Morton's Neuroma and Forefoot Ultrasound: Technique and Measurement Thresholds

Forefoot pain is a common referral, and Morton's neuroma is one of the more satisfying small-parts diagnoses to make on ultrasound — it's superficial, it's dynamic, and a well-performed scan genuinely changes management. It's also easy to get a false negative from if your technique is off, so the details matter more than they might seem to for such a small structure.

What a Morton's Neuroma Actually Is

Despite the name, it isn't a true neuroma (a nerve tumor). It's a perineural fibrosis of the common plantar digital nerve, most often at the level of the intermetatarsal ligament, caused by chronic mechanical irritation. It occurs almost exclusively in the third intermetatarsal space (between the third and fourth metatarsal heads) and, less commonly, the second space. It is rare in the first and fourth spaces — a "neuroma" reported there deserves a second look for an alternate diagnosis.

Typical patient: Burning, cramping forefoot pain radiating into the toes, often worse with tight or narrow shoes, sometimes with a sensation of "walking on a pebble" or a fold in the sock.

Anatomy of the Intermetatarsal Space

The common plantar digital nerve runs plantar to the deep transverse intermetatarsal ligament before dividing to supply adjacent toes. This position — just deep to the ligament, at the level of the metatarsal heads — is exactly where a neuroma forms and exactly where you need to look.

Scanning Technique

Probe: High-frequency linear transducer, 12–18 MHz.

Patient position: Supine or seated, foot relaxed in slight plantarflexion, sole flat against the exam table or supported.

Two approaches — use both:

1. Dorsal approach: Probe placed transversely across the dorsum of the forefoot at the level of the metatarsal heads. This is the standard starting view and avoids compressing the space with direct plantar pressure, which can obscure a small neuroma.

2. Plantar approach: Probe on the sole, between the metatarsal heads. Useful for confirming a finding seen dorsally and for very superficial lesions, though direct pressure from the probe can compress and hide a soft neuroma — light contact only.

The maneuver that finds the ones you'd otherwise miss — the toe squeeze (Mulder's sign under ultrasound):

1. Locate the intermetatarsal space transversely from the dorsal approach
2. Have the patient (or you, with your free hand) apply firm medial-to-lateral compression across the forefoot, squeezing the metatarsal heads together
3. Watch the space in real time — a neuroma will bulge plantarward, becoming more conspicuous and sometimes producing a palpable and visible "click"

This dynamic compression view catches neuromas that are equivocal or invisible on a static scan. Do it on every study where the static images are negative or borderline but the clinical suspicion is high.

Sonographic Appearance

• Hypoechoic, ovoid or lobulated mass centered at the plantar aspect of the intermetatarsal space, typically at or just distal to the metatarsal heads
• Longitudinal orientation — appears elongated when scanned along the long axis of the nerve, rounder in the transverse plane
• Usually avascular or hypovascular on color Doppler — this helps distinguish it from an inflamed bursa
• No posterior acoustic enhancement pattern typical of a simple cyst; internal echotexture is solid, not fluid

Measurement and the Threshold That Matters

Measure the neuroma in its largest transverse dimension (short axis).

• > 5 mm is generally considered a size threshold that correlates with a higher likelihood of clinically significant, symptomatic neuroma.
• < 5 mm neuromas can still be symptomatic — size correlates with but does not define clinical significance. Correlate with the location of the patient's pain and reproducibility with the toe-squeeze maneuver, not size alone.

Document three dimensions when possible, and always compare to the contralateral foot if the patient has any pain there too — small, asymptomatic neuromas are a common incidental finding.

The Key Differential: Intermetatarsal Bursitis

This is the most important thing to get right, because management differs and the two can look similar on a quick glance.

A bursa with adjacent inflammation can coexist with a neuroma in the same space — describe both if present, since a bursa alone won't respond to neuroma-directed treatment (injection targeting, alcohol ablation, or surgical excision).

Other Differentials to Keep in Mind

• Metatarsal stress fracture — cortical irregularity/step-off, point tenderness, no soft mass
• Plantar plate tear — at the metatarsophalangeal joint itself, not the intermetatarsal space; associated with toe malalignment
• Ganglion cyst — anechoic, compressible, posterior enhancement

Reporting Language

Positive finding: "A 7 × 4 mm hypoechoic, avascular, ovoid mass is identified in the plantar aspect of the third intermetatarsal space at the level of the metatarsal heads, corresponding to the patient's site of pain. The mass becomes more prominent with the toe-squeeze maneuver. Findings are consistent with Morton's neuroma."

Negative but clinically suspicious: "No discrete mass identified in the second or third intermetatarsal spaces on static or dynamic (toe-squeeze) imaging. Clinical correlation is recommended; a small or early neuroma cannot be entirely excluded."

Key Images to Capture

1. Dorsal transverse view of the symptomatic space, static
2. Same view during toe-squeeze compression
3. Longitudinal view of the mass, if identified, with measurements
4. Color Doppler over the mass
5. Contralateral foot for comparison if bilateral symptoms

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SonoBuddy's MSK guides include shoulder, knee, ankle, and now forefoot ultrasound protocols.