Corpus: Extensor retinaculum

The extensor retinaculum is considered a reinforcement of the antebrachial fascia with its transverse fiber bundles. It forms the equivalent of the flexor retinaculum on the palmar side of the hand, known as the palmar manus, with its six compartments containing the tendons of the extensor muscles and their associated tendon sheaths.

Tendon Compartments

The tendon compartments of the extensor retinaculum are formed by connective tissue fibers that extend deep to the periosteum of the carpal bones. When viewed from radial to ulnar, the six compartments and their contained tendons are as follows:

• In the first tendon compartment, located most radially, are the tendons of the abductor pollicis longus and the extensor pollicis brevis muscles.
• Adjacent, in the second tendon compartment, lie the tendons of the extensor carpi radialis longus and the extensor carpi radialis brevis muscles.
• The tendon of the extensor pollicis longus muscle can be found in the third tendon compartment.
• In the fourth tendon compartment lie the tendons of the extensor digitorum and the extensor indicis muscles.
• The tendon of the extensor digiti minimi muscle can be found in the fifth tendon compartment.
• In the sixth and most ulnarly located tendon compartment lies the tendon of the extensor carpi ulnaris muscle.

The extensor retinaculum, with its tendon compartments, serves as a movable fixation for the extensor tendons of the forearm muscles. This means that the extensor retinaculum holds the tendons in their compartments during extension, without reducing their function. This prevents lifting of the tendons and ensures optimal mobility.