Enthesis

An enthesis (plural: entheses) is the connective tissue between a tendon or ligament and bone. More specifically, it refers to the site where these soft tissues attach to the skeleton. Entheses transmit mechanical forces between muscle and bone, enabling movement and providing stability.

There are two main types of entheses:

• Fibrous entheses: These attachments are direct, with the tendon or ligament inserting directly into the bone. Fibrous entheses are typically found in areas where significant stress is applied. The insertion is often reinforced by Sharpey's fibers, which are collagen fibers that penetrate the bone matrix.

• Fibrocartilaginous entheses: These attachments involve an indirect insertion via a zone of fibrocartilage between the tendon or ligament and the bone. This fibrocartilage transition zone helps to distribute stress and reduce peak loading at the insertion site, which is particularly important in areas that experience high tensile forces or compressive loads. These entheses undergo a gradual transition from tendon/ligament to fibrocartilage to mineralized fibrocartilage and then finally to bone.

Enthesopathies are disorders or diseases affecting the entheses. These conditions can be inflammatory, degenerative, or traumatic. Enthesitis, specifically, refers to inflammation at the enthesis. Enthesopathies are commonly associated with conditions such as spondyloarthritis, including ankylosing spondylitis and psoriatic arthritis, and can also be seen in overuse injuries. Diagnosis of enthesopathies often involves physical examination, imaging studies (such as ultrasound or MRI), and sometimes blood tests to assess for inflammatory markers.