Image by x1987x via Flickr

by Benny Arce

The Basics of Bones

Bones are often referred to as the framework of the human body. Without them, we could neither sit upright nor stand. Bones are made up of a hard outer layer, called the cortical or compact bone, which is filled with a porous network of rod- and plate-like elements variously called the trabecular, cancellous, or spongy bone.

Like other cells in the human body, bone cells are continually in a state of flux. There is a continuous uptake of old bone (resorption) followed by the deposit of new one. Cells that resorb old bone are called osteoclasts, while those that lay down new bone are called osteoblasts. When osteoclasts outrun osteoblasts, the density of bones decreases.

Bone density, or bone mineral density (BMD), refers to the amount of matter, such as calcium, salts, alkaline, and phosphate, per square centimeter of bone. This directly correlates with bone strength and load-bearing capacity, thus by measuring it, the risk of bone fractures can be predicted.

Measuring Bone Mineral Density

Technology has provided quick, non-invasive methods of BMD scanning, also called bone densitometry.

The most common is the Dual Energy X-ray Absorptiometry or DEXA which uses low levels of radiation exposure. As the name implies, DEXA uses two x-ray beams, one is low energy while the other is high. The x-rays that pass though the bone are different for the two beams. These two are compared and based on this, BMD is calculated. Often, DEXA machines are used to measure the density of the hip and spine.

There are also small DEXA peripherals which are used to measure the density of the heel, shin bone, and kneecap. They use the same method as the bigger DEXA machine.

Quantitative Computed Tomography (QCT) is also used. This is a standard CT scanner but it has a higher radiation level, not to mention costs, than DEXA machines.

Ultrasound is a new method used for measuring BMD. An advantage of this method is that it does not use radiation. Ultrasound beam is aimed at a bone, and their scattering and absorption is analyzed. However, their results are not as precise as the DEXA machines.

BMD measurement results are presented as a T-score or Z-score. T-score compares the BMD of a patient to a healthy 30-year old of the same gender and race, and is used for post-menopausal women and men aged 50 and over. Z-score is a comparison of the average BMD of persons of the same age, gender, and race. It is used in pre-menopausal women, men below 50, and children.

A T-score of -1.0 or higher means normal BMD, less that -1.0 to -2.5 indicates osteopenia, while lower than -2.5 points to osteoporosis.

Who Should be Tested

All women beyond 65 years old should undergo BMD scanning. Those below 65 who are post-menopausal and have risk factors for osteoporosis should also undergo the test.

The risk factors include the following: advanced age, small bone structure, a history of fractures both personally and within the family, late start of menstrual cycles, early menopause.

Certain medical conditions and drugs also predispose a person to osteopenia or osteoporosis. Hyperparathyroidism, hyperthyroidism, a deficiency in estrogen levels and Vitamin D are known causes of low BMD. The immunosuppressant prednisone and antiepileptic phenytoin also contribute to bone loss.

In addition to all these, cigarette smoking, excessive caffeine and alcohol intake, and lack of physical activity aggravate the condition.

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