Enthesis

When a muscle generates force, that force travels through a tendon to the bone. Ligaments connect bones to one another and stabilize joints. An enthesis is the site where the end of a tendon or ligament meets and connects to bone. Entheses are found throughout the body, around joints involved in movement, such as the elbows, shoulders, knees, and heels.

The similarly named myotendinous junction is the boundary where muscle meets tendon, while an enthesis is the boundary where a tendon or ligament meets bone. Both transmit force, but their locations and tissue characteristics differ.

Tendons are tough yet flexible, while bones are hard. If these two tissues met directly, force could easily become concentrated at their interface. In many entheses, the tissue therefore changes gradually in this sequence:

1. Fibrous tissue of the tendon or ligament 2. Fibrocartilage (tissue that is harder than tendon and more flexible than bone) 3. Calcified fibrocartilage 4. Bone

As stiffness changes in stages, force is distributed evenly rather than concentrated in one spot. The principle is similar to placing a cushioning section with a gradual transition between a rubber band and a metal rod, rather than tying them directly together.

Not all entheses are the same. At sites that attach to the end of a bone, such as the Achilles tendon or rotator cuff, the fibrocartilage layers described above are distinct. At sites closer to the shaft of a bone, the fibrous tissue may instead connect with the periosteum, the membrane covering the bone. Their form varies according to their location and role.

Entheses receive force very directly. In addition to the pulling force of muscles, they also experience compression as tendons press against the edges of bones, depending on the angle at which a joint bends. On days when these areas are used heavily, they may therefore feel achy or tender when pressed.

• Heel — This is where the plantar fascia and Achilles tendon attach.
• Outside and inside of the elbow — These are attachment sites for the tendons that move the wrist. Lateral epicondylitis and medial epicondylitis are discussed in relation to these areas.
• Front of the knee — This is where the patellar tendon attaches.

Entheses are also notable sites in inflammatory conditions affecting the areas around joints, such as ankylosing spondylitis. Although discomfort often occurs at entheses, this does not mean that the attachment site itself is the cause of the problem. Entheses are normal structures that everyone has. Seeing an enthesis on an imaging test does not mean there is a problem, either. Discomfort arising in this area is covered separately in Enthesopathy.

Tendons, ligaments, and bones are tissues that gradually become stronger in response to the forces placed on them (mechanotransduction). Entheses also adapt more comfortably when loads increase gradually rather than when they are suddenly subjected to large forces.

• Try increasing your exercise volume or walking distance gradually over several weeks rather than all at once.
• Before exercise, warm up by gently moving the muscles connected to the tendons, such as those in your calves and forearms.
• On days when you repeat one movement for a long time, take breaks and alternate the body parts you use.
• Rather than rubbing firmly over the site where a tendon attaches to bone, gently relaxing the muscles connected to the tendon tends to feel more comfortable.

If tenderness when pressing on a tendon’s attachment to bone continues for several weeks, or if the first movements in the morning or after rest are repeatedly unusually stiff and painful, consider which activities may have placed strain on the area and consult a professional.