What is periosteum?
Periosteum [from Greek peri = surrounding; osteon = bone] is a fibrous membrane that covers the outer surface of the bones [1,16].
Periosteum covers all the bones, except the kneecap (patella) and certain small bones in the wrists, hands and feet, which are embedded within the tendons . The periosteum also does not cover the cartilaginous parts of the bones in the joints 17,35. The periosteum that covers the skull is called pericranium.
Besides the periosteum, the bones are covered by two other membranes:
- Endosteum covers the inner surfaces (cavities) of bones.
- Perichondrium covers the cartilage, except the articular cartilage in the joints.
The outer, fibrous periosteal layer consists mainly of the dense irregular connective tissue, made of superficial collagen (type I) and deeper elastin fibers, and rare connective tissue cells (fibroblasts) . This layer also contains arteries, veins, lymphatic vessels and sensory nerves [16,31]. The inner, cellular or osteogenic periosteal layer, also called cambium, which has got name from the cambium between the bark and wood of a tree, is in direct contact with the cortical bone; it consists of progenitor cells that develop into osteoblasts, which produce the bone tissue. The cambium layer is thickest in a newborn and thins with age [16,31].
In the growing individuals, the periosteum of the long bones is 70-150 micrometers (0.07-0.15 millimeters) thick; it is thicker toward the ends of the bones, and it thins with age . The periosteum of the nasal bones is about 0.8-1.8 mm thick  and alveolar periosteum (under the gingiva) is about 0.5 mm thick .
During periosteal reaction due to an irritating bone or systemic disorder, the periosteum can become several millimeters thick .
Endosteum covers the inner surfaces of the long bones. The endosteum is thinner than the periosteum since it has very little fibers.
Sharpey’s perforating fibers run from the periosteum through the bone to the endosteum and firmly attach both membranes to the bone . They are thin (<40 micrometers) but strong fibers made mainly of collagen and elastin . They are especially abundant at the sites of the ligament and tendon attachments to bones and in the alveolar bone, from which the teeth grow . Sharpey’s fibers become stronger with activity and weaker with lower blood estrogen levels .
Picture 1. Periosteum location.
The periosteum covers the outer surfaces of the bones,
except in the joints, where the bone is covered by the cartilage.
Functions of the Periosteum
1. Attachment Sites for the Muscle Tendons and Ligaments
Certain tendons (bands that connect muscles to bones) and ligaments (which connect the bones in the joints) attach to the periosteum at the insertion sites, which are called entheses or osteotendinous or osteoligamentous junctions [16,35]. Some tendons and ligaments attach directly to the bones (not via the periosteum) . In the joints, the periosteum continues into the articular cartilage and into the fibrous layer of the joint capsule .
Picture 2. Sharpey’s fibers connect periosteal attachments
(tendons, ligaments, joint capsule) firmly to the bone.
2. Nourishment of the Bones and Muscles
Periosteum contains periosteal arteries, which run within the outer periosteal layer and supply blood to the periosteum and outer one third of the cortical bone . Larger periosteal arteries run above the periosteum and communicate with the muscle arteries and contribute to the blood supply of the part of muscles adjacent to bones . When the periosteum is detached from the bone due to injury, surgery or during obtaining periosteal grafts, it does not affect the blood supply to the muscles and it only minimally affects the blood supply to the bone, because the bone also gets blood from the arteries that run inside the bone . The periosteum also contains veins, which collect the blood, and lymphatic vessels, which collect the lymph from the bones [1,16].
3. Bone Sensitivity (Innervation of Periosteum)
Sensory nerves in the outer, fibrous layer of the periosteum make it sensitive to tension and tearing [1,17]. Some nerve endings from the periosteum penetrate into the bone canals and thus make the bones sensitive to pain . The periosteum also contains the sympathetic nerves, which regulate the diameter of the periosteal arteries and thus the blood flow through the periosteum .
Nerve fibers in the periosteum or bones of the skull and in the dura mater (the outermost brain membrane) may be involved in migraine pain . Periosteal nerves contain substance P, which is involved in pain mediation .
4. Bone Growth and Repair
In children [1,16], the osteoblasts in the inner layer of the periosteum (cambium) produce new bone and thus enable the growth of the bone in the width or periosteal or appositional bone growth 1,16. Periosteal bone growth, to some extent, probably also occurs in adults . After a bone fracture, the progenitor cells in the cambium develop into osteoblasts and chondroblasts that contribute to the bone repair and healing by forming a soft periosteal callus [1,16,48]. The bone healing potential of the periosteum decreases with age and is greater in long bones, such as the tibia, than in flat bones, such as the skull bones . The periosteum may also contribute to bone wasting (resorption); for example, after a tooth loss, the alveolar bone under the gingiva shrinks .
Small bones, like wrist bones, that have only small amount of the periosteum, have spherical growth plates inside them .
In the growing males, low amounts of estrogens are needed for periosteal bone expansion .
5. Periosteal Hinge
In certain fractures of long bones, the periosteum remains intact and acts as a hinge — it holds two pieces of the broken bone together .
Periosteum In Various Bones
1. Periosteum in the Skull (Pericranium)
Pericranium is the periosteum that covers the outer surface of the bones in the skull.
The inner side of the skull bones is covered by the outermost brain membrane or meninge called the dura mater, which has the periosteal layer, which is firmly attached to the skull bones, and the inner meningeal layer. At certain points, the two layers separate and form canals called cerebral sinuses or dural venous sinuses through which the venous blood from the brain flows.
Meningeal arteries, which supply blood for the dura mater, run between the periosteal layer of the dura mater and the skull bones, in the so-called epidural space . A break of a meningeal artery, usually caused by a break of one of the skull bones, results in an epidural hematoma.
2. Periosteum In the Eye Orbit
The periosteum in the eye orbit is continuous with the dura mater ; it is loosely attached to the bone, so surgeons often elevate it when removing tumors in the orbit to avoid damage of the eye content . A subperiosteal hematoma, usually in the roof of the orbit, can occur after a blunt trauma to the frontal area of the head (a fall or direct blow) .
3. Periosteum In the Spinal Vertebrae
The periosteum in the vertebrae is well innervated, which may be one of the reasons for pain during injection of anesthetic for epidural anesthesia .
Conditions Related to the Periosteum
Periosteal tear refers to a detachment of a part of the periosteum from the bone at the site where a ligament or tendon is attached to it . It commonly occurs in football players when they suddenly twist a leg or foot. The main symptom is local tenderness and pain during stretching the affected tendon .
Subperiosteal hematoma or periosteal bruise is a collection of blood between the periosteum and the bone, usually due to direct blow to the bone or bone fracture. Symptoms include local pain, a noticeable tender lump and skin bruise within minutes of injury.
Shin splints or medial tibial stress syndrome refers to an overuse injury of the shin area with pain in the lower two thirds of shins triggered by exercise [6,11].
Periosteal ganglion is a ganglion cyst arising from the periosteum, mostly in adults [12,13,14,15]. It can develop after an injury. It can cause local pain, for example, in the wrist  or shin .
Periostitis refers to a periosteal inflammation, and periosteal reaction to formation of new bone or bone hypertrophy under the periosteum in response to bone irritation [2,3,17]. The terms are often used as synonyms. Symptoms may be absent or may include pain, tenderness and swelling of the affected limb [21,41]. Causes include shin splints, bone infection (osteomyelitis), vasculitis , benign tumors, fluorosis (especially in vertebrae) , cancer , chronic lung or liver disease and inflammatory bowel disease (IBD) . Main causes in infants include congenital syphilis and physiological periostitis . Diagnosis can be confirmed by X-ray, CT or MRI .
Picture 1. X-ray of a periosteal reaction (long arrow)
triggered by an osteoid osteoma (short arrow)
in the right thighbone (femur) of the 7-month old girl
(source: springer.com, Creative Commons licence)
Periosteal necrosis–death of a part of the periosteum–can result from burns , contact with bone implants  or osteomyelitis.
Periosteal Flaps and Grafts
Periosteal grafts can be used to treat defects in the connective tissue, bone, cartilage and muscles. This is possible because periosteal cells can differentiate into connective tissue cells (fibroblasts), bone cells (osteoblasts), cartilage cells (chondrocytes) or skeletal muscle cells (myocytes). An autologous periosteal graft, which is taken from the same person in whom the graft is used, for example a galea-periosteum graft from the scalp  or a graft from the shinbone (tibia) can be used to treat:
- Knee cartilage defects caused by osteochondritis, osteoarthritis or injuries [24,25]
- Muscle hernia, for example, anterior tibial muscle hernia 
- Sternal clefts  or maxillary clefts  in infants
- The gingival recession defects in the gums 
- Frontal sinus defects 
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