Hartknochen

Hartknochen, German for compact bone, is the dense outer layer of bone tissue that constitutes the cortical bone. It forms the sturdy shell around a bone's interior and contrasts with the trabecular, spongy bone inside. It provides mechanical strength and protection and is present in all bones, with variations in thickness.

Hartknochen is organized into osteons (Haversian systems) in many regions. Each osteon has concentric lamellae surrounding a central Haversian canal with vessels and nerves. Osteocytes reside in lacunae linked by canaliculi. The extracellular matrix is rich in type I collagen and hydroxyapatite, giving rigidity.

Distribution and function: In long bones, Hartknochen forms the diaphysis' thick cortex; in flat bones it composes the outer compact plates. It supports body weight, enables muscle leverage, protects marrow, and provides attachment sites for ligaments and tendons. Periosteum covers the surface; endosteum lines internal surfaces.

Remodeling and metabolism: Hartknochen is dynamic, remodeled by osteoclasts and osteoblasts in response to loading and hormonal signals, maintaining mineral homeostasis and repairing microdamage. Blood vessels run through Haversian and Volkmann's canals; nerves accompany them.

Clinical notes: Cortical bone density can decrease with age and disease, affecting fracture risk. Imaging shows cortical thickness and continuity.