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1
Department of Radiology, University of Michigan Medical Center, 1500 E.
Medical Center Dr. TC-2910G, Ann Arbor, MI 48109-0326.
2
Department of Radiology, Henry Ford Hospital, 2799 W. Grand Blvd., Detroit, MI
48202.
Received June 9, 1999;
accepted after revision November 12, 1999.
Address correspondence to J. A. Jacobson.
Abstract
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MATERIALS AND METHODS. Thirty intermetatarsal spaces were evaluated with sonography to diagnose Morton's neuromas. The prospective sonographic reports were correlated with surgical and pathologic findings. Each sonogram was then blindly and retrospectively reviewed to characterize mass echotexture, location, size, and continuity with the plantar digital nerve when present.
RESULTS. Surgery revealed 27 Morton's neuromas, one synovial cyst with infarcted tissue, one ganglion cyst, and one giant cell tumor of the tendon sheath. The prospective sonographic reports correctly identified neuromas in 85% of the cases. Retrospectively, 79.2% (19/24) of the neuromas were characterized as hypoechoic compared with muscle, whereas 12.5% (3/24) were of mixed echotexture and 8.3% (2/24) were anechoic. One half (50%) of the neuromas were located dorsal to the plantar aspect of the metatarsal heads, and 50% were both dorsal and plantar to this level. No statistical difference in height and width was found between neuromas and nonneuromas; however, nonneuromas were statistically greater in length than neuromas. All 15 masses in which presumed plantar digital nerve continuity with the mass was identified were neuromas.
CONCLUSION. Sonography can reveal a Morton's neuroma in 85% of cases. Identification of the presumed plantar digital nerve in continuity with the mass improves diagnostic confidence. The finding of an interdigital mass greater than 20 mm in length should raise suspicion of an abnormality other than a neuroma.
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Morton's neuroma can be diagnosed clinically with history of pain radiating from the mid foot to the toes [4]. Characteristic physical findings also exist, such as Tinel's sign and Mulder's sign (a palpable click when pressure is applied to the sole of the foot and the metatarsals are subsequently squeezed together) [5, 6]. Imaging correlation is particularly useful when the presentation is atypical. Preoperative localization of the neuroma often is warranted and imaging provides evaluation for the presence of multiple Morton's neuromas and for other causes of metatarsalgia.
Sonography has been used successfully for the diagnosis of Morton's neuroma with reported sensitivities of 95-98% [3, 7, 8]. The sonographic appearance of a Morton's neuroma is described as a hypoechoic intermetatarsal mass [7]. In our experience, we have noted variability in the echotexture and location of Morton's neuromas. In addition, we have identified presumed digital nerve continuity with several Morton's neuromas.
We retrospectively analyzed sonograms of Morton's neuromas to further characterize the sonographic features. We also reviewed the prospective sonographic reports to determine the ability of sonography to diagnose Morton's neuromas.
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The sonographic evaluations were performed by one of five staff radiologists using 7.5- and 10-MHz linear array transducers (5200S, Acoustic Imaging, Phoenix, AZ; and HDI 3000, Advanced Technology Laboratories, Bothel, WA). All patients were scanned from the plantar aspect of the foot. Equal pressure on the dorsal aspect of the foot from the sonographer's nonimaging finger assisted in viewing the intermetatarsal space by splaying the metatarsals. Liberal sonographic transmission gel was used in place of a standoff pad. Imaging from the dorsal aspect of the foot was not performed. Images were acquired in both transverse and longitudinal planes relative to the metatarsal shafts. In each case, images were labeled by the sonographer to identify the metatarsal space being evaluated.
Prospective Report Review
We believe the prospective sonographic report, rather than retrospective
review of static sonograms, is the best representation of the real-time
sonographic diagnosis. Therefore, the information in the original sonographic
report was compared with surgical results to determine the ability of
sonography to provide a correct diagnosis. The prospective sonographic reports
were reviewed by one radiologist who was unaware of the surgical results and
who categorized the report as positive, negative, or equivocal for the
presence of a Morton's neuroma.
Retrospective Image Review
Retrospective review of the sonograms was then completed to characterize
Morton's neuromas and other intermetatarsal abnormalities discovered at
surgery. Sonograms were reviewed by a single fellowship-trained
musculoskeletal radiologist with extensive experience in musculoskeletal
sonography who had not performed any of the prospective real-time
examinations. The reviewer had knowledge of the research study and knew that
all patients had undergone surgery. The images were reviewed for the presence
of an interdigital mass for each intermetatarsal space location. The
intermetatarsal space, if homogeneously hyperechoic, representing normal fatty
tissue, was classified as being without mass
(Fig. 1). The echotexture of
the mass was characterized as predominately anechoic, hypoechoic, mixed
echogenicity (anechoic and hypoechoic), or isoechoic relative to muscle. The
location of the mass was determined in respect to the plantar surface of the
metatarsal heads and characterized as dorsal to this level, ventral to this
level, or both. The width, depth, and length of the mass was determined
whenever possible. Finally, the presence of the presumed plantar digital nerve
longitudinally entering the mass was noted by identifying a linear, 2-3 mm,
relatively hypoechoic structure in continuity with the intermetatarsal mass.
Because color Doppler images were not obtained in all cases, this information
was not used in our study.
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Surgical Data
Surgery was performed by one of four orthopedic surgeons. The decision for
surgery was based on clinical findings and on the prospective sonographic
report. The pathologic report was reviewed for each case, and the operative
report was reviewed when available. The surgical results were considered
positive or negative on the basis of the pathologic findings. These results
were then compared with the prospective sonographic reports and the
retrospective sonograms. Statistical analysis was performed using the
Student's t test and Fisher's exact test to determine both the
ability of sonography to provide a correct diagnosis and whether any
characteristic sonographic features of Morton's neuromas might allow
differentiation from normal cases or other abnormalities.
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Surgical results of the 30 interspaces revealed 27 Morton's neuromas. One synovial cyst with infarcted tissue, one ganglion cyst, and one giant cell tumor of the tendon sheath were excised from the three other metatarsal spaces in question.
The prospective sonographic reports were categorized as 24 positive, two negative, and four equivocal for Morton's neuroma. Based on surgical and pathologic results, 23 sonographic results for Morton's neuroma were true-positive (Fig. 2A,2B). Two sonographic results were false-negative, and Morton's neuromas were revealed at surgery. The one false-positive sonographic result revealed a ganglion cyst at surgery (Fig. 3A,3B). No true-negative sonographic results were surgically verified. Of the four sonographic studies interpreted as equivocal, two were positive for neuroma. Of the remaining two equivocal sonographic results, a giant cell tumor (Fig. 4A,4B) and a synovial cyst with infarcted tissue were found surgically. The prospective reports correctly identified neuromas in 85% of the cases.
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Retrospective analysis of the sonograms identified an interdigital mass in 24 of the 27 cases in which neuromas were present at surgery. A mass was also retrospectively identified in the three cases revealing masses other than a Morton's neuroma at surgery. In three surgically positive cases, no mass was retrospectively identified.
Echotexture
Nineteen neuromas were characterized sonographically as hypoechoic relative
to muscle (Fig.
2A,2B),
three were mixed, and two were anechoic. Focal anechoic areas were seen within
the predominantly hypoechoic neuromas (Fig.
2A,2B).
None of the masses was isoechoic. Of the three other masses not representing a
Morton's neuroma, two masses were mixed echogenicity, and one was hypoechoic
on sonography. No statistically significant relationship was found (p
= 0.16) between the echotexture of the mass and surgical diagnosis.
Location
All nine of the masses identified as dorsal to the plantar aspect of the
metatarsal heads were neuromas. Twelve masses were located both dorsal and
plantar to this level (Fig.
2A,2B),
representing nine neuromas and the three other intermetatarsal masses. None of
the masses was solely located plantar to the metatarsal heads. In six cases,
mass location relative to the metatarsal heads could not be assessed. The
results of mass location were not statistically significant (p =
0.23).
Size
Statistical analysis was performed using only the masses in which
measurements of all three dimensions were obtained. This included 16 neuromas
and all three of the nonneuromas. The measured neuromas had an average width
of 6 ± 2 mm. The average height (craniocaudal) was 9 ± 5 mm. The
average length was 13 ± 6 mm. The nonneuromas had an average width of 8
± 2 mm. The height was 11 ± 2 mm and the average length 23
± 10 mm. Only the difference in length was statistically significant
(p = 0.02) (width, p = 0.14; height, p = 0.34). All
of the Morton's neuromas were less than 20 mm in length (Fig.
2A,2B).
Nerve Continuity
Presumed plantar digital nerve continuity with the interdigital mass was
present in 15 neuromas (Fig.
5). Nerve continuity was not identified in seven neuromas or in
the three false-positive cases. Continuity could not be assessed in five of
the cases. The results were statistically significant (p = 0.05).
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Patients typically experience a sharp, burning pain originating in the region of the metatarsal heads and radiating to the toes. Tingling or paresthesias in the same distribution may also be the predominant symptom. Symptoms are exacerbated by walking and relieved by rest. Women are more commonly affected than men. This female predilection has been attributed to the more pliable nature of the female foot and wearing high-heeled, narrow-toed shoes [9].
Although the cause remains uncertain, the treatment of Morton's neuromas has advanced dramatically from the "vigorous bloodletting" that Morton [9] recommended for acute exacerbations of pain. Conservative treatment includes the use of orthotic devices and local corticosteroid injections. In cases where symptoms are resistant to these forms of treatment, surgery often provides an effective cure [12]. Excision of the plantar digital nerve through a dorsal approach has replaced other, more aggressive techniques. Unfortunately, 20-30% of patients experience recurrent symptoms [13, 14]. Failures have been attributed to recurrent neuroma formation, scar tissue, and inadequate resection as well as to other causes of pain including referred symptoms from neuromas in adjacent interspaces.
These surgical failures emphasize the importance of imaging correlation in Morton's neuromas. Localization of neuromas and evaluation for multiple lesions with sonography can be performed when clinical findings are indeterminate. This can prevent surgical exploration of adjacent interspaces, which may lead to severe complications [7]. Other causes of pain can also be excluded [4, 11]. The differential diagnosis of metatarsalgia includes stress fracture, tendon sheath ganglion, foreign body reaction, nerve sheath tumors, dislocation of the metatarsophalangeal joint, and intermetatarsal bursitis [15,16,17]. Imaging findings are characteristic in many of these disorders.
Many studies advocate MR imaging for the evaluation of metatarsalgia, although reports have been conflicting [4, 5, 15, 18,19,20]. Zanetti et al. [4] prospectively diagnosed 13 of 15 surgically excised neuromas using MR imaging, advocating T1-weighted images for anatomic localization of the neuroma and T2-weighted images for limiting the differential diagnosis. Terk et al. [15] showed the usefulness of gadolinium enhancement and fat suppression in identification of neuromas using MR imaging.
Sonography has also been proposed as an accurate means of revealing Morton's neuromas. Resch et al. [20] dismissed the usefulness of sonography for this purpose because they found that false-negative results were common in their small series of patients. However, other studies, using larger patient groups, have identified 95-98% of Morton's neuromas prospectively and up to 100% retrospectively [3, 7, 8]. Our results corroborate these findings. In our study, 85% (23 of 27) of neuromas were identified prospectively. This increases to 93% if the two equivocal cases in which Morton's neuroma was suggested in the differential diagnosis are included.
Missed neuromas may be attributed to variability in sonographer experience and technique (a hypoechoic mass was identified retrospectively on sonograms in one false-negative case) and to the small size of a Morton's neuroma. The diameter of the normal plantar digital nerve is 1-2 mm at the level of the intermetatarsal heads and is not readily identifiable on sonography [3, 7, 11]. Most symptomatic neuromas are larger than 5 mm and this diameter has been proposed as a threshold value for symptomatology [7, 17]. However, size and symptomatology are not absolutely related as shown by Pollack et al. [8], and it is possible that some symptomatic lesions are too small to be revealed on sonography.
The appearance of Morton's neuromas sonographically has been characterized as a well-defined hypoechoic mass at the level of the metatarsal heads (Fig. 2A,2B). In our study, the majority of neuromas were indeed hypoechoic, although three were characterized as being both hypoechoic and anechoic. Shapiro and Shapiro [3] described hyperechoic areas within some neuromas and proposed that this was related to envelopment of fat within the mass.
In our study, the location of intermetatarsal masses relative to the metatarsal heads was surprising. In nine neuromas, we found the mass to lie completely dorsal to the plantar aspect of the metatarsal head or directly between the metatarsals. These findings not only contradict past literature, they also are anatomically counterintuitive. The plantar digital nerve courses plantar to the intermetatarsal ligament, which bridges the plantar aspects of the metatarsal heads. Therefore, at least a portion of a neuroma should be located plantar to the metatarsal heads in the region of the neurovascular bundle. There are two possible explanations for the observation of a dorsally located intermetatarsal mass. First, the Morton's neuroma may be located more dorsally than expected because of increased intermetatarsal space caused by abnormal metatarsal mobility [6]. Second, the mass could represent an enlarged intermetatarsal bursa coexisting with an occult Morton's neuroma because both may appear hypoechoic. Identification of an anechoic region with posterior acoustic enhancement suggests the presence of a coexisting bursa (Fig. 2A,2B). The plantar method of scanning does not appear to account for this unexpected position of the mass because equal pressure routinely is placed on the dorsal aspect of the intermetatarsal space by the sonographer's nonimaging fingers to assist in imaging between the metatarsals.
In nine other cases, the mass was characterized as both plantar and dorsal to the plantar aspect of the metatarsal heads, often having a bilobed appearance that is more typical of previous Morton's neuroma descriptions (Fig. 2A,2B). A coexisting, enlarged intermetatarsal bursa is likely in these cases; the size and extent of the masses could not be attributed solely to a Morton's neuroma. We propose that we often are imaging both a Morton's neuroma and an enlarged intermetatarsal bursa. This theory could not be confirmed in this study because the intermetatarsal bursa frequently is not removed or is destroyed during surgery. Additionally, the pathologists did not identify the bursa when confirming a Morton's neuroma.
The intermetatarsal bursa normally is present in each interspace dorsal to the deep transverse intermetatarsal ligament and extends distal to this structure only in the second and third interspaces [21]. Physiologic fluid distention of the intermetatarsal bursa, typically less than 3 mm in transverse dimension, has been noted in asymptomatic patients using MR imaging [17]. However, the intermetatarsal bursa often is found to be intimately associated with Morton's neuromas surgically, and inflammation of the bursa has even been proposed as a factor in the pathogenesis of these lesions [6, 10]. A significant relationship between fluid in the intermetatarsal bursa and Morton's neuromas has been described by MR imaging in the third interspace [17]. Shapiro and Shapiro [3] used sonography to reveal fluid in the intermetatarsal bursa in 10% of patients with surgically proven neuromas measuring 2-4 mm along the dorsal margin of the neuroma. The average size of neuromas imaged in our study was significantly larger than this, perhaps relating to measurement of both the neuroma and an associated bursa.
The typical appearance of Morton's neuromas is fusiform enlargement of the nerve resulting in an ovoid shape with the greatest dimension being length. Although no statistical significance was found with regard to measurements of height and width in our study, the difference in length between neuromas and nonneuromas was significant. The average length of the nonneuromas (23 ± 10 mm) (Figs. 3A,3B and 4A,4B) was much greater than that of the measured neuromas (13 ± 6 mm) (Fig. 2A,2B). No neuroma was greater than 20 mm in length.
Sonography in the sagittal plane often allows identification of presumed plantar digital nerve continuity with the mass (Fig. 5). Although the normal plantar digital nerve typically is not identified on sonography, the focal presence of a neuroma permits detection of what appears to represent the plantar digital nerve entering the neuroma. Perhaps the presence of nerve edema improves conspicuity [4]. Although we were not able to correlate with pathologic findings, we believe that this may represent an abnormal portion of the nerve. The proximal portion of the plantar digital nerve usually is more identifiable than portions distal to the mass where the nerve bifurcates.
The presumed finding of plantar digital nerve continuity with an intermetatarsal mass was present in 15 of our cases retrospectively; all were Morton's neuromas. In none of the other intermetatarsal masses was the digital nerve shown to enter the mass. Therefore, we believe that this finding (although not confirmed) raises the specificity for Morton's neuroma in the presence of an interdigital mass.
There are several limitations to this study. First, a selection bias exists because the most symptomatic patients proceeded to surgery. Additionally, surgeons were aware of and often acted on the sonographic results. Second, sensitivity and specificity could not be calculated because true-negative sonographic results could not be confirmed. Third, retrospective review of sonograms was limited to what was imaged prospectively. Complete characterization of Morton's neuromas could not be performed in several cases because of the lack of sufficient images. Missing images from other surgically investigated intermetatarsal spaces presents another limitation. Finally, given the low number (three) of soft-tissue masses not representing Morton's neuromas, it may be difficult to apply the differentiating sonographic features to larger populations.
In conclusion, sonography can prospectively indicate a diagnosis of Morton's neuroma in 85% of cases. Consistent with previous studies, we found a hypoechoic, interdigital mass in a high percentage of surgically proven neuromas. The finding of an interdigital mass longer than 20 mm should increase suspicion of an abnormality other than a neuroma. Identification of what appears to represent plantar digital nerve continuity with the mass should increase confidence that a neuroma is present. Finally, we raise the question of whether a Morton's neuroma is reliably distinguished from an adjacent enlarged intermetatarsal bursa. The presence of an adjacent intermetarsal bursa may cause the true size of the Morton's neuroma to be overestimated on sonography. A prospective study with detailed comparison to surgical results may be required to answer this question.
Acknowledgments
We thank Mignon Schley for compiling data and Gordon B. Jacobsen for
statistical analysis.
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