Morphological characteristics of the flexor hallucis longus-associated part of Kager's fat pad

The deep, flexor hallucis longus (FHL)-associated part of Kager's fat pad (KFP-F) is adjacent to structures implicated in the pathophysiology of posterior ankle impingement syndrome (PAIS), and pathological changes within Kager's fat pad itself are also observed in PAIS. However, the detailed topographical relationships of these structures in the deep ankle remain insufficiently understood. This study aimed to provide a comprehensive anatomical description of the KFP-F and its relationship with surrounding neurovascular and musculoskeletal structures.

Fifty legs from 28 Japanese cadavers (28 male, 22 female; mean age at death, 78 years) were examined. The presence or absence of direct contact and positional relationship between the KFP-F with FHL, tibial nerve, posterior tibial artery, posterior tibial vein, talocrural joint capsule, and subtalar joint capsule was systematically evaluated. The width of the contact interface of KFP-F with the talocrural and subtalar capsules were measured and compared.

The KFP-F demonstrated direct contact with the FHL and the talocrural and subtalar joint capsules in all specimens. In 45 ankles (90%), the KFP-F also demonstrated direct contact with the tibial nerve, posterior tibial artery, and posterior tibial vein. Notably, the KFP-F was observed to circumferentially envelop the superficial and deep aspects of the FHL and neurovascular structures. The width of the contact interface with the talocrural joint capsule was significantly greater than that with the subtalar joint capsule (p < 0.01).

The KFP-F forms circumferentially enveloping relationship with the FHL and the posterior tibial neurovascular bundle. This detailed topographical description provides a crucial foundation for improving the interpretation of diagnostic imaging and refining surgical approaches for deep posterior ankle pain.