PRACTICAL CONSIDERATIONS : THE CARPAL JOINTS. 331 



the hollow formed in the anterior and inferior surfaces of the bones of the first row 

 (Humphry). 



The joints between the individual bones of the carpus allow of but little motion, 

 and much force is needed to produce displacement of those bones. In the order of 

 frequency the os magnum, semilunar, scaphoid, pisiform, trapezium, trapezoid, and 

 unciform have been reported as separately dislocated. It is interesting to note in 

 relation to the order of frequency that the middle finger is the longest, and is the 

 one most exposed to injury and to force applied to the fingers ; its metacarpal bone 

 is the longest ; it articulates directly with the strongest carpal bone, — the os 

 magnum, — and it, in its turn, with the semilunar, which unites with the scaphoid in 

 connecting the hand with the forearm. In reported cases the pisiform was thought 

 to be dislocated secondarily after the rupture of the tendon of the flexor carpi 

 ulnaris below the bone. 



The other separate carpal luxations have but little anatomical interest. 



Disease of the mid-carpal joint is usually tuberculous, and is apt to begin in or 

 extend to the os magnum because — i. It is the bone most exposed to traumatism 

 {vide supra), receiving the effects of injury to three metacarpal bones. 2. The joint 

 participates in the movements of flexion and extension of the wrist, which are partly 

 limited by the portion of the oblique fibres (both radial and ulnar) of the anterior 

 annular ligament (page 325) and by some of the radial fibres of the weak posterior 

 ligament, which are attached to the os magnum. 3. The slight rotation permitted in 

 the mid-carpal joint is around a vertical axis drawn through the head of the os mag- 

 num. A very slight enlargement of the bone would tend to pinch and bruise the 

 synovial membrane between it and the trapezoid, those two being more closely bound 

 together than any of the other bones. It has been noticed (Mundell) that the point 

 of greatest tenderness in these cases of carpal tuberculosis was in a line between the 

 index- and middle fingers, corresponding to the junction of the os magnum and the 

 trapezoid. Harwell says that in tuberculosis of the wrist-joint the point of special 

 tenderness is on the outer side of the extensor indicis tendon. This is on the same 

 line, and, in cases in which the carpus has become involved, would correspond to 

 the same point of junction. 



Dislocations of the metacarpal bones from the carpus usually involve single 

 bones, are incomplete, and are in the backward direction. The wavy, irregular out- 

 line of the distal edge of the carpus, the dovetailing of the metacarpals and carpals 

 by means of the alternating convexities and concavities, and the strength of the 

 interosseous and transverse metacarpal ligaments sufificiently explain the infrequency 

 of dislocation of the metacarpus as a whole. 



Dislocations of the metacarpo-phalangeal and interphalangeal joints amount to 

 " nearly thirty per cent, of all dislocations" (Stimson). Backward displacement of 

 the proximal phalanx of the thumb is the most frequent and the most important. 

 The cause is usually exaggerated extension of the phalanx, which carries its proximal 

 end up onto the dorsum of the metacarpal bone above the articular surface. The 

 relation to the muscles of the thumb is so important that the luxation will be 

 described in that connection (page 617). 



Dislocations between the phalanges usually occur at tne first phalangeal joint, 

 and in the backward direction, as the cause is commonlv a fall upon the palmar 

 surface of the finger in extension. 



