Understanding different types of bones, especially in the context of their unique characteristics and locations, is fundamental to human anatomy. The question asks to identify which of the given options is NOT a sesamoid bone. Let's delve into what sesamoid bones are and analyze each option provided.

Sesamoid Bone Definition

A sesamoid bone is a small, independent bone or nodule of cartilage developed in a tendon where it passes over an angular structure, typically in the hands and feet. The name 'sesamoid' comes from the Greek word 'sesamoeides', meaning 'like a sesame seed', due to their small size. These bones serve to protect the tendon and to improve the mechanical advantage of the muscle pulling on that tendon, often by changing the angle of the tendon's pull.

Calcaneum: The Non-Sesamoid Bone

The Calcaneum, also known as the heel bone, is the largest of the tarsal bones in the foot. It forms the foundation of the rearfoot and connects to the talus bone above it and the cuboid bone in front. Crucially, the calcaneum is a primary load-bearing bone of the foot and is classified as an irregular bone, not a sesamoid bone. It is part of the main structure of the foot, forming a significant portion of the ankle joint and providing attachment points for various muscles and ligaments.

Patella: A Prominent Sesamoid Bone

The Patella, commonly known as the kneecap, is the largest sesamoid bone in the human body. It is embedded within the quadriceps femoris tendon, where this tendon crosses the knee joint. Its primary function is to protect the knee joint and to increase the mechanical advantage of the quadriceps muscle by providing a longer lever arm. This allows the quadriceps to exert more force on the tibia, aiding in knee extension.

Fabella: A Lesser-Known Sesamoid Bone

The Fabella is a small sesamoid bone found in the lateral head of the gastrocnemius muscle tendon behind the knee joint. It is present in a significant portion of the population (estimates vary, but often cited around 10-30% in humans). Like other sesamoid bones, its presence is thought to offer mechanical advantage to the gastrocnemius muscle and protect the tendon from compression at the knee.

Cyamella: An Occasional Sesamoid Bone

The Cyamella is another small sesamoid bone. While more commonly found in certain animal species (like dogs), it can occasionally be present in humans. When present, it is typically located within the popliteus tendon, which runs across the posterior aspect of the knee joint. Its function, similar to other sesamoid bones, would be to reduce friction and improve the leverage of the popliteus muscle, which helps unlock the knee joint during flexion.

Conclusion on Bone Classification

Based on the definitions and characteristics of each bone:

• Patella is a sesamoid bone.
• Fabella is a sesamoid bone.
• Cyamella is a sesamoid bone (though rare in humans).
• Calcaneum is an irregular tarsal bone, forming the heel, and is not a sesamoid bone.

Therefore, the bone that is NOT a sesamoid bone among the given options is the Calcaneum.