Some kids are born with a bony connection between the radius and ulna. This bony "bridge" can also develop after a trauma in an adult. When that happens, the bones are no longer separate and the ability to rotate the forearm is not present. Because the shoulder is so mobile, we can make up for some loss of forearm rotation with shoulder movement. It is pretty easy to pull arm away from body- abduct- and the hand assumes a palm down position. It is less easy to move the shoulder in such a way to allow the palm up position.
The bottom line is that kids adapt amazingly well to radioulnar synostosis. Often, families do not even realize this condition is present until after kids start school. There is no pain and, as noted above, some rotation can be achieved through the wrist. When both sides are affected, it may be a little more challenging to adapt. Additionally, it matters in which position the forearm is fused. The forearm can be "stuck" in full palm up (supination), full palm down (pronation), or anywhere in between. The best position is half- way between (the clapping position) which allows the patient to use the shoulder to help accommodate in both directions. Palm down is better than palm up as so many of life's activities are palm down (keyboarding, etc).
In a small number of kids, the forearm position causes trouble with activities. In those, a small surgery can be done to reposition the forearm in a better position for function. The pictures below show a patient positioned with both forearm in mid rotation. He has no issues with function despite his limited rotation.
|Synostosis with full elbow extension|
|Synostosis with full elbow flexion|
|Synostosis with attempted supination- palm up|
|Synostosis with attempted pronation- palm down|
|Xray showing synostosis near elbow. The two bones are joined together.|