I believe you are referring to a "fusion" or "fused joint"
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The answer given in my book is Synarthroses.
Synarthrosis
synarthrosis
By definition a joint (articulation) is the site where two bones come together. There are many classifications depending on the amount of movement that occurs between the bones and the age of the person. Some are freely moveable (your shoulder) and some are immovable (bones in the adult cranium).
suture
suture
The joints between bones of the skull are immovable and called sutures.
Sutures are immovable, wavy joints between the bones of skull.
flexion decreases in angle between articulating bones extension increases in angle between articulation bones
The immovable joints in the human body are the cranial bones. They are separate bones that are sutured together as a person grows. They do not move, and therefore are immovable joints.
The bones in the skull and the bones in the sacrum come to mind . . .
An arthrosis is any articulation or joint between bones, or any disease of such a joint, such as osteoarthritis.
suturesutureThe part of the skull that is a example of an immovable fibrous joint would be a suture.
The bones of the skull when they fuse in an adult form an immovable joint.
The bones of the cranium or the brain box are immovable. They are 8 in number.