Bone density (or bone mineral density) is a medical term referring to the amount of mineral matter per square centimeter of bones. Bone density is used to determine ones fracture risk by determining if someone has osteoporosis or osteopenia. This medical bone density is not the true physical “density” of the bone, which would be computed as mass per volume. It is measured by a procedure called densitometry. The measurement is painless and non-invasive and involves low radiation exposure. Measurements are most commonly made over the lumbar spine and over the upper part of the hip, unless one has had a joint replacement. Fractures of the legs and pelvis due to falls are a significant public health problem, especially in elderly women, leading to much medical cost, inability to live independently, and even risk of death. Bone density measurements are used to screen women and men for osteoporosis risk and to identify those who might benefit from measures to improve bone strength.