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 subtibiale, with a prevalence of approximately 1%. It is a secondary ossification center of the distal tibia that appears during the first year of life, and which in most people fuses with the shaft at approximately 15 years in females and approximately 17 years in males. Os subfibulare, with a prevalence of approximately 0.2%.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. Type 1: An os tibiale externum is a 2–3 mm sesamoid bone in the distal posterior tibialis tendon. Usually asymptomatic.
In most cases, the first three fuse at 15–18 years, whereas the base part fuses to the scapular spine at 12 years. Such failure to fuse occurs in between 1% and 15% of cases. It rarely causes pain. Vertebral column
An Oppenheimer ossicle is found in approximately 4% (range 1–7%) of individuals. It is associated with the facet joints of lumbar spines. It usually occurs as a single, unilateral ossicle at the inferior articular processes, but can also occur at the superior articular processes. Knee
The fabella is present in 10% to 30% of individuals. See also
Accessory muscle


== References ==