longbone

Long bones are a class of bones whose length greatly exceeds their width. They function as levers and provide structural support for movement, while also serving as major sites for mineral storage and hematopoiesis. Typical long bones have a cylindrical shaft called the diaphysis, two expanded ends called the epiphyses, and a dense outer layer of compact bone surrounding cancellous bone at the ends. The outer surface is covered by a fibrous periosteum that contains nerves and blood vessels, while the inner surfaces are lined by the endosteum. The central medullary or marrow cavity in the diaphysis contains bone marrow.

The growth and repair of long bones rely on specialized regions and tissues. In children, growth occurs at the epiphyseal plates (growth plates), which enable longitudinal bone elongation; in adults these plates close, leaving an epiphyseal line. The inner cancellous bone forms a lattice-like network that helps absorb stress, while the cortex provides rigidity. Blood supply typically enters through a nutrient artery via a nutrient foramen, with additional vessels serving the metaphyses and periarticular regions.

Development and metabolism are closely linked in long bones. Long bones arise primarily through endochondral ossification, with primary ossification centers developing in the diaphysis before birth and secondary centers in the epiphyses after birth. Growth ceases when the growth plates fuse during late adolescence.

Common examples of long bones include the femur, tibia, fibula, humerus, radius, and ulna. Clinically, long bones are frequent sites of fractures, which heal through a process involving callus formation and remodeling; systemic conditions like osteoporosis increase fracture risk.