periosteum

STRUCTURE: Two layers. (1) Outer fibrous layer: dense irregular collagen (type I) with fibroblasts, blood vessels, and nerves. (2) Inner cambium (osteogenic) layer: osteoprogenitor cells, osteoblasts, and mesenchymal stem cells. Sharpey fibers (perforating collagen fibers) anchor the periosteum to the outer circumferential lamellae of bone and are the attachment points for tendons and ligaments. The endosteum is the analogous thin membrane lining the medullary cavity, trabeculae, and Haversian/Volkmann canals.

ABSENT: Periosteum is absent over articular surfaces (covered by hyaline cartilage), and at sites of tendon/ligament insertion and sesamoid bones.

FUNCTION: (1) Appositional growth: osteoblasts in the cambium layer lay down new bone circumferentially, increasing bone diameter/width (intramembranous ossification process). Longitudinal growth occurs at the physis (growth plate), not the periosteum. (2) Blood supply: outer third of cortex is supplied by periosteal arteries; inner two-thirds by the nutrient artery (endosteal supply). (3) Fracture healing: periosteal osteoprogenitor cells form the external (hard) callus via intramembranous and endochondral ossification. (4) Sensation: richly supplied by sensory nerves, so it is very pain-sensitive, whereas bone itself has few pain fibers. (5) Attachment of muscles/tendons.

CLINICAL SIGNIFICANCE: - Periosteal reaction on imaging (X-ray): elevation/new bone formation suggests pathology. A solid or thin, uniform reaction suggests a benign/slow process (e.g., healing fracture, osteoid osteoma, chronic osteomyelitis). Aggressive patterns suggest malignancy or rapid process: "onion-skin" (lamellated) layering in Ewing sarcoma; "sunburst" pattern and Codman triangle in osteosarcoma (Codman triangle = elevated periosteum with reactive new bone at the edge of the lesion; also seen in Ewing sarcoma and osteomyelitis). - Osteomyelitis: pus can lift periosteum, disrupting cortical blood supply and causing sequestrum (necrotic bone) with involucrum (new periosteal bone formation around it). - Child abuse: periosteal elevation/new bone formation from subperiosteal hemorrhage; classic metaphyseal "corner" or "bucket-handle" fractures. Children have a thick, loosely attached periosteum, so greenstick and buckle (torus) fractures occur, and subperiosteal hematomas can occur with minimal trauma. - Scurvy (vitamin C deficiency): impaired collagen synthesis causes subperiosteal hemorrhages and painful limbs (and perifollicular hemorrhages, gingival bleeding); radiographic Wimberger ring and Frankel line. - Hypertrophic osteoarthropathy (digital clubbing, periostitis of long bones, arthritis): associated with lung cancer, cyanotic heart disease, IBD; linear periosteal new bone formation. - Shin splints (medial tibial stress syndrome): periostitis of the tibia from repetitive stress. - Infantile cortical hyperostosis (Caffey disease) and chronic venous insufficiency can also show periostitis. - Surgery: periosteum is often preserved for bone graft/healing; periosteal flaps/grafts used for reconstruction. - Osteosarcoma and periosteal osteosarcoma arise from/near the periosteum; subperiosteal resorption of the phalanges is a classic radiographic sign of hyperparathyroidism (osteitis fibrosa cystica).

HIGH-YIELD PEARLS: Periosteum = outer, pain-sensitive, osteogenic, appositional growth, fracture callus. Endosteum = inner lining. Sharpey fibers = anchoring. Codman triangle and onion-skin = aggressive periosteal reactions.