An enthesis (plural: entheses), also known, more informally, as a footprint, generally refers to the anatomic junction where connective tissue (e.g. ligament, tendon, joint capsule, bursa or a combination thereof) attaches to bone.

Entheses are commonly classified into two types ¹:

• fibrous

• fibrocartilage

Histology

Fibrous

In a fibrous enthesis, the collagen fibres comprising ligament or tendon attach directly to periosteum or bone cortex ^1,2. They are usually associated with short tendons, e.g. gluteus maximus insertion onto the femur ¹. This usually occurs along a relatively wide area of a bone shaft ¹.

Fibrocartilage

A fibrocartilage enthesis represent a more complex attachment between tendon and bone. They generally occur at locations where tendon inserts onto a portion of bone lacking periosteum, e.g. an epiphysis.

They are classically divided into four zones:

1. collagen zone - derived from tendon or other soft tissue structure

2. non-calcified fibrocartilaginous zone - variable thickness where chondrocytes predominate

3. calcified fibrocartilaginous zone - an abrupt transition ("tidemark" or "blue line")

4. subchondral bone zone

The zonal organisation is thought to result in decreased stress on the interface ¹.

Related pathology

• enthesopathy: a pathological condition affecting the enthesis

• enthesitis: an inflammatory condition causing enthesopathy

• enthesophyte: bony projection at an enthesis in response to enthesopathy