These joints may be subdivided into three sets: (1) those of the proximal row of carpal bones; (2) those of the distal row of carpal bones; and (3) those of the two rows with each other.
1. THE JOINTS OF THE PROXIMAL ROW OF CARPAL BONES
These are plane joints. The scaphoid, lunate and triquetral bones are connected by dorsal, palmar and interosseous ligaments.
The dorsal and palmar ligaments, two of each, are placed transversely between the bones of the first row; they connect the scaphoid and lunate bones, and the lunate and triquetral bones. The palmar ligaments are weaker than the dorsal.
The interosseous ligaments (fig. 542) are two narrow bundles, one connecting the lunate and scaphoid bones, the other the Innate and triquetral bones. They are on a level with the proximal surfaces of these bones, and form part of the convex articular surface of the radiocarpal joint.
The pisiform bone articulates with the palmar surface of the triquetral bone, and the ligaments of the joint are : a capsular, a pisohamate and a pisometacarpal ligament.
The capsular ligament is thin, and surrounds the joint; its synovial membrane is distinct from that of the other carpal joints.
The pisohamate ligament connects the pisiform to the hook of the hamate bone, and the pisometacarpal ligament joins the pisiform to the base of the fifth metacarpal bone (fig. 540). Both ligaments are continuous with the tendon of insertion of the flexor carpi ulnaris.
These also are plane joints; the bones are connected by dorsal, palmar and interosseous ligaments.
The dorsal and palmar ligaments, each three in number, extend transversely from one hone to another; one connects the trapezium and trapezoid (greater and lesser multangular) bones, a second the trapezoid and capitate bones, and a third the capitate and hamate bones.
The three interosseous ligaments are much thicker than those of the proximal rote : one unites the capitate and hamate bones, a second the capitate and trapezoid bones, and a third the trapezium and trapezoid bones. The first is the strongest; the third is sometimes wanting.
3. JOINTS OF THE TWO ROWS OF CARPAL BONES WITH EACH OTHER.
The joint between the scaphoid, lunate and triquetral bones on the one hand, and the second row of carpal bones on the other; is named the mid-carpal joint, and is made up of three portions : in the center the head of the capitate bone and the proximal surface of the hamate bone articulate with the deep cup-shaped cavity formed by the scaphoid and lunate bones, and constitute a sort of ball-and-socket joint; on the radial side the trapezium and trapezoid articulate with the scaphoid bone, and on the ulnar side the hamate bone articulates with the triquetral bone.
The dorsal and palmar ligaments consist of short, irregular bundles passing between the bones of the first and second rows. On the palm ar surface the fibers radiating from the head of the capitate bone to the surrounding bones are sometimes termed the ligamentum carpi radiatum.
The lateral and medial ligaments are very short : the one is placed on the radial, the other on the ulnar side of the carpus : the former, the stronger and more distinct, connects the scaphoid bone and the trapezium, the latter the triquetral and hamate bones; they are continuous with the collateral ligaments of the wrist-joint. In addition to these ligaments, a slender, interosseous band sometimes connects the capitate and scaphoid bones.
The synovial membrane of the carpus is very extensive (fig. 542), and bounds a cavity of very irregular shape. The proximal part of the cavity intervenes between the distal surfaces of the scaphoid, Innate and triquetral bones and the proximal surfaces of the bones of the second row. It sends two prolongations upwards-between the scaphoid and Innate bones, and between the lunate and triquetral bones-and three downwards between the four bones of the second row. The prolongation between the trapezium and the trapezoid, or that between the trapezoid and capitate bone, is, owing to the absence of the interosseous ligament often continuous with the cavity of the carpometacarpal joints. sometimes of the second, third; fourth, and fifth metacarpal bones, sometimes of the second and third only. In the latter condition the joint between the hamate bone and the fourth and fifth metacarpal bones has a separate synovial membrane. The synovial cavities of these joints are prolonged for a short distance between the bases of the metacarpal bones. There is a separate synovial cavity between the pisiform and triquetral bones.
Movements.-The chief movements permitted in the mid-carpal joint are flexion and extension, flexion being freer than extension. Avery slight amount of rotation is also permitted, the head of the capitate bone rotating round a vertical axis drawn through its own center.
Applied Anatomy.–The radiocarpal joint is rarely dislocated; its strength depending mainly upon the numerous strong tendons which surround the articulation. Its security is further provided for by the number of small bones of which the carpus is made up, and which are united by very strong ligaments. The slight movements which take place between the several bones serve to break the jars that result front falls or blows to the hand. Dislocation backwards, which is the more common, simulates to a considerable extent, Colles’ fracture of the radius, and is liable to be mistaken for it. The differential diagnosis can be easily made by observing the relative positions of the styloid processes of the radius and the ulna.
The grasp of the hand is strongest when the radiocarpal joint is extended slightly; the wrist, therefore, should be kept in this position during the treatment of any disease or injury likely to lead to ankylosis of the joint.
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