Kager's fat pad

Kager’s fat pad is a distinct, triangular mass of adipose tissue located in the posterior ankle, occupying the anatomical space known as Kager’s triangle. This triangle is bounded posteriorly by the Achilles tendon, anteriorly by the flexor hallucis longus (FHL) muscle and tendon, and inferiorly by the superior cortex of the calcaneus. The fat pad is encapsulated by a double-layered fascia derived from the union of the leg fascia and the flexor and peroneal retinacula, which helps compartmentalize and stabilize the structure.

Kager’s fat pad is subdivided into three regions: an Achilles-associated part, an FHL-associated part, and a calcaneal wedge:

• The Achilles part is adherent to the posterior aspect of the tendon for most of its length, providing cushioning and protecting the vascular supply to the tendon.

• The FHL part envelops the muscle and tendon, and is in direct contact with the tibial neurovascular bundle, talocrural, and subtalar joint capsules, often circumferentially.

• The calcaneal wedge projects into the retrocalcaneal bursa during plantarflexion, minimizing pressure changes and facilitating lubrication and debris removal from the bursa.

Kager’s fat pad is metabolically active, with histological features including lobular white adipose tissue surrounded by connective septa and a synovial lining. It undergoes significant deformation during ankle motion, acting as a shock absorber and modulating pressure at the Achilles enthesis. Pathological changes, such as edema or inflammation, are associated with posterior ankle pain and can be visualized on MRI.