Morton's Neuroma or Intermetatarsal Bursitis-A Prospective Diagnostic Study of Intermetatarsal Pain

Abstract

Background: Intermetatarsal bursitis (IMB) is emerging as a diagnostic consideration for patients with forefoot pain. However, few investigations have been conducted into the incidence of IMB among patients with forefoot pain. The symptoms of IMB are described as mimicking those of Morton's neuroma (MN). Currently, the best method to differentiate between MN and IMB is radiological evaluation. Based on this, the aim of this study was to investigate the incidence of IMB and MN in a prospective cohort of patients with intermetatarsal pain diagnosed with radiological evaluation and compared to a control group. Methods: This study included 26 patients and 13 controls. All participants underwent magnetic resonance imaging (MRI) and ultrasound (US) of one forefoot. Results: Among the 26 patients, 5 (19.2%) had MN and 14 (53.8%) had IMB on MRI compared to US, with which 25 (96.2%) cases of IMB and 0 with MN were identified. In the control group, both modalities found asymptomatic web space pathology in four cases (30.8%), and US identified normal intermetatarsal bursas in five cases. Additionally, our results indicate that MN patients have more severe pain and a longer history of pain compared to IMB patients. Conclusions: Based on our MRI results, we conclude that IMB is frequent in patients with intermetatarsal pain. Differentiation between MN and IMB with US is complex and should be performed with caution and an understanding of both conditions. Normal intermetatarsal bursas are also visible on US as hypoechoic but non-expansive masses.

Keywords: Morton’s neuroma; diagnostic criteria; intermetatarsal bursitis/bursa; magnetic resonance imaging; prospective study; ultrasound; web space.

Figure 1

Schematic diagram of the magnetic resonance imaging (MRI) method to differentiate between Morton’s neuroma (MN) and intermetatarsal bursitis (IMB). All scans are in the transverse plane. P = phalanx; M = metatarsal. Please note that the images were selected to illustrate typical features of each sequence and originate from different patients.

Figure 2

Flowchart of the inclusion process (patients/controls).

Figure 3

Schematic diagram of the anatomic landmarks used in the differentiation between MN and IMB on MRI. Both conditions have a proximal extent into the space between the metatarsal heads; here, they are separated by the deep transverse metatarsal ligament (DTML) in the dorso-plantar direction. The space is narrow and limits the information from radiological evaluation as demonstrated in (a) (where the open arrowhead points at MN, * marks fluid in the intermetatarsal bursa, and the thin arrow points at the DTML) and (d) (where the open arrow points at IMB); note the plantar location of MN and the dorsal location of IMB. More distally at the level of the metatarsophalangeal joints and between the proximal phalanges, we obtain the best view of the entities ((b), where the open arrowhead points at MN, and (e), where the open arrow points at IMB). Without the DTML to separate them, they have overlapping anatomic locations. The references for the proximal–distal location of the axial scans are shown on (c) for MN and (f) for IMB. M = metatarsus; P = phalanx.

Figure 4

A 63-year-old woman with MN (marked with a dotted line) in the second web space on the left foot. (a) Axial MRI T1-weighted FSE image showing a hypointense process extending plantar. (b) Axial MRI T2-weighted FSE image showing the process with a hypointense appearance. (c) Axial MRI T1-weighted FSE Flex contrast-enhanced image showing the contrast enhancement of the process. (d,e) Sagittal ultrasound of the second web space in the same patient. Note the expansive appearance of the hypoechoic mass as marked with the dashed line. (f) The process measures 11.5 mm × 19.0 mm. M = metatarsus, P = phalanx.

Figure 5

A 57-year-old man diagnosed with IMB (arrows) in the third web space on the right foot. (a) Axial MRI T1-weighted FSE image showing a hypointense process extending both dorsally and plantarly. (b) Axial MRI T2-weighted FSE image showing the process inhomogeneous, but predominantly hyperintense. (c) Axial MRI T1-weighted FSE Flex contrast-enhanced image showing the peripheral enhancement of the process. (d,e) Sagittal ultrasound of the third web space in the same patient. Note the expansive appearance of the hypoechoic mass as marked with the dashed line. (f) The process measures 18.7 mm × 17.1 mm. M = metatarsus, P = phalanx.

Figure 6

Examples of asymptomatic web space findings from the control group. (a–c) A 27-year-old female’s second web space, left foot, with a visible but nonpathological bursa on sagittal ultrasound measuring 12.9 mm (a,b) and axial MRI with reported IMB ((c), white arrow). (d–f) A 52-year-old female’s second web space, right foot, diagnosed with IMB on both sagittal ultrasound (d) and axial MRI ((f), open arrow). The hypoechoic mass measures 15.7 mm (e).

Figure 7

Examples of visible intermetatarsal bursas on ultrasound in our control group. (A,a) A 47-year-old woman’s third web space, left foot; ultrasound reported IMB measuring 16.2 mm. (B,b) A 65-year-old man’s second web space, right foot; ultrasound reported a visible but normal bursa measuring 14.4 mm. (C,c) A 53-year-old woman’s second web space, left foot; ultrasound reported a visible but normal bursa measuring 12.3 mm. All scans were obtained in the sagittal plane in a dorso-plantar direction. Left is proximal and right is distal. Note the presence of concave sides and a non-expansive appearance of the hypoechoic process, as marked with the dashed line. The MRI scans for all three patients were normal.

Figure 8

Frequency of (A) symptoms, (B) alleviating factors, and (C) aggravating factors, as reported by the patients. * “Sensation” refers to altered feeling in the foot, e.g., feels like walking on cottonwool, cold feeling, walking on gravel, etc. ** “Shoes” refers to the use of a specific shoe or insole. Subgrouping was based on MRI diagnosis.

Figure 9

Boxplots of duration of pain and VAS score of patients with IMB and MN according to MRI diagnosis. * Indicates significant values (p < 0.05).