Nerves around ankle

[Linh]

Dear Steve,

I got a referral today from a specialist asking for a lower limb nerve study: check for sural and anterior tiobial nerves, check retinaculae, to 4th-5th toes ?perineural scarring.

What is your scanning protocol for this sort of study? Are there areas we need to look carefully?

Best regards,
Linh

[Stephen Bird]

Hi Linh,

This referral is a little odd really,

The dermatome for the sural nerve is the lateral aspect of the hindfoot, so if the patient has sensory symptoms around the lateral calcanea area and along the area of cuboid / 5th metatarsal I would be very interested in the sural nerve.
The sural nerve is easy to find with ultrasound. If you start in the middle of the calf scanning in the midline you can see the small saphenous vein in the subcutaneous space. The sural nerve is immediately adjacent to the small saphenous vein. You can follow it distally where is passes just lateral to the Achilles tendon on it way to the hind foot. The sural nerve is easy to damage due to its superficial / subcutaneous location.

The next nerve you mention is the anterior tibial nerve. But there is no anterior tibial nerve!
There are two branches of the peroneal nerve (superficial and deep). The Deep peroneal nerve is the one that lives in the anterior tibial neuromuscular bundle. You can see it on the anterior aspect of the interosseous membrane in the calf and then follow it onto the dorm of the foot. Once it reaches the foot it does pass underneath the extensor retinaculum which has several bands passing over the hindfoot and mid foot. Irritation from the retinaculum may happen, however OA change in the mid foot with osteophyte formation at the tall-navicular, navicular-cuneiform joints are more common compression sites. The last place this nerve can come to grief is due to compression by the extensor hallucis brevis tendon which passes over the nerve almost exactly at the level of the anterior Lisfranc ligament which is between the medial cuneiform and the base of the 2nd metatarsal. The problem I have is that this nerve has a very specific dermatome and causes sensory symptoms only in the webbing between the big toe and the second toe so it is not implicated in the 4th or 5th metatarsal area.

The superficial branch of the peroneal nerve however does however supply the area of the 4th and 5th metatarsals. You can follow this starting from the common peroneal nerve. Find the muscle belly of short head of biceps femurs just proximal to the head of the fibula at the knee level. On the media aspect of the short head of biceps femurs you will see the nerve (common peroneal), follow it distally and it wraps around the neck of the fibula which is a spot where it can be compressed (healing fracture etc). Follow it distally and it divides into 2 branches. Follow the branch that passes down the lateral calf between the peroneal muscles and the extensor digitorum longus. About 2/3 of the way down the calf this branch surfaces into the subcutaneous space. At this level you can see traction injuries following ankle inversion. It is also a spot I have seen several neuromas. Follow the superficial peroneal nerve distally and it divides into terminal branches as it approaches the lateral aspect of the ankle / foot. You can follow the terminal branches down onto the dorm of the foot looking for any compression / deviation / swelling etc.

I suspect the superficial branch of the peroneal nerve is the one that was asked for as this fits the lateral foot pain scenario best.

The question about the extensor retinaculum I am uncertain about. You can see it easily enough as it flows on from the inferior peroneal retinaculum at the level of the peroneal retinaculum and passes medially across the foot. The problem is the superficial peroneal nerve branches are superficial to it so it is unlikely to cause compression.

I would have done a sural nerve study and a superficial branch of peroneal nerve study as described and then checked for other pathology like stress fracture / plantar fasciitis of the lateral band at the base of 5th metatarsal insertion, calcanea-cuboid arthritis / ligament injury

Sorry it is a complicated answer.

Let’s discuss it further!

Steve

[Linh]

Dear Steve,

Thank you so much for such a detailed reply. It took me some time to do extra reading + your answer to make sense for me.

Sural nerve wasn’t too bad for me to assess at the time but I spent a long time just trying to navigate with the peroneal nerves (I did annotate Deep and Superficial Pero nerves as I cant seem to find info on anterior tibial nerve anywhere!).

I found the superficial pero nerve at the level of the ankle (it sits in subcutaneous layer) and traces it down to either side of the 4th and 5th toes. At this point, it really is the digital nerve already with many branches. My question is: do we attempt to look for neuromas from its origin at the fibular head all the way down to the toes? If I cannot find any neuromas from ankle to toes, I shouldn’t call it NAD?

Happy New year to you Steve!

Linh

[Stephen Bird]

Yes indeed,

My preference for a nerve study is to do the length of the nerve visible with ultrasound.
Martinoli call it the elevator technique which I quite like. Scan it is short axis from the Fibula head to the foot in short axis. That way you can’t miss anything. You never know where a neuroma / nerve sheath tumour will be , nor a compression site.
If you do this and find nothing the answer is NAD

Steve

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