The synovium is a thin lining on the inside of the joint. There are a very large number of blood vessels in the synovium. This is one of the reasons why joint bleeding is so common in hemophilia. When there is bleeding in the joint, the synovium absorbs the blood in an attempt to remove it. The iron in the blood accumulates in the synovium.
Doctors think the iron causes the synovial lining to get thicker.
The thicker the synovium is, the more blood vessels it contains. This, in turn, makes bleeding easier. There are two types of cartilage. The most important is the one which forms a smooth hard cap on the ends of the bones which form the joint. They allow the bones to rub and glide over each other without friction.