The plantar aponeurosis is located in the sole of the foot. It performs important static and protective functions.
What is the plantar aponeurosis?
An aponeurosis is a plantar tendon or tendon plate. The term plantar is a place name and comes from planta pedis = sole of the foot. The compound name plantar aponeurosis accordingly denotes a tendon plate in the area of the sole of the foot. In a narrower sense, the term aponeurosis is not used quite correctly here, because another characteristic is missing. By definition, tendons are the connective tissue origin and attachment structures of muscles. They attach the muscle to the bone. Planar tendons occur on flat muscles, for example, in the oblique abdominal muscles. Although the tendon of origin of the flexor digitorum brevis muscle on the calcaneus is fused with the plantar aponeurosis, it still forms an independent connective tissue structure with independent functions. However, the tissue composition and tissue properties correspond to those of a firm fibrous tendon. Some authors rather assign it to fascial tissue and therefore use the term plantar fascia.
Anatomy and structure
Similar to muscles, tendons, and ligaments, aponeuroses have a systematic structure of bundle-like units. In addition to the ground substance (matrix) and fat bodies, there are massive accumulations of collagen fibers in the fiber bundles, which give the structure tremendous tensile strength. They are oriented according to the direction of traction and are additionally arranged in layers in the plantar aponeurosis. The tendon plate originates on the underside of the calcaneus, at the tuber calcanei. From there, it initially runs as a path toward the toes. In the middle area of the sole of the foot, it divides into 5 fibrous strands that run in a delta shape toward the toes. The closed formation dissolves there and spaces develop between the 5 bundles. The base of the plantar aponeurosis is in the area of the metatarsophalangeal joints, where the tracts radiate into the joint capsules, ligaments and tendons of the toe flexors. At the level of the heads of the metatarsals, 2 transverse fibrous tracts regularly occur, linking the longitudinal tracts and providing a connection to the inner and outer edges of the foot. Externally, the plantar aponeurosis is firmly fused to the skin by connective tissue bridges. In the area of these connections, hollow chambers are formed in which fatty tissue is embedded. In this way, a relatively thick cushion-like tissue is formed.
Function and tasks
The plantar aponeurosis is a very important structure on the foot with many functions. Together with the skin, it forms the protective covering to the outside. The penetration of foreign bodies and pathogens into deeper sensitive layers is prevented or made more difficult. The special connection between the skin and the plantar aponeurosis with the pressure pad construction provides an effective buffer when standing and walking. Loads are not transferred as quickly or as intensively to the deeper structures, especially to the bones. The firm cross connections prevent the otherwise usual displacement of the skin, it is fixed. This mechanism has a positive effect on stability and is an important stability component. Another important function of the plantar aponeurosis is to protect the underlying structures. In addition to the muscle bellies and tendons of the muscles that pull along there, these are primarily vessels and nerves. These run for the most part under the covered surfaces of the plantar aponeurosis. They emerge in the interstices and reach their respective supply areas. Mechanically, the most important function of the plantar aponeurosis is its participation in the arch structure of the foot, which is composed of longitudinal and transverse arches. The supports of this architecture are the 3 contact points at the heel, the ball of the big toe and the ball of the little toe. The structure consists of 3 layers. The inner part is formed by the bones of the foot skeleton, the middle by ligaments and the outer by the plantar aponeurosis with the muscles and tendons running there. Because of their extension and the associated better leverage, their efficiency on the longitudinal arch is greater than that of the other structures. For the transverse arch, the transverse connections provide only a small additional function.
Diseases
A typical overuse syndrome in which painful irritation of the plantar aponeurosis develops is plantar fasciitis.In rare cases, tears can also form in the tissue. Relatively often, this condition occurs in running athletes, especially when poor footwear is used and the surface is very hard and not very springy. The development of a heel spur can be the result of prolonged or recurrent irritation. In the acute phase, regular walking is not possible or only possible to a limited extent due to the pain. As a result, unfavorable loading moments are generated in the knee and hip joints and in the spinal column. The most important complex of complaints affecting the plantar aponeurosis or even caused by it are foot deformities such as fallen arches, splay feet and flat feet. In flat foot, the longitudinal arch is flattened or absent, in splayfoot the same is true for the transverse arch, and in flat foot both constructions are affected. There are various triggers for this problem such as axial misalignments of the knee joints or fractures with defect healing in the area of the tarsal and ankle joints. In people with congenital connective tissue weakness, all supporting ligaments, tendons and also the plantar aponeurosis are too lax and can no longer optimally support the arches, they sink. A significant amplifier of this process is obesity, which significantly increases the load on the supporting structures. To a certain extent, the decay of the arches can be stopped or slowed down by appropriate training of the muscles involved. However, when the process has progressed to the point where the inner tarsal row is slipping away from the outer one, active measures can no longer do anything. Orthotics are then usually prescribed to reduce discomfort and prevent adverse static changes in other joints and the spine.
