Cortical bone

Last revised by Bálint Botz on 15 Jul 2020

The outer shell of compact bone is called cortical bone or cortex. It is formed by compact bone which is one of the two macroscopic forms of bone, the other being cancellous bone

Cortical bone contains Haversian systems (osteons) which contain a central Haversian canal surrounded by osseous tissue in a concentric lamellar pattern. The Haversian canals contain vessels which supply blood to the cellular elements of the osteon (e.g. osteoblasts, osteocytes, and osteoclasts).

Two fibrovascular layers surround the cortical bone which contain cells that remain osteogenic. The outermost layer (between the outer surface of the bone and soft tissue) is periosteum and the innermost layer (between compact bone and the medullary space containing spongiosa) is endosteum 1.

The thickness of the cortex is from subperiosteal deposition of bone. In children, the periosteal layer loosely adheres to the cortex. The periosteum becomes thicker, vascular, and active with age and increased activity.  

The periosteum surrounds the cortex except for the portions which are intra-articular, covered by cartilage, or covered by synovial membrane. The periosteal layer is intimate with connecting tendons and ligaments at sites known as entheses (see enthesopathy) via Sharpey's fibers. The periosteal layer contains afferent sensory neurons which compress easily by relatively low-threshold mechanical stimuli 2.

Cortical bone is the most dense tissue, and represents 80% of the total skeletal mass in healthy adults 4

Cortical bone appears radiopaque (white) on radiographs as the outermost layer of bone. It is best visualized in long bones. The periosteum is not well-visualized on radiographs in the absence of underlying pathology (see periosteal reaction). The endosteum appears at the interface between the cortical bone and the medullary cavity in long bones and with pathology, may appear scalloped (see endosteal scalloping).

Cortical thickness is measured via dual-energy x-ray absorptiometry (DEXA) 3 at various locations (e.g. lumbar spine, femoral neck, distal radius) to determine bone density and evaluate for bone insufficiency.

Cortical bone appears radiopaque (white) on computed tomography. Fractures are more easily identified on CT than radiographs.

The outermost layer of cortex contains few mobile protons. The result is low signal (black) on all pulse sequences, except those with ultrashort-TE 4. The periosteum is normally not visible with MR, but periosteal reactions may be detected sooner than with radiographs.

Fracture lines, including stress fractures, appear hyperintense on T2-weighted images within the cortex.

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