An accessory bone or supernumerary bone is a bone that is not normally present in the body, but can be found as a variant in a significant number of people. It poses a risk of being misdiagnosed as bone fractures on radiography.

Wrist and hand

Os ulnostyloideum

The os ulnostyloideum is an ulnar styloid process that is not fused to the rest of the ulna bone. On X-rays, an os ulnostyloideum is sometimes mistaken for an avulsion fracture of the styloid process. However, the distinction between these is extremely difficult. It is alleged that the os ulnostyloideum has a close relationship with or is synonymous with the os triquetrum secundarium.

Os centrale

The os carpi centrale (also briefly os centrale) is, where present, located on the dorsal side of the wrist between the scaphoid, the trapezoid and capitate, radially to the deep fossa of the capitate. The bone is present in almost every human embryo of 17–49 mm length, but then usually fuses with the ulnar side of the scaphoid. Sometimes it fuses with the capitate or the trapezoid. The literature also refers to an os centrale at the palm of the carpus, but this existence is questioned. In most primates, including orangutans and gibbons, the os centrale is an independent bone that is attached to the scaphoid by strong ligaments. Conversely, in African apes and humans, the os centrale normally fuses to the scaphoid early in development. In chimpanzees, the bone fuses with the scaphoid first after birth, while in gibbons and orangutans this occurs first at older age. A good number of scholars have construed the scaphoid-centrale fusion as a functional adaptation to knuckle-walking, since a fused morphology would better cope with the increased shear stress on this joint during this kind of quadrupedal locomotion. The results from a simulation study have shown that fused scaphoid-centrales show lower stress values as compared to non fused morphologies, thus supporting a biomechanical explanation for the scaphoid-centrale fusion as a functional adaptation for knuckle-walking.

Ankle

Accessory bones at the ankle mainly include: Os trigonum (further described below) may also be seen on an ankle X-ray.

Foot

Accessory navicular

An accessory navicular bone, also called os tibiale externum, occasionally develops in front of the ankle towards the inside of the foot. This bone may be present in approximately 2–21% of the general population and is usually asymptomatic. When it is symptomatic, surgery may be necessary. The Geist classification divides the accessory navicular bones into three types.

Os trigonum

The os trigonum or accessory talus represents a failure of fusion of the lateral tubercle of the posterior process of the talus bone. Is estimated to be present in 7–25% of adults. It can be mistaken for an avulsion fracture of lateral tubercle of talus (Shepherd fracture) or a fracture of the Stieda process. In most cases, Os Trigonum will go unnoticed, but with some ankle injuries it can get trapped between the heel and ankle bones which irritates the surrounding structures, leading to Os Trigonum Syndrome.

Less common accessory bones

[Image also including the locations of rare accessory bones of the foot:

1=Os cuneometatarsale I plantare, 2=os uncinatum, 3=os sesamoideum tibialis posterior, 4=os sesamoideum peroneum, 5=os cuboideum secundarium, 6=os trochleare calcanei, 7=os in sinus tarsi, 8=os sustentaculum tali, 9=os talocalcaneale posterius, 10=os aponeurosis plantaris, 11=os subcalcaneum, 12=os sesamoideum tibialis anterior, 13=os cuneometatarsale I tibiale, 14=os intermetatarsale I, 15=os cuneometatarsale II dorsale, 16=os paracuneiforme, 17=os cuneonaviculare, 18=os intercuneiforme, 19=os intermetatarsale IV, 20=/os talonaviculare, 21=os vesalianum pedis, 22=os tibiale externum, 23=os talotibiale dorsale, 24=os supratalare, 25=os calcaneus secundarius, 26=os subtibiale, 27=os subfibulare, 28=os retinaculi, 29=os calcaneus accessorius, 30=os trigonum, 31=os supracalcaneum, 32=os tendinis calcanei | upload.wikimedia.org/wikipedia/commons/8/8d/Tarsalia///accessoria.png]

Other locations

Neck

Shoulder

Vertebral column

Knee