The development and maturation of most human bones is very predictable. For example, orthopedic surgeons and others engaged in osteology (the study of bones) know that there are six different growth centers in the elbow, each initiating ossification (turning from baby cartilage into mature bone) at different ages depending on the child’s gender. During maturation, most bones go through a transition from cartilage to bone while at the same time grow in size.
Some of the bones in our bodies appear to not always follow a predictable development, and some do not always develop in every person. One particular class of bone that is notorious for this behavior is the sesamoid bones. The term sesamoid means sesame-like, and relates to their general appearance, often small and semi-round. When found, they are always present within the substance of a tendon where the tendon crosses over a joint.
The sesamoid bone is intended to hold the tendon slightly away from the joint in order to shift the mechanics and thereby improve the action of that tendon on the joint. A few of these sesamoid bones develop predictably, the most common and ubiquitous is the patella or kneecap. Another common set of sesamoid bones are found under the big toe.
Due to the unique positioning of these bones and the high stress they experience, they are at risk for developing problems often related to growing. When a child grows, it is actually their bones (thigh and leg bones, for example) that are increasing in length. The muscles and other surrounding structures have to be stretched out to accommodate this length change, and it often results in muscle tightness. This muscle tightness then increases the strain on the sesamoid bones.
When the child is somewhere between 2 and 6 years old, their cartilage patella starts forming a center of bone. Often, the kneecap will start to form bone at multiple centers within the cartilage. About 5 percent of the time, some of these bone centers do not fuse together with the main bone center. When this happens, it is called a bipartite patella, meaning a two-part kneecap. Whereas, these are often not painful; occasionally, kids can develop pain at this nonfused site on the patella. It can be preceded by an injury to the knee, but often it is only due to growth of the child.
The sesamoid bones of the big toe are also at risk for the development of bipartite bones and can often experience pain. When this happens in the foot (regardless of the presence of a bipartite sesamoid), it is called sesamoiditis or inflammation of the sesamoid.
No matter which sesamoid is inflamed, foot or knee, the treatment tends to be the same, at least initially. Modification of activity is the first treatment and often involves immobilization of the joint. Casting is uncommonly performed, but could be necessary. In children, if X-ray demonstrates the presence of a bipartite sesamoid, and the above mentioned treatment fails to resolve symptoms, then an MRI should be obtained to confirm the diagnosis.