Tendon

Tendo

Also known as: sinew, aponeurosis, myotendinous junction, enthesis

A cord or sheet of dense regular collagenous tissue that transmits the force of muscle to bone. A flat, sheet-like tendon is called an aponeurosis.

Sarcomere & Sliding Filaments

Overview

Main collagenType I (≈95%)

Failure strain≈8–10% (damage begins at ≈4–5%)

Tendons are made of tightly packed, parallel bundles of type I collagen. The cells between the bundles are rows of flattened fibroblasts (tenocytes). The collagen is organised in a hierarchy: fibrils → fibres → fascicles (surrounded by endotenon) → the whole tendon (surrounded by epitenon and then a loose paratenon). Where tendons run through tunnels or around bony pulleys (wrist, ankle, fingers), they lie inside synovial tendon sheaths that reduce friction. Tendons are stiff in tension and stretch only ≈4–5% before damage. They store and return elastic energy (e.g. the calcaneal tendon in running).

At the myotendinous junction, muscle fibres end in finger-like folds of the sarcolemma that greatly increase the contact area with the collagen. This is the commonest site of strains. At the bony attachment (enthesis), the tendon either inserts through four zones (tendon, uncalcified fibrocartilage, calcified fibrocartilage, bone: a fibrocartilaginous enthesis) or anchors directly to periosteum through Sharpey fibres (a fibrous enthesis). Tendons have relatively few blood vessels and a low metabolic rate. They heal slowly, and some have watershed zones (supraspinatus, calcaneal tendon).

Parts & subdivisions

Myotendinous junction
Interdigitating junction between muscle and tendon. It is the weakest link in the muscle–tendon unit and the commonest site of strains.
Enthesis
The site where tendon or ligament attaches to bone, either fibrocartilaginous or fibrous. Enthesitis is a hallmark of spondyloarthropathies.
Aponeurosis
A flat, sheet-like tendon, e.g. the external oblique aponeurosis, palmar aponeurosis and epicranial aponeurosis.
Synovial sheath
A double-layered synovial tube around tendons in fibro-osseous tunnels. Inflammation causes tenosynovitis (e.g. De Quervain).

Anatomy

Arterial supply: From the muscle (at the myotendinous junction), the bone (at the enthesis) and the paratenon or vincula (inside sheaths). Watershed zones lie between these territories.
Histology: Parallel, wavy (crimped) bundles of acidophilic collagen with rows of elongated, flattened tenocyte nuclei between them. There is little ground substance and few vessels.

Function

Transmits muscle force to bone and concentrates it on a small attachment site
Stores and releases elastic strain energy (spring function)
Lets a muscle belly act at a distance (e.g. long finger flexors in the forearm)
Contains Golgi tendon organs that sense tension

Clinical significance

Tendinopathy

Overuse causes failed healing with collagen disorganisation, neovascularisation and mucoid degeneration ("tendinosis") rather than true inflammation. Common sites are the calcaneal tendon, the patellar tendon, the common extensor origin (lateral epicondylitis) and the supraspinatus.

Fluoroquinolones and steroids

Fluoroquinolone antibiotics and corticosteroid injections increase the risk of tendon rupture, particularly of the calcaneal tendon in older adults.

Test yourself

After a fall on an outstretched hand, a patient is tender in the anatomical snuffbox. Which tendons form its boundaries?

FCR and palmaris longus
APL and EPB laterally (anterior); EPL medially (posterior)
ECRL and ECRB
Extensor digitorum and extensor indicis

Answer: APL and EPB laterally (anterior); EPL medially (posterior)

The anterior (lateral) border is abductor pollicis longus with extensor pollicis brevis, and the posterior (medial) border is extensor pollicis longus. The scaphoid forms the floor and the radial artery crosses it, so snuffbox tenderness suggests a scaphoid fracture.

What structure does the Golgi tendon organ monitor?

Muscle length
Rate of change of muscle length
Muscle tension
Joint angle

Answer: Muscle tension

GTOs lie in series with muscle fibres at the musculotendinous junction and signal tension through Ib afferents. Muscle spindles lie in parallel with the fibres and signal length and its rate of change through Ia and II afferents.

A runner felt a "kick" in the back of the ankle, cannot stand on tiptoe, and squeezing the calf produces no plantarflexion. What is the diagnosis?

Plantaris rupture
Calcaneal (Achilles) tendon rupture
Medial gastrocnemius strain
Tibialis posterior dysfunction

Answer: Calcaneal (Achilles) tendon rupture

A positive Thompson (Simmonds) test with a palpable gap indicates rupture of the calcaneal tendon, usually 2–6 cm above its insertion where the blood supply is poorest.