MRI for Tarsal Tunnel Syndrome

What Is Tarsal Tunnel Syndrome?

Tarsal tunnel syndrome (TTS) is a compression neuropathy of the posterior tibial nerve as it passes through the tarsal tunnel — a fibro-osseous channel on the inner (medial) side of the ankle, behind the medial malleolus (the bony bump on the inside of your ankle). It's the foot's equivalent of carpal tunnel syndrome in the wrist.

The tarsal tunnel is roofed by the flexor retinaculum (laciniate ligament), a thick band of connective tissue. Through this tunnel pass the posterior tibial nerve, the posterior tibial artery and vein, and the tendons of tibialis posterior, flexor digitorum longus, and flexor hallucis longus (remembered by the mnemonic "Tom, Dick, And Not Harry" — tibialis posterior, flexor digitorum longus, artery, nerve, flexor hallucis longus).

Within or just beyond the tarsal tunnel, the posterior tibial nerve divides into three branches: the medial plantar nerve, the lateral plantar nerve, and the medial calcaneal nerve. The specific branch compressed determines the pattern of symptoms.

Tarsal tunnel syndrome is less common than carpal tunnel syndrome and is probably under-diagnosed. Exact UK prevalence is uncertain, but it accounts for a notable proportion of chronic medial ankle and plantar foot pain cases that don't respond to standard treatment. It affects both men and women, with a slight female predominance, typically between ages 30 and 60.

Why MRI Is Used for Tarsal Tunnel Syndrome

Clinical diagnosis of TTS can be challenging — symptoms (burning, tingling, and numbness in the sole of the foot) overlap with plantar fasciitis, diabetic neuropathy, and lumbar radiculopathy. MRI serves several critical functions:

• Identifying the cause of compression: In about 60-80% of TTS cases, MRI can identify a specific structural cause of nerve compression. These include ganglion cysts, varicose veins (dilated venae comitantes), accessory muscles (flexor digitorum accessorius longus), tendon sheath swelling (tenosynovitis), lipomas, schwannomas, tarsal coalition, and post-traumatic scarring.
• Ruling out alternative diagnoses: Conditions that mimic TTS — stress fractures, plantar fasciitis, posterior tibial tendon dysfunction, inflammatory arthritis — are all visible on MRI.
• Pre-surgical planning: If tarsal tunnel release surgery is planned, the surgeon needs to know the exact location and nature of the compressive lesion, whether the nerve is compressed at the tunnel entrance, within the tunnel, or at the bifurcation into plantar branches.
• Assessing nerve changes: Advanced MRI can show direct signs of nerve injury — swelling, increased signal within the nerve, and changes in the muscles supplied by the nerve (denervation oedema).
• Evaluating post-surgical failure: If symptoms persist after tarsal tunnel release, MRI can identify scar tissue, incomplete release, or a missed compressive lesion.

MRI Sequences Used

• T1-weighted axial: Shows the tarsal tunnel anatomy beautifully — the nerve, artery, tendons, and retinaculum are all individually identifiable. Space-occupying lesions are characterised by their T1 signal: ganglion cysts appear dark, lipomas appear bright, schwannomas appear intermediate.
• T2-weighted fat-saturated axial: The key diagnostic sequence. Oedema around or within the nerve appears bright. Ganglion cysts are uniformly bright. Tenosynovitis shows as fluid around the tendons. Varicose veins appear as dilated, serpentine bright structures.
• STIR coronal and sagittal: Provides overview of the ankle, detecting bone marrow oedema, stress fractures, and broader inflammatory changes.
• Proton density (PD) axial: Excellent for nerve and tendon assessment. The fascicular pattern of the tibial nerve (multiple small nerve bundles within the epineurial sheath) can be appreciated on high-resolution PD images at 3T.
• T1 post-gadolinium fat-saturated: Used when a nerve sheath tumour (schwannoma, neurofibroma) is suspected, or to assess the degree of nerve enhancement (which indicates nerve injury or inflammation).
• MR neurography protocols (if available): Specialised sequences (3D STIR SPACE, SHINKEI) that use diffusion preparation and blood suppression to selectively visualise the nerve, producing images that trace the nerve along its course — essentially "nerve-only" images.

What Radiologists Look For

• Space-occupying lesion within the tunnel: The most common identifiable cause. Ganglion cysts (arising from the ankle joint or tendon sheaths) are the most frequent culprit, followed by varicose veins of the posterior tibial venae comitantes, and accessory muscles.
• Nerve swelling: The posterior tibial nerve normally measures 4-5mm in diameter at the tunnel. Enlargement suggests compression. Swelling may be focal (at the point of compression) or diffuse.
• Increased nerve signal on T2: The nerve should be isointense (similar signal) to muscle on T2. Increased T2 signal within the nerve indicates oedema from compression or injury.
• Loss of normal fascicular pattern: A healthy nerve shows a neat "honeycomb" pattern of fascicles on axial images. Loss of this pattern suggests injury.
• Denervation changes in supplied muscles: In acute/subacute denervation, the muscles supplied by the compressed nerve (abductor hallucis, abductor digiti minimi, flexor digitorum brevis, quadratus plantae) show increased T2 signal (oedema). In chronic denervation, fatty atrophy appears (bright on T1). These indirect signs support the diagnosis even when the nerve itself appears unremarkable.
• Tendon pathology: Posterior tibial tendon tenosynovitis (fluid around the tendon), tears, or spring ligament injury — all of which can be associated with or mimic TTS.
• Tarsal coalition: Bony or fibrous bridging between tarsal bones can narrow the tarsal tunnel. MRI shows both osseous (bone-to-bone) and non-osseous (cartilaginous or fibrous) coalitions.

How to Prepare for the Scan

• No special preparation — eat and drink normally
• Remove shoes, socks, ankle supports, and any toe or ankle jewellery
• Wear shorts or trousers that can roll above the knee
• If you use custom orthotics, you don't need to bring them for the scan (but mention them to your doctor as they may be relevant to treatment)
• The scan involves only the foot and ankle entering the scanner — claustrophobia is rarely an issue

What Happens During the Scan

You'll lie on your back with your foot positioned in a dedicated foot and ankle coil. The affected foot enters the scanner first. The scan takes approximately 25-40 minutes, depending on whether a standard ankle protocol or a dedicated nerve protocol is used.

Keeping your foot still is particularly important for this scan — the nerve and its branches are small structures, and any movement blurs the fine detail needed for diagnosis.

The scan is comfortable for most people. Because only the foot and lower leg enter the bore, the rest of your body remains outside, making it one of the least confining MRI experiences.

Contrast may be used if a nerve tumour or inflammatory cause is suspected, but most TTS evaluations are done without contrast.

What MRI Can't Show

• Nerve function: MRI shows nerve anatomy but not nerve conduction. Nerve conduction studies (NCS) and electromyography (EMG) remain important complementary tests, measuring the speed and strength of electrical signals through the nerve. However, NCS sensitivity for TTS is modest (50-90% depending on the study), and a normal NCS does not exclude the diagnosis.
• Idiopathic compression: In 20-40% of TTS cases, no structural cause is identified on MRI. The compression may be from dynamic factors (wrist position during walking), subtle fibrosis, or venous engorgement that isn't present during the scan.
• Dynamic compression: Weight-bearing and certain ankle positions may worsen nerve compression. Standard MRI captures the ankle in a neutral, non-weight-bearing position and may not reproduce the compression that occurs during activity.
• Very small branch involvement: The terminal branches of the plantar nerves (distal to the tarsal tunnel) are tiny. Compression at these distal sites (e.g. Baxter's neuropathy, Jogger's foot) may be beyond standard MRI resolution, though 3T scanners with dedicated coils are improving this.

Treatment Pathways After Diagnosis

• Conservative management: Activity modification, proper footwear with arch support, custom orthotics (medial posting to reduce hindfoot valgus), and anti-inflammatory medication. Night splints maintaining the ankle in neutral position can reduce nocturnal symptoms.
• Physiotherapy: Nerve gliding exercises (neurodynamic mobilisations), ankle strengthening, and addressing biomechanical factors (flat feet, excessive pronation) that contribute to tunnel compression.
• Corticosteroid injection: Ultrasound-guided injection into the tarsal tunnel (around the nerve) can reduce local inflammation and provide diagnostic and therapeutic relief. Response to injection supports the diagnosis.
• Tarsal tunnel release surgery: Division of the flexor retinaculum to decompress the nerve. If a specific lesion is identified on MRI (ganglion cyst, varicose veins), this is excised at the same time. Success rates are higher when a definable compressive lesion is identified on MRI (around 80-90%) compared to idiopathic cases (50-70%).
• Excision of space-occupying lesion: Ganglion cysts, lipomas, and schwannomas are removed. For varicose veins, ligation and excision of the dilated vessels is performed.
• Treatment of underlying condition: If TTS is secondary to inflammatory arthritis, diabetes, or hypothyroidism, treating the underlying condition is essential alongside local management.

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