Bone is an active living tissue that is constantly being renewed through a process known as remodelling. Cells called osteoclasts break down old, damaged bone to make way for new bone laid down by cells called osteoblasts. Over time, this bone is mineralised, forming a hard and strong skeleton. Under normal circumstances the amount of bone removed is balanced by the amount of bone laid down. In Paget’s disease the osteoclasts are larger than usual and break down bone more rapidly. The osteoblasts respond to this by depositing new bone at an increased rate. This activity leads to an increase in bone size which has an irregular and weakened structure.
You can watch a video of bone remodelling and Paget's disease by following this link to an external website