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The patella is classified as a sesamoid bone because it forms within a tendon and is typically found in locations where a tendon crosses over a joint. Sesamoid bones are unique in that they help to protect tendons from stress and wear, and they also serve to improve the mechanical leverage of muscles that cross the joint. The patella, specifically, functions to enhance the strength of the quadriceps muscle during knee extension by providing a smoother surface for the tendon to glide over, thus facilitating movement and providing stability.

In contrast, the carpal bones are classified as short bones, comprising the wrist's structure and aiding in its function. The vertebrae are irregular bones, designed for structural support and protection of the spinal cord. The scapula is a flat bone that forms the shoulder girdle, contributing to the structure and movement of the upper limbs. Each of these bones has a distinct classification based on their shape and function, unlike the patella, which fits the definition of a sesamoid bone due to its specific characteristics and role in the body.