no articular facets on the sides of its base (for intermetacarpal articulations) because it articulates exclusively with the trapezium. Ossification The metacarpal bone of the thumb has two centres of ossification: a primary centre in the shaft and a secondary centre in the base. This contrasts to the other four metacarpal bones where the secondary centre is found in the head. The ossification process begins in the shaft during the ninth week of prenatal life, and in the base during the second year of life in girls and the third year in boys. When the both sexes were considered together, the ossification of base of first metacarpal was seen to start between 13 and 41 months. These centres unite before fifteenth year in girls and seventeenth year in boys. The thumb has several associated accessory bones. When present, these bones are usually found near the base and trapezium and are the products of an additional centre of ossification, usually of the trapezium, that failed to fuse with the associated bone. Named accessory bones include: os trapezium secundarium - between the ulnar base of the first metacarpal and the distal margin of the trapezium os praetrapezium - between the thumb metacarpal and the distal aspect of the trapezium os paratrapezium - between the radial base of the first metacarpal and the distoradial aspect of the trapezium Fractures Fractures to metacarpal bones account for 30-40% of all hand fractures, of which 25% occur in the first metacarpal (second to fractures to the fifth metacarpal bone). 80% of fractures to the first metacarpal occur at its base. Common fractures to the thumb metacarpal include Bennett's fracture and Rolando's fracture Additional images Notes References Skeletal system Bones of the hand Metacarpus