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Kager’s fat pad

Kager’s fat pad, also known as the pre-Achilles fat pad or Kager’s triangle, is a triangular adipose space located in the posterior ankle. It occupies a strategic anatomical position between the Achilles tendon, calcaneus, and posterior tibia, acting as a gliding and cushioning structure that facilitates smooth motion of the Achilles tendon during ankle movement.

It is an important landmark in ankle anatomy, musculoskeletal imaging, trauma assessment, and evaluation of posterior ankle disorders.

Synonyms

  • Pre-Achilles fat pad

  • Kager’s triangle

Location

  • Situated in the posterior ankle region

  • Anterior to the Achilles tendon

  • Posterior to the tibia and talus

  • Superior to the calcaneus

  • Inferior to the posterior ankle joint capsule

Anatomical components

  • Triangular adipose space

  • Filled with fat and traversed by small vessels

  • Surrounded by musculoskeletal structures:

    • Achilles tendon

    • Flexor hallucis longus tendon

    • Posterior joint capsule

  • Enclosed by fascial planes

Relations

Posteriorly:

  • Achilles tendon

Anteriorly:

  • Posterior surface of the tibia and talus

  • Posterior ankle joint capsule

Inferiorly:

  • Superior surface of the calcaneus

Superiorly:

  • Distal posterior tibia

Medially:

  • Flexor hallucis longus tendon

Laterally:

  • Posterior talofibular region

X-ray appearance

Lateral ankle radiograph:

  • Kager’s fat pad: Seen as a well-defined triangular radiolucent area

  • Boundaries: Outlined by the Achilles tendon posteriorly and calcaneus inferiorly

  • Clinical utility: Loss or distortion of this fat pad contour may suggest posterior ankle pathology

CT appearance

Non-contrast CT:

  • Fat pad: Low-attenuation (fat-density) triangular region

  • Margins: Well-defined between tendon and bone

  • Surrounding structures: Achilles tendon and calcaneus clearly delineated

  • Spatial detail: Excellent depiction of its size and shape

MRI appearance

T1-weighted images:

  • Fat pad: High signal intensity consistent with fat

  • Margins: Sharp delineation from surrounding tendons and bones

T2-weighted images:

  • Fat pad: High signal intensity

  • Adjacent fluid: Higher signal than fat, allowing differentiation

STIR:

  • Fat pad: Signal suppressed due to fat saturation

  • Surrounding soft tissues: Well visualized against suppressed fat background

Proton density fat-saturated (PD FS):

  • Fat pad: Suppressed signal

  • Adjacent structures: Enhanced visibility of Achilles tendon, joint capsule, and FHL tendon

MRI image

Kager’s fat pad mri anatomy labelled image-img-00000-00000_00001

MRI image

Kager’s fat pad mri anatomy labelled image-img-00000-00000

CT image

Kager’s fat pad CT