Enthesis

An enthesis is the attachment of tendon, ligament or joint capsule to bone. The fibrocartilaginous type, found at the Achilles, rotator cuff and patellar insertions, is a graded transition across roughly a millimetre: aligned tendon, then uncalcified fibrocartilage, then an abrupt basophilic tidemark, then calcified fibrocartilage anchored into bone. The point of the gradient is mechanical. Stiffness rises by orders of magnitude between tendon and bone, and an abrupt junction between materials of such different moduli would concentrate stress catastrophically.

The clinically important fact is that the gradient is developmental and does not come back. Surgically repaired tendon-to-bone attachments heal with disorganised fibrovascular scar, with no tidemark and no fibrocartilage transition, and that interface is the mechanical weak point. It is a major reason rotator cuff repairs re-tear at rates that, depending on tear size and how retear is defined on imaging, have been reported from around a fifth to the large majority in massive tears.

Entheses are also a site of primary disease. Enthesitis, rather than synovitis, is the characteristic lesion of spondyloarthritis, reflecting resident cells at the insertion that respond to interleukin-23, which is why the enthesis appears in rheumatology as often as in sports medicine.

The misreading with real consequences is generalising tendon-healing data to insertions. Nearly all animal tendon experiments transect the mid-substance, which heals by a different and more forgiving mechanism than a bone interface. A compound that improves mid-substance rat Achilles strength has said nothing about whether an enthesis will rebuild its graded zone, and enthesis regeneration remains an unsolved problem that no drug has been shown to solve in humans.