Section 2 - Bone Classification

Section 2 Learning Objectives

By the end of this section, you will be able to:

• Classify bones according to their shapes
• Describe the function of each category of bones

At birth, a baby’s skeleton consists of 270 to 300 bones, many of which are primarily composed of cartilage, a flexible connective tissue. As the body develops, some of these bones gradually fuse together, ultimately forming the 206 bones found in the adult skeleton (Betts et al., 2022).

While bones stop growing in length typically between the ages of 16 and 18, bone density—the overall amount of bone tissue—continues to increase gradually until a person reaches their late 20s (Betts et al., 2022). This ongoing bone development is essential for maintaining strength and structural integrity.

The 206 bones of the adult skeleton are classified into five categories based on their shape. These distinct shapes are directly related to their function, ensuring that each type of bone serves a specific role in supporting movement, protecting organs, or enabling other physiological functions (Tortora & Derrickson, 2021).

Figure 6.6 Classifications of Bones are classified according to their shape.

Types of Bones and Their Functions:

Bones come in a variety of shapes, each designed to serve specific structural and functional roles within the skeletal system. These shapes help determine a bone's movement, support, and protection capabilities.

Long Bones

Long bones are elongated and cylindrical, meaning they are longer than they are wide. This classification is based on shape rather than size. These bones function as levers, working with muscles to facilitate movement. They are found in the arms (humerus, radius, ulna), legs (femur, tibia, fibula), and even in the fingers and toes (metacarpals, metatarsals, and phalanges) (Moore et al., 2020).

Short Bones

Short bones are cube-like, with nearly equal length, width, and thickness. These bones are primarily found in the wrists (carpals) and ankles (tarsals) and provide stability and support, allowing for limited movement.

Flat Bones

Although the term "flat bone" suggests that these bones are completely flat, they are often thin yet slightly curved. Examples include the skull bones (cranial bones), scapulae (shoulder blades), sternum (breastbone), and ribs. Flat bones serve as attachment points for muscles and play a crucial role in protecting internal organs (Drake et al., 2020).

Irregular Bones

Bones that do not fit into any specific shape category are classified as irregular bones. These bones typically have complex shapes, such as the vertebrae, which provide structural support and protect the spinal cord from compressive forces. Additionally, many facial bones, particularly those with sinuses, also fall under this category.

Sesamoid Bones

Sesamoid bones are small, round bones that resemble sesame seeds. These bones develop within tendons, the fibrous tissues that connect muscles to bones, and are exposed to high levels of pressure and mechanical stress at joints. Their primary function is to protect tendons by reducing friction and absorbing compressive forces. While the number and location of sesamoid bones vary among individuals, the patella (kneecap) is the only sesamoid bone that is consistently present in all people.

Table 6.1 Bone Classifications