Near infrared spectroscopic assessment of loosely and tightly bound cortical bone water

Evaluation method for proteoglycans using near-infrared spectroscopy

Cartilage is a connective tissue composed of mainly water, collagen (COL) and proteoglycans (PGs) including chondroitin sulfate (CS). Near-infrared (NIR) spectroscopy is adequate for examination of soft and hard tissues with large amount of water non-destructively and non-invasively. We measured tablets containing CS and COL using NIR spectroscopy to develop an evaluation method for PGs in cartilage non-destructively and non-invasively. Calibration curves were constructed using the NIR spectra of the tablets that show the quantitative linear relationship between the concentration and height of the second derivative at 4260 cm-1 for COL and at 5800 cm-1 for COL and CS. An equation to calculate the CS-to-COL ratio was derived from the calibrated slopes at 5800 and 4260 cm-1, and the utility of the equation was demonstrated by the evaluation of tablets. Moreover, we conducted an evaluation of the CS-to-COL ratio in the aqueous nucleus pulposus and annulus fibrosus, and the results were consistent with the glycosaminoglycans (GAGs)-to-COL ratios obtained through Raman spectroscopy of the same specimens. Thus, this method is adequate for evaluating PGs with large amount of water non-destructively, non-invasively and with less damage.

The Multifactorial Relationship Between Bone Tissue Water and Stiffness at the Proximal Femur

Bone mechanical function is determined by multiple factors, some of which are still being elucidated. Here, we present a multivariate analysis of the role of bone tissue composition in the proximal femur stiffness of cadaver bones (n = 12, age 44-93). Stiffness was assessed by testing under loading conditions simulating a sideways fall onto the hip. Compositional properties of cortical and trabecular tissues were quantified in femoral neck cross sections by Fourier transform infrared (FTIR) spectroscopy and near infrared (NIR) spectroscopy. In addition, cross-sectional areas and cortical thickness and tissue mineral density (TMD) were measured at the femoral neck. Pearson correlation analysis showed a significant (p < 0.05) negative relationship between bone stiffness and cortical and trabecular water content, both total (r = -0.63) and tightly bound to matrix and mineral (r = -55). Additionally, significant (p < 0.05) positive correlations were found between stiffness and bone area, both total (r = 0.67) and trabecular (r = 0.58). However, linear regression using each of these properties to predict bone stiffness resulted in weak models (R2 = 0.36-0.48). Interestingly, we found markedly stronger models (cross-validated R2 = 0.80-0.92) by using partial least squares (PLS) regression to predict stiffness based on combinations of bone properties. The models with highest R2 values were found when including bone water parameters as explanatory variables, both total and tightly bound, in cortical and trabecular. This study provides new insights by revealing a multifactorial relationship in which higher bone water content across different tissue compartments contributes to lower bone stiffness, highlighting bone water as a potential biomarker of bone quality and proximal femur mechanical function.

Estrogen deficiency induces changes in bone matrix bound water that do not closely correspond with bone turnover

Postmenopausal osteoporosis, marked by estrogen deficiency, is a major contributor to osteoporotic fractures, yet early prediction of fractures in this population remains challenging. Our goal was to explore the temporal changes in bone-specific inflammation, oxidative stress, bone turnover, and bone-matrix water, and their relationship with estrogen deficiency-induced modifications in bone structure and mechanical properties. Additionally, we sought to determine if emerging clinically translatable imaging techniques could capture early bone modifications prior to standard clinical imaging. Two-month-old female Sprague Dawley rats (n = 48) underwent ovariectomy (OVX, n = 24) or sham operations (n = 24). A subgroup of n = 8 rats per group was sacrificed at 2-, 5-, and 10-weeks post-surgery to assess the temporal relationships of inflammation, oxidative stress, bone turnover, bone matrix water, mechanics, and imaging outcomes. OVX rats exhibited higher body weight compared to sham rats at all time points. By 5-weeks, OVX animals showed elevated markers of inflammation and oxidative stress in cortical bone, which persisted throughout the study, while cortical bone formation rate did not differ from sham until 10-weeks. DXA outcomes did not reveal differences between OVX and sham at any time point. Bound water, assessed using ultrashort echo time magnetic resonance imaging (UTE MRI), was lower in OVX at the earliest time point (2-weeks) and reduced again at 10-weeks with no difference at 5-weeks. These data demonstrate that bound water assessment using novel UTE MRI technology was lower at the earliest time point following OVX. However, no temporal relationship with bone turnover, inflammation, or oxidative stress was observed at the time points assessed in this study. These findings underscore both the increased need to understand bone hydration changes and highlight the usefulness of UTE MRI for non-invasive bone hydration measurements.

Application of Optical Photothermal Infrared (O-PTIR) Spectroscopy for Assessment of Bone Composition at the Submicron Scale

The molecular basis of bone structure and strength is mineralized collagen fibrils at the submicron scale (∼500 nm). Recent advances in optical photothermal infrared (O-PTIR) spectroscopy allow the investigation of bone composition with unprecedented submicron spatial resolution, which may provide new insights into factors contributing to underlying bone function. Here, we investigated (i) whether O-PTIR-derived spectral parameters correlated to standard attenuated total reflection (ATR) Fourier transform infrared spectroscopy spectral data and (ii) whether O-PTIR-derived spectral parameters, including heterogeneity of tissue, contribute to the prediction of proximal femoral bone stiffness. Analysis of serially demineralized bone powders showed a significant correlation (r = 0.96) between mineral content quantified using ATR and O-PTIR spectroscopy, indicating the validity of this technique in assessing bone mineralization. Using femoral neck sections, the principal component analysis showed that differences between O-PTIR and ATR spectra were primarily attributable to the phosphate ion (PO4) absorbance band, which was typically shifter toward higher wavenumbers in O-PTIR spectra. Additionally, significant correlations were found between hydrogen phosphate (HPO4) content (r = 0.75) and carbonate (CO3) content (r = 0.66) quantified using ATR and O-PTIR spectroscopy, strengthening the validity of this method to assess bone mineral composition. O-PTIR imaging of individual trabeculae at 500 nm pixel resolution illustrated differences in submicron composition in the femoral neck from bones with different stiffness. O-PTIR analysis showed a significant negative correlation (r = -0.71) between bone stiffness and mineral maturity, reflective of newly formed bone being an important contributor to bone function. Finally, partial least squares regression analysis showed that combining multiple O-PTIR parameters (HPO4 content and heterogeneity, collagen integrity, and CO3 content) could significantly predict proximal femoral stiffness (R2 = 0.74, error = 9.7%) more accurately than using ATR parameters. Additionally, we describe new findings in the effects of bone tissue orientation in the O-PTIR spectra. Overall, this study highlights a new application of O-PTIR spectroscopy that may provide new insights into molecular-level factors underlying bone mechanical competence.

Near-infrared spectroscopy for structural bone assessment

Disorders of bone integrity carry a high global disease burden, frequently requiring intervention, but there is a paucity of methods capable of noninvasive real-time assessment. Here we show that miniaturized handheld near-infrared spectroscopy (NIRS) scans, operated via a smartphone, can assess structural human bone properties in under three seconds. A hand-held NIR spectrometer was used to scan bone samples from 20 patients and predict: bone volume fraction (BV/TV); and trabecular (Tb) and cortical (Ct) thickness (Th), porosity (Po), and spacing (Sp). NIRS scans on both the inner (trabecular) surface or outer (cortical) surface accurately identified variations in bone collagen, water, mineral, and fat content, which then accurately predicted bone volume fraction (BV/TV, inner R2 = 0.91, outer R2 = 0.83), thickness (Tb.Th, inner R2 = 0.9, outer R2 = 0.79), and cortical thickness (Ct.Th, inner and outer both R2 = 0.90). NIRS scans also had 100% classification accuracy in grading the quartile of bone thickness and quality. We believe this is a fundamental step forward in creating an instrument capable of intraoperative real-time use.

Poly(acrylic acid)-Assisted Intrafibrillar Mineralization of Type I Collagen: A Review

The mineralization of type I collagen is a biological process occurring in vertebrates by which some hard tissues such as bone and dentin are constructed. Due to the extensive clinical needs for bone defect repair and remineralization of mineral-depleted dentin, biomimetic mineralization of collagen is attracting more and more interests. Synthetic analogs of noncollagenous proteins are necessary for directing the in vitro mineralization. In this paper, the function and mechanism of poly(acrylic acid) (PAA) in regulating the mineralization, especially intrafibrillar mineralization (IM) of collagen are reviewed. As two mineralization patterns (extrafibrillar and intrafibrillar) co-exist in natural hard tissues, differences between them in terms of microstructure, biodegradation, cytocompatibility, osteoinduction in vitro, and performance in vivo are systematically compared. Then the roles of PAA in biomimetic collagen IM within one-analog and two-analog systems are discussed, respectively. Moreover, mineralization of some self-mineralizable collagen matrices is described. Due to the interactions between collagen and PAA play a crucial role in the processes of collagen mineralization, some reference researches are also provided involving the collagen/PAA interactions in some other fields. Finally, this review is ended with an outlook for future potential improvements based on the collection of existing bottlenecks in this field.

Bone hydration: How we can evaluate it, what can it tell us, and is it an effective therapeutic target?

Water constitutes roughly a quarter of the cortical bone by volume yet can greatly influence mechanical properties and tissue quality. There is a growing appreciation for how water can dynamically change due to age, disease, and treatment. A key emerging area related to bone mechanical and tissue properties lies in differentiating the role of water in its four different compartments, including free/pore water, water loosely bound at the collagen/mineral interfaces, water tightly bound within collagen triple helices, and structural water within the mineral. This review summarizes our current knowledge of bone water across the four functional compartments and discusses how alterations in each compartment relate to mechanical changes. It provides an overview on the advent of- and improvements to- imaging and spectroscopic techniques able to probe nano-and molecular scales of bone water. These technical advances have led to an emerging understanding of how bone water changes in various conditions, of which aging, chronic kidney disease, diabetes, osteoporosis, and osteogenesis imperfecta are reviewed. Finally, it summarizes work focused on therapeutically targeting water to improve mechanical properties.

Infrared Spectroscopy-Determined Bone Compositional Changes Associated with Anti-Resorptive Treatment of the oim/oim Mouse Model of Osteogenesis Imperfecta

Applications of vibrational spectroscopy to assess bone disease and therapeutic interventions are continually advancing, with tissue mineral and protein composition frequently investigated. Here, we used two spectroscopic approaches for determining bone composition in a mouse model (oim) of the brittle bone disease osteogenesis imperfecta (OI) with and without antiresorptive agent treatment (alendronate, or ALN, and RANK-Fc). Near-infrared (NIR) spectral analysis using a fiber optic probe and attenuated total reflection Fourier transform infrared spectroscopy (ATR FTIR) mode were applied to investigate bone composition, including water, mineral, and protein content. Spectral parameters revealed differences among the control wildtype (WT) and OIM groups. NIR spectral analysis of protein and water showed that OIM mouse humerii had ∼50% lower protein and ∼50% higher overall water content compared to WT bone. Moreover, some OIM-treated groups showed a reduction in bone water compared to OIM controls, approximating values observed in WT bone. Differences in bone quality based on increased mineral content and reduced carbonate content were also found between some groups of treated OIM and WT bone, but crystallinity did not differ among all groups. The spectroscopically determined parameters were evaluated for correlations with gold-standard mechanical testing values to gain insight into how composition influenced bone strength. As expected, bone mechanical strength parameters were consistently up to threefold greater in WT mice compared to OIM groups, except for stiffness in the ALN-treated OIM groups. Furthermore, bone stiffness, maximum load, and post-yield displacement showed the strongest correlations with NIR-determined protein content (positive correlations) and bound-water content (negative correlations). These results demonstrate that in this study, NIR spectral parameters were more sensitive to bone composition differences than ATR parameters, highlighting the potential of this nondestructive approach for screening of bone diseases and therapeutic efficacy in pre-clinical models.

Applications of Vibrational Spectroscopy for Analysis of Connective Tissues

Advances in vibrational spectroscopy have propelled new insights into the molecular composition and structure of biological tissues. In this review, we discuss common modalities and techniques of vibrational spectroscopy, and present key examples to illustrate how they have been applied to enrich the assessment of connective tissues. In particular, we focus on applications of Fourier transform infrared (FTIR), near infrared (NIR) and Raman spectroscopy to assess cartilage and bone properties. We present strengths and limitations of each approach and discuss how the combination of spectrometers with microscopes (hyperspectral imaging) and fiber optic probes have greatly advanced their biomedical applications. We show how these modalities may be used to evaluate virtually any type of sample (ex vivo, in situ or in vivo) and how "spectral fingerprints" can be interpreted to quantify outcomes related to tissue composition and quality. We highlight the unparalleled advantage of vibrational spectroscopy as a label-free and often nondestructive approach to assess properties of the extracellular matrix (ECM) associated with normal, developing, aging, pathological and treated tissues. We believe this review will assist readers not only in better understanding applications of FTIR, NIR and Raman spectroscopy, but also in implementing these approaches for their own research projects.