This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The wrist-joint is formed by the radius and triangular cartilage above and the navicular (scaphoid), lunate (semilunar), and cuneiform bones below. These are joined by the anterior, posterior, internal and external lateral, and capsular ligaments. The two lateral ligaments are strong, well-defined bands, the anterior and posterior ligaments are weaker and are fused with the capsular ligament.
The internal lateral ligament is attached above to the tip of the styloid process of the ulna and the tip of the triangular cartilage; below it is attached to the border of the cuneiform bone and is continued on to the pisiform bone.
The external lateral ligament is attached above to the tip of the styloid process of the radius and below to the base of the tubercle of the navicular bone.
The capsular ligament of the wrist-joint is composed of an anterior and a posterior portion strengthened by the two lateral ligaments just described. The anterior ligament has the bulk of its fibres running downward and inward from the edge of the radius to the palmar surface of the navicular, lunate, and cuneiform bones. It is stronger than the posterior. The posterior ligament likewise has its fibres running downward and inward to be attached to the first row of carpal bones.
The wrist is classed as a biaxial diarthrosis or condyloid joint. This means that it is a double hinge-joint having movements around two axes, one anteroposterior and the other transverse. A combination of these movements results in circumduction, but it has at least no voluntary movement of rotation.
When rotation of the hand occurs it is accomplished by pronating or supinating the forearm. If the wrist-joint possessed this latter movement it would be a ball-and-socket or enarthrodial joint. The hand can be flexed and extended through an arc of approximately 140 degrees and adducted and abducted about half as much. The position assumed by the bones in flexion and extension is shown in Figs. 343 and 344.
Adduction or bending toward the ulnar side is much greater than is possible toward the radial side. The fact of the ulna not coming so low as the radius accounts, at least in part, for this. The lateral ligaments check the movements of abduction and adduction, and in addition the contact of the styloid process of the radius with the trapezium prevents further outward movement.
Fig. 343. - Position assumed by the carpal bones in flexion of the wrist.
The extent of the movements of the wrist of course varies much in different individuals. The laxness of the joints in children, women, and those not accustomed to hard manual labor is well known.
The movements of the wrist are performed by two different sets of muscles.
Fig. 344. - Position assumed by the carpal bones in extension of the wrist.
The first set is composed of the flexor carpi radialis and flexor carpi ulnaris, with which we may perhaps include the palmaris longus, - although it properly belongs with the finger muscles, - and of the extensor carpi ulnaris, extensor carpi radialis longior, and extensor carpi radialis brevior. If the fingers are clinched and the extensors of the fingers contract they aid the three carpal extensors to bend the hand backward. If the fingers are held extended and the flexors of the fingers contract they aid the carpal flexors to bend the hand forward. Contraction of the flexor and extensor carpi ulnaris adducts the hand and contraction of the flexor carpi radialis and extensor carpi radialis longior and brevior, aided by the short extensor of the thumb and extensor ossis metacarpi pollicis, abducts the hand.
In the affection known as wrist-drop all the extensor muscles are paralyzed. It is due to injury, usually from pressure on the radial (musculospiral) nerve, either in the groove of the humerus or in the axilla. Although there are a number of synovial bursas around the joint in connection with the tendons none communicate with it.