Published online before print January 15, 2003, 10.1148/radiol.2263011925
(Radiology 2003;226:849-856.)
© RSNA, 2003
Morton Neuroma: MR Imaging in Prone, Supine, and Upright Weight-bearing Body Positions1
Dominik Weishaupt, MD,
Karl Treiber, RT,
Hans-Peter Kundert, MD,
Hans Zollinger, MD,
Patrice Vienne, MD,
Juerg Hodler, MD,
Jürgen K. Willmann, MD,
Borut Marincek, MD and
Marco Zanetti, MD
1 From the Institute of Diagnostic Radiology, University Hospital, Rämistrasse 100, CH-8091 Zurich, Switzerland (D.W., K.T., J.K.W., B.M.); Orthopedic Practice, Zurich, Switzerland (H.P.K., H.Z.); and Departments of Orthopedic Surgery (P.V.) and Radiology (J.H., M.Z.), Orthopedic University Hospital Balgrist, Zurich, Switzerland. Received November 26, 2001; revision requested February 8, 2002; final revision received May 29; accepted July 15. Address correspondence to D.W. (e-mail: dominik.weishaupt@dmr.usz.ch).
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ABSTRACT
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PURPOSE: To assess the effect of prone, supine, and upright weight-bearing body positions on visibility, position, shape, and size of Morton neuroma during magnetic resonance (MR) imaging.
MATERIALS AND METHODS: Eighteen patients with 20 Morton neuromas underwent MR imaging of the forefoot in prone (plantar flexion of the foot), supine (dorsiflexion of the foot), and upright weight-bearing positions. Visibility (3 = good, 2 = moderate, 1 = poor), position relative to the metatarsal bone, shape, and transverse diameter of Morton neuroma were assessed on transverse T1-weighted MR images. Associations between different body positions and variables of interest were calculated with Wilcoxon signed rank test, 
2 test, and paired Student t test.
RESULTS: In the prone position, visibility of all 20 Morton neuromas was rated with a score of 3; visibility in the supine and weight-bearing positions was inferior (mean score, 2.4). All 20 (100%) Morton neuromas changed their position relative to the metatarsal bone between prone and supine and between prone and weight-bearing positions. When compared with the prone position, there was a difference in the shape of all 20 Morton neuromas in the weight-bearing position (P < .001). Between prone (mean transverse diameter of Morton neuroma, 8 mm) and supine (mean transverse diameter of Morton neuroma, 6 mm) positions, the transverse diameter of Morton neuroma significantly decreased by 2 mm (P = .03); between prone and weight-bearing positions, the decrease of the mean transverse diameter was also significant (difference, 2 mm; P = .03).
CONCLUSION: Morton neuroma appears significantly different during MR imaging in prone, supine, or weight-bearing positions. The transverse diameter of Morton neuroma is significantly larger on images obtained in the prone position than it is on images obtained in the supine and upright weight-bearing positions. Visibility of Morton neuroma is best on MR images obtained in the prone position.
© RSNA, 2003
Index terms: Efficacy study • Foot, MR, 465.121411 • Foot, neoplasms, 465.315, 465.369 • Neuroma, 465.315, 465.369
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INTRODUCTION
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Magnetic resonance (MR) imaging has been shown to be accurate in the diagnosis of Morton neuroma (1–3). MR imaging characteristics of Morton neuroma include a well-demarcated mass that is isointense relative to muscle at T1-weighted spin-echo (SE) sequences and homogeneously or inhomogeneously hypointense relative to fat tissue at T2-weighted fast SE sequences (4). Typically, Morton neuromas are located at the plantar side of the deep transverse intermetatarsal ligament (5). Apart from being useful in the diagnosis of Morton neuroma, MR imaging of a clinically suspected Morton neuroma has a major effect on diagnostic thinking and therapeutic decisions of orthopedic surgeons (6). This is mainly explained by the ability of MR imaging to help correctly localize Morton neuroma and assess its size (6). However, there is clinical evidence that the position of Morton neuroma relative to the metatarsal head may vary (7). Morton neuroma can be displaced by manual compression of the involved intermetatarsal space (7). This variation of Morton neuroma with regard to its position may be important when Morton neuromas are investigated with standard MR imagers, considering that patients may be placed in either supine (1,2) or prone position (3,4). To our knowledge, the effect of MR imaging in supine or prone positions on the appearance of Morton neuromas has not been investigated.
Another area of potential interest is the appearance of Morton neuroma in weight-bearing conditions, since pain associated with Morton neuroma is usually aggravated by standing, walking, and wearing shoes (5). The feasibility of weight-bearing MR imaging of the forefoot with an open-configuration MR system has been demonstrated in a preliminary study of 32 asymptomatic volunteers (8). This provides new opportunities in imaging the forefoot, since, for the first time, MR imaging of the forefoot can be performed in a physiologic situation.
The purpose of this study was to assess the effect of prone (plantar flexion of the foot), supine (dorsiflexion of the foot), and upright weight-bearing body positions on visibility, position, shape, and size of Morton neuroma during MR imaging.
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MATERIALS AND METHODS
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Patients
From November 2000 to April 2001, 41 patients suspected of having Morton neuroma were referred to the radiology department of the Orhopedic University Hospital Balgrist in Zurich for MR imaging. Nineteen patients were referred by foot surgeons of an orthopedic university hospital and 22 patients, by two experienced orthopedic foot surgeons (H.P.K., H.Z.) working in private practice. The study was approved by the review board of the hospital, and informed consent was obtained from all patients.
All patients underwent MR imaging of their symptomatic forefoot in the prone position with a 1.0-T imager (hereafter, standard MR imager). After completion of standard MR imaging, the patients were asked to undergo MR imaging with an open-configuration MR system in the supine (dorsiflexion of the foot) and weight-bearing body positions if they fulfilled the following criterion: presence of at least one Morton neuroma 5 mm or larger in its transverse diameter, as measured on standard T1-weighted MR images. Diagnosis of Morton neuroma was based on previously published MR imaging criteria (1–3), and measurement of its diameter was performed similarly as with the technique described by Zanetti et al (3,4). For this purpose, standard MR images were reviewed by one of two experienced musculoskeletal radiologists (J.H., M.Z.) prior to inclusion in the study. Since previous study findings have shown that a Morton neuroma 5 mm or larger in its transverse diameter is more commonly symptomatic than a smaller neuroma (3), and Biasca et al (9) have demonstrated a more favorable clinical outcome for a Morton neuroma of this size, a cutoff value of 5 mm or greater was chosen as the inclusion criterion.
A total of 18 patients (17 women, one man; mean age, 49.6 years; age range, 25–72 years) with 20 Morton neuromas in 19 feet were enrolled in the study. Ten (50%) of 20 Morton neuromas were located in the right forefoot, and an additional 10 (50%) were located in the left forefoot. There was a delay of 4.8 days (range, 0–13 days) between standard MR imaging and MR imaging with the open-configuration MR system.
Diagnosis of Morton neuroma was proven with surgical exploration of the intermetatarsal space in 14 patients with 16 Morton neuromas. The plantar intermetatarsal nerves were excised in all 16 intermetatarsal spaces, and histologic correlation was obtained. In four patients with four Morton neuromas, conservative treatment that included local infiltration of a mixture of anesthetics and a corticosteroid was performed.
Standard MR Imaging
Standard MR imaging was performed with a 1.0-T imager (Expert; Siemens Medical Systems, Erlangen, Germany). The patient was placed in the prone position, with the foot placed in plantar flexion in a circularly polarized, send-receive extremity coil (Fig 1). The imaging protocol consisted of a transverse T1-weighted SE sequence (repetition time msec/echo time msec, 600/15; field of view, 140 mm; imaging time, 3 minutes 30 seconds) and a transverse T2-weighted turbo SE sequence perpendicular to the metatarsal bones (4,500/96; field of view, 140 mm; imaging time, 3 minutes 42 seconds). For both sequences, the section thickness was 3 mm, the intersection gap was 0.3 mm, the image matrix was 256 x 256, and two signals were acquired. Echo train length was seven for the T2-weighted turbo SE sequence. Planning of both of these sequences was performed by using a sagittal T1-weighted SE image (500/15; field of view, 180 mm; imaging time, 2 minutes 46 seconds) that had been acquired first.
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Figure 1a. Different forefoot positions in MR imaging of Morton neuroma. (a) Patient is placed in prone position, with the foot placed in plantar flexion in a circular polarized send-receive extremity coil. (b) Patient is placed in supine position, with dorsiflexion of the foot. The foot is placed in a flexible transmit-receive wraparound surface coil. (c) Positioning device designed for MR imaging of the forefoot in a weight-bearing position. The device consists of a wooden footplate that permits integration of the flexible transmit-receive wraparound surface coil. The device can be placed in the center of the magnet bore of an open-configuration MR system. The cushions used for stabilizing the forefoot within the coil are not shown.
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Figure 1b. Different forefoot positions in MR imaging of Morton neuroma. (a) Patient is placed in prone position, with the foot placed in plantar flexion in a circular polarized send-receive extremity coil. (b) Patient is placed in supine position, with dorsiflexion of the foot. The foot is placed in a flexible transmit-receive wraparound surface coil. (c) Positioning device designed for MR imaging of the forefoot in a weight-bearing position. The device consists of a wooden footplate that permits integration of the flexible transmit-receive wraparound surface coil. The device can be placed in the center of the magnet bore of an open-configuration MR system. The cushions used for stabilizing the forefoot within the coil are not shown.
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Figure 1c. Different forefoot positions in MR imaging of Morton neuroma. (a) Patient is placed in prone position, with the foot placed in plantar flexion in a circular polarized send-receive extremity coil. (b) Patient is placed in supine position, with dorsiflexion of the foot. The foot is placed in a flexible transmit-receive wraparound surface coil. (c) Positioning device designed for MR imaging of the forefoot in a weight-bearing position. The device consists of a wooden footplate that permits integration of the flexible transmit-receive wraparound surface coil. The device can be placed in the center of the magnet bore of an open-configuration MR system. The cushions used for stabilizing the forefoot within the coil are not shown.
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Supine and Weight-bearing MR Imaging
MR imaging in supine and weight-bearing positions was performed with a vertically opened superconducting 0.5-T MR system (Signa SP; GE Medical Systems, Milwaukee, Wis). The 56-cm vertical gap between the two vertically placed ring magnet components provided sufficient patient access to allow weight-bearing MR examinations in addition to imaging in supine or prone body positions.
MR images were first obtained with the patient positioned supine, with dorsiflexion of the foot. For this purpose, the foot was placed in a flexible transmit-receive wraparound surface coil, and transverse T1-weighted SE images (500–600/19) were obtained perpendicular to the metatarsal bones (Fig 1). After completion of MR imaging in the supine position, the forefoot was imaged in the weight-bearing position by using a recently described technique (8). For that purpose, the patients were standing upright in the gap between the two magnet rings. The foot was then placed on a positioning device that consisted of a wooden footplate, which was placed in the center of the magnet bore. This positioning device permits integration of the flexible transmit-receive wraparound surface coil. During MR data acquisition, the patients were asked to shift their body weight onto the forefoot with the presumed Morton neuroma and make sure the hindfoot was still in contact with the footplate. Subsequently, transverse T1-weighted MR images of the forefoot perpendicular to the metatarsal bones were obtained in a similar fashion. By using this technique, high-quality MR images of the forefoot can be obtained (8). This technique is referred to as weight-bearing MR imaging.
Additional imaging parameters for all T1-weighted MR sequences obtained in supine and weight-bearing positions were an image matrix of 256 x 224, a field of view of 14 cm, a section thickness of 3 mm, an intersection gap of 0.3 mm, and acquisition of three signals. Imaging time for each of the T1-weighted sequences was 5 minutes 11 seconds.
Qualitative Image Evaluation
All MR images were evaluated by two musculoskeletal radiologists (D.W., M.Z.) in consensus. The readers were blinded to patient information. T1-weighted SE MR images were evaluated because they were available for all three body positions. For qualitative image evaluation, MR images obtained with all three body positions were available for image evaluation. Visibility of the Morton neuroma at different body positions was rated by using the following four-point scale: score 3, good (good delineation of the Morton neuroma against the surrounding tissue, no motion artifacts); score 2, moderate (radiologist’s judgment between good and poor visibility); score 1, poor (poor delineation of the Morton neuroma or severe motion artifacts); and score 0, none (Morton neuroma not visible).
The position of the Morton neuroma on transverse T1-weighted MR images obtained with the patients in prone (plantar flexion of the foot), supine (dorsiflexion of the foot), and weight-bearing positions was assessed relative to a virtual line paralleling the plantar aspect of the cortex (hereafter, plantar cortical line) of two adjacent metatarsal heads, metatarsal necks, or metatarsophalangeal joint spaces (depending on the location of Morton neuroma). At the level of the metatarsal heads, the plantar line represents the presumed location of the deep intermetatarsal ligament (10), which is usually barely visible on MR images. The observers were asked to assess the position of the center of the Morton neuroma relative to the plantar cortical line. Morton neuroma positioned above this line was referred to as intermetatarsal. The center of the Morton neuroma was defined as the location where its largest transverse diameter was present. Changes in the location of the center of the Morton neuroma with regard to migration along a more distal or a more proximal direction in the axis of the metatarsal ray were also noted among different body positions. Finally, the form assumed by the Morton neuroma in different body positions was assessed by using the following terms: pear shaped, dumbbell shaped, and inverted pear shaped.
Quantitative Image Evaluation
The size of the Morton neuroma was measured as the largest transverse diameter on T1-weighted SE MR images in a similar fashion as described previously (4). All measurements were performed by one of the authors (D.W.) at a separate workstation (Octane; Sun Microsystems, Mountain View, Calif) using software (Provision; Algotec, Raanana, Israel). The measurements were performed separately for all three body positions to the nearest millimeter by using an electronic caliper.
Statistical Analysis
Differences in subjective image-rating analysis between prone, supine, and weight-bearing MR images were evaluated with the Wilcoxon signed rank test. Other qualitative grades were analyzed with the 2 test. For comparison of quantitative measurements, a one-tailed paired Student t test was used. P < .05 was considered to indicate a statistically significant difference. Statistical analysis was performed with software (StatView version 5.0.1; SAS Institute, Cary, NC).
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RESULTS
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MR imaging in prone, supine, and weight-bearing body positions was successful in all 18 patients. No additional Morton neuroma (ie, <5 mm in transverse diameter) was found on any of the MR images. Of 20 Morton neuromas, 15 were located in the third and five in the second intermetatarsal space. One Morton neuroma was present in 16 of 18 patients. Two Morton neuromas within two separate intermetatarsal spaces were present in the same foot in one patient. One Morton neuroma was present in each foot in one patient, who had been referred for MR imaging of both feet.
Qualitative Image Evaluation
The visibility ratings for the Morton neuromas at different body positions are presented in Table 1. In the prone position, all 20 Morton neuromas were rated with the highest possible score of 3 (good visibility). The mean scores obtained with patients in either supine or weight-bearing positions were lower (mean score of 2.4 for both prone and weight-bearing positions) (Fig 2). On MR images obtained with the patient in the weight-bearing position, visibility of two (10%) of 20 Morton neuromas was rated as poor (score 1), whereas all scores obtained with patients either in the prone or the supine position were higher than a score of 1. None of the MR image sets were rated with a score of 0 (not visible). Between the prone and supine positions, eight Morton neuromas that changed their position from below the plantar cortical line of two adjacent metatarsal heads (or metatarsal necks or metatarsophalangeal joint spaces) to above the line (Table 2) were rated with a score of 2, whereas a score of 3 was assigned for the prone position. The higher mean visibility score obtained with patients in the prone position was significant (P = .002) when compared with the mean visibility score (P = .005) obtained with patients in the supine and weight-bearing body positions. There was no significant difference between the visibility scores on MR images obtained with patients in a supine and those obtained with patients in a weight-bearing position (P = .901).
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Figure 2a. Positional dependence of a surgically confirmed Morton neuroma in the third intermetatarsal space in a 60-year-old woman. MP2, MP3, and MP4 are metatarsophalangeal joint spaces of the second, third, and fourth rays, respectively; M2, M3, and M4 are metatarsal heads of the second, third, and fourth rays, respectively; and PP4 is the proximal phalanges of the fourth ray. (a) T1-weighted SE MR image (600/15) obtained perpendicular to the metatarsal bone with the patient in prone (plantar flexion of the foot) position demonstrates a typical Morton neuroma (arrows). The visibility of Morton neuroma was rated as good (score 3). (b) On a T1-weighted SE MR image (500/19) obtained perpendicular to the metatarsal bone with the patient in supine (dorsiflexion of the foot) position, the visibility of Morton neuroma (arrows) was rated as moderate (score 2). (c) On T1-weighted MR image (500/19) obtained with the patient in weight-bearing position, the visibility of Morton neuroma (arrows) was rated as moderate (score 2). Compared with the location of Morton neuroma in prone position, Morton neuroma at both supine and weight-bearing positions shows slight distal migration along the axis of the metatarsal ray.
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Figure 2b. Positional dependence of a surgically confirmed Morton neuroma in the third intermetatarsal space in a 60-year-old woman. MP2, MP3, and MP4 are metatarsophalangeal joint spaces of the second, third, and fourth rays, respectively; M2, M3, and M4 are metatarsal heads of the second, third, and fourth rays, respectively; and PP4 is the proximal phalanges of the fourth ray. (a) T1-weighted SE MR image (600/15) obtained perpendicular to the metatarsal bone with the patient in prone (plantar flexion of the foot) position demonstrates a typical Morton neuroma (arrows). The visibility of Morton neuroma was rated as good (score 3). (b) On a T1-weighted SE MR image (500/19) obtained perpendicular to the metatarsal bone with the patient in supine (dorsiflexion of the foot) position, the visibility of Morton neuroma (arrows) was rated as moderate (score 2). (c) On T1-weighted MR image (500/19) obtained with the patient in weight-bearing position, the visibility of Morton neuroma (arrows) was rated as moderate (score 2). Compared with the location of Morton neuroma in prone position, Morton neuroma at both supine and weight-bearing positions shows slight distal migration along the axis of the metatarsal ray.
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Figure 2c. Positional dependence of a surgically confirmed Morton neuroma in the third intermetatarsal space in a 60-year-old woman. MP2, MP3, and MP4 are metatarsophalangeal joint spaces of the second, third, and fourth rays, respectively; M2, M3, and M4 are metatarsal heads of the second, third, and fourth rays, respectively; and PP4 is the proximal phalanges of the fourth ray. (a) T1-weighted SE MR image (600/15) obtained perpendicular to the metatarsal bone with the patient in prone (plantar flexion of the foot) position demonstrates a typical Morton neuroma (arrows). The visibility of Morton neuroma was rated as good (score 3). (b) On a T1-weighted SE MR image (500/19) obtained perpendicular to the metatarsal bone with the patient in supine (dorsiflexion of the foot) position, the visibility of Morton neuroma (arrows) was rated as moderate (score 2). (c) On T1-weighted MR image (500/19) obtained with the patient in weight-bearing position, the visibility of Morton neuroma (arrows) was rated as moderate (score 2). Compared with the location of Morton neuroma in prone position, Morton neuroma at both supine and weight-bearing positions shows slight distal migration along the axis of the metatarsal ray.
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On transverse T1-weighted MR images obtained with patients in the prone position, the centers of all 20 Morton neuromas were located below the plantar cortical line of two adjacent metatarsal heads (or metatarsal necks or metatarsophalangeal joint spaces) (Table 2). When Morton neuromas were examined individually, the position of the centers of eight Morton neuromas (40%) changed from below the plantar cortical line to above this line between prone and supine positions; the positions of all centers of all 20 (100%) Morton neuromas changed from below the plantar cortical line to above the line between prone and weight-bearing positions (Table 2; Figs 3, 4). Between supine and weight-bearing positions, the positions of the centers of 12 (60%) Morton neuromas changed from below the plantar cortical line to above this line; there was no change in the position of the center of eight (40%) Morton neuromas located above the cortical line in the supine position. The center of eight (40%) of 20 Morton neuromas migrated proximally along the axis of the metatarsal ray between the prone and weight-bearing positions (mean migration distance, 4.3 mm). In four of these eight Morton neuromas, a proximal migration of the center of the Morton neuroma was also observed between the supine and weight-bearing positions (mean migration distance, 4 mm).
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Figure 3a. Positional dependence of the shape and location of a Morton neuroma in the third intermetatarsal space in a 48-year-old woman. Morton neuroma was proven with surgical exploration. M2, M3, and M4 are metatarsal heads of the second, third, and fourth rays, respectively, and MP3 and MP4 are metatarsophalangeal joint spaces of the third and fourth rays, respectively. (a) On a T1-weighted SE MR image (600/15) obtained perpendicular to the metatarsal bone with the patient in prone position, Morton neuroma (arrows) is pear shaped and located below the plantar cortical line (P). (b) On a T1-weighted SE MR image (500/19) obtained perpendicular to the metatarsal bone with the patient in supine position, Morton neuroma (arrows) is dumbbell shaped and located above the virtual plantar cortical line (P). (c) On a T1-weighted SE MR image (500/19) obtained perpendicular to the metatarsal bone with the patient in the weight-bearing position, Morton neuroma (arrows) is inverted pear shaped and located above the plantar cortical line (P).
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Figure 3b. Positional dependence of the shape and location of a Morton neuroma in the third intermetatarsal space in a 48-year-old woman. Morton neuroma was proven with surgical exploration. M2, M3, and M4 are metatarsal heads of the second, third, and fourth rays, respectively, and MP3 and MP4 are metatarsophalangeal joint spaces of the third and fourth rays, respectively. (a) On a T1-weighted SE MR image (600/15) obtained perpendicular to the metatarsal bone with the patient in prone position, Morton neuroma (arrows) is pear shaped and located below the plantar cortical line (P). (b) On a T1-weighted SE MR image (500/19) obtained perpendicular to the metatarsal bone with the patient in supine position, Morton neuroma (arrows) is dumbbell shaped and located above the virtual plantar cortical line (P). (c) On a T1-weighted SE MR image (500/19) obtained perpendicular to the metatarsal bone with the patient in the weight-bearing position, Morton neuroma (arrows) is inverted pear shaped and located above the plantar cortical line (P).
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Figure 3c. Positional dependence of the shape and location of a Morton neuroma in the third intermetatarsal space in a 48-year-old woman. Morton neuroma was proven with surgical exploration. M2, M3, and M4 are metatarsal heads of the second, third, and fourth rays, respectively, and MP3 and MP4 are metatarsophalangeal joint spaces of the third and fourth rays, respectively. (a) On a T1-weighted SE MR image (600/15) obtained perpendicular to the metatarsal bone with the patient in prone position, Morton neuroma (arrows) is pear shaped and located below the plantar cortical line (P). (b) On a T1-weighted SE MR image (500/19) obtained perpendicular to the metatarsal bone with the patient in supine position, Morton neuroma (arrows) is dumbbell shaped and located above the virtual plantar cortical line (P). (c) On a T1-weighted SE MR image (500/19) obtained perpendicular to the metatarsal bone with the patient in the weight-bearing position, Morton neuroma (arrows) is inverted pear shaped and located above the plantar cortical line (P).
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Figure 4a. Positional dependence of a surgically confirmed Morton neuroma located in the third intermetatarsal space in a 31-year-old man. M2, M3, and M4 are the metatarsal heads of the second, third, and fourth rays, respectively, and MP3 and MP4 are metatarsophalangeal joint spaces of the third and fourth rays, respectively. On T1-weighted SE MR images (600/15) obtained perpendicular to the metatarsal bone with the patient in (a) prone and (b) supine positions, the Morton neuroma (arrows) is located below the plantar cortical line (P) between two adjacent metatarsal heads. (c) On T1-weighted SE MR image (500/19) obtained with the patient in a weight-bearing position, the Morton neuroma (arrows) is located above the plantar cortical line (P).
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Figure 4b. Positional dependence of a surgically confirmed Morton neuroma located in the third intermetatarsal space in a 31-year-old man. M2, M3, and M4 are the metatarsal heads of the second, third, and fourth rays, respectively, and MP3 and MP4 are metatarsophalangeal joint spaces of the third and fourth rays, respectively. On T1-weighted SE MR images (600/15) obtained perpendicular to the metatarsal bone with the patient in (a) prone and (b) supine positions, the Morton neuroma (arrows) is located below the plantar cortical line (P) between two adjacent metatarsal heads. (c) On T1-weighted SE MR image (500/19) obtained with the patient in a weight-bearing position, the Morton neuroma (arrows) is located above the plantar cortical line (P).
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Figure 4c. Positional dependence of a surgically confirmed Morton neuroma located in the third intermetatarsal space in a 31-year-old man. M2, M3, and M4 are the metatarsal heads of the second, third, and fourth rays, respectively, and MP3 and MP4 are metatarsophalangeal joint spaces of the third and fourth rays, respectively. On T1-weighted SE MR images (600/15) obtained perpendicular to the metatarsal bone with the patient in (a) prone and (b) supine positions, the Morton neuroma (arrows) is located below the plantar cortical line (P) between two adjacent metatarsal heads. (c) On T1-weighted SE MR image (500/19) obtained with the patient in a weight-bearing position, the Morton neuroma (arrows) is located above the plantar cortical line (P).
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In the prone position, all 20 Morton neuromas were pear shaped (Table 3). These diagnoses changed for nine Morton neuromas (45%) between the prone and the supine position (all dumbbell shaped in the supine position) and for all 20 Morton neuromas between the prone and weight-bearing position (10 inverted pear shaped and 10 dumbbell shaped in the weight-bearing position) (Figs 3, 4).
Between supine and weight-bearing positions, the diagnosis with regard to the shape of Morton neuroma changed in 17 (85%) of 20 instances. Eleven (55%) Morton neuromas were pear shaped in the supine position. A change of diagnosis to dumbbell shaped was made in seven Morton neuromas (35%) and that to inverted pear shaped, in four (20%). Nine (45%) Morton neuromas were dumbbell shaped in the supine position. When changed to the weight-bearing position, six (30%) Morton neuromas were inverted pear shaped. No change was present in three Morton neuromas (dumbbell shape at both body positions).
The change in the shape of Morton neuroma between different body positions was significant (prone vs supine position, P = .03; prone vs weight-bearing position, P = .001; supine vs weight-bearing position, P = .04).
Quantitative Image Evaluation
The transverse diameter of Morton neuroma varied between 5 and 11 mm (mean, 8 mm) in the prone position. In the supine position, the mean transverse diameter of Morton neuroma was 6 mm (range, 4–10 mm) and that in the weight-bearing position was also 6 mm (range, 3–11 mm) (Fig 5). The difference between the transverse diameters of Morton neuromas in prone and supine positions was significant (2-mm difference; P = .03). The difference between the transverse diameters of Morton neuromas between prone and upright positions (2 mm) was also significant (P = .03). No difference in the transverse diameter was found between supine and weight-bearing positions.
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Figure 5a. Positional dependence of the size of Morton neuroma in the third intermetatarsal space in a 50-year-old woman. The neuroma was proven with surgical exploration. MP2, MP3, and MP4 are metatarsophalangeal joint spaces of the second, third, and fourth rays, respectively, and M2, M3, and M4 are the metatarsal heads of the second, third, and fourth rays, respectively. T1-weighted SE MR images (500/19) obtained with the patient in (a) prone, (b) supine, and (c) weight-bearing positions. The transverse diameter (d) of Morton neuroma is 10 mm in prone position, 7 mm in supine position, and 7 mm in weight-bearing position. Between supine and weight-bearing positions, the Morton neuroma slightly migrated proximally along the axis of the metatarsal bone.
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Figure 5b. Positional dependence of the size of Morton neuroma in the third intermetatarsal space in a 50-year-old woman. The neuroma was proven with surgical exploration. MP2, MP3, and MP4 are metatarsophalangeal joint spaces of the second, third, and fourth rays, respectively, and M2, M3, and M4 are the metatarsal heads of the second, third, and fourth rays, respectively. T1-weighted SE MR images (500/19) obtained with the patient in (a) prone, (b) supine, and (c) weight-bearing positions. The transverse diameter (d) of Morton neuroma is 10 mm in prone position, 7 mm in supine position, and 7 mm in weight-bearing position. Between supine and weight-bearing positions, the Morton neuroma slightly migrated proximally along the axis of the metatarsal bone.
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Figure 5c. Positional dependence of the size of Morton neuroma in the third intermetatarsal space in a 50-year-old woman. The neuroma was proven with surgical exploration. MP2, MP3, and MP4 are metatarsophalangeal joint spaces of the second, third, and fourth rays, respectively, and M2, M3, and M4 are the metatarsal heads of the second, third, and fourth rays, respectively. T1-weighted SE MR images (500/19) obtained with the patient in (a) prone, (b) supine, and (c) weight-bearing positions. The transverse diameter (d) of Morton neuroma is 10 mm in prone position, 7 mm in supine position, and 7 mm in weight-bearing position. Between supine and weight-bearing positions, the Morton neuroma slightly migrated proximally along the axis of the metatarsal bone.
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DISCUSSION
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Pain caused by Morton neuroma is a frequent cause of ill-defined pain of the forefoot, and it commonly occurs in the second or third intermetatarsal space but rarely in the first or fourth (11,12). Symptoms of Morton neuroma include numbness and radiating pain or tingling sensation and often increase with activity and wearing of narrow shoes. Removal of shoes or rest reduces the symptoms. Morton neuroma affects women more frequently than it does men. The highest prevalence is in the 5th and 6th decades of life (5).
The diagnosis of Morton neuroma may be established clinically on the basis of patient history and physical examination results (5,13). However, since the clinical localization of a Morton neuroma may be equivocal (4), imaging modalities, including ultrasonography, computed tomography, and MR imaging, are considered useful for both confirmation of the diagnosis and exact localization of a suspected Morton neuroma (4,14–16).
MR imaging has shown high sensitivity (87%) and specificity (100%) in the depiction of Morton neuroma (3) and is therefore considered to be useful for narrowing the broad differential diagnosis of forefoot pain. In addition, MR imaging permits correct localization of Morton neuroma with regard to the intermetatarsal space in cases where precise localization is required preoperatively. Previously, the effect of MR imaging on diagnostic thinking and therapeutic decisions by orthopedic surgeons in patients with clinically suspected Morton neuroma has been shown (6). The authors demonstrated that the clinical diagnosis of Morton neuroma was reversed in 28% of the feet following MR imaging and a change in location or number was made in one-third of the remaining feet (6).
Findings of several studies have demonstrated the importance of the measurement of Morton neuroma size on transverse MR images. Larger (>5-mm-diameter) Morton neuromas are shown to be more commonly symptomatic than smaller neuromas (3,4,16). The importance of size, as measured at MR imaging with regard to surgical management of Morton neuroma, is substantiated by findings in a study of Biasca et al (9). In that study, the authors found that 77% of patients with a Morton neuroma 5 mm or larger in its transverse diameter have had a good outcome following surgical excision. When smaller Morton neuromas (ie, <5 mm in transverse diameter) were excised, the outcome was poorer: Only 17% of the patients had good postoperative outcomes. Finally, in the series by Zanetti et al (6), the transverse diameter (as depicted with MR imaging) of surgically treated Morton neuroma was significantly (P = .003) larger than the transverse diameter of those Morton neuromas that were subjected to conservative treatment.
The results of our study show that the transverse diameter of Morton neuroma as measured on transverse MR images is dependent on the body position at which the imaging is performed. Between prone and supine positions and between prone and weight-bearing positions, differences in the diameters of Morton neuromas were statistically significant (P = .03). The decrease in Morton neuroma diameter in supine and weight-bearing positions is most likely caused by the change in position of Morton neuromas. As the Morton neuroma becomes dislocated more dorsally, it is squeezed between the metatarsals. These findings substantiate the influence of body position on the transverse diameter of Morton neuroma as measured on MR images.
In our study, besides the transverse diameter of Morton neuroma, its shape was also dependent on the position: Nine (45%) of 20 Morton neuromas changed their shape between prone and supine body positions. The change in shape was even more frequent between prone and weight-bearing positions, with all 20 Morton neuromas (100%) changing their shape.
For MR imaging of Morton neuromas, imaging sequences and imaging planes are largely standardized. Most authorities use a circular-polarized send-receive extremity coil and a combination of T1- and T2-weighted standard SE or fast SE sequences in the transverse plane (perpendicular to the metatarsals) (1–4,6). In previous studies, patients suspected of having Morton neuroma were either placed in prone (3,4,6) or supine body positions (1,2) for MR imaging. Although there is, to our knowledge, no intraindividual or interindividual comparison between both body positions with regard to detection of Morton neuroma, Zanetti et al (3) preferred MR imaging in the prone position because of reduced motion artifacts.
The results of our study have shown that the visibility of Morton neuroma was significantly better in the prone position compared with that in the supine position. Several reasons may explain the improved visibility of Morton neuroma during imaging in the prone position. On the basis of our own experience, motion artifacts may be reduced when the patient is placed in the prone position. The visibility of Morton neuroma may also be influenced by its position relative to the metatarsophalangeal joint and the metatarsal bone. In the prone position, the metatarsophalangeal joints are usually in a slightly plantar and flexed position. Conversely, in the supine position, the metatarsophalangeal joints are in a neutral or slightly extended (dorsiflexion of the foot) position. When the metatarsophalangeal joint is flexed, traction forces may pull the neurovascular bundle with the Morton neuroma deeper into the intermetatarsal space. This may result in differences in the position of Morton neuroma relative to the metatarsophalangeal joint and the metatarsal bone between prone and supine positions.
The change in position of the Morton neuroma caused by traction forces during flexion in the metatarsophalangeal joint may also explain the change in the shape of Morton neuroma between different body positions. Hence, since the results of this study demonstrated that visibility of Morton neuroma is best in the prone position, it may be concluded that Morton neuroma should be imaged in the prone position when MR imaging is performed with standard closed-configuration MR systems.
The exact pathogenesis of Morton neuroma remains controversial. Most authorities regard Morton neuroma as an entrapment neuropathy caused by repetitive compression of the nerve against the deep transverse intermetatarsal ligament, with subsequent perineural fibrosis (5). Other suggested causes of Morton neuroma include ischemia and entrapment of the nerve by intermetatarsal bursitis and metatarsophalangeal joint ganglion (17).
In a previous study of 32 asymptomatic volunteers who underwent MR imaging of the forefoot in supine and weight-bearing positions, we were able to demonstrate position-dependent changes of the neurovascular bundle (8). In asymptomatic volunteers, a change in the position of the neurovascular bundle from underneath a plantar cortical line of two adjacent metatarsal heads in the supine position to a location above this line was present in 50 (82%) of 61 neurovascular bundles. Similarly, the present study findings demonstrated that the position of the center of a Morton neuroma with regard to a virtual plantar cortical line of two adjacent metatarsal heads (or metatarsal necks or metatarsophalangeal joint spaces, depending on the location of the Morton neuroma) is dependent on the body position. Changes with regard to the position of the Morton neuroma were most frequent between prone and weight-bearing positions. In the latter position, all centers of all Morton neuromas were positioned above the cortical metatarsal line within the intermetatarsal space. Together with the data obtained in asymptomatic volunteers, it may be concluded that entrapment of the nerve in the intermetatarsal space and compression of the nerve against the transverse metatarsal ligament represent an important biomechanical factor in the pathogenesis of Morton neuroma.
The position-dependent changes of Morton neuroma with regard to the position relative to the metatarsal bone and the changes in shape are of special interest for other peculiarities of the entity. These results may explain the Mulder test, which is commonly used by clinicians in cases of suspected Morton neuroma (5). The test result is considered positive when a painful and palpable "click" (Mulder sign) is present when the metatarsal heads are squeezed together with one hand and the involved intermetatarsal space is simultaneously compressed with the thumb and index finger of the opposite hand from both the dorsal and plantar directions. The palpable click is thought to be caused by the sudden dislocation of the Morton neuroma from the two adjacent compressing metatarsal heads (5,7).
We have to acknowledge that relatively large Morton neuromas have been investigated in this study. Position-dependent changes of a smaller neuroma may be more difficult to visualize. In this study, however, we focused our interest on clearly visible Morton neuromas to make the position-dependent changes more obvious. Moreover, these larger Morton neuromas are considered to be of clinical importance (3,6,9,16). Other limitations are related to the design of the study. MR imaging in the prone position was performed with a different magnetic field strength, different coils, and slightly different imaging parameters than was imaging in the supine and weight-bearing positions. This may have influenced the results with regard to the visualization of Morton neuromas. In addition, the fact that patients were included in the study on the basis of the presence of a Morton neuroma in the prone position might have resulted in an inclusion bias.
In conclusion, our results demonstrate that Morton neuromas show position-dependent changes relative to the metatarsal heads and with regard to the shape during MR imaging in prone, supine, or weight-bearing body positions. Morton neuromas appear to be best visualized in the prone position. For practical reasons, patients suspected of having Morton neuroma should therefore be imaged in the prone instead of the supine position.
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FOOTNOTES
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Abbreviation: SE = spin echo
Author contributions: Guarantors of integrity of entire study, D.W., M.Z.; study concepts, D.W., M.Z., H.P.K., H.Z.; study design, D.W., M.Z., J.H.; literature research, D.W., M.Z.; clinical studies, D.W., K.T., M.Z., P.V.; data acquisition, D.W., K.T., J.K.W., M.Z.; data analysis/interpretation, D.W., M.Z.; statistical analysis, D.W., J.K.W.; manuscript preparation, D.W., M.Z., J.H.; manuscript definition of intellectual content, D.W., M.Z., J.H., H.P.K.; manuscript editing, D.W., M.Z.; manuscript revision/review, D.W., M.Z., J.H., B.M., H.Z.; manuscript final version approval, D.W., M.Z., J.H.
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REFERENCES
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ABSTRACT
INTRODUCTION
