I have heard that when you get a bone fused in your back, the doctor can take some of your own bone or he can take it from someone else. If he takes it from you, where does it come from? And how do they decide what type of graft to use?

Autologous bone grafts are grafts that come from your own bones. Allografts are those that come from someone else.

When deciding whether to use auto or allografts, the surgeon takes into account many issues, the most important being the condition of your own bones. If your bones are weaker than they should be because of a disease, such as osteoporosis, Paget's disease, or hyperparathyroidism, to name a few, then using your own bone would likely not be a good idea. The bone graft may not be strong enough to hold. In that case, an allograft is chosen.

If you are going to be using your own bone, the piece of bone is usually taken from your pelvis, near the hip area. The necessary amount of bone (not much) is shaved off and used in the spinal fusion.