This blog is informed by a lecture on the correct calliper placement in Achilles tendon ruptures by Mr Stephen Bird, a quite brilliant sonographer based in Australia who has a wonderful educational resource well worth checking out and subscribing to https://birdultrasound.com.au
The complexity of the Achilles insertion point over the posterior calcaneum is a carefully thought out design!
The insertional fibres of the Achilles tendon are anchored to the bone by means of a complex of components which are collectively known as an enthesis organ.
Principally, the purpose of the enthesis is to prevent the damage that would be caused to the bone/periosteum under the inherent high levels of stress.
Firstly, it’s worth noting the region between the periosteal fibrocartilage and the sesamoid fibrocartilage, made up of several components, known as the synovio-enthesis complex (SEC).
This recess is lined by synovium which nourishes the enthesis related fibrocartilage and forms a bursa, The retrocalcaneal bursa.
A large fat pad, commonly known as kager’s fat pad, moves in and out of the bursa during movement, acting as a buffer to stop the periosteal and sesamoid fibrocartilages from bashing and rubbing against one another. Interestingly, fat is a liquid at body temperature and this is how it is able to act as a space filler.
Pathophysiology at the Enthesis
As well as the shock absorbing properties of the SEC, the fat pad/adipose tissue at the SEC has many nerve endings which plays a role in proprioception by sensing joint position. However, this nerve rich tissue is also a source of pain.
Furthermore, the fat at the enthesis is rich in macrophages, immune cells which contribute to the inflammatory process at the enthesis, commonly known as enthesitis.
With overload and increased traction at the point of the enthesis or indeed due to repetitive inflammation, bony proliferations/spurs can arise which are orientated along the direction of pull/mechanical stress. These are known as enthesophytes, and can be differentiated from osteophytes, which are bony protrusions, typically seen at joint margins, often as a consequence of degenerative change.
Therefore location is key when differentiating between enthesophytes, which occur at the attachment of a ligament or tendon.
Ultrasound appearances
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