Bone Classification

Learning Objectives

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

The 206 bones that compose the adult skeleton are divided into five categories based on their shapes (Figure 1). Their shapes and their functions are related such that each categorical shape of bone has a distinct function.

This illustration shows an anterior view of a human skeleton with call outs of five bones. The first call out is the sternum, or breast bone, which lies along the midline of the thorax. The sternum is the bone to which the ribs connect at the front of the body. It is classified as a flat bone and appears somewhat like a tie, with an enlarged upper section and a thin, tapering, lower section. The next callout is the right femur, which is the thigh bone. The inferior end of the femur is broad where it connects to the knee while the superior edge is ball-shaped where it attaches to the hip socket. The femur is an example of a long bone. The next callout is of the patella or kneecap. It is a small, wedge-shaped bone that sits on the anterior side of the knee. The kneecap is an example of a sesamoid bone. The next callout is a dorsal view of the right foot. The lateral, intermediate and medial cuneiform bones are small, square-shaped bones of the top of the foot. These bones lie between the proximal edge of the toe bones and the inferior edge of the shin bones. The lateral cuneiform is proximal to the fourth toe while the medial cuneiform is proximal to the great toe. The intermediate cuneiform lies between the lateral and medial cuneiform. These bones are examples of short bones. The fifth callout shows a superior view of one of the lumbar vertebrae. The vertebra has a kidney-shaped body connected to a triangle of bone that projects above the body of the vertebra. Two spines project off of the triangle at approximately 45 degree angles. The vertebrae are examples of irregular bones.

Figure 1. Classifications of Bones. Bones are classified according to their shape.

Long Bones

A long bone is one that is cylindrical in shape, being longer than it is wide. Keep in mind, however, that the term describes the shape of a bone, not its size. Long bones are found in the arms (humerus, ulna, radius) and legs (femur, tibia, fibula), as well as in the fingers (metacarpals, phalanges) and toes (metatarsals, phalanges). Long bones function as levers; they move when muscles contract.

Short Bones

A short bone is one that is cube-like in shape, being approximately equal in length, width, and thickness. The only short bones in the human skeleton are in the carpals of the wrists and the tarsals of the ankles. Short bones provide stability and support as well as some limited motion.

Flat Bones

The term flat bone is somewhat of a misnomer because, although a flat bone is typically thin, it is also often curved. Examples include the cranial (skull) bones, the scapulae (shoulder blades), the sternum (breastbone), and the ribs. Flat bones serve as points of attachment for muscles and often protect internal organs.

Irregular Bones

An irregular bone is one that does not have any easily characterized shape and therefore does not fit any other classification. These bones tend to have more complex shapes, like the vertebrae that support the spinal cord and protect it from compressive forces. Many facial bones, particularly the ones containing sinuses, are classified as irregular bones.

Sesamoid Bones

A sesamoid bone is a small, round bone that, as the name suggests, is shaped like a sesame seed. These bones form in tendons (the sheaths of tissue that connect bones to muscles) where a great deal of pressure is generated in a joint. The sesamoid bones protect tendons by helping them overcome compressive forces. Sesamoid bones vary in number and placement from person to person but are typically found in tendons associated with the feet, hands, and knees. The patellae (singular = patella) are the only sesamoid bones found in common with every person. Table 1 reviews bone classifications with their associated features, functions, and examples.

Table 1. Bone Classifications
Bone classification Features Function(s) Examples
Long Cylinder-like shape, longer than it is wide Leverage Femur, tibia, fibula, metatarsals, humerus, ulna, radius, metacarpals, phalanges
Short Cube-like shape, approximately equal in length, width, and thickness Provide stability, support, while allowing for some motion Carpals, tarsals
Flat Thin and curved Points of attachment for muscles; protectors of internal organs Sternum, ribs, scapulae, cranial bones
Irregular Complex shape Protect internal organs Vertebrae, facial bones
Sesamoid Small and round; embedded in tendons Protect tendons from compressive forces Patellae

Self-Check Questions

Take the quiz below to check your understanding of Bone Classification: