American Academy of Professional Coders (AAPC) Practice Exam

What type of bone is formed within tendons and is initially cartilaginous in early life?

1. Flat

2. Irregular

3. Sesamoid

4. Long

The correct answer is: Sesamoid

Sesamoid bones are unique in that they develop within tendons, serving to protect the tendon and improve mechanical advantage during joint movement. They begin as cartilaginous structures during early development and can later ossify into bone. A well-known example of a sesamoid bone is the patella, or kneecap, which forms within the tendon of the quadriceps. In contrast to sesamoid bones, flat bones (such as those in the skull) are primarily involved in protection and have a different formation process, being typically developed through intramembranous ossification. Irregular bones, like vertebrae, have complex shapes but do not specifically form within tendons. Long bones (like the femur and humerus) are characterized by their length and include structures like epiphyses and diaphyses for growth and function. Therefore, the defining characteristics of sesamoid bones, including their formation within tendons and early cartilaginous state, make this answer the most accurate.