epiphysis and diaphysis to form a synostosis.

The attachment of the first rib to the sternum becomes a synostosis with age, also the skull of a baby

this is called a synostosis

It derives from the Greek word for 'bone" (ostoun) + syn=with/together, so I presume it must be a medical term for bones that get stuck or bonded together?

Donald Russell Crawford has written:

'Synostosis in the spinal column of the rainbow trout' -- subject(s): Spine, Fishes, Anatomy, Trout