The tarsus connects the lower leg to the midfoot. It has a prominent mechanical role in load transfer.
What is the tarsal?
The tarsus consists of 7 bones that can be divided into 2 sections. In the near-body (proximal) section, the two largest bones are found, the talus (ankle bone) and the calcaneus (heel bone). The second row is formed by the navicular bone (Os naviculare), the cuboid bone (Os cuboideum) and the 3 sphenoid bones (Os cuneiforme mediale, intermedium and laterale). The talus is connected to the ends of the two lower leg bones and forms the upper ankle joint with them. It rests on the calcaneus, which is the only one of the 7 bones in contact with the ground. Together with the Os naviculare, the two bones form the lower ankle joint. The 3 ossa cuneiformia and the cuboid bone articulate with the bases of the 5 metatarsals. All tarsal bones form the hindfoot, which is joined distally by the metatarsus and finally the toes.
Anatomy and structure
The underside of the tibia and the insides of the two ankles, which form the malleolar fork, unite with the talus pulley to form the upper ankle joint. Because of the shape and the strong tension in this system, only movements in one plane are possible there, lifting (dorsiflexion) and lowering (plantar flexion) of the foot. The largest tarsal bone, the calcaneus, is located under the talus and together with it forms the posterior chamber of the lower ankle joint. The head of the talus (caput tali) projects like a rounded cylinder into the distal region of the tarsus. It has 2 convex articular surfaces with which the calcaneus and the os naviculare join it to form the anterior chamber of the lower ankle joint. Combined rotational movements of the foot can be performed here. All other bony connections of the tarsal bones to each other and to the metatarsal bones are so strongly secured by taut ligaments that only slight displacements are possible (amphiarthroses). The calcaneus and the os cuboideum form the foundation of the longitudinal arch of the foot. The talus and all other tarsal bones rest on these two, secured by bone and ligaments, and form the beginning of the bridge construction, which continues in the midfoot and ends at the metatarsophalangeal joints.
Function and tasks
The movements of the foot are largely determined by the upper and lower ankle joints and the controlling muscles. In the swing leg phase, during walking and running, a combination of dorsiflexion in the upper ankle and elevation of the inner edge (supination) in the lower ankle brings the foot into a position that allows unimpeded guidance of the free leg. During jumping, rapid plantar flexion occurs via the powerful calf muscles that attach to the cusp of the calcaneus. The remaining joints of the tarsal bones and metatarsals, which are only slightly displaceable, give the foot a certain stability overall, but still allow it to adapt to unevenness when stepping. On the one hand, the bony construction of the longitudinal arch is supported by strong ligamentous tractions under the sole of the foot, the ligamentum plantare longum and the plantar aponeurosis. On the other hand, the tendons of the toe flexors run partially on the inside under the bridge arch and also help with this function. This creates a buffer system that is able to absorb shocks and heavy weight loads in a springy manner and protect the joints of the foot, legs and spine. The tarsal bones are the most massive of the foot skeleton. This equips them very well for the task of bearing the weight of the body. The unique design of the tarsus distributes the load very favorably and significantly reduces stress on the individual parts. Due to its central position, the talus is the switching and distribution center in this process. The weight that comes from above is transferred to it via the tibia. A large part is passed on to the massive calcaneus and reaches the ground from there. The remaining load is transferred through the anterior chamber of the lower ankle joint to the adjacent tarsal bones and further through the arch structure to the forefoot. This creates a load distribution over many elements with a low stress on the individual parts.
Diseases
All tarsal bones are at risk of fracture due to trauma that happens by direct or indirect force.The calcaneus is affected when falls from great heights involve landing on it, such as occupational accidents and suicide attempts. Fractures of the talus can occur when great force is applied to the ankle. Such injuries are typical sports injuries in which the affected person twists their ankle with simultaneous lateral opposition or fixation of the foot. Similar injury mechanisms can also cause fractures in the other tarsal bones. Bone healing problems often develop as a result. Either unevenness remains, such as in the talus, with subsequent osteoarthritis formation, or metabolic disorders cause a loss of substance of the bone material. The sphenoid bones in particular can be affected by so-called fatigue fractures. They occur as a result of overloading during sporting or occupational activities. In contrast to acute fractures, their problem develops gradually and is often not recognized at the beginning because the symptoms are very unspecific. A flattening of the longitudinal arch, the so-called flat foot naturally affects the tarsal bones. The ligamentous support below the arch gives way due to too much stress and too little resistance, and the arch gradually becomes flatter. In the final stage, the entire row of tarsal bones, which rest on the calcaneus and the os cuboideum, slips off. The bottom of the 3 cuneiform bones and the os naviculare reach the ground and enter the zone of compressive stress. This stress causes severe pain and must be passively corrected with appropriate orthotics.
