Entheses

Entheses are the sites at which tendons, ligaments, or joint capsules attach to bone. They act as a transition from soft tissue to mineralized bone, helping to transmit muscular forces while distributing loads and reducing stress on the attachment.

There are two main forms of enthesis: direct (fibrocartilaginous) enthesis and indirect enthesis. In direct entheses, the tendon inserts into bone through a specialized fibrocartilaginous zone. Indirect entheses attach to bone primarily through fibrous tissue that integrates with the periosteum and cortical bone via Sharpey's fibers. In direct entheses, histology typically shows four transitional zones: tendon, nonmineralized fibrocartilage, mineralized fibrocartilage, and bone.

The concept of the enthesis organ extends beyond the simple insertion to include adjacent structures such as bursae, the synovial sheath, and small fat pads that together help absorb and distribute mechanical loads at the enthesis.

Clinical relevance includes enthesopathy and enthesitis. Enthesitis denotes inflammation of the enthesis and is a hallmark of several rheumatic diseases, notably seronegative spondyloarthropathies. Enthesopathy refers to a broad range of disorders affecting entheses, including common overuse injuries such as plantar fasciitis, lateral epicondylitis, and Achilles tendinopathy.

Diagnosis is typically aided by imaging methods such as ultrasound or MRI, which can reveal thickening, erosions, calcifications, or enthesophytes. Treatment focuses on reducing inflammation and pain, addressing mechanical load, and rehabilitating the affected area as appropriate.