AJR 2003; 180:1121-1123
© American Roentgen Ray Society
Dynamic Sonography of the Forefoot: The Sonographic Mulder Sign
Martin Torriani1 and
Susan V. Kattapuram
1 Both authors: Department of Radiology, Division of Musculoskeletal Radiology,
Massachusetts General Hospital, 15 Parkman St.–WACC 515, Boston, MA
02114.
Received July 15, 2002;
accepted after revision September 3, 2002.
Address correspondence to M. Torriani.
Introduction
Morton's neuroma is a common cause of interdigital foot pain. Although
diagnosis is most often based on history and physical examination, clinical
findings are occasionally inconclusive
[1,
2,
3]. Sonography of the forefoot
is frequently requested to investigate causes of interdigital foot pain and is
performed using dorsal or plantar approaches
[1,
2,
3,
4,
5,
6,
7]. Interdigital pressure is
applied opposite the surface being scanned to splay the metatarsal bones and
improve visualization [1,
4,
7]. Mulder's clinical test is a
well-known maneuver in which compression of the metatarsal heads produces a
palpable click due to displacement of an intermetatarsal mass (e.g., a
neuroma) [8]. To our knowledge,
the application of Mulder's test during sonographic assessment for Morton's
neuroma has not been described. The test is a useful adjunct to routine
sonography of the forefoot and takes advantage of real-time imaging
capabilities.
Technique
Two sonographic techniques are used for assessment of Morton's neuroma. The
first consists of a dorsal approach in which the intermetatarsal spaces are
scanned with plantar flexion of the toes
[1,
5,
6]. Pressure can be applied to
the plantar aspect of the intermetatarsal soft tissues to improve
visualization [1]. The second
technique consists of scanning the plantar surface of the foot with
dorsiflexion of the toes [2,
3]. Pressure applied with a
nonimaging finger on the dorsal aspect of the intermetatarsal spaces causes
splaying of the metatarsal heads, improving the visualization of soft tissues
and increasing conspicuity of masses in this region
[4,
7].
Mulder's clinical test was first described in 1951
[8]. It consists of clasping
the metatarsal heads with the fingers of the left hand while the thumb of the
right hand exerts pressure on the sole of the foot at the site of the
suspected neuroma. The examiner should feel a mass that is pressed between the
metatarsal heads with the thumb and displaces to the plantar surface with
lateral compression of the metatarsals. When this maneuver is performed with
more strength, the mass may be compressed between the metatarsal heads before
it is displaced. In patients with a neuroma, this movement may elicit
characteristic pain coinciding with a palpable click caused by the tumor
escaping from the intermetatarsal space (the Mulder sign)
[8].
To perform Mulder's test during sonography, the examiner should direct the
patient to lie prone with the feet resting on the examining table. This
position provides stability for the sonographer's nonimaging hand, which holds
the foot during examination. The patient's foot should be held with the
metatarsal heads along an imaginary line between the hypothenar region and the
middle finger of the sonographer's hand
(Fig. 1). The transducer is
then applied to the plantar aspect of the intermetatarsal region, and the
intermetatarsal spaces are individually assessed in the coronal and sagittal
planes.
The normal inferior intermetatarsal spaces are predominantly composed of
fat. They contain the intermetatarsal neurovascular bundles, which are not
easily seen on sonography [4].
Therefore, if lateral compression is performed, displacement of hyperechoic
fat will be seen when the metatarsal heads are squeezed together. If findings
suggestive of Morton's neuroma (hypoechoic mass in the intermetatarsal space)
are identified, Mulder's test can be performed to increase diagnostic
sensitivity. Initially, some degree of pressure on the plantar surface should
be applied with the transducer in the coronal plane, lodging the mass between
the metatarsal heads. This movement should be followed by squeezing the
metatarsals together while relieving pressure on the transducer. At this
moment, the mass displaces toward the plantar surface, and a click may be felt
(Fig. 2A,
2B,
2C,
2D). If a neuroma is present,
characteristic symptoms may be elicited. For this reason, we recommend gradual
increase of metatarsal compression to avoid significant discomfort, especially
in more symptomatic patients. The same maneuver may be performed with the
transducer in the sagittal plane. However, the coronal plane provides better
landmarks (i.e., metatarsal heads) for revealing displacement of the mass.
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Fig. 2A. —56-year-old woman with Morton's neuroma. Sonogram of third
intermetatarsal space obtained in coronal plane without compression shows
hypoechoic mass (open arrows) with posterior acoustic enhancement
(solid arrow) located between hyperechoic metatarsal heads (3 and 4).
P = plantar.
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Fig. 2B. —56-year-old woman with Morton's neuroma. Sonograms obtained
during Mulder's test with increasing lateral pressure from B through
D show plantar displacement of neuroma as metatarsal compression
narrows interspace (arrows, D). Click and typical pain (Mulder
sign) were elicited between C and D.
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Fig. 2C. —56-year-old woman with Morton's neuroma. Sonograms obtained
during Mulder's test with increasing lateral pressure from B through
D show plantar displacement of neuroma as metatarsal compression
narrows interspace (arrows, D). Click and typical pain (Mulder
sign) were elicited between C and D.
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Fig. 2D. —56-year-old woman with Morton's neuroma. Sonograms obtained
during Mulder's test with increasing lateral pressure from B through
D show plantar displacement of neuroma as metatarsal compression
narrows interspace (arrows, D). Click and typical pain (Mulder
sign) were elicited between C and D.
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Discussion
Morton's neuroma is a painful entity of unknown cause that arises from the
plantar nerve. Histologic evidence points to a non-neoplastic enlargement with
varied degrees of perineural fibrosis, local vascular proliferation, edema of
the endoneurium, and axonal degeneration
[1,
4]. The plantar digital nerve
of the third intermetatarsal space is most frequently involved. The condition
is most commonly seen in middle-aged women, presumably because the foot is
more pliable in these patients and because they are likely to have worn
high-heeled, narrow-toed shoes on a regular basis
[4]. Clinical findings include
a history of pain radiating from the midfoot to the toes, Tinel's sign
(radiating pain when pressure is applied to the web space), and the Mulder
sign [4]. More than one web
space may be involved, and bilateral involvement is common
[1]. Differential diagnostic
considerations of intermetatarsal pain include bursitis, metatarsophalangeal
synovitis, ganglion cyst, stress fracture, and flexor tenosynovitis
[1].
Sonography has been proposed as an accurate means of detecting Morton's
neuroma. Several studies have identified 85–98% of neuromas
prospectively and as many as 100% retrospectively
[4,
5,
7]. MR imaging has shown good
results in diagnosing Morton's neuroma, but sonography has proven to be a
specific diagnostic tool that is accurate, well tolerated, and readily
available [1,
2,
3,
5,
6].
The normal plantar nerve at the level of the metatarsal heads measures
1–2 mm in diameter; it is not easily seen on sonography
[4,
5,
6]. It seems reasonable to
adopt 3 mm as a threshold for considering hypoechoic intermetatarsal areas as
abnormal [1,
3,
5]. Although most symptomatic
neuromas are larger than 5 mm in diameter
[5,
7], size and symptoms are not
absolutely related, and some symptomatic lesions may be too small to be
revealed on sonography [4].
Detection of arterial pulsation as well as identification of continuity with
the interdigital nerve facilitate localization and increase diagnostic
confidence. Adding metatarsal compression to the sonographic investigation for
Morton's neuroma may allow visualization of small or otherwise occult lesions.
In addition, the displaced mass is more reliably measured and evaluated.
Morton's neuromas may be in two locations relative to the plantar aspect of
the metatarsal heads [4]: both
plantar and dorsal, often having a bilobate appearance, or completely dorsal
(between the metatarsal heads). In one study of 30 intermetatarsal spaces, 50%
of confirmed neuromas were located directly between the metatarsals
[4]. Although these masses were
identified using a routine sonographic protocol, performing Mulder's clinical
test while scanning may provide increased conspicuity and diagnostic
confidence, regardless of the location of the mass.
Other intermetatarsal abnormalities may mimic the sonographic appearance of
Morton's neuroma, including schwannomas, cysts, and intermetatarsal bursitis.
The latter is frequently seen with Morton's neuroma
[1,
4,
7] and has been implicated in
the pathogenesis of this lesion
[4,
7]. Further experience with the
proposed sonographic maneuver is necessary to determine its value in
differentiating intermetatarsal masses.
Conclusion
Sonography is a low-cost, expeditious, and accurate method for diagnosing
Morton's neuroma and intermetatarsal masses. Performing Mulder's clinical test
during sonographic assessment of the forefoot may increase diagnostic
confidence and overall accuracy.
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