint, so a combination of blocks is always required.

Is it possible to do a motor sparing adductor canal block?

  • Yes
  • The more distal you go, the more you'll selectively target the sensory saphenous nerve
  • However there will always be some proximal spread of local anaesthetic
  • The more proximal the spread, the more motor blockade you get
  • The nerve to vastus medialis is not simply a motor nerve, it also carries important articular sensory fibres
  • This is why some adductor canal blocks work better than others, it depends on whether they covered these articular branches

Femoral nerve block, femoral triangle block and adductor canal block exist on the same spectrum of increasingly distal blockade.

Femoral nerve block

  • Most proximal
  • Best analgesia
  • Most quadriceps weakness

Femoral triangle block

  • Intermediate
  • May capture medial femoral cutaneous nerve and nerve to vastus medialis
  • Balance of analgesia and motor preservation

Adductor canal block

  • Most distal
  • Primarily sensory
  • Best motor preservation

Moving distally generally preserves more motor function while potentially sacrificing some articular branch coverage.

What are the advantages and disadvantages of sciatic nerve blockade for knee surgery?

Advantages

  • Excellent posterior knee analgesia
  • Reliable coverage of tibial articular branches
  • Reduced opioid requirements

Disadvantages

  • Foot drop
  • Hamstring weakness
  • Delayed mobilisation
  • Increased falls risk
  • Potential masking of perioperative nerve injury

Motor block prevents early neurological assessment for potential surgical nerve injury.

Can't I just throw in a spinal?

Nice - single shot familiar technique, takes two minutes, no ultrasound needed, covers everything you need, job done.

Bosh.

You absolutely can use a spinal as your anaesthetic technique but there are a couple of issues:

  • It might be contraindicated (anticoagulation, spinal pathology, haemodynamic instability)
  • It doesn't last very long, so gives minimal postoperative analgesia
  • If you shove opiates in to prolong the analgesia, you get more labile blood pressures, slower mobility, more itching and sickness, and it still won't last as long as a good peripheral block

So yeah, you can use a spinal as part of your anaesthesia for the operation, but it's not really going to cut the mustard in terms of postop analgesia.

Unfortunately our horse riding man with a smashed tibial plateau is going to need some seriously good peripheral blocks instead.

Which local anaesthetic should I use?

As with everything in anaesthetics, your choice of drug is of course going to depend on your institution, consultant, personal experience and maybe what phase the moon is currently in.

However there are usually two main candidates for peripheral limb blocks:

  • Levobupivacaine
  • Ropivacaine

Because they have a nice long duration of action and the fact that they kick in slowly (takes a good 20 minutes at least to start working) usually isn't an issue if you're doing a spinal or general anaesthetic as well.

There appears to be some preference for ropivacaine due to its magical motor sparing tendencies, but to be completely honest it's as much about how you use the local anaesthetic as it is about which one you choose to employ.

What on earth is an IPACK block?

IPACK stands for infiltration of local anesthetic between the popliteal artery and capsule of the knee.

It was conceived as a motor sparing technique targeting:

  • Terminal sensory branches
  • Popliteal plexus

And thus giving excellent posterior capsule analgesia,

But not blocking

  • The tibial nerve
  • The common peroneal nerve

Hence it gets hailed for its motor-sparing properties.

💡
IPACK + adductor canal is a pretty ideal block combo.

Here's an awesome video on the IPACK block.

I'd like several relevant in depth articles please

Well of course.

Curated by the legendary Dr Jeff Gadsden.

Tell me about genicular nerves

Technically 'genicular' means 'of the knee' so any nerve supplying the knee is a genicular nerve.

The term is used to reference the terminal sensory branches accompanying the genicular arteries, in a characteristic distribution:

  • Superolateral
  • Superomedial (probably most important to block)
  • Inferomedial

They arise variably from the femoral, tibial, common peroneal and posterior obturator nerves.

There is also inferolateral but it's so close to the common peroneal that you don't want to try and block it, because you'll get foot drop and then might as well have just done a popliteal block.

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There's a trend of moving from blocking the large motorways (femoral, obturator, sciatic) to the smaller B roads (popliteal plexus and genicular nerves) for the motor sparing benefits.

And yes, we're quite pleased with that motorways pun.

Why don't we just do genicular blocks on their own?

There's way too much overlap of sensory innervation to the knee, with sensation also received through:

  • Nerve to vastus medialis
  • Saphenous nerve
  • Obturator branches
  • Popliteal plexus
  • Tibial articular branches

You'd be exceptionally lucky to catch all of these with local genicular blocks.

Don't forget to infiltrate

Amongst all this chat about clever nerve blocks, it's easy to forget a very simple but very effective technique that our surgical colleagues can help with - local infiltration.

Large volumes of dilute ropivacaine (or equivalent) can provide an excellent adjunct to whatever techniques you choose to employ.

Videos you have to watch

There's no substitute for video when it comes to learning regional anaesthesia, and so here are our favourite videos (shared with permission) on this topic that will be the single best use of your time on this topic.

It is seriously worth taking the time to watch these in full.

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References and Further Reading

Regional Anaesthesia - Anaestheasier
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