The junctions of the rounded and flat surfaces are marked by longitudinal ridges that serve as the attachments for the fibrous part of the digital tendon sheath (see Fig. The volar aspect of the shaft is flat from side to side and rounded dorsally. The shafts of the phalanges serve as anchors for the long digital flexor tendons. The proximal and middle phalanges, like the metacarpals, are bowed dorsally along their long axis and thus contribute to the longitudinal arch of the hand. ![]() The hand contains 14 phalanges the thumb has only 2, whereas each of the other digits has 3. This arch is larger than the proximal arch and merely reflects the orientation of the metacarpals and the fact that the metacarpal heads are farther apart than their bases. The distal transverse arch is at the level of the metacarpal heads and is also a dorsal convexity. Since the metacarpal bases and distal carpals are positioned very close to one another and are held tightly together, they collectively form a dorsal convexity and thus a side-to-side arch. The bases of the metacarpals as well as the distal row of carpals are wedge-shaped in cross section, and the apex of each wedge is directed volarly. The proximal arch is at the level of the distal row of carpal bones and the bases of the metacarpals. The shapes of the metacarpals also contribute to the proximal and distal transverse arches of the hand (see Fig. Prominent dorsal tubercles are found dorsally on each side of the head, just proximal to the articular surface. And importantly, the surface extends farther onto the volar aspect of the bone than dorsally. The side-to-side dimension is considerably shorter than the length from dorsal to palmar, but it is wider on the palmar aspect than it is dorsally. The articular surface is rounded, both from side to side as well as dorsal to palmar. The heads of all the metacarpals are similar. The transverse and longitudinal arches of the hand. (Keep in mind that this bone is rotated about 90 degrees relative to the other metacarpals and this description is based on the anatomic position.) The most medial aspect of the base protrudes more proximally than the rest of the base and thus presents a triangular beak. The concave surface is oriented from medial to lateral the convex from anterior to posterior. The somewhat flattened proximal surface is in the shape of a shallow saddle, all of which is articular surface. The base of the thumb metacarpal is significantly different. Articular surface is found on the sides as well as the proximal aspect of the base. The bases of the four medial metacarpals are irregular in shape and less wide volarly than dorsally, thus contributing to the proximal transverse arch ( Fig. The mechanical advantage of these muscles is also enhanced by the metacarpal shape their lines of pull are located volar to the flexion–extension axes of the metacarpophalangeal (MCP) joints. The dorsal convexities of the metacarpals along with their triangular cross sections provide significant room for the soft tissue of the palm, the bulk of which consists of the intrinsic interossei muscles and the more volarly positioned long digital flexor tendons and accompanying intrinsic lumbrical muscles. The overall shape of each metacarpal (along with that of the phalanges) contributes to the longitudinal arch of the hand. ![]() This concentration of dense bone reflects the significant compressile force on the flexor side of the bone. The shaft of each metacarpal is triangular in cross section, with the apex of this triangle directed volarly and composed of more dense bone than the dorsal aspect of the shaft. Note the dorsal convexities of the metacarpal and proximal and middle phalanges. Lateral view of the middle finger and the capitate.
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