DOI:10.2214/AJR.05.1066
AJR 2007; 188:W348-W354
© American Roentgen Ray Society
Retinacula of the Foot and Ankle: MRI with Anatomic Correlation in Cadavers
Numphung Numkarunarunrote1,
Amaar Malik1,2,
Rodrigo O. Aguiar1,3,
Debra J. Trudell1 and
Donald Resnick1
1 Department of Radiology, Veterans Affairs Medical Center, University of
California, San Diego, 3350 La Jolla Village Dr., San Diego, CA 92161.
2 Present address: Nevada Imaging Centers-Siena, Henderson, NV.
3 Present address: Department of Radiology, Federal University of Rio de
Janeiro, Rio de Janeiro, Brazil.
Received June 21, 2005;
accepted after revision October 11, 2006.
Address correspondence to N. Numkarunarunrote
(numphung36{at}yahoo.com).
WEB This is a Web exclusive article.
Abstract
OBJECTIVE. The retinacula of the ankle are regions of localized
thickening of superficial aponeurosis that provide mechanical strength to
prevent tendon bowstringing. The purpose of this study was to define the foot
and ankle retinacula as seen on MRI with anatomic correlation in cadavers.
MATERIALS AND METHODS. Ten fresh foot and ankle specimens from
humans were imaged with 1.5-T MRI. T1- and intermediate-weighted images were
obtained in the axial, coronal, and sagittal planes. Specimens then were
sectioned into 3-mm-thick sections in either the axial or the coronal plane to
correspond with the MR images. Two radiologists interpreted the MR images and
sections by consensus for the anatomic landmarks and best imaging planes for
identification of the retinacula and discernment of their shape, thickness,
and relations to adjacent tendons.
RESULTS. Normal retinacula of the ankle appeared as bands of low
signal intensity in both MRI sequences. The bony landmarks were helpful in
localization of the attachment sites of the retinacula. The superior extensor
retinaculum and superior and inferior peroneal retinacula were optimally
visualized on axial images. Their thicknesses averaged 0.9, 1.0, and 0.8 mm,
respectively. The flexor retinaculum and three root components (medial,
intermediate, and lateral) of the stem ligament of the inferior extensor
retinaculum were well seen in the coronal plane. The average thicknesses of
these structures were 0.9, 1.5, 1.0, and 0.9 mm, respectively.
CONCLUSION. MRI in standard orthogonal planes is a useful technique
for visualizing the attachment sites, signal intensity, and normal thickness
of foot and ankle retinacula.
Keywords: anatomy • ankle • MRI • musculoskeletal imaging • retinacula
Introduction
The retinacula of the ankle are distinct structures defined as
regions of localized thickening of the superficial aponeurosis covering the
deep structures of the distal portion of the leg, ankle, and foot. These
retinacula are sites of reinforcement of the superficial aponeurosis,
maintaining approximation of tendons to the underlying bone. The retinacula
act as a pulleylike mechanism that appears to represent an adaptation of the
body to provide both a smooth gliding surface and the mechanical strength to
prevent tendon bowstringing.
The ankle retinacula include the superior and inferior extensor and flexor
retinacula and the superior and inferior peroneal retinacula. The retinacula
have three histologic layers: an inner gliding layer; a thick middle layer
that contains collagen bundles, fibro-blasts, and interspersed elastin fibers;
and an outer layer of loose connective tissue that contains vascular channels.
Retinacula throughout the body have this basic three-layer histologic
composition [1].
As described in previous studies
[2-5],
congenital or traumatic absence, laxity, or disruption of the superior
peroneal retinacula allows acute or chronic subluxation of the peroneal
tendons over the sharp edge of the fibula. Injuries to the superior peroneal
retinaculum, particularly in the acute phase, often are clinically mistaken
for other causes of lateral ankle pain. Undetected superior peroneal
retinacular injuries can lead to increased friction of the tendons as they
slide in and out of the peroneal groove.
As in the case of the superior peroneal retinaculum, it is likely that
abnormalities of all the retinacula around the ankle have consequences for or
at least occur in association with nearby tendon abnormalities. To our
knowledge, the normal MRI features of these retinacula have not been described
in detail. We performed MRI and anatomic dissection of cadaveric ankle
specimens for complete analysis of the MRI appearance of these structures. Our
objective was to define all components of the ankle retinacula visualized with
MRI and to correlate the findings by anatomic study of cadavers.
Materials and Methods
Ten fresh foot and ankle specimens were harvested from eight unembalmed
cadavers (six men, two women; age range, 75-94 years; mean age, 83.8 years).
The specimens were prepared by sectioning through the distal portions of the
tibia and fibula. Before MRI, the specimens were deep frozen at -40°C for
at least 3 days and allowed to thaw for 24 hours at room temperature.
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Fig. 1A —Superior extensor retinaculum in cadaver. Axial T1-weighted image
(A) and corresponding photograph of gross section (B) 1.5 cm
above tibiotalar joint show medial and lateral attachments of superior
extensor retinaculum at anterior crest of tibia (black arrow) and
lateral malleolus (white arrow).
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Fig. 1B —Superior extensor retinaculum in cadaver. Axial T1-weighted image
(A) and corresponding photograph of gross section (B) 1.5 cm
above tibiotalar joint show medial and lateral attachments of superior
extensor retinaculum at anterior crest of tibia (black arrow) and
lateral malleolus (white arrow).
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Fig. 1C —Superior extensor retinaculum in cadaver. Axial T1-weighted image
shows separate tunnel (arrows) formed by dissociation of superior
extensor retinacular fibers for anterior tibialis tendon.
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MR images were obtained with a 1.5-T unit (Signa, GE Healthcare). A knee
coil was centered just anterior to the tibiotalar joint with the ankle in mild
plantar flexion. MR images were acquired in the axial, coronal, and sagittal
planes. A T1-weighted spin-echo sequence was performed with the following
parameters: TR/TE, 600/22; number of excitations, 2; section thickness, 2.5 mm
with 0.5-mm interspace thickness; field of view, 12 x 12 cm; matrix
size, 512 x 256. The T1-weighted sequence was followed by an
intermediate-weighted fast spin-echo sequence with the following parameters:
1,100/24; number of excitations, 2; section thickness, 2.5 mm with 0.5-mm
interspace thickness; field of view, 12 x 12 cm; matrix size, 512
x 256.
After MRI, all specimens were immediately positioned with the ankle in mild
plantar flexion and were frozen at -40°C for at least 24 hours. A band saw
was used to section the specimens into 3-mm-thick sections in either the axial
or coronal plane to correspond closely with the MR images. The sections were
examined with a special radiographic unit (Faxitron X-ray system 43805N,
Hewlett-Packard) designed to provide images with high spatial resolution.
Anatomic sections also were photographed with a 3-megapixel digital camera
(Nikon 990, Nikon).
Two radiologists experienced in musculoskeletal imaging interpreted the MR
images and anatomic sections by consensus. The appearance of all visualized
retinacula was agreed on before the results were recorded. The images and
sections were evaluated for anatomic landmarks and best imaging planes for
identification of all ankle retinacula and for shape, thickness, and relation
of the retinacula to the adjacent tendons.
Results
The normal retinacula of the ankle appeared as bands of low signal
intensity on both T1- and intermediate-weighted images. In general, the axial
and coronal T1-weighted spin-echo MR images best depicted the anatomic
features of the retinacula because of the orientation of these images and the
contrast provided by the low signal intensity of the retinacula and high
signal intensity of the surrounding fat. Axial images were best for
identification of the superior extensor retinaculum, the superomedial oblique
band of the inferior extensor retinaculum, and the peroneal retinacula. The
coronal MR images were best in depicting the flexor retinaculum. The stem
portion of the inferior extensor retinaculum was not well visualized in any
imaging plane. However, the lateral roots of the inferior extensor retinaculum
within the sinus tarsi were well delineated only in the coronal plane.
Sagittal images were useful primarily in identification of the inferomedial
oblique band of the inferior extensor retinaculum. In many cases, bony
landmarks were helpful in localization of the attachment sites of the
retinacula.
Superior Extensor Retinaculum
The superior extensor retinaculum is a transverse, roughly rectangular band
located above the tibiotalar joint. It attaches laterally on the lateral crest
of the lower fibula and the lateral surface of the lateral malleolus and
medially on the anterior crest of the tibia and the medial malleolus (Figs.
1A and
1B). The structures that pass
deep in relation to this retinaculum include the tibialis anterior, extensor
hallucis longus, extensor digitorum longus, and peroneus tertius tendons; the
anterior tibial blood vessels; and the deep peroneal nerve. The superior
extensor retinaculum was optimally visualized on axial MR images, appearing
approximately 6-9 mm above the tibiotalar joint as a band of low signal
intensity in both sequences. In our study, the average width of the superior
extensor retinaculum was 0.9 mm (range, 0.7-1.3 mm). According to previous
findings [6], a separate tunnel
for the tibialis anterior tendon is present in approximately 25% of cases.
This tunnel is formed by dissociation of the fibers into a superficial and a
deep layer. Our cadaveric study revealed this tunnel in four of 10 cases
(Fig. 1C).
Superior Peroneal Retinaculum
The superior peroneal retinaculum has been thought to be the primary
restraint preventing subluxation and dislocation of the peroneal tendons as
they pass behind the retromalleolar groove and through the peroneal tunnel.
The origin of this retinaculum was described by Eckert and Davis
[7] in 1976 as the periosteum
of the posterolateral ridge of the fibula. Davis et al.
[8] later found five distinct
insertional variations and widths of the superior peroneal retinaculum.
The superior peroneal retinaculum is a transverse rectangular band that
originates in the lateral border of the retromalleolar groove and tip of the
lateral malleolus. The medial aspect is contiguous with the superficial and
deep aponeuroses of the posterior segment of the distal portion of the leg
(Figs. 2A and
2B). This retinaculum was best
evaluated in the axial plane, appearing as a structure of low signal intensity
in both sequences. The average thickness of the superior peroneal retinaculum,
measured from the fibular tip, was approximately 1.0 mm (range, 0.7-2.6 mm).
Davis et al. [8] described five
variants of the superior peroneal retinaculum, including an inferior oblique
band. This band was identified in 40% of our cases and was similar in
thickness to the transverse band. In two cases, the band attached to the
calcaneus posterior to the calcaneofibular ligament (Figs.
2C and
2D). In the other two cases,
the band attached to the deep aponeurosis in the posterior aspect of the
ankle.
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Fig. 2A —Superior peroneal retinaculum in cadaver. Axial T1-weighted image
(A) and corresponding photograph of gross section (B) show
superior peroneal retinaculum attaching laterally to lateral malleolus and
medially in continuity with superficial (white arrows) and deep
(black arrows) aponeuroses of posterior compartment of lower portion
of leg.
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Fig. 2B —Superior peroneal retinaculum in cadaver. Axial T1-weighted image
(A) and corresponding photograph of gross section (B) show
superior peroneal retinaculum attaching laterally to lateral malleolus and
medially in continuity with superficial (white arrows) and deep
(black arrows) aponeuroses of posterior compartment of lower portion
of leg.
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Fig. 2C —Superior peroneal retinaculum in cadaver. Axial T1-weighted image
(C) and corresponding photograph of gross section (D) show
superior peroneal retinaculum (thin arrows) attaching to lateral wall
of calcaneus just posterior to attachment site of calcaneofibular ligament
(thick arrow).
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Fig. 2D —Superior peroneal retinaculum in cadaver. Axial T1-weighted image
(C) and corresponding photograph of gross section (D) show
superior peroneal retinaculum (thin arrows) attaching to lateral wall
of calcaneus just posterior to attachment site of calcaneofibular ligament
(thick arrow).
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Inferior Peroneal Retinaculum
The inferior peroneal retinaculum is an oblique rectangular band that
originates in the posterior segment of the lateral rim of the sinus tarsi and
extends downward and posteriorly to insert just inferoposterior in relation to
the trochlear process on the retrotrochlear eminence (Fig.
3A,
3B). The origin of the inferior
peroneal retinaculum was not clearly identified because of obscuration by the
adjacent fibers of inferior extensor retinaculum at the lateral wall of the
sinus tarsi. The inferior peroneal retinaculum was best visualized in the
axial plane, appearing as a low-signal-intensity structure in both sequences.
The mean thickness of the inferior peroneal retinaculum was 0.8 mm (range,
0.7-1.0 mm).
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Fig. 3A —Inferior peroneal retinaculum in cadaver. Axial T1-weighted image
(A) and corresponding photograph of gross section (B) at level
of trochlear process (thick arrow) show inferior peroneal retinaculum
(thin arrows) attaching to retrotrochlear eminence and covering
peroneus tendons.
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Fig. 3B —Inferior peroneal retinaculum in cadaver. Axial T1-weighted image
(A) and corresponding photograph of gross section (B) at level
of trochlear process (thick arrow) show inferior peroneal retinaculum
(thin arrows) attaching to retrotrochlear eminence and covering
peroneus tendons.
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Flexor Retinaculum
The flexor retinaculum is located medially, enclosing the tarsal tunnel. It
has a roughly triangular shape and extends from the medial malleolus to the
posterosuperior aspect of the calcaneus. Anatomically, the posterior border
extends medially from the tip of the medial malleolus to the posterosuperior
aspect of the calcaneus. There is no clear demarcation between the posterior
border and the deep aponeurosis of the lower portion of the leg. The anterior
border of the flexor retinaculum corresponds to a vertical line drawn from the
anterior border of the medial malleolus to the medial border of the foot,
which is in continuity with the dorsal aponeurosis of the foot. The base of
the flexor retinaculum corresponds to the superior border of the abductor
hallucis muscle and is continuous with the plantar aponeurosis.
The coronal plane at the level of the susten-taculum tali of the calcaneus
was the best plane and level for visualizing the flexor retinaculum (Fig.
4A,
4B,
4C,
4D,
4E,
4F,
4G,
4H), which appeared as a
low-signal-intensity structure in both sequences. In our study, the thickness
of the flexor retinaculum averaged 0.9 mm (range, 0.7-1.0 mm).
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Fig. 4A —Flexor retinaculum in cadaver. Sequential coronal T1-weighted MR
images (A, C, E, and G) and corresponding photographs of gross
sections (B, D, F, and H) from anterior to posterior aspects
show vertical orientation of flexor retinaculum (white arrow), which
extends inferiorly from medial malleolus to superior aspect of fascia of
abductor hallucis muscle. Intimate tibialis posterior (arrowhead,
H) and flexor digitorum longus (black arrow, D and
H) tendons are evident.
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Fig. 4B —Flexor retinaculum in cadaver. Sequential coronal T1-weighted MR
images (A, C, E, and G) and corresponding photographs of gross
sections (B, D, F, and H) from anterior to posterior aspects
show vertical orientation of flexor retinaculum (white arrow), which
extends inferiorly from medial malleolus to superior aspect of fascia of
abductor hallucis muscle. Intimate tibialis posterior (arrowhead,
H) and flexor digitorum longus (black arrow, D and
H) tendons are evident.
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Fig. 4C —Flexor retinaculum in cadaver. Sequential coronal T1-weighted MR
images (A, C, E, and G) and corresponding photographs of gross
sections (B, D, F, and H) from anterior to posterior aspects
show vertical orientation of flexor retinaculum (white arrow), which
extends inferiorly from medial malleolus to superior aspect of fascia of
abductor hallucis muscle. Intimate tibialis posterior (arrowhead,
H) and flexor digitorum longus (black arrow, D and
H) tendons are evident.
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Fig. 4D —Flexor retinaculum in cadaver. Sequential coronal T1-weighted MR
images (A, C, E, and G) and corresponding photographs of gross
sections (B, D, F, and H) from anterior to posterior aspects
show vertical orientation of flexor retinaculum (white arrow), which
extends inferiorly from medial malleolus to superior aspect of fascia of
abductor hallucis muscle. Intimate tibialis posterior (arrowhead,
H) and flexor digitorum longus (black arrow, D and
H) tendons are evident.
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Fig. 4E —Flexor retinaculum in cadaver. Sequential coronal T1-weighted MR
images (A, C, E, and G) and corresponding photographs of gross
sections (B, D, F, and H) from anterior to posterior aspects
show vertical orientation of flexor retinaculum (white arrow), which
extends inferiorly from medial malleolus to superior aspect of fascia of
abductor hallucis muscle. Intimate tibialis posterior (arrowhead,
H) and flexor digitorum longus (black arrow, D and
H) tendons are evident.
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Fig. 4F —Flexor retinaculum in cadaver. Sequential coronal T1-weighted MR
images (A, C, E, and G) and corresponding photographs of gross
sections (B, D, F, and H) from anterior to posterior aspects
show vertical orientation of flexor retinaculum (white arrow), which
extends inferiorly from medial malleolus to superior aspect of fascia of
abductor hallucis muscle. Intimate tibialis posterior (arrowhead,
H) and flexor digitorum longus (black arrow, D and
H) tendons are evident.
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Fig. 4G —Flexor retinaculum in cadaver. Sequential coronal T1-weighted MR
images (A, C, E, and G) and corresponding photographs of gross
sections (B, D, F, and H) from anterior to posterior aspects
show vertical orientation of flexor retinaculum (white arrow), which
extends inferiorly from medial malleolus to superior aspect of fascia of
abductor hallucis muscle. Intimate tibialis posterior (arrowhead,
H) and flexor digitorum longus (black arrow, D and
H) tendons are evident.
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Fig. 4H —Flexor retinaculum in cadaver. Sequential coronal T1-weighted MR
images (A, C, E, and G) and corresponding photographs of gross
sections (B, D, F, and H) from anterior to posterior aspects
show vertical orientation of flexor retinaculum (white arrow), which
extends inferiorly from medial malleolus to superior aspect of fascia of
abductor hallucis muscle. Intimate tibialis posterior (arrowhead,
H) and flexor digitorum longus (black arrow, D and
H) tendons are evident.
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Inferior Extensor Retinaculum
The inferior extensor retinaculum is a Y-shaped retaining structure located
on the anterior aspect of the tarsus and the ankle. It is a complex structure
that has four components: the stem, or frondiform, ligament; the oblique
superomedial band; the oblique inferomedial band; and the oblique
superolateral band [6]. The
stem ligament acts as a sling, retaining the tendons of the extensor digitorum
longus and peroneus tertius muscles against the talus and calcaneus. This
ligament has three roots: lateral, intermediate, and medial
[9,
10] (Figs.
5 and
6A,
6B,
6C,
6D,
6E,
6F,
6G,
6H,
6I). The lateral root is
superficial, originating laterally in the sinus tarsi. The intermediate root
arises from the sinus tarsi just posterior in relation to the origin of the
cervical ligament. The medial root has three components, two calcaneal and one
talar, which ascend upward and medially against the lateral aspect of the
talar neck. These three root components combine to complete the sling for the
extensor digitorum longus and peroneus tertius tendons.
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Fig. 5 —Schematic of sinus tarsi shows cervical ligament (c) and nearby root
components of stem ligament of inferior extensor retinaculum: medial root (1),
intermediate root (2), and lateral root (3).
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Fig. 6A —Inferior extensor retinaculum in cadaver. Sequential coronal
T1-weighted MR images (A, C, and E) and corresponding
photographs of gross sections (B, D, and F) at level of sinus
tarsi show position of cervical ligament (curved arrow, A and
B) and nearby medial root (thin arrows, A and
B), intermediate root (thick arrow, A and B),
and lateral root (chevron, A-F) of stem ligament of
inferior extensor retinaculum.
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Fig. 6B —Inferior extensor retinaculum in cadaver. Sequential coronal
T1-weighted MR images (A, C, and E) and corresponding
photographs of gross sections (B, D, and F) at level of sinus
tarsi show position of cervical ligament (curved arrow, A and
B) and nearby medial root (thin arrows, A and
B), intermediate root (thick arrow, A and B),
and lateral root (chevron, A-F) of stem ligament of
inferior extensor retinaculum.
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Fig. 6C —Inferior extensor retinaculum in cadaver. Sequential coronal
T1-weighted MR images (A, C, and E) and corresponding
photographs of gross sections (B, D, and F) at level of sinus
tarsi show position of cervical ligament (curved arrow, A and
B) and nearby medial root (thin arrows, A and
B), intermediate root (thick arrow, A and B),
and lateral root (chevron, A-F) of stem ligament of
inferior extensor retinaculum.
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Fig. 6D —Inferior extensor retinaculum in cadaver. Sequential coronal
T1-weighted MR images (A, C, and E) and corresponding
photographs of gross sections (B, D, and F) at level of sinus
tarsi show position of cervical ligament (curved arrow, A and
B) and nearby medial root (thin arrows, A and
B), intermediate root (thick arrow, A and B),
and lateral root (chevron, A-F) of stem ligament of
inferior extensor retinaculum.
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Fig. 6E —Inferior extensor retinaculum in cadaver. Sequential coronal
T1-weighted MR images (A, C, and E) and corresponding
photographs of gross sections (B, D, and F) at level of sinus
tarsi show position of cervical ligament (curved arrow, A and
B) and nearby medial root (thin arrows, A and
B), intermediate root (thick arrow, A and B),
and lateral root (chevron, A-F) of stem ligament of
inferior extensor retinaculum.
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Fig. 6F —Inferior extensor retinaculum in cadaver. Sequential coronal
T1-weighted MR images (A, C, and E) and corresponding
photographs of gross sections (B, D, and F) at level of sinus
tarsi show position of cervical ligament (curved arrow, A and
B) and nearby medial root (thin arrows, A and
B), intermediate root (thick arrow, A and B),
and lateral root (chevron, A-F) of stem ligament of
inferior extensor retinaculum.
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Fig. 6G —Inferior extensor retinaculum in cadaver. Sequential axial
T1-weighted MR images at level of tibiotalar joint show oblique superomedial
band (arrows) of inferior extensor retinaculum coursing upward and
attaching to anterior crest of tibia under tibialis anterior tendon.
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Fig. 6H —Inferior extensor retinaculum in cadaver. Sequential axial
T1-weighted MR images at level of tibiotalar joint show oblique superomedial
band (arrows) of inferior extensor retinaculum coursing upward and
attaching to anterior crest of tibia under tibialis anterior tendon.
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Fig. 6I —Inferior extensor retinaculum in cadaver. Sequential axial
T1-weighted MR images at level of tibiotalar joint show oblique superomedial
band (arrows) of inferior extensor retinaculum coursing upward and
attaching to anterior crest of tibia under tibialis anterior tendon.
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In our study, the stem ligament was not well visualized in the standard
orthogonal planes. On the coronal images, however, the medial, intermediate,
and lateral roots that compose this ligament were well visualized in the sinus
tarsi. The inferior extensor retinaculum appeared as an area of low signal
intensity in both sequences.
The roots of the stem ligament of the inferior extensor retinaculum were
measured in three areas. The thickness averaged 1.5 mm (range, 1.1-2.0 mm) for
the medial root, 1.0 mm (range, 0.6-1.4 mm) for the intermediate root, and 0.9
mm (range, 0.7-1.3 mm) for the lateral root. The oblique superomedial band
continues in the direction of the stem, passing over the tendon of the
extensor hallucis longus muscle and under the tendon of the tibialis anterior
muscle, and inserts on the anterior aspect of the medial malleolus. The
superomedial band was 0.8 mm (range, 0.7-1.0 mm) thick. The oblique
inferomedial band arises from the apex of the stem ligament, advances
inferomedially, and reaches the medial border of the foot at the level of
cuneonavicular joint. The thickness of this band averaged 1.1 mm (range,
0.7-1.5 mm).
Discussion
The work of Sarrafian [6]
and others has defined the retinacula of the ankle. The location, size,
extent, and variations of these structures have been well described in gross
cadaveric studies. In addition, a few imaging publications have described the
superior peroneal retinaculum in correlation with pathologic findings in the
peroneal tendons. Rosenberg et al.
[2] stated that in the case of
chronic peroneal tendon dislocations, assessing the integrity of the superior
peroneal retinaculum is necessary because the tendons may be in a normal
position during imaging, making peroneal tendon position an unreliable sign.
It also is likely that assessment of the integrity of all of the ankle
retinacula, which function to prevent tendon bowstringing and dislocation, is
important to rule out underlying tendon abnormalities. Our objective was to
define these structures on MRI, the most widely used method of imaging the
soft tissues of the extremities, to elucidate their normal appearance and,
therefore, to be able to clinically identify abnormal ankle retinacula.
We found that for identification of the retinacula around the ankle,
non-fat-suppressed T1- or intermediate-weighted images and proper imaging
planes are necessary. Identification of bone landmarks and adjacent intimate
structures such as tendons, vessels, and nerves also is helpful. Knowledge of
the relation of the roots of the inferior extensor retinaculum to the sinus
tarsi and ligaments of the sinus tarsi is crucial. In general, we found that
the axial plane is suitable for visualizing the superior extensor retinaculum
and the superior and inferior peroneal retinacula. The coronal plane was
helpful for visualizing the flexor retinaculum. The stem portion of the
inferior extensor retinaculum was difficult to localize and evaluate in the
standard orthogonal planes. However, the lateral roots of the inferior
extensor retinaculum were well visualized on coronal images. The superomedial
oblique band of the inferior extensor retinaculum was best evaluated in the
axial plane, and the inferomedial oblique band was best seen in the sagittal
plane.
All retinacula, including the roots of the inferior extensor retinaculum,
were found to be of low signal intensity on both T1- and intermediate-weighted
MR images. The shapes of the retinacula, particularly the inferior extensor
retinaculum, superior peroneal retinaculum, and flexor retinaculum, were
difficult to delineate with MRI because of their very thin structure and
orientation in the standard orthogonal planes. However, with knowledge from
gross cadaveric studies and careful assessment of the MR images and gross
sections, rough definition of these structures is possible. For similar
reasons, the width of the retinacula was difficult to delineate with MRI.
Overall, the thickness of the ankle retinacula averaged approximately 1 mm.
The thin-nest retinacular measurement was 0.7 mm, and this thickness involved
all five retinacula in different specimens. The thickest measurement was 2.4
mm, and this thickness involved not a proper retinaculum but the medial root
of the inferior extensor retinaculum within the sinus tarsi. Only three
specimens had retinacular thicknesses greater than 2.0 mm, and in all three
instances, the medial root of the inferior extensor retinaculum was involved.
If the roots of the inferior extensor retinaculum are excluded from the study,
the range of retinacular thickness was 0.7-1.5 mm. On the basis of our
observations, it appears that normal roots of the inferior extensor
retinaculum tend to be as thick as or, in many cases, thicker than the actual
retinacula.
In most cases, the retinacula were in continuity with their respective
aponeuroses. De-lineation between retinaculum and aponeurosis was based on the
relative change in thickness of the aponeuroses as they formed retinacula. The
best evidence of this distinction was the continuity of the flexor and
superior peroneal retinacula with the superficial and deep aponeuroses of the
distal part of the leg (Fig.
3A,
3B). These structures form a
basket around the Achilles tendon.
Our study had several limitations. Because of the small sample size, we did
not encounter or describe many anatomic variations of the retinacula. Because
we conducted a cadaveric study and did not have clinical data, the results may
not correspond to in vivo findings. Certain retinacula, such as the inferior
extensor retinaculum, which has a complex shape and extends in more than one
plane, were not well depicted in the orthogonal planes used.
The ankle and foot retinacula can be depicted with MRI in detail similar to
that revealed in anatomic studies of cadavers. The thickness of the retinacula
averaged 1 mm, the roots of the inferior extensor retinacula generally
appearing slightly thicker. More in-depth study is needed to asses the
clinical significance of retinacular thickening, which may relate to
abnormality of the underlying tendons or be a source of tendon abnormality.
Our findings suggest that thickened retinaculum is a width of 2 mm or greater.
The results of our study should serve as a foundation for future
investigations dealing with pathologic conditions of the retinacula.
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Abstract
Introduction
