Published online before print January 14, 2008, 10.1148/radiol.2463070196
(Radiology 2008;246:863-870.)
© RSNA, 2008
Fibrosis and Adventitious Bursae in Plantar Fat Pad of Forefoot: MR Imaging Findings in Asymptomatic Volunteers and MR Imaging–Histologic Comparison1
Ueli Studler, MD,
Bernard Mengiardi, MD,
Beata Bode, MD,
Philip B. Schöttle, MD,
Christian W. A. Pfirrmann, MD,
Juerg Hodler, MD, and
Marco Zanetti, MD
1 From the Departments of Radiology (U.S., B.M., C.W.A.P., J.H., M.Z.) and Orthopedic Surgery (P.B.S.), Orthopedic University Hospital Balgrist, Forchstrasse 340, CH-8008 Zurich, Switzerland; and Department of Pathology, Zurich University Hospital, Zurich, Switzerland (B.B.). Received January 30, 2007; revision requested April 3; revision received April 24; accepted May 8; final version accepted August 1.
Address correspondence to U.S. (e-mail: ulrich.studler{at}balgrist.ch).
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ABSTRACT
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Purpose: To retrospectively evaluate plantar fat pad (PFP) signal intensity alterations in magnetic resonance (MR) imaging studies of asymptomatic volunteers and to compare PFP alterations with histopathologic findings in cadavers and patients.
Materials and Methods: After appropriate institutional review board approval and any required informed consent were obtained, MR imaging studies of 70 asymptomatic volunteers (35 women, 35 men; mean age, 45 years; range, 21–69 years) obtained for another investigation were retrospectively analyzed by two musculoskeletal radiologists in consensus. The location, signal intensity, margin, extent, and size of PFP alterations were determined. MR imaging–histopathologic comparison was performed in six cadaveric feet and six feet of symptomatic patients (one woman, five men; mean age, 43 years; range, 31–60 years). For volunteers, the relationship between PFP alterations and Morton neuroma, age, and sex was analyzed by using the Fisher exact test, Spearman rank correlation, and the Wilcoxon rank sum test, respectively. Bonferroni correction was applied, and P < .01 was considered to indicate a significant difference.
Results: Fifty-nine (84%) volunteers had PFP signal intensity alterations. Forty-nine (70%), six (9%), one (1%), four (6%), and 43 (61%) volunteers had alterations beneath the first, second, third, fourth, and fifth metatarsal heads, respectively. Ninety-four (91%) of 103 signal intensity alterations were in the form of hypointensity on T2-weighted images. Blurred margins were present in 90 (87%) alterations. Ninety percent of all PFP alterations in asymptomatic volunteers were 14 mm or smaller. The relationship between PFP alterations and Morton neuroma, age, and sex was not statistically significant. In cadaveric forefeet, PFP alterations corresponded histopathologically to a variable amount of fibrosis. In nine PFP alterations, development of fluid-containing spaces resembling bursae was present. Among the six patients with PFP alterations, histopathologic examination revealed fibrosis and adventitious bursae in two, fibrosis with inflammation in three, and a soft-tissue chondroma in one.
Conclusion: PFP signal intensity alterations are commonly seen in asymptomatic volunteers under the first and fifth metatarsal heads. At histologic examination, PFP signal intensity alterations correspond most commonly to fibrosis and adventitious bursae.
© RSNA, 2008
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INTRODUCTION
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Forefoot pain involving the metatarsal region, also called metatarsalgia, is a common clinical problem. Magnetic resonance (MR) imaging has the potential to enable a specific diagnosis in these patients. This is particularly true for disorders such as Morton neuroma, intermetatarsal bursitis, stress fractures, tenosynovitis, and plantar plate failure of the metatarsophalangeal joints (1–5). In the absence of such specific diagnoses, the radiologist may be faced with plantar fat pad signal intensity alterations at the level of the metatarsophalangeal joints whose clinical importance is equivocal. If there is no history of direct trauma, overuse may cause such alterations. MR imaging has been used to evaluate both functional fat pad properties and structural abnormalities of the plantar soft tissue in patients with diabetes (6,7). There is no study, to our knowledge, of the spectrum of plantar fat pad signal intensity alterations at MR imaging in a large series of asymptomatic healthy volunteers or of the histologic features of such abnormalities in nondiabetic feet. On the basis of our experience with routine MR imaging, signal intensity alteration of the plantar fat pad is a common but rather variable feature at forefoot examinations in patients with metatarsalgia. In only a few instances, where symptoms are unambiguously related to MR imaging abnormalities, is surgical excision of areas of plantar fat pad signal intensity alteration performed. Thus, little is known about the histologic features of symptomatic plantar fat pad alterations. The aim of our study, therefore, was to retrospectively evaluate plantar fat pad signal intensity alterations on MR imaging studies in asymptomatic volunteers and to compare plantar fat pad alterations with histopathologic findings in cadavers and patients.
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MATERIALS AND METHODS
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For the various components of our study, appropriate institutional review board approval and any required informed consent were obtained.
Volunteer Study
Seventy asymptomatic volunteers with an even age distribution (seven men and seven women per decade for the 2nd through 6th decades; mean age, 45 years; range, 21–69 years) were recruited. The right forefoot from each volunteer was imaged and analyzed (70 forefeet). Criteria for inclusion were (a) no prior forefoot surgery, (b) no recent or remote foot pain, (c) no trauma to the foot, and (d) no inflammatory joint disease or other systemic disease that may affect the forefoot. Thirty-four of the volunteers were healthy members of the hospital staff. The remaining 36 volunteers were patients who were undergoing MR imaging of other joints or the spine. The volunteer data set was acquired for a previously published investigation (2). The images were read again in terms of plantar fat pad signal intensity alterations for the current study. Already existing data on the presence of Morton neuromas were reused for the current study. Our current retrospective study was approved by the institutional review board of Orthopedic University Hospital Balgrist, with waiver of the need to obtain informed consent.
MR imaging protocol.—MR imaging was performed with a 1.0-T system (Impact; Siemens Medical Solutions, Erlangen, Germany). Volunteers were examined in the prone position with the right foot placed in a plantar-flexed position in a dedicated circularly polarized send-receive extremity coil. Both T1-weighted spin-echo (repetition time msec/echo time msec, 400/15; image matrix, 256 x 256; acquisition time, 1 minute 52 seconds; number of signals acquired, two) and T2-weighted fast spin-echo (4500/119; image matrix, 240 x 256; acquisition time, 2 minutes 28 seconds; number of signals acquired, three) images were obtained in the coronal plane. The section thickness was 3 mm with an intersection gap of 0.3 mm, and the field of view was 14 cm for both sequences.
Analysis of MR images.—All MR images were analyzed in consensus by two experienced musculoskeletal radiologists (M.Z., with 12 years of experience in musculoskeletal MR imaging, and C.W.A.P., with 7 years of experience). Plantar fat pad alterations were defined as signal intensity alterations underneath the metatarsal heads, located within a distance of 10 mm proximally from the metatarsophalangeal joint space (Fig 1). To avoid misinterpreting normal vertical fibers extending from the flexor sheaths to the aponeurosis as abnormal fat pad signal intensity alterations, alterations were considered to be abnormalities only when the shape was oval or round and the signal intensity alteration was seen on at least two adjacent sections. Well-demarcated T2-weighted hypointense spindle-shaped masses centered in the region of the intermetatarsal neurovascular bundle were diagnosed as Morton neuromas (2,8).
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Figure 1a: MR imaging findings of plantar fat pad signal intensity alterations in 35-year-old asymptomatic female volunteer. (a) Coronal T1-weighted image (400/15) shows continuous signal intensity alterations with blurred margins (arrows) in plantar fat pad under first and fifth metatarsal heads of right forefoot. (b) Coronal T2-weighted image (4500/119) depicts same signal intensity alterations (arrows).
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Figure 1b: MR imaging findings of plantar fat pad signal intensity alterations in 35-year-old asymptomatic female volunteer. (a) Coronal T1-weighted image (400/15) shows continuous signal intensity alterations with blurred margins (arrows) in plantar fat pad under first and fifth metatarsal heads of right forefoot. (b) Coronal T2-weighted image (4500/119) depicts same signal intensity alterations (arrows).
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The signal intensity of fat pad abnormalities was compared with that of plantar skeletal muscle on T1-weighted spin-echo and T2-weighted fast spin-echo images. When both high and low signal intensities were equally seen in a heterogeneous signal intensity alteration, the signal alteration was defined as mixed intensity. The margins of the abnormalities were subjectively classified as either well defined or blurred. The alterations were described as being contiguous with the skin, contiguous with the flexor tendon sheath, or continuously extending from the skin to the flexor tendon sheath (hereafter referred to as continuous). The greatest dimension of plantar fat pad abnormalities was measured on coronal (perpendicular to the long axis of the metatarsals) T1-weighted images by a fellow trained in musculoskeletal radiology (U.S., 1st year of fellowship).
Cadaveric Forefoot Examinations
Institutional review board approval and informed consent from the donor (prior to death) or the appropriate family member were available for this component of our study. Clinical information regarding forefoot pain of the cadavers was not available. Six forefeet excised from six frozen human cadavers (four women [two left and two right feet] and two men [one right and one left foot]; mean age at death, 78 years; age range, 68–91 years) were examined. MR images were obtained with a 1.5-T system (Symphony; Siemens Medical Solutions) by using a standard extremity coil. All cadaveric feet were placed in a neutral position. T1-weighted spin-echo (470/20) and T2-weighted fast spin-echo (4000/91) MR images were obtained in the coronal plane. A 3-mm section thickness, a 10-cm field of view, and a matrix size of 256 x 512 were used. The number of signals acquired was two. After imaging, the forefoot plantar fat pad, including the intact skin, was dissected from the metatarsal bone by an orthopedic surgeon (P.B.S., with 4 years of experience). The cadaveric forefoot specimens were fixed in 4% formalin and then sliced in the coronal plane that best corresponded to the MR imaging planes. The slices were 3 mm thick. The MR images were evaluated by using the same criteria described above for the volunteers. The histologic specimens were analyzed by a board-certified pathologist (B.B., with 7 years of experience in musculoskeletal pathology).
Specimens from Symptomatic Patients
Patients' rights are protected by a law in our country requiring that patients be informed that their charts and images might be reviewed for research purposes. Patients have the opportunity to forbid such use of their data. All patients included in our study agreed to the use of their data. We were able to include six patients (one woman, five men; mean age, 43 years; age range, 31–60 years) who had been referred to our radiology department for MR imaging of the forefoot between January 2003 and September 2005 and who subsequently underwent surgical excision (four patients) or ultrasonographically (US) guided biopsy (two patients) of areas of plantar fat pad signal intensity alterations. All patients presented with forefoot pain, which was associated with plantar fat pad signal intensity alterations on MR images. All patients had no history of any forefoot disorder (eg, trauma, infection, inflammatory joint disease, prior surgery). The six forefeet were examined according to our standard protocol, including the acquisition of coronal and sagittal T1-weighted spin-echo (600/15; number of signals acquired, two), coronal T2-weighted fast spin-echo (4500/96; number of signals acquired, two), and transverse short inversion time inversion-recovery (4600/23; inversion time, 150 msec; number of signals acquired, two) sequences. Coronal T1-weighted spin-echo sequences (735/15) with fat suppression were performed after intravenous administration of 0.1 mmol gadopentetate dimeglumine (Magnevist; Bayer Schering Pharma, Berlin, Germany) per kilogram of body weight. The MR images were evaluated by using the same criteria described above for volunteers. In addition, the plantar plate of each metatarsophalangeal joint was assessed on sagittal T1-weighted images for signs (hyperintensity, disruption) of failure (5). Results of histopathologic analysis (B.B.) of tissue specimens obtained from these six patients were included in this study.
Statistical Analysis
The frequency and location of plantar fat pad alterations in asymptomatic male and female volunteers were noted and shown as histograms. Medians, ranges, and 10th and 90th percentiles were calculated for the size of the fat pad alterations. The relationship between the presence of fat pad alterations and Morton neuroma was tested by using the Fisher exact test. The correlation between the presence of fat pad alterations and age (as a continuous variable) for each metatarsal was analyzed by using Spearman rank correlation. The conditional probability for the coexistence of signal intensity alterations beneath the first and fifth metatarsals in the same patient was calculated. Sex differences with respect to the presence and size of signal intensity alterations were tested for each metatarsal by using the Fisher exact test and the Wilcoxon rank sum test, respectively. The Bonferroni correction was applied to compensate for multiple testing (each metatarsal). P < .01 was considered to indicate a statistically significant difference on a local level. All analyses were performed with software (SPSS for Windows, release 10.1.3; SPSS, Chicago, Ill).
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RESULTS
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Volunteer Study
Fifty-nine (84%) of 70 right forefeet in asymptomatic volunteers showed a total of 103 plantar fat pad signal intensity alterations. In 49 (70%), six (9%), one (1%), four (6%), and 43 (61%) volunteers, alterations were beneath the first, second, third, fourth, and fifth metatarsal heads, respectively (Fig 2). When fat pad alterations were present beneath the first metatarsal, the probability of alterations beneath the fifth metatarsal was 66%. Fat pad alterations solely beneath the fifth metatarsal head occurred in 11 (16%) of volunteers. When compared with skeletal muscle, 103 (100%) of 103 fat pad signal intensity alterations consisted of isointensity on T1-weighted images. Ninety-four (91%) alterations were hypointense, eight (8%) were isointense, and one (1%) was hyperintense compared with muscle on T2-weighted images. Blurred margins were seen in 90 (87%) fat pad alterations (Fig 1). Ninety-one (88%) alterations were continuous from the flexor tendon to the skin, 11 (11%) were contiguous with the flexor tendon sheath, and one (1%) was contiguous with the skin. Four and five of 11 signal intensity alterations that were contiguous with the flexor tendon sheath were located under the first and fifth metatarsals, respectively.
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Figure 2: Histogram shows numbers of asymptomatic male and female volunteers with plantar fat pad signal intensity alterations under the metatarsal heads. The majority of fat pad signal intensity alterations were located under the first and fifth metatarsals.
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When all 103 signal intensity alterations were pooled, the 90th percentile was 14 mm, and the 50th percentile was 9 mm. Table 1 shows results for the measurement of signal intensity alterations according to each metatarsal. The prevalence of Morton neuromas was 30% (21 of 70). Fat pad signal intensity alterations were coexistent in volunteers with Morton neuromas in 81% of cases (17 of 21). There was no relationship between the presence of plantar fat pad alterations and Morton neuromas (P = .99). Women were found to have fat pad signal intensity alterations under all metatarsal heads more often. Among a total of 103 fat pad signal intensity alterations, 62 (60%) occurred in women and 41 (40%) occurred in men. The sizes of signal intensity alterations were greater in men than in women. After Bonferroni adjustment, there was no significant difference (P 
.01) in the prevalence and size of fat pad alterations between male and female volunteers (Table 2). There was no significant correlation between increasing age and the presence of plantar fat pad alterations (Table 2).
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Table 1. Tenth, 50th (Median), and 90th Percentiles and Ranges of Sizes of Plantar Fat Pad Alterations under First through Fifth Metatarsal Heads in 70 Asymptomatic Volunteers
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Table 2. Relationships between Plantar Fat Pad Parameters at MR Imaging and Demographic Variables for Each Metatarsal in 70 Volunteers
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Histopathologic Comparison with Cadaveric Forefeet
In four cadaveric forefeet, a total of 11 fat pad alterations were found. Five alterations were located under the first, one was located under the third, one was located under the fourth, and four were located under the fifth metatarsal head. All 11 fat pad alterations were isointense on T1-weighted and hypointense on T2-weighted images when compared with adjacent muscle. Blurred margins were seen in eight of 11 fat pad alterations. Eleven (100%) alterations were continuous. The median and 90th percentile of the greatest diameter of the 11 fat pad alterations were 11 and 13 mm, respectively. MR images showed no fat pad alterations in two cadaveric feet. The histologic examination revealed normal plantar fat structures in these two cadavers. Fat pad alterations seen on MR images corresponded to variable amounts of fibrosis histopathologically. In nine fat pad alterations, a cavity resembling a bursa was present within the area of fibrosis (Fig 3).
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Figure 3a: MR imaging–histopathologic comparison of plantar fat pad signal intensity alterations in right cadaveric forefoot. (a) Coronal T1-weighted MR image (470/20) shows fat pad signal intensity alterations (arrows) with blurred margins under first metatarsal head. (b) Histopathologic specimen reveals fibrosis (arrowheads) and a slitlike cavity within collagen sheets that represents an adventitious bursa (arrow). (Hematoxylin-eosin stain; original magnification, x1.)
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Figure 3b: MR imaging–histopathologic comparison of plantar fat pad signal intensity alterations in right cadaveric forefoot. (a) Coronal T1-weighted MR image (470/20) shows fat pad signal intensity alterations (arrows) with blurred margins under first metatarsal head. (b) Histopathologic specimen reveals fibrosis (arrowheads) and a slitlike cavity within collagen sheets that represents an adventitious bursa (arrow). (Hematoxylin-eosin stain; original magnification, x1.)
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Symptomatic Patients
In the six symptomatic patients, three signal intensity alterations were located under the first, two were located under the second, and one was located under the third metatarsal head. The median and 90th percentile of the largest diameter of the six fat pad alterations in symptomatic patients were 20 and 27 mm, respectively. The two largest signal intensity alterations were predominantly hyperintense on T2-weighted images. Histologic analysis revealed fibrous capsules surrounding cavities with fibrin-lined papillary projections in both patients (Fig 4). The fibrous capsule consisted of different layers of collagen sheets. The collagen sheets were microscopically similar to those found in cadavers. One abnormality proved to be a partially calcified chondroma and manifested as mixed signal intensity on T2-weighted images (Fig 5). In the remaining three patients, fibrosis and mild inflammation were found at histologic examination (US-guided biopsy: n = 2; complete resection: n = 1). Of the six signal intensity alterations, three each showed blurred and well-defined margins. Five signal intensity alterations were continuous, and one (the chondroma) was contiguous with the flexor tendon sheath. No signs of plantar plate rupture were seen. Five of six fat pad signal intensity alterations displayed mostly peripheral enhancement after intravenous contrast material administration (Fig 4). The chondroma enhanced inhomogeneously after contrast material administration (Fig 5). None of the signal intensity alterations showed a nodular or homogeneously enhancing pattern.
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Figure 4a: MR imaging–histopathologic comparison of plantar fat pad signal intensity alterations representing adventitious bursa in symptomatic 59-year-old man. (a) Coronal T1-weighted image (600/15) shows a signal intensity alteration (arrows) with indistinct margins in plantar fat pad beneath first metatarsal head of right forefoot. (b) On a T2-weighted image (4500/96), the majority of the signal intensity alteration (arrows) is hyperintense. Bandlike structures of low signal intensity (arrowhead) are apparent within the fat pad alteration. (c) T1-weighted contrast material–enhanced fat-suppressed image (735/15) displays peripheral enhancement (arrows). (d) Photomicrograph of histologic specimen shows fibrous collagen bundles (*). Within the cavity, fibrin-lined papillary projections (arrowhead) are seen and correspond to the bandlike structures at T2-weighted MR imaging. (Elastin-van Gieson stain; original magnification, x32.)
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Figure 4b: MR imaging–histopathologic comparison of plantar fat pad signal intensity alterations representing adventitious bursa in symptomatic 59-year-old man. (a) Coronal T1-weighted image (600/15) shows a signal intensity alteration (arrows) with indistinct margins in plantar fat pad beneath first metatarsal head of right forefoot. (b) On a T2-weighted image (4500/96), the majority of the signal intensity alteration (arrows) is hyperintense. Bandlike structures of low signal intensity (arrowhead) are apparent within the fat pad alteration. (c) T1-weighted contrast material–enhanced fat-suppressed image (735/15) displays peripheral enhancement (arrows). (d) Photomicrograph of histologic specimen shows fibrous collagen bundles (*). Within the cavity, fibrin-lined papillary projections (arrowhead) are seen and correspond to the bandlike structures at T2-weighted MR imaging. (Elastin-van Gieson stain; original magnification, x32.)
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Figure 4c: MR imaging–histopathologic comparison of plantar fat pad signal intensity alterations representing adventitious bursa in symptomatic 59-year-old man. (a) Coronal T1-weighted image (600/15) shows a signal intensity alteration (arrows) with indistinct margins in plantar fat pad beneath first metatarsal head of right forefoot. (b) On a T2-weighted image (4500/96), the majority of the signal intensity alteration (arrows) is hyperintense. Bandlike structures of low signal intensity (arrowhead) are apparent within the fat pad alteration. (c) T1-weighted contrast material–enhanced fat-suppressed image (735/15) displays peripheral enhancement (arrows). (d) Photomicrograph of histologic specimen shows fibrous collagen bundles (*). Within the cavity, fibrin-lined papillary projections (arrowhead) are seen and correspond to the bandlike structures at T2-weighted MR imaging. (Elastin-van Gieson stain; original magnification, x32.)
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Figure 4d: MR imaging–histopathologic comparison of plantar fat pad signal intensity alterations representing adventitious bursa in symptomatic 59-year-old man. (a) Coronal T1-weighted image (600/15) shows a signal intensity alteration (arrows) with indistinct margins in plantar fat pad beneath first metatarsal head of right forefoot. (b) On a T2-weighted image (4500/96), the majority of the signal intensity alteration (arrows) is hyperintense. Bandlike structures of low signal intensity (arrowhead) are apparent within the fat pad alteration. (c) T1-weighted contrast material–enhanced fat-suppressed image (735/15) displays peripheral enhancement (arrows). (d) Photomicrograph of histologic specimen shows fibrous collagen bundles (*). Within the cavity, fibrin-lined papillary projections (arrowhead) are seen and correspond to the bandlike structures at T2-weighted MR imaging. (Elastin-van Gieson stain; original magnification, x32.)
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Figure 5a: MR imaging–histopathologic comparison of soft-tissue chondroma of plantar fat pad in symptomatic 61-year-old man. (a) Coronal T1-weighted image (600/15) shows well-defined signal intensity alteration (arrows) in plantar fat pad under third metatarsal bone of right forefoot. (b) On a T2-weighted image (4500/96), the alteration (arrows) shows mixed signal intensity, corresponding to that of cartilage and calcification. (c) On a T1-weighted contrast-enhanced fat-suppressed MR image (735/15), the alteration shows faint inhomogeneous enhancement (arrows). (d) Histologically, a well-circumscribed tumor consisting of lobules of partly calcified hyaline cartilage (arrow) with low cellularity without atypia was found. (Hematoxylin-eosin stain; original magnification, x50.)
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Figure 5b: MR imaging–histopathologic comparison of soft-tissue chondroma of plantar fat pad in symptomatic 61-year-old man. (a) Coronal T1-weighted image (600/15) shows well-defined signal intensity alteration (arrows) in plantar fat pad under third metatarsal bone of right forefoot. (b) On a T2-weighted image (4500/96), the alteration (arrows) shows mixed signal intensity, corresponding to that of cartilage and calcification. (c) On a T1-weighted contrast-enhanced fat-suppressed MR image (735/15), the alteration shows faint inhomogeneous enhancement (arrows). (d) Histologically, a well-circumscribed tumor consisting of lobules of partly calcified hyaline cartilage (arrow) with low cellularity without atypia was found. (Hematoxylin-eosin stain; original magnification, x50.)
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Figure 5c: MR imaging–histopathologic comparison of soft-tissue chondroma of plantar fat pad in symptomatic 61-year-old man. (a) Coronal T1-weighted image (600/15) shows well-defined signal intensity alteration (arrows) in plantar fat pad under third metatarsal bone of right forefoot. (b) On a T2-weighted image (4500/96), the alteration (arrows) shows mixed signal intensity, corresponding to that of cartilage and calcification. (c) On a T1-weighted contrast-enhanced fat-suppressed MR image (735/15), the alteration shows faint inhomogeneous enhancement (arrows). (d) Histologically, a well-circumscribed tumor consisting of lobules of partly calcified hyaline cartilage (arrow) with low cellularity without atypia was found. (Hematoxylin-eosin stain; original magnification, x50.)
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Figure 5d: MR imaging–histopathologic comparison of soft-tissue chondroma of plantar fat pad in symptomatic 61-year-old man. (a) Coronal T1-weighted image (600/15) shows well-defined signal intensity alteration (arrows) in plantar fat pad under third metatarsal bone of right forefoot. (b) On a T2-weighted image (4500/96), the alteration (arrows) shows mixed signal intensity, corresponding to that of cartilage and calcification. (c) On a T1-weighted contrast-enhanced fat-suppressed MR image (735/15), the alteration shows faint inhomogeneous enhancement (arrows). (d) Histologically, a well-circumscribed tumor consisting of lobules of partly calcified hyaline cartilage (arrow) with low cellularity without atypia was found. (Hematoxylin-eosin stain; original magnification, x50.)
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DISCUSSION
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Our study results demonstrate that plantar fat pad signal intensity alterations are commonly (84%) found in MR imaging studies in asymptomatic forefeet. Characteristically, the signal intensity alterations were present under the first (70%) or the fifth (61%) metatarsal heads. Women tended to have more fat pad signal intensity alterations (62 of 103) than did men (41 of 103), although the difference was not significant. We found no influence of age on the prevalence of plantar fat pad signal intensity alterations.
The histologic examination of the cadaveric forefeet showed that MR signal intensity alterations of the plantar fat pad underneath the metatarsal heads corresponded to fibrosis and cavities resembling bursae. Bursae are classified either as naturally occurring synovial bursae or as secondarily developed adventitious bursae (9). Subcutaneous synovial bursae reduce friction between skin and subjacent bony protuberances. Typical examples include bursae located superficially to the patella and olecranon (10). In addition to naturally occurring bursae, adventitious bursae may form in adulthood at sites where subcutaneous tissue is exposed to high friction and pressure (11). Subcutaneous cavities arise following a sequence of connective tissue degeneration. Adventitious bursae always lack an epithelial lining (12). We believe that our observations of adventitious bursae of the forefoot are comparable to those found in patients with below-the-knee amputations with prosthesis-related problems (12,13).
It is plausible to believe that iso- to hypointense plantar fat pad signal intensity alterations on both T1- and T2-weighted MR images in asymptomatic volunteers also represent reactive fibrous tissue induced by mechanical stress. According to our comparison of MR imaging and histopathologic findings in cadaveric forefeet, these plantar fat pad signal intensity alterations correspond to various degrees of fibrosis in the fatty tissue. Because we did not perform follow-up examinations of asymptomatic volunteers, we do not know whether fat pad alterations could be regarded as an early stage in the development of bursae or whether these signal intensity alterations represent a normal reaction to mechanical stress. A hyperintense signal alteration was present in only 1% (one of 103) of asymptomatic fat pad signal intensity alterations at MR imaging, whereas signal intensity alterations in symptomatic patients were often mixed to hyperintense on T2-weighted images (three of six). Fat pad signal intensity alterations in symptomatic patients were larger (median, 20 vs 9 mm) and proportionately more often under the second or third metatarsal (three of six vs seven of 103) when compared with findings in asymptomatic volunteers. In two symptomatic patients, large fluid-filled cavities at MR imaging corresponded to adventitious bursae at histologic examination. In two patients in whom US-guided biopsy of the signal intensity alterations was performed, histologic examination revealed only nonspecific reactive tissue alterations, including fibrosis and inflammatory tissue. It is possible that the biopsy sample was not representative and that adventitious bursitis was also present in these patients.
The distribution of signal intensity alterations and the predominance of fibrosis within the signal intensity alterations suggest that MR alterations of the plantar fat pad in asymptomatic volunteers are related to weight-bearing pressure. According to the "tripod model" (14), the pressure on the foot is supported at the heel and at the head of the first and fifth metatarsals. However, other authors (15,16) have questioned the presence of a distal transverse metatarsal arch in asymptomatic volunteers. Alternatively, a possible explanation for the distribution of plantar fat pad alterations in our study is that the pattern reflects a direct mechanical effect from footwear rather than a weight-dependent mechanism. This possible explanation may account for part of the sex difference found in our investigation. Because plantar fat pad alterations in symptomatic patients were more often seen under the second and third metatarsals, these signal intensity alterations may have a different pathogenesis. Plantar plate failure is hypothesized to be a common cause of metatarsalgia of the second and third metatarsophalangeal joints (5). However, we did not observe findings of plantar plate rupture at MR imaging in our limited number of patients.
Deep soft-tissue masses of the foot are uncommon and include nonneoplastic signal intensity alterations (eg, those caused by rheumatoid nodules) and benign (eg, nerve sheath tumors) and malignant (eg, synovial sarcoma) neoplasms (17–19). Some signal intensity alterations occur in a specific anatomic location. For example, Morton neuromas are located within the intermetatarsal space (2), and plantar fibromatosis is related to the plantar aponeurosis (20). In our series of symptomatic signal intensity alterations, one abnormality proved to be a soft-tissue chondroma. This benign tumor demonstrated subtle differences compared with the other symptomatic signal intensity alterations. First, the abnormality was contiguous with the flexor tendon sheath. Second, there was only faint and more-speckled contrast material enhancement, whereas the fibrous and bursal signal intensity alterations showed a predominantly peripheral enhancement. None of the plantar fat pad signal intensity alterations showed solid or homogeneous enhancement after contrast agent administration.
We acknowledge the following study limitations. The signal intensity of the plantar fat pad alterations may not be identical in cadavers and volunteers and may not be directly comparable. According to our experience, freezing of cadaver body parts results in loss of cell nuclei. We do not believe that this phenomenon compromised the MR imaging–histopathologic comparisons in our study, because the spectrum of tissue analyzed consisted mainly of fibrous alterations. A fluid-filled cavity was never seen on T2-weighted MR images obtained in cadavers in our study. The most likely explanation for this lack of presumed fluid pockets is that fluid escapes from plantar fat pad cavities during preparation of cadaveric forefeet and therefore goes undetected with fluid-sensitive MR imaging sequences. Because only a small number of patients underwent biopsy or surgical excision of symptomatic signal intensity alterations, the spectrum of histologic findings may differ substantially in other patient groups (eg, in rheumatoid nodules in patients with rheumatoid arthritis). However, to our knowledge, ours is the first report of a comparison between histologic and MR imaging findings of signal intensity alterations centered in the plantar fat pad.
In summary, we found a high prevalence of alterations in the plantar fat pad at MR examination of volunteers without foot pain. Considering the high prevalence of metatarsalgia in the general population, the presence of plantar fat pad alterations on MR images in a patient with metatarsalgia may be coincidental. The typical location under the first and fifth metatarsal heads, the small size (<14 mm), the low to intermediate signal intensity on T2-weighted MR images, and the blurred margins of plantar fat pad alterations in asymptomatic volunteers can help differentiate clinically nonrelevant alterations from symptomatic abnormalities. At histologic examination, plantar fat pad alterations correspond most commonly to fibrosis and adventitious bursae.
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ADVANCES IN KNOWLEDGE
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- Plantar fat pad signal intensity alterations are commonly found on MR imaging studies in asymptomatic volunteers under the first (70%) and the fifth (61%) metatarsal heads.
- At histologic examination, plantar fat pad alterations (as assessed in cadavers and symptomatic patients) correspond most commonly to fibrosis and adventitious bursae.
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IMPLICATION FOR PATIENT CARE
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- Familiarity with normal MR imaging findings in the plantar fat pads at the metatarsal head level may help differentiate these signal intensity alterations from symptomatic abnormalities.
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FOOTNOTES
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Guarantors of integrity of entire study, U.S., J.H., M.Z.; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; manuscript final version approval, all authors; literature research, U.S.; clinical studies, U.S., B.B., M.Z.; experimental studies, U.S., B.M., B.B., P.B.S.; statistical analysis, U.S., M.Z.; and manuscript editing, U.S., B.B., C.W.A.P., J.H., M.Z.
Authors stated no financial relationship to disclose.
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ABSTRACT
INTRODUCTION
