The metatarsals form the center of the foot skeleton. They have a significant static function.
What is the metatarsal bone?
The foot skeleton consists of 3 parts with at least 26 bones, the tarsus (root of the foot), the metatarsus (midfoot), and the digiti (toes). The tarsal bones form the part of the foot close to the body (proximal), the hindfoot, while the toes represent the part far from the body (distal), the forefoot. The 5 metatarsals are articulated to the other parts and form the link between them. Analogous to the toes, they are arranged next to each other and together with them form the so-called rays, which slightly diverge towards the front. Like the bones, these are numbered from 1 to 5 from the inside out. Accordingly, the first ray is the 1st metatarsal together with the big toe and the fifth is the little toe and the 5th metatarsal. This construction has important functional significance in locomotion and statics.
Anatomy and structure
All 5 metatarsals have a uniform structure with three parts, base, corpus, and head. The bases are articulated to the adjacent tarsal bones and to each other. The articular surfaces in this region are all relatively planar, so there is no distinct socket and no distinctly shaped head. Above and below, there are numerous small ligaments that secure the joints and allow little movement. Toward the sole of the foot, more powerful ligaments extend to hold all the metatarsal bones in a bridging tension. Continuing on, the elongated and thinner bodies follow, with gaps between them lined with connective tissue. At the distal end are the broader heads, which together with the proximal phalanges form the metatarsophalangeal joints. The articular surfaces of the metatarsals are convex here, while those of the proximal phalanges are concave. Anatomically, these are ball and socket joints with 3 degrees of freedom. Functionally, however, only movements in 2 planes are possible, since the rotation is not actively feasible, because no muscles with a corresponding course exist. On the 1st and 5th metatarsal bones, there are proximal roughnesses that serve as attachment surfaces for muscles that come from the lower leg and pull there. Regularly, 2 sesamoid bones are found on the underside of the head of the 1st metatarsal in the area of the metatarsophalangeal joint.
Function and tasks
The metatarsus has little mobility due to its severe bracing, yet slight upward, downward, and lateral displacements are possible. Toward the toes, mobility increases slightly. This mobility gives the foot the ability to adapt to uneven ground, an important function in maintaining balance. At the base of the first metatarsal bone, the tibialis anterior muscle attaches, which is responsible for lifting the foot with rotation of the inner edge. This function ensures that the foot remains above the ground during the swing leg phase. Pulling on the underside of the base of the 5th metatarsal is the peronaeus brevis muscle. It pulls the outer edge of the foot downward and rotates it in the process. Especially when standing, this function gives the foot good stability. The first metatarsal is the strongest of the 5 parts. This is due to its function during walking. Together with the big toe, this is where the foot is pushed off the ground at the end of the stance leg phase. The most important function of the metatarsals is their participation in the arch construction of the foot. The tarsus and metatarsus are arranged so that the inner components rest on the outer ones. This results in 2 strands, of which only the outer one is in contact with the ground, the inner one spans out like a bridge between the calcaneus and the heads of metatarsals 1 – 3. This forms the bony base of the longitudinal arch of the foot. The strong ligamentous support under the metatarsal and tarsal bones forms the basis of the transverse arch of the foot, which ensures that distally the heads 1 and 5 are the main points of contact. The arch structure acts as a shock absorber and is a very important static component. Shocks are buffered and the load on the joints of the legs close to the body and the spine is significantly reduced.
Diseases
A common functional impairment is insufficiency of the arch structure, in which the metatarsals play a major role.Due to various factors, the longitudinal or transverse arch or both can sink and partially or completely lose their buffer function. If the longitudinal arch is affected, this is known as fallen arches, and if the transverse arch is affected, it is known as splayfoot, because the metatarsal bones and the toes move apart laterally. On the one hand, this event affects walking, but above all it affects the load on the areas of the body above. Knee, hip and spinal joints are subjected to significantly more stress because impacts are transmitted to them much more directly. Different degrees on the left or right side can lead to changes in the leg axis or to a pelvic obliquity with a one-sided spinal load. The metatarsals, with their tubular structure, are basically at risk of fracture. Weights from above, for example a kick with the foot or a falling object, can lead to metatarsal fractures, which often affect several bones. These injuries have drastic consequences for the people affected, since the metatarsus must not be loaded during the healing phase. So-called marching fractures are also very common. These are fatigue fractures that develop due to overloading of the bones. The symptoms develop only gradually and initially appear as non-specific pain on exertion, which is often not associated with a fracture. Only a specific X-ray can provide clarity in this case. The typical deformity of the big toe, hallux valgus, has its origin in a deviation of the 1st metatarsal. In a splayfoot, this bone moves further inward. The articular surfaces of the metatarsophalangeal joint come into a different position relative to each other and the big toe deviates outward.
