With myeloma, the cancerous plasma cells (called myeloma cells) make chemicals called osteoclast activating factors (OAFs). These OAFs tell the osteoclasts to break down bone faster than usual, so old bone is broken down faster than new bone is made.
This causes bone lesions, and they can make your bones weak and break more easily.
About 80% of people with myeloma will have a broken bone caused by myeloma. This is called a pathologic fracture. About 1 out of every 3 people with myeloma learn they have the disease when a bone breaks.