CLASSIFICATION OF BONES: ACCORDING TO SHAPE, SIZE, STRUCTURE, FUNCTION - ANATOMY AND PHYSIOLOGY - 2025

The classification of bones is a practical method to simplify the study and better understand the functioning of these anatomical structures typical of vertebrates.

Bones are rigid organs that make up the skeleton. They fulfill mechanical and protective functions, in addition to having a fundamental role in the formation of blood cells and serve as a storage site for essential minerals.

Femur. Jordiferrer

The set formed by the bones, muscles, ligaments and shock-absorbing elements, forms the locomotor skeletal system, which guarantees the movement, balance, plasticity and elasticity of the body. All types of bone are distributed in the body in no specific order, rather shaping a functional and practical system.

Bone cells differentiate early in the fetus, forming primitive bones that remain in their final position during the first trimester of gestation. However, as the individual continues to increase in size until adulthood, bones have the ability to lengthen through a process mediated by hormones.

Regardless of their size and shape, all bones are made up of a type of bone tissue that can be completely solid (compact bone) or have hollow sponge-shaped spaces (cancellous or trabecular bone) or both.

Bone cells have a particular characteristic that is to remove and replace bone tissue throughout life. This process is known as bone remodeling. Thus, under normal conditions, the complete skeleton of an adult is replaced every 10 years by the cellular activity of bone.

Classification of bones

To achieve the harmonic composition of the skeleton, there are several types of bone that differ both in size and shape and in the distribution of bone cell tissue.

Therefore, we recognize different types of bones depending on where they are located.

According to their size they are classified as long and short; according to its shape in planes, sesamoids and irregular; and according to its structure in compact and spongy.

- Types of bone according to their size

Long

Long bones are all those that are taller than they are wide, regardless of their size. Some examples are the femur, tibia, and clavicle. The bones that make up the fingers are also considered long since they are taller than they are wide.

These types of bones are essential to balance the body, although they are also part of the joints providing movement. Many of them bear a significant weight load, such as the bones of the legs.

Long bones. Anatomography is provided by DBCLS

During the growth stage they are lengthened by a process mediated by growth hormone that is secreted by the pituitary gland, which is found in the brain. Anatomically, several parts are distinguished in the long bone, these are:

- Epiphysis: the ends.

- Diaphysis: the body.

- Metaphysis: junction area between the epiphysis and the diaphysis.

- Articular cartilage: is the portion of soft tissue that joins two bones, forming a joint.

- Periosteum: external sheet that covers the bone, very fibrous and resistant with a high content of blood vessels and neurological endings.

- Endostium: surface that covers the internal part of the bone.

- Artery: called the nutritive artery, it constitutes the blood supply for the metabolic activities of bone cells. Each bone has an artery that nourishes it.

- Medullary cavity: space that contains a tissue rich in triglycerides and that constitutes an important energy reservoir.

Anatomy of a long bone. OpenStax College

Short

Short bones are those whose dimensions, in height and width, are equal or almost equal, adopting a cuboidal shape. They provide stability, cushioning and support to the skeleton, but not mobility.

Right posterior human distal radius and ulna. Brian C. Goss

Examples of these are some bones of the hand and foot, specifically those that make up the carpus and tarsus.

- Types of bone according to its shape

Blueprints

They are those that have a flattened shape and where their height and extension predominate over their thickness.

Its main function is to protect organs, such as the ribs, the sternum, the bones of the pelvis and those of the skull. They also provide a sufficient surface area for the insertion of some muscles.

Flat bones. Anatomography is provided by DBCLS

They are composed mainly of spongy bone tissue, which contains the bone marrow, which is the organ responsible for the formation of blood cells.

Because of this content, rich in medullary tissue, they are preferred when performing bone marrow biopsies or extracting that material for transplantation. One of the bones most used for these procedures is the ischium or pelvic bone.

Sesamoids

They are small bones that are embedded within a tendon or muscle.

Its function is to act as a pulley, providing a smooth surface on which soft structures, such as tendons, can slide without encountering friction.

Sesamoid bones on the metatarsophalangeal of the big toe. AngelHM

They are present in several locations, but they are easy to reveal on radiography, next to the joint of the first toe. Calcification of the sesamoid bones is an important finding as it indicates that the patient has reached puberty.

Irregular

Irregular bones are those that, due to size or shape, do not fall into any of the previous categories. They are peculiar in shape and fulfill different functions depending on the place where they are.

Irregular bones Anatomography is provided by DBCLS

The vertebrae of the spinal column are an example of irregular bones that protect the spinal cord; the hyoid bone, which is located in the neck, is an attachment surface for the muscles of chewing and swallowing.

- Types of bone according to their structure

The structure of bone refers to the type of tissue that makes it up. It can be compact or fluffy. These tissues are arranged in all the bones of the body to a greater or lesser extent, depending on the shape and function of each one.

Therefore, the terms compact bone or cancellous bone refer specifically to the internal structural part of the anatomy of each bone and not to an additional type to those previously described.

Compact bone

Also called cortical, it is a strong, dense and hard tissue that represents about 80% of the total bone mass of the body. It is very resistant to torsion and compression.

It is formed by firm layers of lamellae that are arranged in the form of a column, giving the characteristic high density of this fabric. Within the compact bone are the most important bone cells, the osteocytes.

Histological structure of a compact bone. OpenStax Anatomy and Physiology

It is located mainly in the body of long bones, diaphysis, and on the outside of flat bones. There is also compact tissue in short bones depending on their location.

This type of tissue contains the nutritional vessels of the bone, and forms canaliculi through which the bone cells receive their blood supply.

The metabolic activity of compact bone is low but they have a great capacity to form new bone, therefore it is an essential tissue when injuries such as fractures occur. This process is known as bone remodeling.

Spongy bone

Also known as trabecular, it is a type of very porous tissue that is located at the ends of the long bones, epiphyses, in the vertebrae and inside the flat bones.

It is composed of trabeculae or hard partitions that form empty spaces, within which the red bone marrow is distributed.

Scheme of a cancellous bone. Modified from Pbroks13

During movement, it is responsible for cushioning the force received by the bone, transmitting it towards the surface of compact tissue, which is more resistant.

Within this tissue, the bone marrow carries out the formation and release of blood cells into the stream. This process is named as Hematopoiesis.

Bone anatomy

The bone is the main anatomical unit of the skeleton. The adult body has 213 bones. They are composed of a specialized tissue that contains various types of collagen and a large amount of calcium, which gives it its characteristic rigidity.

Together with the ligaments and muscles they form the skeletal system, a supporting and protective apparatus of the body.

Bones contain living tissue called bone marrow, which is responsible for the creation and release of blood cells into circulation. This process is named as Hematopoiesis.

The main function of bones is to support soft tissues and protect vital organs and elements. In addition to this, they are storage sites for mineral salts, such as calcium and phosphate, so they play a fundamental role in maintaining the body's balance.

In the bone, a complex process of formation and reabsorption of bone mass occurs, directly mediated by the cells found inside.

The bone-forming cells are called osteoblasts, and those that reabsorb it are the osteoclasts. These are specialized cells that have the ability to recognize states of stress and trauma, making it possible to eliminate poorly functional tissue and to form new bone.

During growth, bone cells are activated to make tissue that lengthens bone. This happens in a specific part of the bones which is not fully solidified during childhood and adolescence.

References

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4. Oftadeh, R; Perez-Viloria, M; Villa-Camacho, J. C; Vaziri, A; Nazarian, A. (2015). Biomechanics and mechanobiology of trabecular bone: a review. Journal of biomechanical engineering. Taken from: ncbi.nlm.nih.gov
5. Osterhoff, G; Morgan, E. F; Shefelbine, S. J; Karim, L; McNamara, L. M; Augat, P. (2016). Bone mechanical properties and changes with osteoporosis. Injury, 47 Suppl 2. Taken from: ncbi.nlm.nih.gov