Yes. The knee-cap or patella (a sesamoid bone which protects and stabilizes the anterior surface of the knee joint) is one of the few bones in the body that is completely contained within a tendon. Ossification of the patella occurs between the ages of 2 and 6 years. It is formed through the process of endochondral ossification, in which cartilage is replaced with bone, rather than intramembranous ossification, in which bone is formed as a fetus develops.
The patella generally develops from one ossification center, but in some cases a separate second center of bone formation is present. If these two centers of bone formation don't eventually fuse, a situation arises where there is an accessory bone from the second center of ossification that is attached to the patella by fibrous or cartilage tissue. This is called a bipartate patella.