Bone Mineral Density is the amount of calcium and similar minerals in each square cm of bone. This is a 2 dimensional picture. The bone is actually 3 dimensional. This means that we must estimate the size and thickness of the back half of the spine and hope it is not arthritic.
Over age 70 more than half of patients have enough arthritis in their spine to invalidate the computer generated BMD readings from a DXA machine. A Certified Clinical Densitometrist can recognize this and correct the reading.
A “normal” Bone Mineral Density does not eliminate the possibility of an individual having Osteoporosis. That is because BMD is g/cm2 of mineral (mostly calcium). A Vertebral Compression Fracture in the spine squashes the vertebra so that it has a smaller area (cm2) on the DXA. Decreasing the cm2 without changing the grams of calcium, increases the calculated BMD. The computer does not recognize the crushed vertebra as crushed. A Certified Clinical Densitometrist can recognize this and correct the reading.
Bone Mineral Density does NOT measure the volume of bone matrix protein. This protein part of bone actually is more important for bone quality and resistance to fracture than is calcium alone. Researchers are still years away from being able to measure the volume of bone matrix protein easily and cheaply enough to be widely used.
Bone strength (resistance to fracture) is Bone Mineral Density plus Bone Quality (amount and microstructure of bone matrix proteins). BMD is not the whole story. It is the part of the story that we can measure the most easily. Vertebral Fracture Assessment or VFA is another part of the story.
Bone Mineral Density is about 30% of the calculation of overall fracture risk in FRAX. The Risk of Future Fracture is actually far more important in deciding on therapy than BMD alone.
Jay Ginther, MD
2008 / Revised November 2010