Accessory bones, i.e. additional bones that occur in only a few people, are particularly common on the hand and foot. Most of these bones are bones that have been lost in the course of evolution and are considered obsolete, i.e. they are no longer needed by humans. One speaks of so-called atavisms.
The accessory bones do not always have to be congenital, they can also develop during growth or bone maturation. Accessory bones are often confused with fracture fragments, which is why it is important for radiologists and orthopaedic surgeons to know these variants in order to avoid drawing the wrong conclusions. In most cases, the discovery of an accessory bone is a chance diagnosis, because it usually does not cause any symptoms. Accessory bones are to be distinguished from the so-called sesamoid bones, which sometimes only occur in some people. But unlike accessory bones, sesamoid bones have a function in which they are embedded within a tendon and protect it, as well as avoiding unnecessary friction.
Besides the hand, the foot is the most common place where accessory bones are found. They are found in a significant percentage of the population. For example, the os tibiale externum is found in one-fifth of all people.
In addition, the Os trigonum is relatively common with a proportion of 3-15%. In about 50% of the cases these bones are found on both sides. Some people also have several accessory bones in the tarsal region.
The accessory bones usually do not cause any symptoms. However, if the bones protrude outwards, this can lead to pressure points, chafing of the shoe or similar. The Os tibiale externum is a variation of the scaphoid of the foot (Os naviculare).
In some cases, the os tibiale externum can cause pressure or stress pain, especially in people who also have a flat foot. In these cases a special shoe inlay is made to reduce the symptoms. In most cases, however, the Os tibiale externum is asymptomatic.
Hand bones are very often found on the hand. Examples are the Os styloideum, Os vesalianum, Os hypolunatum, Os triangulare, Os epilunatum, Os radiale externum, Os centrale. The additional bones of the hand are actually always asymptomatic.
They often raise questions when diagnosing a fracture. An important distinguishing feature between fracture and accessory bone is the shape. Accessory bones of the hand are always round, while fracture fragments have irregular contours and sclerosis.
In the area of the elbow, the formation of accessory bones is very rare. The most common accessory bone found in the elbow is the so-called Sesamum cubiti. This accessory bone is often confused with a fracture of the elbow. As a rule, the additional bone in the joint is not accompanied by restricted mobility.