Video: Types of bones

Hello, and welcome to Kenhub! In today's tutorial, we'll explore the different types of bones in the human skeleton. There are five distinct types of bones, classified by their shape and structure. These are: long bones, short bones, flat bones, irregular bones, and sesamoid bones.

Here on the right, you can see the femur, a typical example of a long bone. Most long bones have a characteristic structure with a number of common parts. The diaphysis, equivalent to the body or shaft, is tubular with a thick outer layer of compact bone. Contained within it is the medullary cavity, which is bordered by a small amount of spongy bone tissue surrounding the bone marrow.

Found at the ends of the long bone are the epiphyses, which are usually wider and more prominent. They consist primarily of spongy bone surrounded by a thinner layer of compact bone. They develop from a center of ossification separate to that of the diaphysis and are partly covered by a layer of articular curtilage where joints form with other bones.

Between the diaphysis and epiphysis lies the metaphysis, where longitudinal growth occurs. This growth is driven by the epiphyseal plate, a cartilage layer that ossifies over time, forming the epiphyseal line once growth is complete.

Finally, we also have apophyses which are bony outgrowths which usually function as sites for tendon and ligament attachment such as the greater and lesser trochanters of the femur as you can see here. They also develop separately to the rest of the bone before fusing with it later in growth.

Long bones are formed through endochondral or indirect ossification. This process involves the transformation of cartilage into bone, rather than intramembranous or direct formation from connective tissue. Key examples of long bones include the humerus, ulna, and radius in the upper limb, and the femur, tibia, and fibula in the lower limb. Additionally, the metacarpal bones, metatarsal bones, and the phalanges of the fingers and toes are all classified as long bones. The clavicle is another important example.

Similar to long bones, short bones consist largely of spongy bone encased in a thin layer of compact bone. This type of bones is mainly found in the hands and feet. Examples include the carpal bones and tarsal bones.

Flat bones are found in areas subjected to various mechanical forces and often serve to provide protection to soft tissues deep to them. They feature a sandwich-like arrangement with strong outer and inner layers of compact bone, which encase spongy bone and bone marrow. These bones form through an intramembranous or desmal ossification, a process involving the gradual transformation of embryonic connective tissue known as mesenchyme into bone. Examples of flat bones include some of the skull bones, in particular the neurocranium, and also the ribs, the sternum in the thoracic cage, and the scapula.

Irregular bones, as their name suggests, vary in shape and structure and therefore do not fit into any other category. They often have a fairly complex shape, which helps protect internal organs. Examples of this group include several bones of the skull like the sphenoid and ethmoid bones as well as the vertebrae and the hip bone.

Sesamoid bones are small, round bones embedded within a tendon, typically found in locations where a tendon passes over a joint. These bones function to reduce friction, modify pressure, and enhance the mechanical efficiency of muscle-tendon systems by acting as pulleys.

Common examples of this type include the sesamoid bones of the thumb and big toe as well as the patella, which is the largest sesamoid bone in the human body. The patella is embedded in the tendon of the quadriceps femoris muscle, where it serves two main functions: it extends the lever arm of the muscle and reduces the sliding resistance of the tendon.

And that's it for today! Thank you for watching and see you next time!