The forearm typically has two bones, a radius and an ulna. The benefit of having two bones is that it allows the radius to rotate around the ulna. This means we can position the hand palm up and down. This is functionally helpful but not functionally mandatory as there are many ways to make up for limited or absent forearm rotation.
One of the most common conditions that affects forearm rotation is a joining of the two bones- a radioulnar synostosis. Developmentally, the bones just don’t ever completely separate. So the condition is present at birth but may not be picked up on until later. If the forearm bones are stuck with the palm all the way down or up and if both right and left sides are both involved, it is more likely to be obviously at a young age. However, if only one side is affected and if the forearm is fused in neutral (i.e., hand clapping position), the condition may not be diagnosed until 7-9 years of age or even later.
Here is a great kid explaining his radioulnar synostosis. It affects only his left side. There is NO pain. His function is great but not perfect as you can hear him describe. We are going to watch him for now. If problems develop over time, there may be treatment but, as I explained to this family, we are unable to restore motion of the forearm. He will no limitations with activities.