Extensor retinaculum of wrist
It is important to appreciate the exact attachments of this ligament. A ribbon-like band, less than 2.5 cm wide, it lies obliquely across the extensor surface of the wrist joint. Its proximal attachment is to the radius at the anterolateral border above the styloid process, at the lateral border of pronator quadratus. It is not attached to the ulna; its upper border passes by the styloid process of the ulna. It is attached to the pisiform and triquetral bones. It is a thickening in the deep fascia of the forearm. Immediately proximal to it the deep fascia is attached to the styloid process of the ulna but the oblique fibers which pass from here to the radial side are not attached to the radius, being held away from the bone by the extensor muscles of the thumb.
If the extensor retinaculum were attached to both bones of the forearm it would be over 30% longer in pronation than in supination, as is shown by measurement on the bones. Such elongation is obviously not possible, and in fact the retinaculum, attached to the radius and carpus and free from the ulna, maintains a constant tension throughout the whole range of pronation and supination, while the radius and carpus move together around the lower end of the ulna. Thus the retinaculum holds down the extensor tendons, like a wrist strap, in all positions of the forearm bones.
From the deep surface of the extensor retinaculum fibrous septa pass to the bones of the forearm, dividing the extensor tunnel into six compartments. The most lateral compartment lies over the lateral surface of the radius at its distal extremity, and through it pass the tendons of abductor pollicis longus and extensor pollicis brevis, each usually lying in a separate synovial sheath. The next compartment extends as far as the dorsal tubercle, and conveys the tendons of the radial extensors of the wrist (longus and brevis) each lying in a separate synovial sheath. The groove on the ulnar side of the radial tubercle lodges the tendon of extensor pollicis longus, which lies within its own compartment invested with a synovial sheath. Between this groove and the ulnar border of the radius is a shallow depression in which all four tendons of extensor digitorum lie, crowded together over the tendon of extensor indicis. All five tendons in this compartment are invested with a common synovial sheath. The next compartment lies over the radioulnar joint and transmits the double tendon of extensor minimi digiti in a synovial sheath. Lastly, the groove near the base of the ulnar styloid transmits the tendon of extensor carpi ulnaris (in its synovial sheath) which then passes into its own compartment on the dorsal surface of the carpus to reach the fifth metacarpal bone.