Bone defects are commonly caused by injury, disease, or surgical interventions. Defects up to about 1/3 inch in a large bone can heal by themselves in a healthy non-smoker, but bone defects larger than this, or even smaller in small bones never heal completely. This is schematically depicted below.

In order to facilitate a more complete and faster bone healing, many surgeons use bone replacement or bone enhancement materials to fill the defect. These are usually categorized as allografts and xenografts, autografts, or synthetic materials.