<P> Heterotopic ossification is a process resulting in the formation of bone tissue that is often atypical, at an extraskeletal location . Calcification is often confused with ossification . Calcification is synonymous with the formation of calcium - based salts and crystals within cells and tissue . It is a process that occurs during ossification, but not vice versa . </P> <P> The exact mechanisms by which bone development is triggered remains unclear, but it involves growth factors and cytokines in some way . </P> <Table> <Tr> <Th> Time period </Th> <Th> Bones affected </Th> </Tr> <Tr> <Td> Third month of fetal development </Td> <Td> Ossification in long bones beginning </Td> </Tr> <Tr> <Td> Fourth month </Td> <Td> Most primary ossification centers have appeared in the diaphyses of bone . </Td> </Tr> <Tr> <Td> Birth to 5 years </Td> <Td> Secondary ossification centers appear in the epiphyses </Td> </Tr> <Tr> <Td> 5 years to 12 years in females, 5 to 14 years in males </Td> <Td> Ossification is spreading rapidly from the ossification centers and various bones are becoming ossified </Td> </Tr> <Tr> <Td> 17 to 20 years </Td> <Td> Bone of upper limbs and scapulae becoming completely ossified </Td> </Tr> <Tr> <Td> 18 to 23 years </Td> <Td> Bone of the lower limbs and os coxae become completely ossified </Td> </Tr> <Tr> <Td> 23 to 25 years </Td> <Td> Bone of the sternum, clavicles, and vertebrae become completely ossified </Td> </Tr> <Tr> <Td> By 25 years </Td> <Td> Nearly all bones are completely ossified </Td> </Tr> </Table> <Tr> <Th> Time period </Th> <Th> Bones affected </Th> </Tr>

When do the bones of the sternum become completely ossified