Athro, Limited Biology Evolution Panda's Thumb

The front of the human knee showing the patella inside the patellar tendon.

Most bones grow on a framework of cartilage. Sesamoid bones, however, grow inside connective tissues. Tendons and ligaments are strips of connective tissue. Tendons are the bits of connective tissue that form the backbone of muscles and attach them to bones. Ligaments are similar bits of connective tissue, but attach bones to each other to stabilize joints. Sesamoid bones can also form in other connective tissues around joints.

The human kneecap bone, the patella, is a familiar sesamoid bone. It forms within the patellar tendon which links the quadriceps muscle of the upper leg to the bones of the lower leg. Other familiar sesamoid bones are found in the ends of chicken drumsticks.

Sesamoid bones form where tendons cross joints. Why?

Sesamoid bones do two things: 1) they act like pulleys to increase mechanical advantage 2) they strengthen connective tissues by reducing the ability of tears to propagate across tendons. Tendons will tear most easily where they are bent (put under shear stresses) at a joint. Having a bone form within a tendon near the place it is often flexed makes it into a composite material that is better able to resist tearing. Tendons are largely made up lots of tiny strings of a protein known as collagen. These strings are linked together and are quite strong when pulled on (put under tension), but are much weaker when bent sideways (put under shear stress). Tendons are thus good at resisting tension, adding a bone to a tendon will help prevent it from tearing.