Published online before print September 24, 2001, 10.1148/radiol.2212010469
(Radiology. 2001;221:478-484.)
© RSNA, 2001
Intermetatarsal Spaces: Analysis with MR Bursography, Anatomic Correlation, and Histopathology in Cadavers1
Nicolas H. Theumann, MD,
Christian W. A. Pfirrmann, MD,
Christine B. Chung, MD,
Aurea V. R. Mohana-Borges, MD,
Parviz Haghighi, MD,
Debra J. Trudell, RA and
Donald Resnick, MD
1 From the Departments of Radiology (N.H.T., C.W.A.P., C.B.C., A.V.R.M.B., D.J.T., D.R.) and Pathology (P.H.), Veterans Administration Medical Center, San Diego, Calif; and Department of Radiology, Centre Hospitalier Universitaire vaudois, Rue du Bugnon 46, BH 10, CHUV, 1011 Lausanne, Switzerland (N.H.T.). Received February 15, 2001; revision requested April 3; revision received April 30; accepted May 2. Supported by the Swiss Radiological Society and the Swiss National Science Foundation. Address correspondence to N.H.T. (e-mail: Nicolas.Theumann@chuv.hospvd.ch).
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ABSTRACT
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PURPOSE: To describe the normal magnetic resonance (MR) imaging–depicted anatomy of the intermetatarsal spaces, with emphasis on the MR imaging appearance of the intermetatarsal bursae, and to correlate the MR findings with those seen in anatomic sections and at histopathologic analysis.
MATERIALS AND METHODS: Conventional radiography and pre- and postcontrast T1-weighted and fat-saturated T1-weighted spin-echo MR imaging were performed in 32 intermetatarsal spaces in eight human cadaveric feet. The cadaveric specimens were sectioned in planes corresponding to those at MR imaging for anatomic correlation. The intermetatarsal space anatomy was analyzed. Histopathologic examinations of the bursae were performed.
RESULTS: The intermetatarsal spaces were located in the forefoot between two metatarsal heads, below and above the deep transverse metatarsal ligament (DTML) that separated the spaces into two levels. The superior level contained the synovial bursa, the plantar and dorsal interosseous muscles and tendons, and the collateral ligament complexes of the metatarsophalangeal joints. The inferior level contained lumbrical muscles and neurovascular bundles. The bursae extended distally to the DTML in the second and third spaces close to the neurovascular bundles and did not extend beyond the DTML in the first and fourth spaces. In the first intermetatarsal space, the bursa had a specific appearance as it coursed along the adductor hallucis tendon as a tendon sheath. Histopathologic examination of the bursae revealed a single layer of attenuated cells.
CONCLUSION: MR bursography provided detailed information about the intermetatarsal anatomy, especially the intermetatarsal bursae.
Index terms: Bursitis, 465.251 • Foot, anatomy, 465.91 • Foot, MR, 465.121411, 465.12143 • Neuroma, 465.315
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INTRODUCTION
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In the anatomic literature (1,2), the intermetatarsal spaces are described as being separated into two levels by the deep transverse metatarsal ligament (DTML). The superior level is delineated below by the DTML and bilaterally by the capsular structures of the adjacent metatarsophalangeal joints. The superior level contains the interosseous tendons, the adductor hallucis tendon in the first intermetatarsal space, and the intermetatarsal bursae (Fig 1). The inferior level, or fibrous channel, is delineated above by the DTML, below by the STML, and bilaterally by the perforating fibers. This level contains the lumbrical muscle and the neurovascular bundle.
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Figure 1. Drawing illustrates coronal view of the inferior and superior intermetatarsal spaces. 1 = dorsal interosseous tendons, 2 = plantar interosseous tendons, 3 = lumbrical muscles, 4 = adductor hallucis tendon, 5 = deep transverse metatarsal ligament, 6 = superficial transverse metatarsal ligament (STML), 7 = perforating fibers, 8 = lateral sesamoid bone, 9 = neurovascular bundles, 10 = flexor digitorum tendons. Metatarsal heads 1-5 (M1-M5) are depicted.
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A potential diagnostic problem in MR imaging is caused by fluid in the intermetatarsal bursae. This fluid may occur independently or be associated with a Morton neuroma (3). Although intermetatarsal bursitis is a cause of painful forefoot (1,4), this condition has received little attention in the radiology literature (5). The presence of these bursae provides an anatomic explanation for such clinical entities as acute metatarsalgia and revives the debate as to the precise pathogenesis of Morton metatarsalgia (1). To our knowledge, the MR appearance of the intermetatarsal structures has not been described in detail.
The purpose of our cadaveric study was to describe the normal MR anatomy of the intermetatarsal spaces, with emphasis on the magnetic resonance (MR) imaging appearance of the intermetatarsal bursae, and to correlate the findings with those seen in anatomic sections and at histopathologic analysis.
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MATERIALS AND METHODS
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Cadavers and Histopathologic Specimen Preparation
Anteroposterior radiographs of 30 fresh human cadaveric forefeet were obtained to exclude the presence of traumatic or articular disorders. Twenty-two feet were excluded from the study. The 32 intermetatarsal spaces of the eight remaining specimens were imaged. These specimens were harvested from legs cut through the distal portions of the tibia and fibula of seven nonembalmed cadavers (one woman, 6 men; age range at death, 75–86 years; mean age at death, 78 years). The specimens were immediately deep frozen at -40°C (Forma Bio-Freezer; Forma Scientific, Marietta, Ohio). All specimens were allowed to thaw for 24 hours at room temperature before conventional radiography. Anteroposterior radiography was performed before conventional radiography of the bursae, or bursography, and MR imaging.
Nonenhanced MR Imaging
MR imaging studies were obtained with a 1.5-T unit (Signa; GE Medical Systems, Milwaukee, Wis) by using a dedicated extremity coil. The forefoot was placed prone in the center of the gantry, and the specimens were immobilized with foam pads.
Transverse and coronal T1-weighted spin-echo MR images (500/12 [repetition time msec/echo time msec], 2-mm section thickness, 0.5-mm intersection space, two signals acquired, 6 x 6-cm field of view, 512 x 256 matrix, and acquisition time of 4 minutes 23 seconds) were obtained. Both sequences were performed twice: first with the first and second intermetatarsal spaces placed in the center of the field and then with the third and fourth intermetatarsal spaces placed in this position. Sagittal T1-weighted spin-echo MR images (500/12, 2-mm section thickness, 0.5-mm intersection space, two signals acquired, 6 x 6-cm field of view, 512 x 256 matrix, and acquisition time of 8 minutes 43 seconds) also were obtained.
Bursography
The feet were placed prone on the table. In each foot, with fluoroscopic guidance and a dorsal approach, a 22-gauge (0.7 x 40.0-mm) needle was inserted directly through the skin and advanced into the expected position of the intermetatarsal bursae, which was immediately adjacent to the plantar aspect of the metatarsal heads and midway between the metatarsal heads. The position of the needle tip was verified with a test injection of a small amount of iodinated contrast agent (iohexolum [Omnipaque 350]; Nycomed-Amersham, Princeton, NJ). Subsequently, approximately 1–3 mL of a solution of 1 mL of gadopentetate dimeglumine (Magnevist; Schering, Berlin, Germany) diluted in 250 mL of saline and mixed with an equal amount of iohexolum was injected with fluoroscopic control. Conventional radiographs were then obtained in the anteroposterior projection.
MR Bursography
MR imaging was performed within 30 minutes after contrast agent injection. MR sequences identical to those used before contrast material enhancement of the bursae were performed.
T1-weighted fat-saturated spin-echo MR images (500/12, 2.5-mm section thickness, 0.6-mm intersection space, two signals acquired, 6 x 6-cm field of view, 512 x 256 matrix, and an acquisition time of 4 minutes 50 seconds) were successively obtained in transverse and coronal planes. Each sequence was performed twice in the same positions as those described earlier.
Anatomic Inspection
After imaging, all cadaveric specimens were immediately frozen at -40°C for at least 24 hours and subsequently cut with a band saw into 2-mm-thick sections that corresponded to the thickness of the MR images along one of the imaging planes—that is, coronal (three sections), sagittal (two sections), or transverse (three sections)—according to lines drawn on the specimen at the time of imaging. Photographs of each section were obtained with the specimen thawed.
MR Imaging–Anatomic Correlation and Analysis
Nonenhanced MR images and MR bursographic images were interpreted by means of consensus among three musculoskeletal radiologists (N.H.T., C.W.A.P., A.V.R.M.B.). The intermetatarsal spaces were analyzed on the conventional images of the bursae and on the MR images obtained before and after contrast material enhancement of the bursae, and the findings were correlated with those in the anatomic sections. The relationships between the intermetatarsal bursae and the following structures were analyzed (Fig 1): (a) structures at the superior intermetatarsal level—that is, the plantar and dorsal interosseous tendons, the adductor hallucis tendon in the first space, and the collateral ligament complexes (lateral and accessory ligaments); (b) DTML; and (c) structures at the inferior intermetatarsal level—that is, the lumbrical muscles and tendons, the STML, the perforating fibers, and the neurovascular bundles. On the basis of MR image analysis results, the same three observers reached a consensus regarding the location, extension, and appearance of each bursa on the transverse and coronal MR bursographic images. Any communications between the intermetatarsal bursae and the metatarsophalangeal joints were recorded. The MR imaging–depicted anatomic appearance of the other intermetatarsal structures also were reported.
Histopathologic Analysis of the Bursae
After anatomic inspection, histopathologic analysis of the intermetatarsal bursae was performed in each of the four spaces in two specimens. The samples were suspended in 10% formalin solution, embedded in paraffin, and cut further into 5-µm-thick sections. The histopathologic sections were stained and then analyzed at light microscopy (magnification, x2 to x100) by an orthopedic pathologist (P.H.). The characteristics of the bursae were reported.
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RESULTS
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Anatomic Inspection
All of the listed structures of the intermetatarsal spaces were clearly identified in all specimens. At anatomic inspection, there were 32 intermetatarsal bursae, which corresponded to one bursa per intermetatarsal space. Owing to the orientation of all intermetatarsal bursae, evaluation in the sagittal plane was difficult.
Relationships of the intermetatarsal bursae through the second to fourth web spaces were initially reported. In the coronal plane (n = 9: three web spaces in three coronal sections), the collateral margins of the bursae were limited by the interosseous tendons and collateral ligament complexes. The inferior margins were limited by the DTML; and the superior margins, by abundant fat tissue. In the transverse plane (n = 9), the collateral margins of the bursae were limited by the interosseous tendons and collateral ligament complexes and proximally and distally by fat tissue.
Different collateral relationships were delineated for the first intermetatarsal bursa (n = 8). The bursa margins were limited medially by the adductor hallucis tendon and the collateral ligament complex of the first metatarsophalangeal joint and laterally by the medial dorsal interosseous tendon and the collateral ligament complex of the second metatarsophalangeal joint. The other margins of the first intermetatarsal bursa corresponded to those that were described earlier, through the second to fourth intermetatarsal spaces.
The DTML consisted of a short wide and flattened fibrous band that extended between the plantar plate from the first to fifth rays and separated the intermetatarsal space into two levels. The structures at the superior level were described earlier. The structures at the inferior level were the neurovascular bundles in close relationship with the DTML and the lumbrical muscles and tendons arising from the perforating fibers and adjacent to the medial side of the flexor digitorum tendons.
Imaging-Anatomic Correlation and Analysis
Intermetatarsal bursae at the superior intermetatarsal level.—The intermetatarsal bursae were not visualized on precontrast MR images of any of the studied specimens. MR bursography depicted all bursae in the transverse and coronal planes, with best visualization at T1-weighted fat-saturated spin-echo imaging. However, it was difficult to evaluate the bursae in the sagittal plane.
At MR bursography, in the second, third, and fourth intermetatarsal spaces, the bursae were located between the interosseous tendons and the collateral ligament complexes. In this location, the bursae had a similar appearance: elliptical or almost oval, with the long axis proximodistal and dorsoplantar (Figs 2, 3). In 29 (91%) cases, the superior borders of the intermetatarsal bursae were not above the level of the interosseous tendons. However, high signal intensity on T1-weighted fat-saturated spin-echo images was seen above the level of the metatarsal heads in three (9%) cases. At correlation with anatomic sections and histopathologic analysis, this high signal intensity corresponded to a region of contrast agent extravasation (Figs 4, 5a, 5b).
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Figure 2. Coronal T1-weighted spin-echo MR bursographic image (500/12) obtained at the level of metatarsal heads 3 (M3), 4 (M4), and 5 (M5) in the foot of a human cadaver. The intermetatarsal bursae (straight black arrows) lie immediately above the DTML (white arrowheads) and between both interosseous tendons (straight white arrows). The inferior intermetatarsal space is delineated above by the DTML and below by the STML (black arrowheads). The inferior intermetatarsal space contains the lumbrical muscles (curved white arrow) and the neurovascular bundle (curved black arrow).
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Figure 3. Transverse fat-saturated T1-weighted spin-echo MR bursographic image (500/12) obtained at the level of the third metatarsal head (M3) in the foot of a human cadaver. The intermetatarsal bursae (straight arrows) lie between both interosseous tendons (curved arrows). Note the distal extension of the second bursa between the proximal phalanges.
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Figure 4a. (a) Coronal fat-saturated T1-weighted spin-echo MR bursographic image (500/12) obtained at the level of the third (M3) and fourth (M4) metatarsal heads, with (b) anatomic specimen correlation. The intermetatarsal bursae (thin straight black arrows) lie between both interosseous tendons (straight white arrows). Note the contrast agent extravasation (curved white arrow in a). The collateral accessory ligaments (white arrowheads) arise in the depressions on the medial and lateral sides of the metatarsal head and extend to the sides of the plantar plates (curved arrows in b) below the interosseous tendons. The curved black arrow in a points to the neurovascular bundle, and the black arrowheads in b point to the flexor digitorum longus. In a, the vertical line on the right (thick straight white arrow) represents a 2-cm scale.
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Figure 4b. (a) Coronal fat-saturated T1-weighted spin-echo MR bursographic image (500/12) obtained at the level of the third (M3) and fourth (M4) metatarsal heads, with (b) anatomic specimen correlation. The intermetatarsal bursae (thin straight black arrows) lie between both interosseous tendons (straight white arrows). Note the contrast agent extravasation (curved white arrow in a). The collateral accessory ligaments (white arrowheads) arise in the depressions on the medial and lateral sides of the metatarsal head and extend to the sides of the plantar plates (curved arrows in b) below the interosseous tendons. The curved black arrow in a points to the neurovascular bundle, and the black arrowheads in b point to the flexor digitorum longus. In a, the vertical line on the right (thick straight white arrow) represents a 2-cm scale.
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Figure 5a. (a) Coronal fat-saturated T1-weighted spin-echo MR bursographic image (500/12) obtained at the level of the first metatarsal head in the foot of a human cadaver, with (b) anatomic specimen correlation. (c) Transverse fat-saturated T1-weighted spin-echo MR bursographic image (500/12) obtained at the level of the metatarsal heads in the foot of a human cadaver, with (d) anatomic specimen correlation. Note the specific shape of the first intermetatarsal bursa (straight black arrows) between the adductor hallucis tendon (thin white arrow) and the medial dorsal interosseous tendon (thick white arrow in b-d) of the second ray. In a, contrast agent leakage (curved arrow) is seen. The black arrowheads in b point to the DTML. In c and d, note the distal extension of the second intermetatarsal bursa (curved arrow), with distal insertions of the interosseous tendons (arrowheads).
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Figure 5b. (a) Coronal fat-saturated T1-weighted spin-echo MR bursographic image (500/12) obtained at the level of the first metatarsal head in the foot of a human cadaver, with (b) anatomic specimen correlation. (c) Transverse fat-saturated T1-weighted spin-echo MR bursographic image (500/12) obtained at the level of the metatarsal heads in the foot of a human cadaver, with (d) anatomic specimen correlation. Note the specific shape of the first intermetatarsal bursa (straight black arrows) between the adductor hallucis tendon (thin white arrow) and the medial dorsal interosseous tendon (thick white arrow in b-d) of the second ray. In a, contrast agent leakage (curved arrow) is seen. The black arrowheads in b point to the DTML. In c and d, note the distal extension of the second intermetatarsal bursa (curved arrow), with distal insertions of the interosseous tendons (arrowheads).
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Figure 5c. (a) Coronal fat-saturated T1-weighted spin-echo MR bursographic image (500/12) obtained at the level of the first metatarsal head in the foot of a human cadaver, with (b) anatomic specimen correlation. (c) Transverse fat-saturated T1-weighted spin-echo MR bursographic image (500/12) obtained at the level of the metatarsal heads in the foot of a human cadaver, with (d) anatomic specimen correlation. Note the specific shape of the first intermetatarsal bursa (straight black arrows) between the adductor hallucis tendon (thin white arrow) and the medial dorsal interosseous tendon (thick white arrow in b-d) of the second ray. In a, contrast agent leakage (curved arrow) is seen. The black arrowheads in b point to the DTML. In c and d, note the distal extension of the second intermetatarsal bursa (curved arrow), with distal insertions of the interosseous tendons (arrowheads).
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Figure 5d. (a) Coronal fat-saturated T1-weighted spin-echo MR bursographic image (500/12) obtained at the level of the first metatarsal head in the foot of a human cadaver, with (b) anatomic specimen correlation. (c) Transverse fat-saturated T1-weighted spin-echo MR bursographic image (500/12) obtained at the level of the metatarsal heads in the foot of a human cadaver, with (d) anatomic specimen correlation. Note the specific shape of the first intermetatarsal bursa (straight black arrows) between the adductor hallucis tendon (thin white arrow) and the medial dorsal interosseous tendon (thick white arrow in b-d) of the second ray. In a, contrast agent leakage (curved arrow) is seen. The black arrowheads in b point to the DTML. In c and d, note the distal extension of the second intermetatarsal bursa (curved arrow), with distal insertions of the interosseous tendons (arrowheads).
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In the anatomic sections, the first intermetatarsal bursa had various characteristics. It was located between the medial dorsal interosseous tendon and the collateral ligament complex of the second metatarsophalangeal joint and between the tendon of the adductor hallucis muscle and the lateral collateral ligament complex of the first metatarsophalangeal joint. The shape of the bursa also was different: It was oriented along the adductor hallucis tendon like a tendon sheath (Fig 5).
At MR bursography, the relationship between the intermetatarsal bursae and the DTML was better evaluated in the coronal plane. The bursae were in immediate contact with the DTML inferiorly. In the transverse plane, all bursae in the second and third spaces reached the distal insertion of the interosseous tendons. The bursae also extended beyond the distal border of the DTML, becoming intermetatarsophalangeal bursae. In all cases, a close relationship between the bursae and the neurovascular bundles was identified distal to the DTML at MR bursography (Fig 6). This relationship was confirmed on coronal anatomic sections in six cases and at histopathologic analysis in four cases. This intimacy was never seen in the first and fourth spaces. In the fourth space, the bursae originated proximally to the DTML, did not extend beyond it, and remained solely intermetatarsal in location.
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Figure 6a. (a) Coronal T1-weighted spin-echo (500/12) and (b) coronal fat-saturated T1-weighted spin-echo (500/12) MR bursographic images obtained at the level of the phalangeal bases beyond the DTML, with (c) anatomic and (d, e) histopathologic correlation (hematoxylin-eosin stain; magnification, x2). The intermetatarsal bursa (straight black arrow) lies between both interosseous tendons (straight white arrows in a-c). Note the close relationship between the neurovascular bundle (curved arrow in a-c and e) and the bursa smaller than 1 mm. In d and e, the intermetatarsal bursa has an extremely attenuated cell lining on both sides, with elongated flattened nuclei (arrowheads in d). Proximal phalanges of the third (PP3) and fourth (PP4) rays are shown.
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Figure 6b. (a) Coronal T1-weighted spin-echo (500/12) and (b) coronal fat-saturated T1-weighted spin-echo (500/12) MR bursographic images obtained at the level of the phalangeal bases beyond the DTML, with (c) anatomic and (d, e) histopathologic correlation (hematoxylin-eosin stain; magnification, x2). The intermetatarsal bursa (straight black arrow) lies between both interosseous tendons (straight white arrows in a-c). Note the close relationship between the neurovascular bundle (curved arrow in a-c and e) and the bursa smaller than 1 mm. In d and e, the intermetatarsal bursa has an extremely attenuated cell lining on both sides, with elongated flattened nuclei (arrowheads in d). Proximal phalanges of the third (PP3) and fourth (PP4) rays are shown.
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Figure 6c. (a) Coronal T1-weighted spin-echo (500/12) and (b) coronal fat-saturated T1-weighted spin-echo (500/12) MR bursographic images obtained at the level of the phalangeal bases beyond the DTML, with (c) anatomic and (d, e) histopathologic correlation (hematoxylin-eosin stain; magnification, x2). The intermetatarsal bursa (straight black arrow) lies between both interosseous tendons (straight white arrows in a-c). Note the close relationship between the neurovascular bundle (curved arrow in a-c and e) and the bursa smaller than 1 mm. In d and e, the intermetatarsal bursa has an extremely attenuated cell lining on both sides, with elongated flattened nuclei (arrowheads in d). Proximal phalanges of the third (PP3) and fourth (PP4) rays are shown.
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Figure 6d. (a) Coronal T1-weighted spin-echo (500/12) and (b) coronal fat-saturated T1-weighted spin-echo (500/12) MR bursographic images obtained at the level of the phalangeal bases beyond the DTML, with (c) anatomic and (d, e) histopathologic correlation (hematoxylin-eosin stain; magnification, x2). The intermetatarsal bursa (straight black arrow) lies between both interosseous tendons (straight white arrows in a-c). Note the close relationship between the neurovascular bundle (curved arrow in a-c and e) and the bursa smaller than 1 mm. In d and e, the intermetatarsal bursa has an extremely attenuated cell lining on both sides, with elongated flattened nuclei (arrowheads in d). Proximal phalanges of the third (PP3) and fourth (PP4) rays are shown.
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Figure 6e. (a) Coronal T1-weighted spin-echo (500/12) and (b) coronal fat-saturated T1-weighted spin-echo (500/12) MR bursographic images obtained at the level of the phalangeal bases beyond the DTML, with (c) anatomic and (d, e) histopathologic correlation (hematoxylin-eosin stain; magnification, x2). The intermetatarsal bursa (straight black arrow) lies between both interosseous tendons (straight white arrows in a-c). Note the close relationship between the neurovascular bundle (curved arrow in a-c and e) and the bursa smaller than 1 mm. In d and e, the intermetatarsal bursa has an extremely attenuated cell lining on both sides, with elongated flattened nuclei (arrowheads in d). Proximal phalanges of the third (PP3) and fourth (PP4) rays are shown.
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The bursae were always covered by abundant fat tissue. No communications between the bursae and the adjacent metatarsophalangeal joints were noted (Fig 7).
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Figure 7. Anteroposterior radiograph of the forefoot of a human cadaver shows the locations of the intermetatarsal bursae (arrows). Note the contrast agent leakage (arrowheads) distal to the second, third, and fourth bursae and proximal to the fourth bursa.
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Other structures at the superior intermetatarsal level.—At MR imaging, the dorsal and plantar interosseus tendons were seen as hypointense linear structures parallel to the long axis of the toes. The plantar interosseous tendons (Fig 8) coursed along the medial side of the third to fifth metatarsal bones and inserted into the ipsilateral aspect of the proximal phalanx. The dorsal interosseous tendons extended along the lateral side of the second to fourth metatarsal bones and the medial side of the second metatarsal bone. They inserted into the ipsilateral side of the base of the proximal phalanx. In the intermetatarsal spaces, they were not distinct from the collateral ligament and the extensor hood at conventional MR imaging and MR bursography.
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Figure 8. Coronal fat-saturated T1-weighted spin-echo MR bursographic image (500/12) obtained at the level of the third metatarsal head (M3). The intermetatarsal bursae (straight black arrows) lie between both interosseous tendons (white arrows). Note the contact between the bursa and the neurovascular bundle (curved arrow).
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At MR imaging and anatomic examination, in the first intermetatarsal space, the adductor hallucis muscle extended along the lateral aspect of the first metatarsal head (Fig 5). This muscle, which is formed by a transverse head running between the DTML and the metatarsal bones and by an oblique head arising from the medial side of the calcaneus, had a very short tendon that was composed of both heads joined at the level of the neck of the metatarsal bone. This tendon had a thick attachment to the lateral aspect of the lateral sesamoid bone. It continued distally to the lateral plantar base of the proximal phalanx and blended with the capsule.
The interosseous tendons and the adductor hallucis muscle and tendon were well visualized at all MR sequences, both before and after contrast enhancement of the bursae and in both the transverse and coronal planes.
DTML.—The DTML was identified as an irregular linear structure that connected the plantar plates and had low to intermediate signal intensity at all MR sequences. In the first space, the DTML was superficial to the transverse head of the adductor hallucis muscle and a prolongation of the deep layer of the plantar aponeurosis.The DTML could be visualized in only the coronal plane at MR imaging and MR bursography. MR bursography improved visualization of the dorsal aspect.
Inferior intermetatarsal space.—At MR bursography, MR imaging, and anatomic inspection, the inferior intermetatarsal space was delineated above by the DTML, below by the STML, and laterally by the perforating fibers. The STML was parallel to the DTML and superficial to the tendons of the flexor digitorum longus and brevis. The STML could be described as a fine and regular hypointense line seen on T1-weighted spin-echo MR images. No STML was visualized in the first intermetatarsal space. The perforating fibers extended from the DTML to the STML, around the flexor digitorum common sheath. No relationship of the STML and perforating fibers with the intermetatarsal bursae was identified.
At MR bursography, MR imaging, and anatomic inspection, the inferior intermetatarsal space also contained the lumbrical muscles and the neurovascular bundles (Fig 2). The lumbrical muscles arose from the perforating fibers along the medial side of the flexor digitorum tendons. The neurovascular bundles were beneath the plantar aspect of the DTML, above a fat pad. In the second and third spaces, at the distal margin of the DTML, the neurovascular bundle extended into the web space close to the intermetatarsal bursa, as described earlier. In the first and third spaces, no close relationship between the neurovascular bundle and the bursae was noted. In the 32 spaces, one 3 x 2-mm Morton neuroma was detected in the second space in the coronal plane. Visibility of the structures at the inferior level was best in the coronal plane.
Histopathologic Analysis of the Bursae
At histopathologic analysis, the bursae appeared as a space with an extremely attenuated cell lining on both sides and elongated flattened nuclei that was subtended by a vascular connective tissue layer and easily seen above the adjacent tendons and ligaments (Fig 5d, 5e).
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DISCUSSION
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The intermetatarsal spaces are divided into inferior and superior levels that are separated by the DTML (1). In the superior space, musculotendinous and ligamentous structures lie close together, whereas in the inferior space, the lumbrical muscle is adjacent to the neurovascular bundle. Intermetatarsal bursae are described as being between the interosseous tendons (6), because bursae are generally found where tendons move against each other or glide over bone surfaces (7). Because of these bursae, the metatarsal heads can move easily in a dorsoplantar direction and thus allow the forefoot to adapt to irregularities of the surfaces on which it is placed. Although functionally important, these bursae and their MR imaging characteristics are rarely mentioned in the literature (3,5).
The intermetatarsal bursae lie immediately above the DTML and are located in the space between the interosseous tendons. The first intermetatarsal bursa has a unique anatomy. Our study described it as a tendon sheath covering the adductor hallucis tendon, as reported by Awerbuch et al (6). The bursae in the second and third intermetatarsal spaces extend distally beyond the DTML, but there is no such extension of the bursae in the first and fourth spaces. Beyond the DTML, in both central spaces, the neurovascular bundles lie close to the inferior aspect of the bursae, as reported in anatomic studies (1,2,4).
Intermetatarsal bursitis is a well-known cause of a painful forefoot (4). The pathogenesis of the disease remains unclear, but it may be similar to that described for Morton neuroma. Awerbuch et al (6) have suggested that an inflamed bursa compresses and alters the interdigital nerve, whereas Bossley and Cairney (4) have proposed that bursa inflammation causes secondary neurofibrosis. However, according to the distal extension of individual bursae, neural irritation by an enlarged bursa is less likely in the first and fourth intermetatarsal spaces than in the central spaces. This observation is in agreement with that in previous studies and confirms the higher potential of the second and third intermetatarsal spaces to develop a Morton neuroma (3,8,9). The bursae never extend below the DTML, whereas a Morton neuroma is located on the plantar side of the DTML (10). Therefore, dorsal to the DTML, a diagnosis of bursitis is more likely than is Morton neuroma.
Although injecting contrast agent into the intermetatarsal bursae is technically difficult, enhancement of all bursae was possible in our study. Contrast agent extravasation was common, however. A potential benefit to performing intrabursal injection is the diagnostic or therapeutic injection of local anesthetics and corticosteroid medication in patients with suspected intermetatarsal bursitis. Fluoroscopically guided contrast material administration allowed verification of the intrabursal injection. No communication with the metatarsophalangeal joints was observed, unlike in the study by Bossley and Cairney (4), who reported such a communication. In an MR imaging study, Zanetti et al (3) observed that fluid collections in the first three intermetatarsal bursae with a transverse diameter of 3 mm or smaller were frequent and could be considered physiologic. Our study was affected by incomplete bursal distention or contrast agent leakage during the procedure, so bursal diameter measurements in the three orthogonal planes would not have been reliable and thus were avoided.
There were several limitations to this study. It is well known that bursal cavities vary greatly in location and size. Owing to the small sample size, further investigations are required to address the results of our study. All of our injections were performed in elderly cadavers. The extent and appearance of the bursae may be different in younger populations. Furthermore, limited clinical data were available; this is a common problem in cadaveric studies. Conventional radiography, however, enabled the exclusion of substantial osseous abnormalities.
In conclusion, MR bursography provided detailed information about the anatomy of the intermetatarsal structures, especially the intermetatarsal bursae. An intimate relationship between the intermetatarsal bursae and the neurovascular bundles was depicted in the second and third intermetatarsal spaces at MR bursography. Detailed knowledge of this anatomy should be helpful clinically in differentiating Morton neuromas from bursal abnormalities.
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FOOTNOTES
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Abbreviations: DTML = deep transverse metatarsal ligament,
STML = superficial transverse metatarsal ligament
Author contributions: Guarantors of integrity of entire study, N.H.T., C.W.A.P., A.V.R.M.B., D.R.; study concepts, N.H.T., C.W.A.P., D.R.; study design, N.H.T., A.V.R.M.B., D.R.; literature research, N.H.T.; experimental studies, N.H.T., D.J.T., P.H.; data acquisition, N.H.T., C.W.A.P., A.V.R.M.B., D.J.T.; data analysis/interpretation, N.H.T., C.W.A.P., A.V.R.M.B., P.H.; manuscript preparation, N.H.T., A.V.R.M.B.; manuscript definition of intellectual content, N.H.T., D.R.; manuscript editing, D.R.; manuscript revision/review, D.R., C.B.C.; manuscript final version approval, D.R., N.H.T.
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REFERENCES
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ABSTRACT
INTRODUCTION
