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High-Res CT Scan May Better Predict Fracture Risk


 

TORONTO — For the first time, the microarchitecture of cortical and trabecular bone can be visualized noninvasively using a high-resolution CT scanner, Dr. Pierre D. Delmas reported at a world congress on osteoporosis.

Measurement of bone density by dual-energy x-ray absorptiometry (DXA) captures only part of the fracture risk in postmenopausal women, Dr. Delmas said. Treatment decisions based on DXA alone can miss a large proportion of women who may have fractures later.

This development may ultimately permit a fuller understanding of the mechanisms involved in osteoporosis and provide more accurate fracture risk assessment and disease monitoring capability, he said.

The device, a 3-D peripheral quantitative computed tomography (pQCT) scanner (XtremeCT, Scanco Medical AG, Bassersdorf, Switzerland), has greater resolution than conventional CT scanners, and permits visualization of trabecular number, thickness, and separation, as well as cortical thickness, said Dr. Delmas, professor of medicine, Claude Bernard University in Lyon, France.

As an example of the capability of this device, he described a case-control study that included 57 women who had experienced fragility fractures during 13 years of follow-up and were evaluated with pQCT. “We found that for every standard deviation decrease in total and trabecular density, and cortical and trabecular thickness, there was a significant increase in risk of fracture,” he said. Moreover, the increases remained significant after adjusting for bone mineral density (BMD) as measured by DXA, suggesting that the two approaches are independent measurements of bone strength, said Dr. Delmas, who is president of the International Osteoporosis Foundation, the sponsor of the meeting. A further capability of the high-resolution pQCT device is that, unlike DXA, it can reveal localized abnormalities in bone structure, he said.

During a press briefing at the meeting Dr. Delmas was asked who might be an appropriate candidate for this type of evaluation. “We need to show in prospective studies that we can do better than with DXA. If we do so you could argue that any postmenopausal woman who is at risk of fracture should have the test, but it's too early to say,” he replied.

He also commented that he and his colleagues plan to conduct studies using pQCT to measure the efficacy of therapy, and to gain insight into how antiosteoporosis drug therapy works. “The paradox is that we have a variety of drugs that are useful in decreasing the risk of fracture, but we're not always sure what is really causing the improved bone strength,” he said. Changes in bone among premenopausal women, important for the overall understanding of skeletal fragility, also are being investigated.

In another presentation at the meeting, Dr. Stephanie Boutroy described a cross-sectional study that included 235 women aged 19–50 years who underwent a scan at the distal radius and tibia with the high-resolution device.

Among the age-related changes seen were small increases in the radius and tibia cross-sectional areas without a correlating change in cortical thickness. “This reflects periosteal apposition and endosteal resorption of similar magnitude,” explained Dr. Boutroy, also of Claude Bernard University. No correlation was seen in density and architecture parameters with age at the radius, but at the tibia there was an age-related 17% decrease in trabecular density, she said. This was explained by a decrease in trabecular number, separation, and distribution.

Age-related changes in bone architecture had been noted previously in another study by Dr. Boutroy and her colleagues that included 108 premenopausal and 148 postmenopausal women. Lumbar spine and femoral neck BMD were measured using DXA; volumetric BMD and microarchitecture were measured at the distal radius and tibia using high-resolution pQCT.

By using the high-resolution device, they were able to detect significant changes in trabecular density, number, and thickness, as well as in cortical thickness. They also found that, unlike with DXA, they were able to discriminate between women with osteopenia who had had a previous fracture and those who had not (J. Clin. Endocrinol. Metab. 2005;90:6508–15).

The cost of the device is currently high, noted Dr. Delmas, at approximately $300,000, and there are only about 15 in use around the world. “But I expect that if our technique is validated by other studies and the device is produced on a larger scale the price will come down,” he said, adding that he has no financial interest in the company.

Shown are a 3-D peripheral quantitative CT scan of a tibia in a healthy woman (left) and a 3-D pQCT scan of a tibia in an osteoporotic woman (right). Photos courtesy Dr. Stephanie Boutroy

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