I’ve always known my knee is in two pieces but it’s never bothered me. Last week, I smacked my knee into a sharp object. Now it’s very painful and swollen. Is it possible I broke it into three pieces?

You may have a condition called bipartite patella. Sometimes it’s also called patella bipartite. As you have described, the patella or kneecap is made up of two bones instead of a single bone. Normally, the two bones would fuse together during adolescence. But in patella bipartite, they remain as two separate bones.

About two to three per cent of the general population have this condition. It remains silent until direct trauma or repetitive (overuse) injury cause pain and/or swelling. The cartilage between the two pieces of bone has a limited ability to heal. There may be a limited blood supply to this area. And constant pull on the patella from the quadriceps tendon causes a traction force. These factors added together can result in limited healing.

The only way to know for sure if there has been a change is to have an X-ray taken. CT scans or bone scans are useful when a fracture is suspected but doesn’t show up on the X-rays. An MRI may be ordered to look at the condition of articular cartilage at the patellar-fragment interface. The lack of bone marrow edema helps rule out a bone fracture.