Accessory bone

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 trigonum (further described below) may also be seen on an ankle X-ray. ==Foot== by dorsoplantar projection, with most common accessory and sesamoid bones of the foot. == Other locations ==

. Neck • Nodules in the posterior margin of the nuchal ligament form bone tissue in approximately 11% of males and 3–5% in females after the third decade of life, and may then be regarded to be sesamoid bones. Shoulder • An os acromiale forms when any of its four ossification centers fail to fuse. These four ossification centers are called (from tip to base) pre-acromion, meso-acromion, meta-acromion, and basi-acromion. 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 ==