The metatarsus (Latin: ossa metatarsalia; French: les métatarses) consists of a series of five somewhat cylindrical bones. Articulated with the tarsus behind, they extend forward, nearly parallel with each other, to their anterior extremities, which articulate with the toes, and are numbered according to their position from great toe to small toe. Like the corresponding bones in the hand, each presents for examination a three-sided shaft, a proximal extremity termed the base, and a distal extremity or head. The shaft tapers gradually from the base to the head, and is slightly curved longitudinally so as to be convex on the dorsal and concave on the plantar aspect.
A typical metatarsal bone
The shaft [corpus] is compressed laterally and presents for examination three borders and three surfaces. The two borders, distinguished as medial and lateral, are sharp and commence behind, one on each side of the dorsal aspect of the tarsal extremity, and, gradually approaching in the middle of the shaft, separate at the anterior end to terminate in the corresponding tubercles. The inferior border is thick and rounded and extends from the under aspect of the tarsal extremity to near the anterior end of the bone, where it bi- furcates, the two divisions terminating in the articular eminences on the plantar aspect of the head.
Of the three surfaces, the dorsal is narrow in the middle and wider at either end. It is directed upward and is in relation -with the extensor tendons. The medial and lateral surfaces, more extensive than the dorsal, corresponding with the interosseous spaces, are separated above, but meet together at the inferior border; they afford origin to the interosseous muscles. The base is wedge-shaped, articulating by its terminal surface with the tarsus, and on each side with the adjacent metatarsal bones. The dorsal and plantar surfaces are rough for the attachment of ligaments. The head presents a semicircular articular surface for the base of the first phalanx, and on each side a depression, surmounted by a tubercle, for the attachment of the lateral ligaments of the metatarso-phalangeal joint. The inferior surface of the head is grooved for the passage of the flexor tendons and is bounded by two eminences continuous with the terminal articular surface.
The several metatarsals possess distinctive characters by which they can be readily recognized.
The first metatarsal
The first metatarsal is the most modified of all the metatarsal bones, and deviates widely from the general description given above. It is the shortest, the thickest, the strongest, and most massive of the series. The base presents a large reniform, slightly concave facet for the first cuneiform and projects downward into the sole to form the tuberosity, a rough eminence into which the peroneus longus and a slip of the tibialis anterior are inserted. A little above the tuberosity, on its lateral side, there is occasionally a shallow, but easily recognized facet, for articulation with the base of the second metatarsal. The head is marked on the plan- tar surface by two deep grooves, separated by a ridge, in which the two sesamoid bones of the flexor hallucis brevis glide. The shaft is markedly prismatic. The dorsal surface is smooth, broad, and convex, directed obliquely upward; the plantar surface is concave longitudinally and covered by the flexor hallucis longus and brevis, whilst the lateral surface is triangular in outline, almost vertical, and in relation with the first dorsal interosseous and adductor hallucis obliquus. A few fibers of the medial head of the first dorsal interosseous occasionally arise from the hinder part of the surface adjoining the base, or from the border separating the lateral from the dorsal surface. Somewhere near the middle of the shaft, and on its fibular side, is the nutrient foramen, directed toward the head of the bone.
The first (left) metatarsal.
The second metatarsal
The second metatarsal is the longest of the series. Its base is prolonged back- ward to occupy the space between the first and third cuneiform, and accordingly it is marked by facets for articulation with each of these bones. The tarsal surface is triangular in outline, with the base above and apex below, and articulates with the second cuneiform bone. On the tibial side of the base, near the upper angle, is a small facet for the first cuneiform, and occasionally another for the first metatarsal a little lower down. The fibular side of the base presents an upper and a lower facet, separated by a non-articular depression, and each facet is divided by a vertical ridge into two, thus making four in all. The two posterior facets articulate with the third cuneiform and the two anterior with the third metatarsal. The base gives attachment to a slip of the tibialis posterior and the adductor hallucis obliquus, whilst from the shaft the first and second dorsal interosseous muscles take origin. The nutrient foramen is situated on the fibular side of the shaft near the middle and is directed toward the base of the bone.
The third metatarsal
The third metatarsal, a little shorter than the second, articulates by the tri- angular surface of its base with the third cuneiform. On the medial side are two small facets, one below the other, for the second metatarsal, and on the lateral side, a single large facet for the fourth metatarsal. The base gives attachment to a slip of the tibialis posterior and the adductor hallucis obliquus, and from the shaft three interosseous muscles take origin. The nutrient foramen is situated on the tibial side of the shaft and is directed toward the base.
The fourth metatarsal
The fourth metatarsal, smaller in size than the preceding, is distinguished by the quadrilateral facet on the base, for the cuboid. The medial side presents a large facet divided by a ridge into an anterior portion for articulation with the third metatarsal and a, posterior portion for the third cuneiform. Occasionally the cuneiform part of the facet is wanting. On the lateral side of the base is a single facet for articulation with the fifth metatarsal.
The fifth metatarsal
The fifth metatarsal, is shorter than the fourth, but longer than the first. It is recognized by the large nipple-shaped process, known as the tuberosity, which projects on the lateral side of the base. It constitutes the hindmost part of the bone and gives insertion to the -peroneus brevis on the dorsal aspect, and flexor brevis digiti quinti and the occasional – abductor ossis metatarsi quinli on the plantar aspect. The fifth metatarsal articulates behind by an obliquely directed triangular facet with the cuboid, and on the medial side with the fourth metatarsal. The plantar aspect of the base is marked by a shallow groove which lodges the tendon of the abductor digiti quinti, and the dorsal surface, continuous with the superior surface of the shaft, receives the insertion of the peroneus tertius. The head is small and turned somewhat laterally in consequence of the curvature of the shaft in the same direction. The shaft differs from that of any of the other metatarsals in being compressed from above downward, instead of from side to side, so as to present superior, inferior, and medial surfaces. It gives origin to the lateral head of the fourth dorsal interosseous and the third plantar interosseous muscles. The nutrient foramen is situated on its tibial side and is directed toward the base.
Each metatarsal ossifies from two centers. The primary nucleus for the shaft appears in the eighth week of embryonic life in the middle of the cartilaginous metatarsal. At birth, each extremity is represented by cartilage, and that at the proximal end is ossified by extension from the primary nucleus', except in the case of the first metatarsal. For this, a nucleus appears in the third year.
The distal ends of the four lateral metatarsals are ossified by secondary nuclei which make their appearance about the third year. Very frequently an epiphysis is found at the distal end of the first metatarsal as well as at its base. The shafts and epiphyses consolidate at the twentieth year. The sesamoids belonging to the flexor hallucis brevis begin to ossify about the fifth year.