Periosteum: Structure, Function & Diseases

The periosteum (periosteum) coats each bone of the body except for the articular surfaces. In the skull, the periosteum is called the pericranium. Inner surfaces of bones, for example long bones, are covered by a thinner skin called endost or endosteum. The periosteum is highly innervated and permeated with blood vessels. Its main function is to supply the bone and to support metabolism in and around the bone.

What is periosteum?

The periosteum surrounds each bone in the body and provides the necessary exchange of substances during the constant building and breaking down processes that take place on and in the bones. In addition, the periosteum forms the connection between bones on the one hand and tendons and ligaments on the other. The periosteum is able to absorb the forces occurring at the attachment points of tendons and ligaments because the skin with its outer firm collagen layer is connected to the bones by a multitude of elastic fibers (Sharpey fibers). In addition to performing its important mechanical function, the periosteum must be able to supply the bone with nutrients and absorb degradation products from the conversion processes within the bone, as well as generate pain sensations and other sensory perceptions and transmit them via appropriate nerve pathways. These functions are performed by the lower layer directly adjacent to the bone, the cambium or stratum osteogenicum. To perform these tasks, the cambium is highly innervated and permeated with blood vessels and enriched with pain sensors (nociceptors).

Anatomy and structure

The periosteum consists of an outer, solid, layer (stratum fibrosum) composed of collagenous proteins and the cambium or stratum osteogenicum, which lies directly against the bone. Elastic fibers (Sharpey fibers) arise from the outer stratum fibrosum and their “free” end is firmly fused to the bone, so that tendons and ligaments find the necessary support to fulfill their function by growing against the periosteum. The cambium is highly innervated and interspersed with blood vessels to handle the metabolic processes on and in the bone. Because the bones themselves are insensitive to pain, the cambium is interspersed with, among other things, three different groups of pain sensors (nociceptors) that can respond to strong mechanical stimuli (A-mechanonociceptors), to heat and strong chemical stimuli (A-polymodal nociceptors), or that can respond to all three stimuli (C-polymodal nociceptors) with pain messages. The cells of the cambium are mostly composed of as yet undifferentiated osteoblasts, bone-forming cells that are responsible for thickness growth and for bone healing after a fracture.

Function and tasks

The periosteum fulfills three main functions: 1. To supply the underlying bone with nutrients and other required substances to support the metabolic processes taking place in and on the bone, and to absorb degradation products. The focus here is on the constantly occurring build-up and breakdown processes in the bone via osteoblasts (bone formation) and osteoclasts (bone breakdown), the thickness growth of the bones and the repair of fractures. 2. establishing the mechanical connection between the bones and the tendons and ligaments to absorb and dissipate the mechanical forces required to move the limbs to varying degrees. The length of the levers used to move limbs is usually relatively short to allow the tendons to pass within the area covered by skin even when the arm or leg or other limb is bent. If larger levers were used, for example, the tendons would have to protrude from the back of the knee like tight cords, which would provoke great risks of injury. 3. sensory protection of the bones. Because the bones are not sensory innervated, this function is performed by the cambium of the periosteum. It is essentially concerned with converting dangerous impacts on the bone from mechanical, chemical, thermal or other peak stresses into pain sensations. The main reason for generating graded pain sensations is its warning function. The message is to provide immediate cessation of the current condition, thus avoiding impending damage to the painful area.

Diseases and ailments

The most common disease associated with the periosteum is periosteitis or periostitis.The disease always affects only a part of the periosteum and can be unpleasantly painful. In most cases, periostitis is triggered by overloading, incorrect loading or too frequent repetition of a load. Often affected are periosteal regions where tendons or ligaments have grown over a wide area, such as the shin or forearm. For example, joggers can be affected by periostitis of the tibia because running applies impact and stretching stimuli to the periosteum in the area of the tendon insertions on the lower part of the inner tibia. The symptoms are also known as tibial tendonitis. Most likely, the repetitive pulling and shearing forces on the periosteum cause microlesions to form on the Sharpey fibers, which then cause inflammatory reactions. Periostitis can also be caused by bacterial infection, for example, streptococcus, Staphylococcus aureus, or Mycobacterium tuberculosis. Bacterially triggered periosteitis is usually distinguished from other periostitis by accompanying symptoms such as severe, localized pain, tissue swelling, and impaired general sensation. Regardless of the fact that symptoms of periostitis should be medically clarified, treatment options include immobilization, cold compresses, and administration of anti-inflammatory medications. In case of bacterial infection, the physician will consider treatment with antibiotics.