The structure of the human foot is an adaptation to the upright gait. The bony basis for this requirement is the foot skeleton with its typical structure.
What is the foot skeleton?
The construction of the foot skeleton forms the basis for the physiognomy and function of the foot. It consists of a total of at least 26 bones, which can be divided topographically into 3 sections. The hindfoot is formed by the 7 tarsal bones, which are connected to the ends of the lower leg bones via the talus. The forefoot is formed by the bones of the 5 toes, of which there are 2 in the big toe and 3 in each of the other toes. Between the two mentioned parts are the 5 metatarsal bones. They each move to a phalanx of the toes and form with them the so-called rays. On the foot skeleton, sesamoid bones can occur in variable numbers. On the underside of the first metatarsal in the area of the metatarsophalangeal joint, 2 are regularly found. The 3 sections of the foot skeleton are architecturally designed to optimally compensate for stresses during walking and standing, even though the total mass of all foot bones is very small.
Anatomy and structure
The 7 tarsal bones can be divided into 2 groups. The talus (ankle bone), calcaneus (heel bone), and navicular bone (os naviculare) are involved in the upper and lower ankle joints, respectively. While motion occurs in these joints, all other contact points of the tarsal bones are tight joints (amphiarthroses) with very little mobility. This is also true for the contact points to the bases of the metatarsals, which form the 3 cuneiform bones (ossa cuneiformia) and the cuboid bone (os cuboideum)in addition to the os naviculare. The metatarsals and phalanges are tubular bones divided into 3 basic components, base, body and head. While the metatarsals also have little displacement between them, all other joints are true articulations. From the inside out, the toes and metatarsals are numbered consecutively 1 through 5. Together, they make up the respective rays, with the big toe and metatarsal 1, for example, forming the first ray, and the little toe and metatarsal 5 forming the fifth ray. Except for the big toe, which has only 2, all toes have 3 limbs (phalanges) that are hinged together.
Function and tasks
The foot skeleton is an architectural masterpiece that allows enormous loads to be distributed so favorably that relatively little stress is placed on the individual parts and little bone mass is required. The first key point in this system is the talus. It takes all the weight that is transferred to it via the lower leg bones and distributes it in different directions. Some of it is transferred to the ground via the calcaneus, while other portions are transferred forward via the anterior ankle joint and distributed to the remaining tarsal bones and the midfoot. The load on the individual parts is minimized by this process and weight is saved. This system is ideally supported by the arch construction of the foot with its 3 support points. The tarsus and metatarsus are arranged to form the bony framework of the longitudinal arch of the foot. The inner row, consisting of the os naviculare, the 3 ossa cuneiformia and metatarsals 1 to 3, rests on the outer bones, the calcaneus, the os cuboideum and metatarsals 4 and 5, spanning like a bridge arch from the heel to the metatarsophalangeal joint. The transverse arch of the foot is made by the wedge shape of the bones involved and taut ligaments located under the metatarsal and tarsal bones. It also spans as an arch from the outer edge of the foot to the inner edge of the foot with contact points to the ground at the ball of the big toe and the ball of the little toe. Together with numerous supporting ligaments and muscles, this creates a buffer system that is a firm yet resilient construct that distributes loads ideally across many parts of the bones. The special arrangement of the foot bones also represents the basic condition for rolling when walking. The ankle and toe joints ensure the mobility of the foot, which is important during walking, running, jumping and other motor activities.
Diseases
External force can cause fractures in all areas of the foot skeleton, which can cause painful impairments on the one hand and severe functional limitations on the other.Fractures in this area always result in the foot not being allowed to bear weight for a while, regardless of whether surgical or conservative therapy has been performed. The so-called marching fractures represent a special form. They are not the result of trauma, but fatigue fractures in the metatarsal or tarsal bones that occur as a result of overloading. Although the symptomatology varies, the functional limitations for those affected are the same. Changes in the arch structure often occur as a result of an unfavorable disposition in conjunction with high loads, such as those caused by obesity. In the case of a so-called flat foot, the longitudinal arch drops, in the case of a splay foot the transverse arch and in the case of a flat foot both. The result is that the loads can no longer be optimally buffered and more and more bone points become load-bearing elements. This leads not only to an unfavorable pressure load on the bones, but also to a change in the entire statics with an additional load for knee and hip joints and the spine. Deformities of the toes lead on the one hand to unpleasant pressure discomfort and on the other hand to an impairment of walking. Hallux valgus often develops as a result of the deviation of the first metatarsal in a splayfoot with a change in position in the metatarsophalangeal joint of the big toe. The big toe deviates and is pulled outward. Hammer and claw toes cause toe extension to become increasingly restricted and prevent full rollover.
