All the bony elements of the hand are preformed in cartilage, and in the case of the carpus the cartilaginous units are usually greater in number than the bony. Possibly this may have an atavistic significance : in lower vertebrate forms the typical carpus consists of two rows, a proximal containing three elements, radial, ulnar, and intermediate, and a distal with five " carpalia," each one supporting a metacarpal and numbered from without inwards : between the two rows is placed an " os centrale," round which the others might be said to be grouped (see Fig. 157).
In the human cartilaginous carpus the os radiale is represented by the scaphoid, the intermedium by the semilunar, and ulnare by cuneiform : the pisiform is usually regarded as of the nature of a sesamoid, although this view is not incontestable.
The five carpaha are represented by-(1) Trapezium, (2) Trapezoid, (3) Os Magnum, (4) and (5) Unciform : the double nature of the unciform is indicated by the facts that it supports two metacarpals, is chondrified from two cartilaginous centres, and is said to be ossified also from two centres. The os centrale is represented by a small cartilaginous nodule usually present in the embryo, which either becomes divided and disappears later or joins with one of the other elements, usually the scaphoid.
Many irregular and differing " elements " have been described from time to time in association with the carpus : of these the most constantly found are the separated styloid process of the 3rd metacarpal and (rarely) a small ossicle on the outer side of the scaphoid, the radiale externum. In many cases the additional so-called " elements " are ununited results of old fractures of the carpus.
The trapezium is at first smaller than the trapezoid, and the hook of the unciform is developed in cartilage after the body.
The carpus is cartilaginous at birth, but there is occasionally a centre in the os magnum. This centre appears as a rule a few months after birth, and the whole carpus, with the exception of the pisiform, is ossified by the sixth or seventh year : the pisiform centre does not appear until about ten to twelve years. The centres appear in a more or less definite order, and for practical purposes the times shown in the table below may be taken as approximately correct. It is to be noted that the centres appear from a few months to a year earlier in the female hand :-
Time of Appearance of Centres.
Os magnum .
£ • • • yd „
Scaphoid . . .
f j %n*m ..
The metacarpals are ossified from two centres, one for the shaft and one for a distal epiphysis, so that at birth the heads are cartilaginous, because the shaft centres appear in the ninth week, whereas the epiphysial centres are found in the second year. The thumb metacarpal is an exception, in that its epiphysis is at the proximal end, the centre appearing in the third year : in about 6 per cent, of cases there is also a small distal epiphysis, appearing later and uniting with the shaft in a few years. The normal metacarpal epiphyses join the shafts between fifteen and twenty. Additional centres are sometimes found for the styloid process of the middle metacarpal and for the proximal end of the index metacarpal (3 per cent.).
These are preformed in cartilage with the exception of the ungual tubercle on the last phalanges : at one stage the cartilaginous terminal phalanges are longer than the middle ones. Ossification starts in the shafts, except in the terminal phalanges, where the primary centre is at the distal end, and at birth the proximal ends of each phalanx are cartilaginous : here the epiphysial centres appear about the second year.
The shaft centres for the last phalanges come first, in the seventh to eighth week, then those for the first phalanges, ninth week, and lastly, about the eleventh or twelfth week, those for the middle row. The proximal epiphysial cenlres come first in the first row, and later in the others.
The last phalanx of the thumb is the earliest to present a centre in its shaft : this is the first ossifying nucleus in the hand.
Ossification of epiphyses appears to start about a year earlier in females in the metacarpus and phalanges, and in them the fusion with the shaft is also earlier : in male bones this occurs about eighteen to twenty.