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.
Dislocations sometimes occur toward the dorsal surface. The bases of the second and third metacarpal bones in the uninjured hand form a bony prominence on the dorsum of the hand which may be mistaken for a luxation. This prominence lies in a direct line with the posterior radial (or thecal) tubercle and about 4 cm. (1 1/2 in.) below it.
Fig. 375. - Dislocation of proximal phalanx of little finger. (From author's sketch).
The bases of the metacarpal bones and carpometacarpal joints are best recognized by following up the interosseous spaces by making firm pressure with the fingers between the bones; when the upper limit of the space is reached the joints can be located 1.25 cm. (1/2 in.) above.
These dislocations occur with moderate frequency. Dislocation of the thumb occurs most frequently and is well known. The little finger is next in frequency, while the other three are rarely luxated. When luxation of the proximal phalanx of the little finger occurs it acts precisely as does that of the thumb (as I have seen in one case, Fig. 375). As the thumb dislocation is the most troublesome it alone will be described. Dislocation of the Proximal Phalanx of the Thumb. - This displacement occurs when the thumb is hyperextended on its metacarpal bone (Figs. 376 and 377), and it is often impossible to reduce it without division of the resisting structures. The head of the metacarpal bone is much larger than the shaft immediately behind it and projects especially on its palmar surface toward each side, forming two tubercles.
Fig. 376. - Dorsal luxation of the proximal phalanx of the thumb, showing the position of the bones.
The joint has two lateral ligaments and an anterior or glenoid ligament. These are more firmly attached to the phalanx than to the metacarpal bone, so that in dislocation they are torn from the latter.
Inserting into the outer side of the base of the proximal phalanx are the tendons of the abductor and outer head of the flexor brevis pollicis. They blend with the lateral ligament and have developed in them a sesamoid bone which rides on the tubercle.
Inserting into the inner side of the base of the proximal phalanx are the inner head of the flexor brevis and the adductor obliquus and transversus pollicis muscles. They blend with the lateral ligament and contain a sesamoid bone which rides on the inner tubercle. The flexor longus pollicis tendon passes between the two tubercles and sesamoid bones.
Fig. 377 - Dorsal luxation of the proximal phalanx of the thumb: Division of the tendons of the abductor and flexor brevis pollicis muscles.
When the thumb is hyperextended the glenoid and lateral ligaments are torn loose from the metacarpal bone and carry with them the tendons and sesamoid bones already described. The head of the metacarpal bone projects forward in the palm and can be felt beneath the skin; the flexor longus pollicis tendon slips to the inner side of the bone. As the head pierces the capsule the latter, strengthened by the tendons of the short muscles of +he thumb, contracts behind it like a collar and prevents reduction.
Reduction is to be attempted by extending the phalanx until it is at right angles with the metacarpal bone and dragging its base forward over the head of the metacarpal bone and then flexing.
If this is not successful, then by means of a narrow knife, either through an open wound or subcutaneously, the lateral ligament and tendons on one side (the radial) are loosened from the base of the phalanx, which can then be brought forward. This, of course, divides the tendinous collar which prevents reposition (Fig. 377).
These frequently occur in playing ball games. In attempting to catch the ball the tip of the finger may be struck and the phalanx hyperextended and thereby luxated (Fig. 378).
These luxations are usually readily reduced by simple traction and flexion. Sometimes, however, reduction is not complete, or there is a concomitant fracture, hence the crippled and deformed fingers so often seen in the case of base-ball players.
Fig. 378. - Dislocation of the terminal phalanx, showing the position of the bones. (From author's sketch).