The functional anatomy of Kager's fat pad in relation to retrocalcaneal problems and other hindfoot disorders

Abstract

Kager's fat pad is a mass of adipose tissue occupying Kager's triangle. By means of a combined magnetic resonance imaging, ultrasound, gross anatomical and histological study, we show that it has three regions that are closely related to the sides of the triangle. Thus, it has parts related to the Achilles and flexor hallucis longus (FHL) tendons and a wedge of fat adjacent to the calcaneus. The calcaneal wedge moves into the bursa during plantarflexion, as a consequence of both an upward displacement of the calcaneus relative to the wedge and a downward displacement of the wedge relative to the calcaneus. During dorsiflexion, the bursal wedge is retracted. The movements are promoted by the tapering shape of the bursal wedge and by its deep synovial infolds. Fibrous connections linking the fat to the Achilles tendon anchor and stabilize it proximally and thus contribute to the motility of its tip.We conclude that the three regions of Kager's fat pad have specialized functions: an FHL part which contributes to moving the bursal wedge during plantarflexion, an Achilles part which protects blood vessels entering this tendon, and a bursal wedge which we suggest minimizes pressure changes in the bursa. All three regions contribute to reducing the risk of tendon kinking and each may be implicated in heel pain syndromes.

Quantifying the motion of Kager's fat pad

“…Dynamic US imaging was previously used to measure abnormalities and motion distances on Kager's fat pad 1,6,7,9 and other soft tissue. 14,15 Still images were extracted from video scans to measure sliding distance offline.…”

“…Histologically, Kager's fat pad has three distinctive parts. 7 The Achilles tendon-related is anchored to the tendon and is encapsulated by the paratenon; the deep pad is located between the Achilles tendon-related pad and the FHL muscle; the bursal protruding wedge extends from the deep pad over the calcaneal superior tuberosity and forms the anterosuperior wall (synovial folds) of the retrocalcaneal bursa (Fig. 1).…”

“…4 The pad is contained within a synovial membrane and consists of adipose cells, [7][8][9] which allows the protruding wedge high mobility within the Achilles tendon enthesis. 7,8 It is still not fully explained how Kager's fat pad behaves in vivo; however, literature investigating bovine fat pad showed that when removed, it behaves in motion as a fat tissue. 7 Kager's fat pad has both neural and vascular supplies, suggesting it provides sensory functions and protection to the tendon's vascular supply.…”

“…7,8 It is still not fully explained how Kager's fat pad behaves in vivo; however, literature investigating bovine fat pad showed that when removed, it behaves in motion as a fat tissue. 7 Kager's fat pad has both neural and vascular supplies, suggesting it provides sensory functions and protection to the tendon's vascular supply. [7][8][9][10][11][12] The motion of the protruding wedge is also suggested to minimize pressure changes within the retrocalcaneal bursal space during ankle flexion 7 and to provide subtendinous lubrication, promoting low wear, suggesting that the pad may provide protection to the tendon.…”

“…7 Kager's fat pad has both neural and vascular supplies, suggesting it provides sensory functions and protection to the tendon's vascular supply. [7][8][9][10][11][12] The motion of the protruding wedge is also suggested to minimize pressure changes within the retrocalcaneal bursal space during ankle flexion 7 and to provide subtendinous lubrication, promoting low wear, suggesting that the pad may provide protection to the tendon. 13 The motion has also been suggested to have an immunoprotective role within the retrocalcaneal bursa-removing debris produced by tear and wear of its lining tissues.…”