Osteoporosis, or porous bone, is a disease in which the bones lose density, become weak and brittle, and are more likely to break. In osteoporosis, the normal process of creating new cells and reabsorbing old ones in the bones becomes imbalanced, leading to a gradual thinning of bone tissue. Osteopenia is an intermediate condition in which bone has lost density and strength but has not reached the more fragile state of osteoporosis. You can see this gradual change in Figure 3.