Popliteal block

The popliteal block is used to provide analgesia for operations on the ankle. 
The advantage of this block over the proximal approaches to the sciatic nerve are that the hamstrings are spared. 
The block can be used for forefoot operations but an ankle block is preferred technique as it does not cause foot drop.  

Anatomy:
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1. The popliteal fossa is a diamond shaped region at the back of the knee. The superomedial boundary is made up of semimembranosus and semitendinosis muscle while the superolateral boundary is made by biceps femoris muscle.
The two heads of gastrocnemius form the inferolateral and medial boundary. 
The Neurovascular bundle lies in the fossa covered by skin and subcutaneous tissue.  
2. The popliteal vein and artery are deeper (away from skin) compared to the nerves when looked at from behind the knee.  
3. The sciatic nerve divides into its two terminal (Tibial and Common peroneal) branches 5 to 12 cm above the popliteal crease.  
4. The nerves are always superficial (closer to the skin) than the artery and vein. 
Compared to the common peroneal nerve the Tibial nerve lies closer to the artery.  It is superficial and lateral to the artery. 
The common peroneal nerve lies lateral to the tibial nerve on the inner side of biceps femoris muscle.  
5. The tibial nerve is always larger than the common peroneal nerve.  
6. In most patients (Depending on the circumference of the thigh) the nerves are within 2-4 cm of the skin.  
7. The medial side of the leg and foot is innervated by  the saphenous nerve so that this nerve will need to be blocked separately.

Indications:

1. Surgery on the ankle (e.g. ankle fusion or replacement) and foot.
Although for forefoot operations an ankle block is more appropriate. 
A proximal sciatic block will affect the hamstring muscles, which will be spared with popliteal block. 
A popliteal block will always cause foot drop though.  

Contraindications:
1. Patient refusal
2. Coagulopathy
3. Infection at the site of the block.
4. If patient needs to be mobilized immediately after operation then foot drop may be a problem.

Key points:
1. Start scanning just above the popliteal crease and follow the nerves cephalad.
2. The thigh is not a perfect cylinder and nerves do not travel in a straight line in it. 
In order to get the best picture of nerve the probe will have to be angled cephalad and caudal and rotated clockwise and anticlockwise.
3. Look for the blood vessels first.  The nerves are always superficial and lateral to them.    

Necessary Equipment:
1. Ultrasound machine with high frequency probe, probe cover and ultrasound gel
2. Insulated stimulating needle (I use 100 mm stimulating needle)+/- nerve stimulator
3. Local anaesthetic: I use 20 ml of 0.5% levobupivacaine if block is done for anaesthesia.  For postoperative analgesia 0.25% levobupivacaine is also sufficient  
4. 2% chlorhexidine  to clean the skin

Locating the nerve:
The ultrasound probe is placed transversely across the popliteal fossa, just above the crease. 
Anticipated depth of the nerves is between 2-4 cm and the ultrasound depth is set accordingly. 

Do not try to see the nerves first.
Find the popliteal artery first which will be seen as a pulsating structure. 
Once you have found the popliteal artery you may find the vein above the artery. 
The vein may not be seen as the pressure applied on the ultrasound probe may obliterate the vein. 
Look superficial and lateral to the blood vessels. 
The tibial nerve will be seen as a hyper echoic structure.
If it is not seen immediately, you may have to change the angle the probe makes with the skin to create an ideal angle for reflection of ultrasound beam from the nerve.
Once the tibial nerve is found move the probe cephalad keeping tibial nerve in centre of picture and in focus. 
As the probe is moved cephalad another smaller nerve will be seen coming from lateral side of the screen.  That is the common peroneal nerve. 
These two nerves will eventually join together to form the sciatic nerve.


The artery (Hypo echoic round structure) is at the bottom. 
The tibial nerve(larger) and common peroneal nerve(smaller) are visible more superficial.

Mouseover image for annotation.

You can see the nerves do see-saw with foot movement in the video below.





Performing the block:


If the patient is awake I do this block with patient in lateral position. 
The leg to be blocked is uppermost and the ultrasound machine is placed such that the screen is easily visualized.
  
In an anaesthetized patient, the block is done with patient supine, breathing spontaneously on LMA.
The patients leg rests on a chair.  Note the position of patient, anaesthetist and machine in the picture.


I stimulate both the nerves individually under ultrasound guidance. 
I approach the nerves in plane. 
I approach the nerves at a point where they are near each other.

Use 20 ml of local anaesthetic.
It is important to see the spread of anaesthetic around the nerves.
Always combine ultrasound with nerve stimulation.

Nerve stimulation

Tibial nerve stimulation causes plantar flexion
Common peroneal stimulation causes dorsi flexion
Simultaneous stimulation of both nerves causes inversion.


The following video demonstrates an ultrasound guided popliteal nerve block. 
This video is courtesy of 'Nerve Imaging Group'.