Our three-year old woke up one morning last week and couldn’t straighten her elbow. After two days with a bent elbow, we took her to the pediatrician and found out she has something called congenital synostosis. We saw the X-ray and heard the explanation but can you please explain this to us again?

Congenital proximal radioulnar synostosis is a very rare condition that often goes unnoticed until an injury or trauma of some kind occurs. To explain the condition, let’s take a look at the label.

Congenital means the child was born with this problem. As we said, it isn’t always obvious at first. Sleeping too long with the elbow fully flexed or some type of injury or trauma are the most common reasons the problem gets diagnosed. The child shows up at the orthopedic surgeon’s office with an elbow stuck in flexion and/or with sudden, severe pain. There can be swelling around the joint but often it looks perfectly normal (except it’s bent).

Radioulnar refers to the two bones in your forearm: the radius and the ulna. Proximal tells us the elbow is involved — specifically where the ulna and the radius meet at the elbow. These two bones also meet at the wrist. Synostosis just means there’s an abnormality in the joint.

With congenital proximal radioulnar synostosis, there can be a wide range of deformities contributing to the problem. For example, he joint may be dislocated. In other words, the bones aren’t in the right place to allow for normal joint motion. In some cases, the top of the radial bone (called the radial head) doesn’t form properly.

Instead of being a firm, round bone that moves smoothly to allow elbow motion, it’s made up of a stiff fibrous piece of cartilage. Sometimes the shape of the radial head is all wrong — instead of looking like a miniature drum, it looks like a fat mushroom. An overgrown radial head pushes the annular ligament up and over itself resulting in a rubberband-like effect preventing full elbow extension.

Any change in the size or shape of the radial head is going to mean problems with normal elbow motion. You may recall from looking at the X-rays and listening to the surgeon’s explanation just what is causing your daughter’s synostosis.

If it’s still not clear in your mind, when you go for a follow-up visit, ask the surgeon to show you the X-ray and explain again what’s causing your daughter’s problem. With the information presented here, it may make more sense.