Compact Bone: Definition, Overview, Function

Compact Bone Definition

Compact bone, also known as cortical bone, is a dense and hard material that makes up the outer shell of most bones in the body.

It is composed of a solid matrix filled with organic ground substance and inorganic salts, with only tiny spaces that contain bone cells. Compact bone is designed to withstand compressive forces and is stronger than spongy bone, which makes up the remaining 20% of the human skeleton.

What is Compact Bone?

Compact bone, also known as cortical bone, is a dense and hard type of bone tissue that makes up 80% of the human skeleton. It is composed of a solid bony matrix filled with organic ground substance and inorganic salts, leaving only tiny spaces called lacunae that contain bone cells called osteocytes.

Compact bone is found in the outer layer of most bones in the body, forming a shell around the cancellous bone, which has a spongy appearance with numerous large spaces and is found in the marrow space of a bone.

The basic units of compact bone are called osteons or Haversian systems. These are cylinder-shaped structures that have a mineral matrix and are home to osteocytes that are trapped in the matrix.

Lamellae are formed by osteons that align themselves in a parallel orientation to form Haversian canals that contain nerves and blood vessels. The parallel orientation of osteons along the high-stress areas of compact bone provides strength to help resist bending or fracturing.

Compact bone is the main structure in the body for support, protection, and movement. Due to its strong nature, compared to spongy bone, it is the preferred tissue for strength. Spongy bone is used for more active functions of the bones, including blood cell production and ion exchange.

However, compact bones also serve the function of storing and releasing calcium to the body when needed. The compact bone also provides strong mechanical levers, against which the muscles can create movement.

The function of Compact Bone

Compact bone, also known as cortical bone, is a dense and strong type of osseous tissue that makes up 80% of the human skeleton. It is composed of a solid bony matrix filled with organic ground substance and inorganic salts, leaving only tiny spaces called lacunae that contain bone cells called osteocytes.

Compact bone is strong and can withstand compressive forces, creating protection and mechanical lever systems that make movement easier. It forms a shell around the cancellous bone, which has a spongelike appearance with numerous large spaces and is found in the marrow space of a bone.

Compact bone is made up of cylindrically-shaped units called osteons, which are the functional units of compact bone. Every osteon is made up of a central canal that contains nerves and blood vessels, perforating canals, lamellae (configurations of bone matrix), and osteocyte-containing lacunae.

The central canal is the smallest unit that can carry out one of the organ’s functions[3]. The perforating canals connect the central canals to each other and to the periosteum, the outer layer of the bone.

The lamellae are arranged in concentric circles around the central canal and contain collagen fibers that give the bone its strength. The osteocytes are located in the lacunae and are connected to each other and to the central canal by tiny channels called canaliculi.

Structure of Compact Bone

Bones are considered organs because they contain various types of tissue, such as blood, connective tissue, nerves, and bone tissue. There are two types of bone tissue: compact and spongy. Compact bone, also called cortical bone, is the denser and stronger of the two types of osseous tissue.

It makes up 80% of the human skeleton, while the remainder is cancellous bone, which has a spongelike appearance with numerous large spaces and is found in the marrow space of a bone. Compact bone is solidly filled with organic ground substance and inorganic salts, leaving only tiny spaces (lacunae) that contain the osteocytes, or bone cells.

Compact bone tissue consists of osteons that are aligned parallel to the long axis of the bone, and the Haversian canal that contains the bone’s blood vessels and nerve fibers.

The osteon is the functional unit of compact bone, meaning it is the smallest unit that can carry out one of the organ’s functions. The osteon is made of lamellae, lacunae, and osteocytes arranged around a central canal.

The extracellular matrix around the cells gives compact bone its hardness and rigidity. This matrix is made of both organic and inorganic materials. For example, collagen provides tensile strength and hydroxyapatite crystals provide the bone with compressive strength.

Spongy bone, also known as cancellous bone, forms the inner layer of all bones. It does not contain osteons that constitute compact bone tissue.

Instead, it consists of trabeculae, which are lamellae that are arranged as rods or plates. The small dark ovals in the osteon represent the living osteocytes. The two types of bone tissue work together to provide support, protection, and movement to the body.