DOI:10.2214/AJR.04.1784
AJR 2006; 186:943-947
© American Roentgen Ray Society
Anatomic Variations and MRI of the Intermalleolar Ligament
Chang-Seok Oh1,
Hyung-Sun Won2,
Mi-Sun Hur2,
In-Hyuk Chung2,
Sungjun Kim3,4,
Jin-Suck Suh3 and
Ki-Sun Sung5
1 Department of Anatomy, Samsung Biomedical Research Institute, Sungkyunkwan
University School of Medicine, Suwon, 440-746, Korea.
2 Department of Anatomy and Brain Korea 21 Project for Medical Science, Seoul,
Korea.
3 Department of Diagnostic Radiology, Yonsei University College of Medicine,
Seoul, 120-752, Korea.
4 Present address: Department of Diagnostic Radiology, Hanyang University
College of Medicine, Seoul, 133-791, Korea.
5 Department of Orthopaedic Surgery, Samsung Medical Center, Sungkyunkwan
University School of Medicine, Seoul, 135-710, Korea.
Received November 17, 2004;
accepted after revision February 10, 2005.
Supported by a grant from Samsung Biomedical Research Institute
(B-A3-105).
Address correspondence to C.-S. Oh
(changoh{at}med.skku.ac.kr).
Abstract
OBJECTIVE. The purpose of this study was to identify the
intermalleolar ligament morphologically and to correlate its shape with MR
images.
MATERIALS AND METHODS. Seventy-seven ankles were used in this study.
After the intermalleolar ligament had been located in the posterior ankle
space, its medial and lateral attaching sites were identified, and its length,
width, and thickness were measured. MRI was performed on 26 ankles before they
were dissected (20 specimens) or serially sectioned (six specimens). The
serial sections were taken at a thickness of 2 mm in the sagittal and
horizontal directions.
RESULTS. The intermalleolar ligament was observed in 81.8% of the
specimens and was composed of more than two bundles of fibers in all cases.
The medial arising sites of the ligament were diverse (e.g., from the medial
malleolus to the floor of the fibrous tunnel of the flexor hallucis longus).
The ligament narrowed laterally and attached with the posterior talofibular
ligament to the medial fossa of the lateral malleolus. Their morphologic
shapes were also diverse, depending on their medial arising sites, the number
of the composing fiber bundles, and the degree of bundle compactness. The
intermalleolar ligament appeared as a thick string or as more than two fine
parallel stripes on coronal MR images and as a linear structure on axial
images. On sagittal images, the ligament appeared as scattered dots in the
medial part and as a thin flat or nodular structure in the lateral part.
CONCLUSION. The intermalleolar ligament seemed to be an almost
invariably present anatomic entity with diverse morphologic features on MR
images.
Keywords: ankle • intermalleolar ligament • MRI • posterior impingement syndrome
Introduction
Posterior impingement syndrome, presenting as posterolateral ankle pain
with plantar flexion, occurs commonly in ballet dancers. The anatomic
structures causing posterior impingement syndrome have been described
[1] and include the
intermalleolar ligament (IML), which is also known as the posterior
intermalleolar ligament.
However, the IML has been neglected in the anatomy literature, and drawings
and pictures of it are inconsistently presented
[2,
3]. Moreover, significant
discrepancies in the frequency of occurrence of the ligament have been
reported by dissection and MRI examination
[3]. Hence, confusion has
arisen regarding the IML and the posterior ankle ligaments that include this
structure [4,
5].
Based on observations in the dissection laboratory that the IML is a
distinct and almost constant structure with diverse shapes, we conducted this
study to identify its morphologic characteristics and to correlate its shapes
with MR images.
Materials and Methods
Seventy-seven ankles from 40 cadavers (25 men, 15 women) were used in this
study. The average age of the cadavers was 62 years (range, 20–93
years), and the cause of death did not affect the joint. Morphologic changes
caused by trauma were not observed in the specimens during the dissection.
After removing the Achilles and flexor hallucis longus (FHL) tendons, the
posterior aspect of the ankle joint was exposed. The tendons of the peroneus
longus and brevis were then removed, and the lateral insertion of the IML on
the lateral malleolus was observed. After removing the flexor digitorum longus
(FDL), fibers from the septum between the FDL and the tibialis posterior (TP)
were identified to constitute the IML. Fibers that arose from the tibial
margin, joint membrane covering the posterior process of the talus, or the
floor of the fibrous tunnel of FHL were also found to constitute the ligament.
After the whole IML had been revealed, its longest length was measured between
the lateral and medial malleoli using a digital caliper. The IML was severed
at its midpoint, and its width (superoinferior distance) and thickness
(anteroposterior distance) were measured.
MRI was performed on a 1.5-T scanner (Signa Horizon, GE Healthcare) with a
dedicated extremity coil on the 26 ankles before dissection (20 specimens) or
serial section (six specimens). Ankles were placed in the center of the coil
in a neutral position, and coronal and axial images were obtained using proton
density–weighted and T2-weighted conventional spin-echo (TR/TE,
2,000–2,400/20; 70 msec) sequences and T1-weighted spin-echo
(300–400/13–15) sequences. Sagittal images were obtained only by
conventional proton density–weighted and T2-weighted spin-echo
(2,000–2,400/20; 70 msec) sequences. The slice thickness was 4 mm
without an interslice gap. One signal was acquired with an imaging matrix of
256 x 192 and a field of view of 12–14 cm. All MR images were
reviewed by a musculoskeletal radiologist. Serial sections were taken using an
electrical bandsaw at a thickness of 2 mm in the sagittal and horizontal
directions after fixing the specimens in plaster.
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Fig. 1A —Dissected ankle and coronal MR image shows intermalleolar ligament
(IML) in shape of thick string. IML arises from lateral border of medial
malleolar sulcus (MM) and runs to lateral malleolus (LM) with posterior
talofibular ligament (PTF). FHL = flexor hallucis longus.
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Fig. 1B —Dissected ankle and coronal MR image shows intermalleolar ligament
(IML) in shape of thick string. IML shown as thick string under inferior
transverse ligament (ITL) in posterior ankle space. Oval structure composed of
multiple dots on sagittal MR image of same specimen is noted in circle
(arrow), representing cross-sectioned bundles of fibers composing
ligament.
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Fig. 2A —Cadaveric specimen shows intermalleolar ligament (IML) of band shape
and its coronal MR image. IML is medially continuous with septum between
flexor digitorum longus (FDL) and tibialis posterior and narrows laterally to
lateral malleolus. Some strands of ligament were connected to midpoint of
posterior distal margin of tibia. FHL = flexor hallucis longus, PTF =
posterior talofibular ligament.
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Fig. 2B —Cadaveric specimen shows intermalleolar ligament (IML) of band shape
and its coronal MR image. Coronal MR image of same specimen shown in A.
IML is observed as two stripes, which represent fiber bundles composing
ligament.
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Results
Dissected Specimens
The IML was observed in 81.8% of the specimens. The ligament was composed
of more than two bundles of fine fibers that arose medially from diverse
sites. The medial arising sites of the ligament included the lateral border of
the medial malleolar sulcus (Fig.
1A); the medial border of the medial malleolar sulcus through the
septum between FDL and TP (Figs.
2A and
3A); the middle part, medial
half, or the medial end of the posterior distal margin of the tibia (Figs.
3A and
4A); the joint membrane
covering the posterior process of the talus; and the floor of the FHL tunnel.
The bundles of fibers converged laterally into a discrete cord on the
posterior talofibular ligament (PTF) near the lateral malleolus and then ran
into the medial fossa of the lateral malleolus with the PTF (Figs.
1A,
2A,
3A, and
4A).
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Fig. 3A —Cadaveric specimen shows intermalleolar ligament (IML) wider than in
Figures 2A and
2B and its coronal MR image.
IML arises from medial border of medial malleolar sulcus through septum
between flexor digitorum longus (FDL) and tibialis posterior and from medial
half of posterior distal margin of tibia. Ligament narrows laterally toward
lateral malleolus (LM) with posterior talofibular ligament (PTF). FHL = flexor
hallucis longus.
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Fig. 4A —Dissection of intermalleolar ligament (IML) of band shape and its
axial MR image. IML is connected to two different sites of posterior distal
margin of tibia. Septum between flexor digitorum longus and tibialis posterior
(TP), with which ligament was continuous medially, has been removed to show
TP. FHL = flexor hallucis longus, PTF = posterior talofibular ligament.
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Fig. 3B —Cadaveric specimen shows intermalleolar ligament (IML) wider than in
Figures 2A and
2B and its coronal MR image.
Coronal MR image of same specimen shown in A. IML appears as multiple
stripes, which represent multiple fiber bundles composing ligament.
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Fig. 4B —Dissection of intermalleolar ligament (IML) of band shape and its
axial MR image. Axial MR image of same specimen shown in A. IML runs
medially between flexor digitorum longus (FDL) and TP and attaches laterally
to lateral malleolus (LM). Small portion of posterior talofibular ligament is
indicated by asterisk.
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The shape of the IML was diverse and varied from a thick string to a band
with broad medial and apical lateral parts (Figs.
1A and
3A). These variations depended
on its medial arising sites, the number of composing fiber bundles, and the
degree of the bundle compactness. The average length of the IML between two
malleoli was 39.2 mm (range, 28.2 to 44.93 mm). The mean width and thickness
of the ligament at the midpoint were 3.7 mm (range, 0.79 to 8.72 mm) and 2.8
mm (range, 0.4 to 5.8 mm), respectively.
The fibers from the septum between the FDL and the TP not only constituted
the IML, but also ran to the posterior distal margin of the tibia or the PTF
(Figs. 2A and
4A). They crossed each other
and often formed inner and outer layers, and the inner fibers composed the
IML. In 10.4% of specimens, a thin fascicle was observed to connect the margin
of the tibia or medial malleolus with the PTF. The fascicle was distinguished
from the IML in that it did not laterally converge into a cord separable from
the PTF like the IML, but rather joined the PTF
(Fig. 5).
MRI Findings
The IML in all 26 ankles could be identified on each MR image plane, except
one case of axial plane. On coronal MR images, the IML appeared as a
relatively thick string of low signal intensity
(Fig. 1B) or as more than two
fine parallel stripes in the posterior ankle space (Figs.
2B and
3B) that were partially visible
or visible along their entire lengths. On axial images, the IML appeared as a
linear structure behind the talus, medially continuous to the septum between
the FDL and the TP, and attached to the lateral malleolus with the PTF
(Fig. 4B). On sagittal images,
the medial part of the IML appeared as scattered dots except one case of an
oval structure composed of multiple dots compactly aggregated in front of the
FHL (Figs. 1B and
6B), depending on the number
and degree of compactness of the bundles composing the ligament (Figs.
1A and
6A). The lateral part of the
IML appeared as a thin flat or nodular structure between the inferior
transverse ligament and the PTF (Figs.
7A and
7B).
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Fig. 6B —Cadaveric specimen shows sagittally sectioned intermalleolar
ligament (IML) and its sagittal MR image. Sagittal MR image of same specimen
corresponding to A. Three low-signal-intensity spots
(arrowheads) represent cross-sectioned bundles shown in A.
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Fig. 6A —Cadaveric specimen shows sagittally sectioned intermalleolar
ligament (IML) and its sagittal MR image. Sagittal section of cadaveric ankle
showing three cross-sectioned bundles of fibers (arrowheads)
composing IML. They are located posteriorly between tibia (Tb) and talus (T)
and in front of flexor hallucis longus (FHL).
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Fig. 7A —Same cadaveric specimen shows sagittally sectioned intermalleolar
ligament (IML) in more lateral part than in Figures
6A and
6B and its sagittal MR image.
IML appears as linear structure between inferior transverse ligament (ITL) and
posterior talofibular ligament (PTF). FHL = flexor hallucis longus, T = talus,
Tb = tibia.
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Fig. 7B —Same cadaveric specimen shows sagittally sectioned intermalleolar
ligament (IML) in more lateral part than in Figures
6A and
6B and its sagittal MR image.
Sagittal MR image corresponding to A. IML is linear and in same place
as shown in A.
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Discussion
Impingement of the IML is known to be a cause of posterior impingement
syndrome [1], and the IML has
been resected arthroscopically to treat this syndrome
[5]. However, the IML has been
neglected in most anatomy textbooks and regarded as a portion of a broad PTF
[6], a strip of tissue
extending from the medial malleolus to the fibular attachment of the PTF
[7], or even as a synonym for
the tibial slip [4], which has
caused confusion regarding the anatomy of the posterior ankle ligaments
[4,
5]. The current study shows
that the IML is a distinctive anatomic structure distinguishable from the PTF
or its tibial slip; it was clearly seen in 81.8% (63 of 77 specimens). This
frequency was higher than 55.6% (20 of 36 specimens) and 72.5% (29 of 40
specimens) in previous studies
[3,
8]. Differences in frequency
among studies may be a result of differences in the number of specimens used
or interracial variations.
The lack of information regarding this ligament in the anatomy literature
can possibly be attributed to three reasons. First, the ligament fibers were
found to arise medially from many different sites, although they laterally
converged into a discrete cord. The shape of the IML was determined not only
by the number of the fiber bundles composing the ligament and the degree of
the bundle compactness, but also by the sites and number of its medial
arising. These diverse shapes of the ligament may make it difficult to define
as a discrete structure with uniform shape, and even regarded as different
structures, such as a portion of a broad PTF
[6] or a strip of tissue
extending from the medial malleolus to the fibular attachment of the PTF
[7]. Medial origin sites of the
IML in previous reports have included the posterior margin of the medial
malleolus in dissection samples and the medial posterior tibial cortex on MRI
[3], the most medial aspect of
the posterior tibia on MRI [9],
and the posterior tibial margin in a dissection sample
[2]. Second, fibers from the
septum between the FDL and the TP crossed each other on the joint membrane,
running toward the lateral malleolus and obliquely to the PTF and the
posterior distal margin of the tibia and sometimes formed layers where the IML
fibers ran internally. The intricacy of these septal fibers may complicate the
separation of the IML fibers, as described in the statement that the
visualization of the posterior ankle ligaments in open surgery is not easy
[5]. In addition, this
intricacy of fibers might be one of the reasons for the difference between the
current study and previous studies in terms of the determined occurrence
frequencies of the IML. Third, in the current study, a fascicle was observed
to laterally join the PTF from the margin of the tibia or medial malleolus in
10.4% of subjects. This fascicle was a different structure from the IML and
seemed to correspond to "the tibial slip of PTF" that was
described as connecting the PTF to the medial malleolus
[10]. However, the IML might
have been regarded as the tibial slip of PTF in previous studies, considering
that the IML was used as a synonym for the tibial slip and that the tibial
slip was depicted as a variation of the PTF
[4,
11].
The present study also shows diverse MR images of the IML, which were
achieved by correlating dissection details and MRI observations in cadaveric
specimens. Upon reexamining previous MR images of the posterior ankle
ligaments in the light of the current study results, it became apparent that
the IML is not often noted
[12] and that structures
labeled as transverse tibiofibular ligament on axial and coronal planes
[13,
14], posterior tibiofibular
ligament on axial and sagittal planes
[15,
16], or PTF on axial plane
[17] appear to be the IML.
This failure to visualize the IML by MRI may explain differences in the
frequency of its presence by dissection (55.6%) and MRI (18.6%) in a previous
report [3], and it may also be
related to the report that stated that MRI was limited in terms of properly
imaging the posterior ligamentous complex
[4]. The IML on the sagittal
plane in particular needs to be distinguished from that of a free
intraarticular body by reviewing images on the coronal and axial planes.
In conclusion, the present study clarifies the IML as an anatomic entity.
It is hoped that the MR images provided help reduce confusion and vagueness
associated with the anatomy of this ligament and thus facilitate the
evaluation of patients with posterior impingement syndrome.
Acknowledgments
We thank Mr. Seung-Ryong Choi and Mr. Dong-Su Jang for their help in
preparing the specimens and figures.
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Abstract
Introduction
