MedWire News: Men with idiopathic osteoporosis have significantly reduced bone matrix mineralization, relative to normal mineralized bone, without evidence of increased bone turnover, Austrian researchers report.
"This is in contrast to postmenopausal osteoporosis, where an increase in bone turnover is responsible for the reduction in mineral content," remark Nadja Fratzl-Zelman (Ludwig Boltzmann Institute of Osteology, Vienna) and colleagues.
"It is widely accepted that bone fragility depends not only on bone mass but also on the material properties of the bone tissue," say the researchers who conducted a retrospective study looking at bone material quality in patients with male idiopathic osteoporosis (mIOP).
They used quantitative backscattered electron imaging to measure bone mineralization density distribution (BMDD) - which describes the mineral content of the bone matrix - in bone biopsies from 25 men aged 18-61 years, who sustained fragility fractures, but were otherwise healthy.
The diagnosis of mIOP was based on the presence of fragility fractures and not on decreased bone mineral density (BMD), but all men had a BMD T-score below -1.0, indicating osteopenia or worse.
Compared with previously established adult reference data, the BMDD of cancellous bone from mIOP patients showed a significant shift toward lower mineralization densities.
More specifically, CaMean (weighted mean of calcium [Ca] content), CaPeak (mode of the BMDD), and CaHigh (portion of fully mineralized bone) were a significant 5.6%, 5.9%, and 76.8% lower, respectively, than the reference values.
In addition, CaWidth (heterogeneity in mineralization) and CaLow (portion of low mineralized bone) were a significant 18.5% and 68.8% higher, respectively, than the reference data.
Histomorphometric analysis showed that the men had fewer osteoblasts and osteoclasts than would be expected, indicating a decrease in bone formation and resorption.
"The shift toward lower mineral content in the bone matrix in combination with reduced indices of bone formation and bone resorption suggests an inherent mineralization defect leading to undermineralized bone matrix, which might contribute to the susceptibility to fragility fractures of the patients," Fratzl-Zelman and co-authors suggest.
"The alteration in bone material might be related to osteoblastic dysfunction and seems fundamentally different from that in high bone turnover osteoporosis with a negative bone balance," they conclude in the journal Calcified Tissue International.
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