Morton neuroma was first described by Thomas Morton in 1876 as "a peculiar and painful affection of the fourth metatarsophalangeal articulation" (17
). However, subsequent studies have shown that the third and second spaces are the more common sites for neuroma (5
). Our study has also demonstrated that the third space is the most common site of involvement.
The exact pathogenesis of Morton neuroma remains controversial. It is generally accepted that the development of Morton neuroma may be induced by repetitive compression of the plantar nerve against the deep transverse intermetatarsal ligament, with subsequent perineural fibrosis (18
A tentative diagnosis of Morton neuroma can be made at the time of clinical assessment, but imaging correlation is required for the exact localization of neuroma and for the detection of multiple lesions, and especially when the clinical findings are indeterminate. This can prevent patients from undergoing rather avoidable surgical exploration at the wrong sites, which may lead to inadvertent complications (5
). In addition, other causes of metatarsalgia can also be excluded via imaging modalities. The possible clinical differential diagnoses that can cause metatarsalgia include intermetatarsal bursitis, stress fractures, necrosis of the sesamoid bones, synovitis of the metatarsophalangeal joint, infection and true neoplasm (3
Some differential diagnoses should be considered during the imaging analysis. These include intermetatarsal bursitis, ganglion cyst, synovial cyst, giant cell tumor (GCT) of the adjacent tendon sheath, fibromatosis, nodular fasciitis, pigmented villonodular synovitis and physiologic fluid distention of the intermetatarsal bursa when examining by using US and MRI (4
). An investigation using US suggested that if a mass in the interdigital space is greater than 20 mm in length, then it is an abnormality other than neuroma such as a ganglion cyst, a synovial cyst or a GCT of the adjacent tendon sheath (17
Several sonographic study results have disclosed a prospective sensitivity of 95-98% for Morton neuromas and a retrospective sensitivity of up to 100% (5
). However, a recent study by Quinn et al. (17
) revealed that 85% of neuromas with an average width of 6 mm, an average height of 9 mm and an average length of 13 mm were identified prospectively. Analysis by US in our study showed a relatively low detection rate (79%) of Morton neuroma. There are multiple reasons for the various reported detection rates with using sonography. Sonography is a very operator-dependent technique, for instance, the sonographer's experience is quite important, and so erroneous detection does factor into US examinations. The size of the Morton neuroma is considered as another factor contributing to the variability of detection. The diameter of the normal plantar digital nerve is 1-2 mm at the level of the intermetatarsal heads and the normal nerve is not readily identifiable on sonography (5
). In our study, the mean sizes of the masses were 4.9 mm with using US and 5.1 mm with using MRI, and these values were smaller than those reported in a previous study (6.5 mm for US and 7.4 mm for MRI) (11
MR imaging is a highly sensitive modality (87%) for the detection of Morton neuroma (1
). Our MR imaging evaluation (detection rate: 76%) did not show a better detection result than that expected at the beginning of the study. Four out of 17 neuromas were not detected. In these cases, two neuromas were as small as 4 mm in transverse diameter (one of these is the case presented in ). That these lesions were missed was possibly due to their smaller size, and it was possibly due to the surgeon's subjective diagnosis in the cases of surgical neurolysis in our study.
We should again consider the matter of the lesions' size. In our study, most of the neuromas that were operated upon were 5 mm or less. There was no lesion less than 3 mm detected by either the US or MRI modalities. In this aspect, the size of the lesions that were operated in our study was relatively smaller as compared with previous studies (1
). The measurement of Morton neuroma on the transverse MR image is dependent on the placement of the foot in the scanner. In addition, the size of the Morton neuroma is significantly larger in the prone position with plantar flexion of the ankle than in the supine and upright weight-bearing positions, in which the ankle is dorsiflexed (15
). Our study was done with the foot placed in the supine position, which possibly caused a decreased diameter of the neuroma, although plantar flexion was applied to the ankle. Furthermore, sonographic measurement and patient position are examiner-dependent. The transverse diameter can be affected by the patient's position or by the compression pressure during sonography.
Previous studies have demonstrated that neuromas larger than 5 mm in diameter are more symptomatic than the smaller ones (3
). Biasca et al. (7
) showed that a more favorable clinical outcome can be expected after surgical intermetatarsal neurectomy when a Morton neuroma has a transverse measurement larger than 5 mm on MRI scans; 77% of the patients in that study had a good outcome when the neuroma was greater than 5 mm, compared with only 17% of the patients had a good outcome when their neuromas measured 5mm or less. On the other hand, Sharp et al. (11
) demonstrated that symptoms were not dependent on the size of the neuroma. Moreover, the MR imaging diagnosis of Morton neuroma does not always imply symptomatology; thus, careful correlation between the clinical and MR imaging findings is mandatory (4
). The fluid distention of the intermetatarsal bursa may mimic that of neuromas, but it may not manifest symptoms and it is typically less than 3 mm in transverse dimension (3
There are different opinions regarding the best MR imaging sequences for detecting Morton neuroma (12
). Erickson et al. (16
) reported that T1WI was the most useful sequence because neuromas with decreased signal intensity were well demarcated from the adjacent fat tissue. On the other hand, Zanetti et al. (3
) suggested that T2WI was beneficial in order to exclude other diseases that mimic neuroma, such as intermetatarsal bursitis, because the latter revealed high signal intensities. Terk et al. (12
) stressed that sequences with fat-suppression and contrast-enhancement were reliable sequences for showing high-contrast images, and their study revealed that T1WI alone failed to demonstrate the presence of 50% (3 of 6) of the neuromas. In our study, the T1WI revealed better results for the detection of Morton neuroma than the CEFS T1WI did. In particular, the rarely enhanced neuroma cases shown in and were better discriminated on T1WI than on CEFS T1WI. However, the opposite was also true in when the neuroma was well enhanced. We think that CEFS T2WI is not obligatory for the purpose of detection alone, although it can be necessary for differentiation of neuroma from other possible lesions.
There are several limitations in our study. The falsepositive and true-negative rates could not be obtained because our study group consisted of only surgically proven Morton neuromas (including cases of neurolysis that might have been related to the surgeon's subjectivity). In addition, performing surgical exploration in the asymptomatic intermetatarsal spaces in clinical settings is not ethically possible whether or not a suspected Morton neuroma is found on imaging studies. The second limitation is the retrospective analysis of the US findings, is that, sonography is an operator-dependent method. The third limitation is that the double lesions in one patient were not separately evaluated in the clinical outcome.
In summary, US and MR imaging are compatible modalities for the evaluation of Morton neuroma, and both modalities have relatively high detection rates. Both imaging studies can provide information on the location and size of the neuroma prior to surgery. In addition, the contrast enhanced sequence does not seem essential for the detection of neuroma.