AJR 2004; 182:147-154
© American Roentgen Ray Society
Anatomy of and Abnormalities Associated with Kager's Fat Pad
Justin Q. Ly1,2 and
Liem T. Bui-Mansfield2,3,4
1 Department of Radiology, Wilford Hall Medical Center, San Antonio, TX
78236-5300.
2 Department of Radiology, Brooke Army Medical Center, 3851 Roger Brooke Dr.,
San Antonio, TX 78234.
3 Division of Radiologic Sciences, Department of Radiology, Wake Forest
University School of Medicine, Medical Center Blvd., Winston-Salem, NC
27157-1088.
4 Department of Radiology, Uniformed Services University of the Health Sciences,
Bethesda, MD 20814-4799.
Received March 28, 2003;
accepted after revision May 14, 2003.
Address correspondence to L. T. Bui-Mansfield.
The opinions and assertions contained herein are those of the authors and
should not be construed as official or as representing the opinions of the
Department of the Air Force, Department of the Army, or Department of
Defense.
Presented at the annual meeting of the American Roentgen Ray Society, San
Diego, CA, 2003.
Introduction
Kager's fat pad, also known as the pre-Achilles fat pad, is a lipomatous
structure located in the posterior ankle joint, anterior to the Achilles
tendon. A sound understanding of the anatomy of this fat pad can be useful in
detecting various abnormalities of the ankle joint. In this pictorial essay,
we describe the normal anatomy of Kager's fat pad and show examples of
abnormalities that can affect this triangular structure.
Normal Anatomy of Kager's Fat Pad
On lateral radiographs of the ankle, Kager's fat pad is a sharply
marginated, radiolucent triangle (Fig.
1A). The boundaries of the triangle are formed by three anatomic
structures: the flexor hallucis longus muscle and tendon anteriorly, the
superior cortex of the calcaneus inferiorly, and the Achilles tendon
posteriorly (Fig. 1B). The
posterior ankle joint extends into the anteroinferior corner of Kager's fat
pad. The retrocalcaneal bursa forms the posteroinferior corner of the pad.
Abnormal conditions involving the posterior ankle may result in increased and
ill-defined soft-tissue density in Kager's fat pad or the obliteration or
distortion of its borders. Although sometimes subtle, these conditions often
are detectable radiographically and can signal the presence of an
abnormality.
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Fig. 1A. —Normal anatomy of Kager's fat pad. Lateral radiograph
(A) and illustration (B) of ankle show triangular radiolucency
(A) and structure of Kager's fat pad. On radiograph, anterior border
(arrowheads, A) is posterior aspect of flexor hallucis longus
muscle and tendon, posterior border (asterisk, A) is Achilles
tendon, and floor (arrow, A) is superior surface of
calcaneus.
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Fig. 1B. —Normal anatomy of Kager's fat pad. Lateral radiograph
(A) and illustration (B) of ankle show triangular radiolucency
(A) and structure of Kager's fat pad. On radiograph, anterior border
(arrowheads, A) is posterior aspect of flexor hallucis longus
muscle and tendon, posterior border (asterisk, A) is Achilles
tendon, and floor (arrow, A) is superior surface of
calcaneus.
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Abnormal Conditions
Achilles Tendon
Several abnormal conditions may involve the Achilles tendon, including
tendinosis, peritendinitis, tears (either partial
[Fig. 2A] or complete
[Fig. 2B]), and ossification of
the Achilles tendon (Fig. 2C).
Ossification of the Achilles tendon may occur at the insertion of the tendon
into the calcaneus, such as enthesopathy, or in the tendon itself subsequent
to a trauma. Radiographic findings of a ruptured Achilles tendon include
increased soft-tissue density in Kager's fat pad, thickening of the Achilles
tendon, a positive finding for Arner's sign, and diminished Toygar's angle
[1]. In the positive finding
for Arner's sign, the anterior contour of the ruptured Achilles tendon curves
away from the calcaneus at its insertion zone and shows forward deviation and
nonparallelism in the tendon and the skin surface in the supracalcaneal zone.
The Toygar's angle is the angle of the posterior skin surface adjacent to the
Achilles tendon seen on lateral ankle radiographs. A Toygar's angle smaller
than 150° is considered abnormal and indicative of a rupture of the
Achilles tendon.
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Fig. 2A. —Abnormalities of Achilles tendon. Sagittal fat-suppressed
fast spin-echo T2-weighted image of 26-year-old man shows focal hyperintensity
in thickened Achilles tendon, corresponding to partial tear
(arrowhead) of Achilles tendon, and edema (arrow) in Kager's
fat pad.
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Fig. 2B. —Abnormalities of Achilles tendon. Sagittal spin-echo
T1-weighted image of 42-year-old man shows large area of intermediate signal
in expected location of Achilles tendon, flanked superiorly and inferiorly by
irregular ends of ruptured and retracted Achilles tendon.
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Radiography cannot be used to distinguish a partial tear from a complete
tear, but MRI can reveal detail necessary for making this distinction.
Moreover, MRI can show other causes of Achilles tendon thickening, such as
gouty arthritis, xanthomatosis (Fig.
3), rheumatoid arthritis, and Haglund's disease
(Fig. 4). Haglund's disease is
associated with thickening of the distal Achilles tendon, retro-Achilles
bursitis, and retrocalcaneal bursitis. It is also known as "pump
bumps" because wearing high heels or improperly fitted shoes is a
predisposing factor for this condition
[2]. In cerebrotendinous
xanthomatosis, the Achilles tendon is focally or diffusely infiltrated by
lipid-laden histiocytes produced by hyperlipidemia. On all pulse sequences,
MRI reveals xanthomatosis as a diffuse stippled pattern with many low-signal
rounded structures of equal size, surrounded by high-signal material. The size
of the Achilles tendon may be either normal or enlarged
[3]. Retrocalcaneal bursitis,
bone erosions, and tophi or subcutaneous nodules may be seen with either gouty
or rheumatoid arthritis.
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Fig. 3. —30-year-old woman with cerebrotendinous xanthomatosis. Axial
T2-weighted image of ankle shows heterogeneity and stippling of thickened
Achilles tendon (arrow). Appearance is attributable to hypointense
collagen surrounded by hyperintense foamy histiocytes and inflammation. T=
tibia, F = fibula. (Reprinted from
[3])
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Fig. 4. —32-year-old woman with Haglund's disease. Lateral ankle
radiograph shows thickening of Achilles tendon at insertion
(asterisk), retrocalcaneal bursitis (curved arrowhead), and
retro-Achilles bursitis (double arrows). Triad of findings is
consistent with Haglund's disease.
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Calcaneus
The most common abnormalities of the calcaneus are trauma-induced fractures
(Fig. 5A), stress fractures
(Fig. 5B), or
insufficiency–avulsion fractures in patients with diabetes
(Fig. 5C). Cortical
destruction of the calcaneus by either tumor or infection may obliterate
Kager's fat pad. Disseminated coccidioidomycosis (Fig.
6A,
6B) is seen in approximately
0.5% of patients with pulmonary coccidioidal infection; osseous involvement
occurs in 10–50% of cases in which the infection has been disseminated
[4].
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Fig. 5B. —Calcaneal fractures. Lateral ankle radiograph of 38-year-old
woman who presented with ankle pain shows curvilinear sclerosis
(arrows) exiting into superior aspect of calcaneus, consistent with
stress fracture. Note subtle density in Kager's fat pad
(asterisk).
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Fig. 6A. —21-year-old man with disseminated coccidioidomycosis. Lateral
ankle radiograph shows several radiolucencies with ill-defined borders
(arrowheads) in calcaneus. Abnormal soft-tissue density
(asterisk) is seen in Kager's fat pad.
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Fig. 6B. —21-year-old man with disseminated coccidioidomycosis.
Gadolinium-enhanced sagittal fat-suppressed T1-weighted image reveals abscess
(white arrow) involving anterior aspect of Kager's fat pad as well as
septic tenosynovitis of flexor hallucis longus tendon (white
arrowheads). Osteomyelitis of calcaneus disrupts superior surface of
calcaneus (black arrow) and tarsal navicular bone (black
arrowhead). Chest radiograph (not shown) showed diffuse miliary pattern
of pulmonary coccidioidomycosis.
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Flexor Hallucis Longus Tendon
Os trigonum syndrome is the most common abnormal condition affecting the
flexor hallucis longus tendon. The condition is usually the result of
repetitive microtrauma and chronic inflammation involving the cartilaginous
synchondrosis between the os trigonum and the adjacent lateral talar tubercle
(Fig. 7A,
7B), leading to disruption of
the synchondrosis with accompanying pain and swelling of the posterior ankle.
Flexor hallucis longus tenosynovitis often occurs in association with os
trigonum syndrome [5]. Other
conditions that can affect this tendon include tendinosis, partial or complete
tears, and neoplasms, including giant cell tumor of the tendon sheath and
hemangiopericytoma (Fig. 8).
Hemangiopericytomas are rare vascular malignancies derived from the pericytes
of Zimmerman and typically occur during the fifth and sixth decades of
life.
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Fig. 7B. —30-year-old man with os trigonum syndrome. Sagittal STIR
image shows associated tenosynovitis of flexor hallucis longus tendon
(arrowheads) and edema at anteroinferior aspect of Kager's fat
pad.
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Fig. 8. —28-year-old man with hemangiopericytoma of flexor hallucis
longus muscle. Lateral radiograph of ankle shows irregular peripherally
calcified mass in flexor hallucis longus muscle associated with masslike
prominence (arrowheads) that bulges into upper anterior aspect of
Kager's fat pad.
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Kager's Fat Pad
The accessory soleus muscle (Fig.
9A,
9B) is the most common
accessory muscle in the ankle and the most common soft-tissue mass seen in
Kager's fat pad. Patients may present with pain or a mass in the posteromedial
ankle. The accessory soleus muscle may cause compression neuropathy of the
posterior tibial nerve [6]. On
MRI, the accessory soleus muscle is isointense relative to the surrounding
muscle and courses anteriorly relative to the Achilles tendon.
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Fig. 9A. —24-year-old man with accessory soleus muscle. Lateral
radiograph of left ankle shows linear soft-tissue density
(arrowheads) in Kager's fat pad and in region immediately anterior to
Achilles tendon.
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Rarely, a neoplasm may arise from or extend into Kager's fat pad.
Extraskeletal chondrosarcoma may present as either a noncalcified (Figs.
10A and
10B) or calcified soft-tissue
mass (Fig. 10C). Calcified
masses are common findings in extraskeletal chondrosarcoma, often showing
stippled and ring- or arclike calcifications
[7]. These rare malignancies
grow slowly and have a favorable prognosis. In large joints, they can arise
from the synovium as a primary neoplasm or as a result of malignant
transformation of synovial chondromatosis. Occasionally, a ganglion arising
from a joint or associated with a tear of a ligament or tendon can herniate
into Kager's fat pad (Fig.
11).
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Fig. 10A. —Extraskeletal myxoid chondrosarcomas. Lateral ankle
radiograph (A) and axial fat-suppressed T2-weighted image (B)
obtained in 42-year-old man with extraskeletal myxoid chondrosarcoma of
Kager's fat pad show large mass in fat pad. On radiograph, soft-tissue mass
(asterisks, A) is shown to be completely obliterating fat pad
and eroding calcaneus (arrow, A). T2-weighted image (B)
reveals large mass with predominantly high signal intensity and macrolobulated
margins.
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Fig. 10B. —Extraskeletal myxoid chondrosarcomas. Lateral ankle
radiograph (A) and axial fat-suppressed T2-weighted image (B)
obtained in 42-year-old man with extraskeletal myxoid chondrosarcoma of
Kager's fat pad show large mass in fat pad. On radiograph, soft-tissue mass
(asterisks, A) is shown to be completely obliterating fat pad
and eroding calcaneus (arrow, A). T2-weighted image (B)
reveals large mass with predominantly high signal intensity and macrolobulated
margins.
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Fig. 10C. —Extraskeletal myxoid chondrosarcomas. Lateral ankle
radiograph of 37-year-old woman with extraskeletal chondrosarcoma of ankle
shows somewhat ovoid mass (arrows) containing stippled calcifications
and obliterating Kager's fat pad.
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Fig. 11. —Sagittal STIR image obtained in 28-year-old woman reveals
homogeneously hyperintense bilobular mass in Kager's fat pad consistent with
ganglion cyst, arising from ankle joint and herniating into Kager's fat
pad.
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Retrocalcaneal Bursa
The retrocalcaneal bursa or pre-Achilles bursa is located between the
Achilles tendon and the calcaneus. It may become hypertrophied and inflamed
[8,
9], particularly in association
with arthritic conditions, such as rheumatoid arthritis
(Fig. 12A), ankylosing
spondylitis, psoriasis, and Reiter's syndrome. Radiographic findings of
retrocalcaneal bursitis include absence of the normal radiolucency seen in the
posteroinferior corner of Kager's fat pad with or without associated erosion
of the calcaneus. On STIR or fat-suppressed T2-weighted images, the signal
characteristics of uncomplicated retrocalcaneal bursitis are similar to the
those of joint fluid (Fig.
12B).
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Fig. 12B. —Retrocalcaneal bursitis. Sagittal STIR image of 36-year-old
man shows retrocalcaneal or pre-Achilles bursitis and high signal intensity
within thickened Achilles tendon resulting from partial tear.
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Ankle Joint
The anteroinferior corner of the Kager's fat pad defines the posterior
aspect of the ankle joint. Ankle effusions typically cause the loss of the
anteroinferior corner radiolucency and can be associated with such pathologic
conditions as occult or radiographically subtle ankle fractures (Fig.
13A,
13B), osteochondritis
dissecans, synovial osteochondromatosis
(Fig. 14A), pigmented
villonodular synovitis (Figs.
14B and
14C), and Charcot's joint
(Fig. 15A,
15B).
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Fig. 13A. —26-year-old woman with occult fracture of ankle joint.
Lateral radiograph of ankle shows small amount of ankle effusion
(arrowheads) extending into anteroinferior aspect of Kager's fat pad.
Findings were otherwise unremarkable.
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Fig. 13B. —26-year-old woman with occult fracture of ankle joint. Axial
CT scan of both ankles obtained at level of articular surface of distal tibia
reveals radiolucent fracture line coursing obliquely through medial aspect of
right distal tibia.
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Fig. 14A. —Synovial masses of ankle joint. Lateral ankle radiograph
obtained in 37-year-old man shows large amount of joint effusion and multiple
intraarticular calcified loose bodies (arrows), consistent with
synovial osteochondromatosis.
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Fig. 14B. —Synovial masses of ankle joint. Lateral radiograph (B)
and sagittal T2-weighted image (C) were obtained in 34-year-old woman
with ankle mass. Radiograph shows bulging and increased density in ankle joint
both anteriorly and posteriorly (arrows, B), displacing and
obscuring anterior aspect of Kager's fat pad. On T2-weighted image (C),
mass in ankle joint displays heterogeneous, low to intermediate signal.
Low-signal-intensity foci (arrows, C) are areas of hemosiderin
deposition from recurrent hemorrhage, consistent with pigmented villonodular
synovitis.
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Fig. 14C. —Synovial masses of ankle joint. Lateral radiograph (B)
and sagittal T2-weighted image (C) were obtained in 34-year-old woman
with ankle mass. Radiograph shows bulging and increased density in ankle joint
both anteriorly and posteriorly (arrows, B), displacing and
obscuring anterior aspect of Kager's fat pad. On T2-weighted image (C),
mass in ankle joint displays heterogeneous, low to intermediate signal.
Low-signal-intensity foci (arrows, C) are areas of hemosiderin
deposition from recurrent hemorrhage, consistent with pigmented villonodular
synovitis.
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Fig. 15A. —56-year-old woman with Charcot's joint and ankle joint
effusion. Lateral radiograph shows marked destruction and deformity of tarsal
bones with associated posterior ankle joint effusion (asterisk)
resulting in obscuring of Kager's fat pad.
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Fig. 15B. —56-year-old woman with Charcot's joint and ankle joint
effusion. Anteroposterior radiograph of right foot reveals lateral subluxation
of tarsometatarsal joint complex relative to navicular bone.
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Conclusion
Many abnormal conditions of the ankle can involve Kager's fat pad. Careful
attention to the borders of this triangle and to the angles formed by these
borders may provide signs of the presence of an abnormality.
References
- Cetti R, Andersen I. Roentgenographic diagnoses of ruptured
Achilles tendons. Clin Orthop 1993;286
: 215–221[Medline]
- Pavlov H, Heneghan MA, Hersh A, Goldman AB, Vigorita V. Haglund's
syndrome: initial and differential diagnosis of posterior heel pain.
Radiology1982; 144:83
–87[Abstract/Free Full Text]
- Dussault RG, Kaplan PA, Roederer G. MR imaging of Achilles tendon
in patients with familial hyperlipidemia: comparison with plain films,
physical examination, and patients with traumatic tendon lesions.
AJR 1995;164:403
–407[Abstract/Free Full Text]
- McGahan JP, Graves DS, Palmer PES. Coccidioidal spondylitis: usual
and unusual radiographic manifestations. AJR1980; 136:5
–9
- Hedrick MR, McBryde AM. Posterior ankle impingement.
Foot Ankle1994; 15:2
–8[Medline]
- DosRemedios ET, Jolly GP. The accessory soleus and recurrent tarsal
tunnel syndrome: case report of a new surgical approach. J Foot
Ankle Surg 2000;39:194
–197[Medline]
- Nakashima Y, Unni KK, Shives TC, Swee RG, Sahlin DC. Mesenchymal
chondrosarcoma of bone and soft tissue: a review of 111 cases.
Cancer 1986;57:2444
–2453[Medline]
- Frey C, Rosenberg Z, Shereff MJ, Kim H. The retrocalcaneal bursa:
anatomy and bursography. Foot Ankle1992; 13:203
–207[Medline]
- Bottger BA, Schweitzer ME, El-Noueam KI, Desai M. MR imaging of the
normal and abnormal retrocalcaneal bursae. AJR1998; 170:1239
–1241[Abstract/Free Full Text]
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Introduction
Normal Anatomy of Kager's...
