Introduction

The clinical assessment of bone strength and fracture risk as well in children as in adults has always been a difficult chal­lenge for its practical solution. The reason for that seems to be matter of both – focus and target. The problem concerns not only how or how well to measure, but essentially what to mea­sure, and mostly how to interpret the data. The additional spe­cial challenge is interpretation of children’s data, what is relat­ed to the fact that bone mineral accrual throughout childhood and adolescence involves changes in bone size, geometry, and mineral content. The processes evolve at varying rates in different regions of the skeleton, with appendicular growth preceding spinal mineral acquisition. Trabecular and cortical compartments respond variably to sex steroids, calcium intake, and mechanical loading. The tempo of mineral accrual is more closely linked to pubertal and skeletal maturation than to chronological age, and these processes vary with gender and ethnicity. Additional problem is related to limited access to pe­diatric reference data.

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Infrared microspectroscopic analysis of bone tissue from ani­mal models and humans at equivalent anatomical locations gave great insight to the role of bone quality in determining bone strength. It became feasible to conclusively show differences in bone mineral maturity between normal and osteoporotic bone at equivalent anatomical locations. Even more revealing was the analysis of the spatial variation in pyr and deH-DHLNL collagen cross-links in the same bones. It was shown that the ratio between these two major collagen cross-links was very different when osteoporotic and normal bones were compared in the area of trabecular bone with ac­tively bone forming surfaces. These data are in ex­cellent agreement with recently published clinical observations that homocysteine blood serum level were elevated in patients with increased fracture risk. It is interesting to note that these differences were also observed between normal, and bone biopsies obtained from pre-menopausal women sus­taining spontaneous fractures while having normal BMD and biochemical markers, suggesting that this might be a common factor / cause of fragile bone.

The effect of therapeutic protocols on bone quality has also been investigated. During these studies, it was discovered that when fracture risk and BMD were divergent, both mineral maturity and pyr / deH- DHLNL collagen cross-link ratio was correlating with fracture risk rather than BMD, further emphasizing the contri­bution of bone quality to its mechanical performance.

Future directions

Since the introduction of the Infrared Microspectroscopic analy­sis in the early 1990′s, the debate rages whether it is a diag­nostic tool. Although it provides a plethora of useful outcomes, it is our opinion that it is not well suited to be employed as a mass-screening tool, for the simple reason that it is an invasive technique as a bone biopsy is required. On the other hand, it is ideally suited for cases of fracturing patients whose “classical” risk indicators such as BMD and biochemical markers are nor­mal.

On the other hand, it is a powerful research tool, affording unique insights into the pathophysiology of musculoskeletal diseases such as osteoporosis, osteogenesis imperfecta, Paget’s disease, osteomalacia, ostepetrosis, osteosclerosis, etc. Its outcomes complement ones obtained through analyses such as histology, histomorphometry, biochemical markers, blood analysis, and BMD measurements, to provide detailed information on the mechanisms that result in healthy and dis­eased bone.

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Molecular bonds are not stationary, but rather undergo motion such as twisting, bending, stretching, rotation and vibration. When irradiated with infrared radiation, these vibrational mo­tions absorb at specific wavelengths, characteristic of the over­all configuration of the atoms, and representative of specific functional groups. Moreover, through detailed analysis of the absorption wavelengths, information may be deduced on the subtle interactions with the surrounding atoms of a molecule. FTIR spectra provide information on all tissue components. The protein and mineral constituents produce intense, struc­ture sensitive IR modes.

IR spectroscopy has been extensively utilized in the analysis of bone mineral. Spectroscopic and mathematical analy­sis of the phosphate band by means of techniques such as de- convolution, second derivative spectroscopy, and curvefitting, spectral regions (underlying peaks) were identified and corre­lated with the various chemical environments present in biolog­ical apatites, enabling the monitoring of the calcium phosphate crystal maturity (ionic substitutions, stoichiometry).

The protein Amide I (peptide bond C=O stretch) and Amide II (mixed C-N stretch and N-H in-plane bend) modes near 1650 and 1550 wavenumbers (cm^-1), undergo frequency and inten­sity changes as a result of changes in protein secondary struc­ture. The Amide I band is especially sensitive to secondary structures. In such studies, information on protein struc­tures is extracted from broad envelopes consisting of compo­nent bands arising from the Amide I modes of various sec­ondary structures by applying a technique of resolution en­hancement such as Fourier self-deconvolution, second deriva­tive spectroscopy, and difference FTIR. Although detailed information on mineral maturity and protein secondary structure was obtainable utilizing these techniques, homogenized bone tissue and / or proteins in solution had to be used, thus it was not possible to correlate the findings with the metabolic activity of bone surfaces (tissue age).

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Bone mineral is a poorly crystalline hydroxyapatite [Ca5(PO4)3OH] phase. Ion substitutions are abundant. For example, Na+¹, and Mg+² are substituting Ca+² ions, HPO₄^-2 ions substituting the phosphate ions, Cl^-1 and F^-1 substituting OH^-1, and CO₃^-2 sub­stituting for either phosphate or hydroxyl groups. Once mineral is deposited in bone by osteoblasts, it is not a static moiety, but rather a dynamic one. Since it is bathed in aqueous biological fluids, the type and extent of these substitutions changes with time resulting in alterations of the mineral maturity, which is ac­companied by changes in mineral crystallite size and /or shape.

The contribution of mineral maturity, and crystallite size and shape to bone strength is very apparent in the case of fluoride treated bone.

In both animal models and in humans it has been reported that osteoporotic bone mineral characteristically consists of crystals which are larger and more perfect than in normal bone, smaller and less perfect, or that there are no differ­ences. Typically in these studies, tissues were homoge­nized prior to analysis, concealing the effect of spatial varia­tions in mineral properties. Recently, utilizing techniques such as Small Angle X-ray Scattering (SAXS), and quantitative backscattered electron imaging (qBEI), the analysis of bone mineral (poorly crystalline hydroxyapatite) at the microscopic level and the contribution of mineral crystallinity (crystallite size) and maturity (chemical composition) to bone strength is being actively pursued. Based on such stud­ies, models for the importance of mineral crystallite shape and size in determining bone strength have been put forth.

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Bone

Bone is a composite material, consisting mainly of mineral and collagen. In normal humans, cortical bone constitutes approxi­mately 80% of the human skeletal mass and trabecular bone approximately 20%. Bone surfaces may be undergoing forma­tion or resorption, or they may be inactive. These processes occur throughout life in both cortical and trabecular bone. Bone remodeling is a surface phenomenon and in humans occurs on periosteal, endosteal, Haversian canal, and trabecular surfaces. The rate of cortical bone remodeling, which may be as high as 50% per year in the mid-shaft of the femur during the first two years of life, eventually declines to a rate of 2%-5% per year in the elderly. Rates of remodeling in trabecular bone are proportionately higher throughout life and may normally be 5-10 times higher than cortical bone remodeling rates in the adult. As is evident, tissue age is variable within the same human.

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A substantial body of knowledge regarding the performance of QUS techniques has been gathered. To date, evidence supports the use of QUS for the assessment of fracture risk. Addi­tional clinical applications of QUS, as the assessment of rates of changes for monitoring disease progression or response to treatment, require further investigation. Moreover, QUS tech­nology has tremendous potential for further improvement and refinement. If one takes an optimistic view, it may eventually be possible to develop a truly non invasive method that will allow the investigation of relevant characteristics of skeletal status that can only be studied by invasive histomorphometric meth­ods today. QUS may also improve the evaluation of skeletal properties on a micro level and open new frontiers for more in- depth and more comprehensive investigation of bone metabo­lism, including the effect of therapeutic interventions.

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Although considerable effort has been made to characterize the relationship between QUS and BMD measurement of the same skeletal site, from a clinical point of view, the most impor­tant issue regarding QUS is its ability to predict fracture risk. There is ample evidence documenting the ability of calcaneal QUS to predict osteoporotic fracture risk both in women (19­34) and in men. It is important to emphasize that QUS parameters result independent predictors of osteoporotic frac­ture, even after adjustment for BMD. These studies reported a strong association of calcaneal QUS with vertebral fracture hip fracture and os­teoporotic fractures in general. Logistic regression analysis has shown that the fracture risk usually increases by 1.5-2.5 times for every 1 standard deviation reduction of each QUS parameters. Moreover it has been demonstrated that the fracture risk prediction increases with both the combination of QUS and DXA.

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