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13-10-2011 | Surgery | Article

Distribution of bone mass differs in osteoporosis and osteoarthritis

Abstract

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MedWire News: Osteoporotic bone and osteoarthritic bone are, structurally, fundamentally different, shows research published in the journal Osteoporosis International.

The findings take researchers closer towards understanding the structural reasons for fracture protection in arthritis.

The study's authors, led by Alessandro Rubinacci (San Raffaele Scientific Institute, Italy), used peripheral quantitative computed tomography (pQCT) at the femoral neck to investigate why fracture risk is reduced at arthritic joints independent of bone mass density.

The team analyzed femoral neck specimens from 32 postmenopausal women using pQCT and characterized the trabecular and cortical bone distribution. Nineteen of the women had received hip prostheses for osteoarthritis, and thirteen for femoral fracture. The ratios of the maximum to minimum diameters of the femoral neck cross-sections were 1.022:1.722 and 1.190:1.768 in the osteoarthritis and fracture groups, respectively.

The two groups differed significantly in cortical bone volume and trabecular thickness, with both factors higher in the osteoarthritis group than the fracture group. This is in line with previous research that suggests femoral fracture in postmenopausal women can be characterized by a loss of cortical bone, rather than trabecular bone.

The distribution of bone loss in the cross-sections suggests it follows the pattern of biomechanical demand, Rubinacci and team explain. In the osteoporotic patients, bone mass was preserved in areas parallel to the primary load axis, which meant the other areas of the cross-section were reabsorbed. The section at the base of the femoral neck, or calcar, which is subjected to the most compressive stress during walking, was best maintained.

However, a fall places stress on quadrants other than the calcar, which helps explain why osteoporotic bone is susceptible to fracture. The researchers found that the bone in these quadrants vulnerable during a fall was preserved in the osteoarthritic but not the osteoporotic samples - therefore providing a possible reason for the reduced fracture risk often associated with osteoarthritic joints.

"Although its causal relationship with the osteoarthritic process cannot be established, this shift of bone mass leading to an enhanced thickness and mass of the cortical bone might be the critical effector of fracture risk protection in primary osteoarthritis," say the researchers.

This study, in contrast with previous studies, showed that both groups of specimens had similar volumetric densities. The researchers attribute this to the pQCT method used, whereas most other studies have used dual-energy X-ray absorptiometry. This adds to growing evidence that postmenopausal women with osteoarthritis are not protected from osteoporosis, the researchers say.

"As a consequence, osteoporosis should not be overlooked in women, regardless of whether they were diagnosed with osteoarthritis or not," Rubinacci and colleagues advise.

By Chloe McIvor

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