Enlargement of joints in osteoarthrosis
Osteoarthritis results from the gradual degradation of articular cartilage. When the cartilage is damaged extensively, e.g., all the way through the cartilage and into the bone, an inflammatory response ensues in the bone. This often happens when bone is rubbing against bone.

One of bone's responses to injury and inflammation is the production of new bone. In this case, the bone, called osteophytes or bone spurs, is produced pathologically (i.e., not like the bone laid down normally during growth). Heberden's nodes are the names given to these osteophytes at the distal interphalangeal joints, but the process is basically the same at other joints in the digits, ankle, etc.