[Embryology of the human carpal bones (triangular cartilage, central carpal bone, morphogenesis of the scaphoid)]

Wax reconstructions according to Born's methods were performed from serial 10-micron sections of the wrists of human embryos (B. Tardif) to study the triangular cartilage and central bone of the carpus. A subsequent morphological study of the adult scaphoid bone was conducted in the light of embryological data. The triangular cartilage was observed in 4 of 38 carpi of embryos with a craniocaudal length of 40 to 69 mm, flatten between the ulna and triquetrum, on the inferior surface of the mesenchymal bud of the triangular ligament. The central bone of the carpus is constant in the embryo, between the inferior surface of the radial chondrification centre, capitatum, trapezium and trapezoid. In one case, the two wrists of the same embryo presented two central nuclei, leading to the distinction of two different formations:--the constant C2 embryonic central bone, trapped between capitatum, trapezoid and trapezium, which participates in the constitution of the scaphoid by fusion with the radial chondrification centre;--the C1 central bone, a rare supernumerary bone in adults, close to the inferolateral angle of the lunatum, on the dorsal surface of the carpus. These embryological data support. Westoll's theory concerning the filiation of the thoracic limb since the fin of crossopterygians, with deviation of the mechanical axis related to the development of weightbearing on the ground. The scaphoid bone must to be assimilated to a cube, but to an irregular tetrahedron, composed of two inverted pyramids joined by their bases. The inferior pyramid, articulating with he trapezium, trapezoid and capitatum, is concerned by morphological variations of the bone, which can present variable configurations between the two extreme morphotypes: the massive type and the strangulated type. The inferior pyramid, constituting the scaphoid, is derived from the central cartilage and presents the same relations. Variations of the volume of this inferior part of the bone are related to the more or less marked regression of this chondrification centre of the embryonic carpus. These variations of the bone can be expressed in terms of a scaphoid index, by calculating the relationship between the greatest dimension and the width of the narrowed portion.