Diagnosis of osteoporosis from dental x-rays

Gwynne D. Koch

Osteoporosis — a progressive disease characterized by low bone mass and increased susceptibility to fractures — affects an estimated 10 million Americans. About 50 percent of women and 25 percent of men over age 50 will have an osteoporosis-related fracture in their lifetime.

Dual-energy x-ray absorptiometry, a technique that scans bones using two beams with different energy levels, is a very accurate method for measuring bone mineral density. However, equipment can be scarce, and the technique is too expensive to be used in routine screening and diagnosis of osteoporosis.

Panoramic radiography is widely used by dentists in routine clinical work. Measuring the width of the edge of the lower jaw from dental x-rays could be a more cost-effective and efficient method for identifying patients at risk of osteoporosis. The width of the mandibular cortex is significantly correlated with bone density at the hip, lumbar spine and forearm, the most common sites of fractures related to osteoporosis. Widths of less than 3 mm indicate that a patient should be referred for further testing.

Researchers at the Universities of Manchester in the UK, Athens in Greece, Leuven in Belgium and Malmö in Sweden and at the Academic Center for Dentistry in Amsterdam, the Netherlands, have developed image analysis software that automatically detects and measures the width of the mandibular cortex on dental x-rays. The software, which is based on active shape modeling, combines the measurements with clinical information to produce an index of osteoporotic risk.

The team constructed a point distribution model by manually marking up the mandibular cortex shape on an independent set of x-rays of patients known to have osteoporosis. The model captures the modes of variation of the bone, searches out the typical shape of the cortex and obtains a measurement. This process can be automatic, but the software is helped by the user’s marking four points along the lower jaw edge.

Dental panoramic radiographs were obtained for 652 women with a mean age of 55 years. Women were selected for the study because they are about four times more likely to develop osteoporosis than are men. Images were captured at 70 kV at 8 mA for 15 s and then digitized at a resolution of 25.64 pixels/mm.

The team compensated for magnification errors, which typically range from 20 to 36 percent for most radiography machines, by having patients bite on a plastic block containing a spherical metal ball bearing measuring 3.175 mm in diameter. The known dimensions of the ball bearing were used to scale the bone width measurements on the digital images. Dual-energy x-ray absorptiometry scans were performed to verify the diagnostic test and to determine those individuals who had osteoporosis.

When a width threshold of less than 3 mm was applied to the search data, 119 out of 652 patients “failed” the radiographic test and would have been referred for further testing. Of those 119 patients, 72 were found to have osteoporosis at one site as measured with dual-energy x-ray absorptiometry. The probability that the referred patients had osteoporosis, therefore, was 60.5 percent.

Image analysis software that automatically detects, measures and analyzes the width of the edge of the lower jaw may help dentists identify patients at risk for osteoporosis. The dental panoramic radiographs illustrate the differences in width of the jaw bone of a healthy patient (left) and of a person with osteoporosis (right).

The scientists found that the diagnostic ability of the measurements obtained with the automated image analysis software was comparable to that of two well-established clinical risk indices: the National Osteoporosis Foundation index and the Osteoporosis Risk Assessment Index. Achieving maximum diagnostic accuracy with the technique, however, requires some interaction of the dentist in manually placing four search initialization points on the images, as well as some training in interpreting the computer-generated data.

The researchers concluded that their image analysis software can be used to diagnose osteoporosis with good accuracy and repeatability. Because the software enables additional information to be collected from x-rays taken for dental examination purposes, an osteoporosis assessment can be made without requiring additional radiation exposure for the patient. Computer software-assisted diagnosis will enable dentists to identify and refer patients at high risk for osteoporosis before they develop fractures.

Bone, April 2007, pp. 835-842.