Bones Bones Bones Bones

Types of Bones

Did you know that newborns have over 300 bones in their skeletons? As we grow, some of these bones fuse together, leaving us with 206 bones in adulthood. Understanding the skeletal system is a key focus in anatomy and physiology courses, especially for therapists, such as sports massage therapists, who need to know the structure and potential injuries affecting the bones they work with.

The bones in our bodies are categorized into five main types: Long, Short, Flat, Irregular, and Sesamoid.

Long Bones

Long bones are longer than they are wide and are the primary bones in our limbs. They grow more than other bone types throughout childhood and play a central role in determining our adult height. Long bones have a hollow medullary cavity, which serves as storage for bone marrow. Examples of long bones include the femur, tibia, fibula, metatarsals, and phalanges.

Short Bones

Short bones are nearly equal in length and width and are often cube- or round-shaped. The carpal bones in the wrist and tarsal bones in the foot are examples of short bones.

Flat Bones

Flat bones vary in size and shape but share the characteristic of being thin in one direction. Unlike long bones, flat bones lack a medullary cavity. Examples include the frontal, parietal, and occipital bones in the skull, as well as the ribs and hip bones.

Irregular Bones

Irregular bones don’t fit into the patterns of long, short, or flat bones due to their unique shapes. Examples include the vertebrae, sacrum, and coccyx of the spine, along with the sphenoid, ethmoid, and zygomatic bones in the skull.

Sesamoid Bones

Sesamoid bones develop within tendons that cross over joints to protect the tendon from stress and to improve the efficiency of muscle movement. The patella (kneecap) and pisiform bone in the wrist are the only sesamoid bones included in the standard 206-bone count. Additional sesamoid bones can develop in the hands and feet, though not everyone has them.

Bone Growth Processes

Bone growth occurs through two main processes: intramembranous ossification and endochondral ossification.

Intramembranous Ossification: Flat bones develop through intramembranous ossification, where bone grows from a primary center within fibrous membranes, leaving a small amount of fibrous tissue between developing bones. In the skull, these spaces, called fontanels, allow for flexibility and growth during infancy, gradually hardening into the adult skull as the bones fuse.

Endochondral Ossification: Long bones develop through endochondral ossification, where a primary ossification center grows within a cartilage model, forming most of the bone structure. Secondary centers at the ends (epiphyses) also develop, leaving a strip of hyaline cartilage, called the growth plate, in between. During childhood and adolescence, this growth plate expands under the influence of growth and sex hormones, contributing to height and limb length. At the end of puberty, the growth plate closes, and the bones fuse into a permanent single structure, completing the transition from childhood to adult skeletal proportions.

The difference in height and limb length from infancy to adulthood is largely due to the continued growth in long bones via endochondral ossification.

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