Raman spectral markers of collagen denaturation and hydration in human cortical bone tissue are affected by radiation sterilization and high cycle fatigue damage

Optimizing number of Raman spectra using an artificial neural network guided Monte Carlo simulation approach to analyze human cortical bone

This study presents a novel methodology for optimizing the number of Raman spectra required per sample for human bone compositional analysis. The methodology integrates Artificial Neural Network (ANN) and Monte Carlo Simulation (MCS). We demonstrate the robustness of ANN in enabling prediction of Raman spectroscopy-based bone quality properties even with limited spectral inputs. The ANN algorithms tailored to individual sex and age groups, which enhance the specificity and accuracy of predictions in bone quality properties. In addition, ANN guided MCS systematically explores the variability and uncertainty inherent in different sample sizes and spectral datasets, leading to the identification of an optimal number of spectra per sample for characterizing human bone tissues. The findings suggest that as low as 2 spectra are sufficient for biochemical analysis of bone, with R2 values between real and predicted values of v1/PO4/Amide I and ∼I1670/I1640 ratios, ranging from 0.60 to 0.89. Our results also suggest that up to 8 spectra could be optimal when balancing other factors. This optimized approach streamlines experimental workflows, reduces data and acquisition costs. Additionally, our study highlights the potential for advancing Raman spectroscopy in bone research through the innovative integration of ANN-guided probabilistic modeling techniques. This research could significantly contribute to the broader landscape of bone quality analyses by establishing a precedent for optimizing the number of Raman spectra with sophisticated computational tools. It also sets a novel platform for future optimization studies in Raman spectroscopy applications in biomedical field.

The influence of radiation-induced collagen chain fragmentation, crosslinking, and sequential irradiation on the high-cycle fatigue life of human cortical bone

Both high-cycle fatigue life and fatigue crack propagation resistance of human cortical bone allograft are radiation dose-dependent between 0 and 25 kGy such that higher doses exhibit progressively shorter lifetimes. Recently, we have shown that collagen chain fragmentation and stable crosslink accumulation may contribute to the radiation dose-dependent loss in fatigue crack propagation resistance of human cortical bone. To our knowledge, the influence of these mechanisms on high-cycle fatigue life of cortical bone have not been established. Sequential irradiation has also been shown to mitigate the loss of fatigue life of tendons, however, whether this mitigates losses in fatigue life of cortical bone has not been explored. Our objectives were to evaluate the influence of radiation-induced collagen chain fragmentation and crosslinking on the high-cycle fatigue life of cortical bone in the dose range of 0-15 kGy, and to evaluate the capability of sequential irradiation at 15 kGy to mitigate the loss of high-cycle fatigue life and radiation-induced collagen damage. High-cycle fatigue life specimens from four male donor femoral pairs were divided into 5 treatment groups (0 kGy, 5 kGy, 10 kGy, 15 kGy, and 15 kGy sequentially irradiated) and subjected to high-cycle fatigue life testing with a custom rotating-bending apparatus at a cyclic stress of 35 MPa. Following fatigue testing, collagen was isolated from fatigue specimens, and collagen chain fragmentation and crosslink accumulation were quantified using SDS-PAGE and a fluorometric assay, respectively. Both collagen chain fragmentation (p = 0.006) and non-enzymatic crosslinking (p < 0.001) influenced high-cycle fatigue life, which decreased with increasing radiation dose from 0 to 15 kGy (p = 0.016). Sequential irradiation at 15 kGy did not offer any mitigation in high-cycle fatigue life (p = 0.93), collagen chain fragmentation (p = 0.99), or non-enzymatic crosslinking (p ≥ 0.10) compared to a single radiation dose of 15 kGy. Taken together with our previous findings on the influence of collagen damage on fatigue crack propagation resistance, collagen chain fragmentation and crosslink accumulation both contribute to radiation-induced losses in notched and unnotched fatigue life of cortical bone. To maximize the functional lifetime of radiation sterilized structural cortical bone allografts, pathways other than sequential radiation should be explored to mitigate collagen matrix damage.

Physiochemical effects of acid exposure on bone composition and function

Bone is primarily composed of collagen and apatite, two materials which exhibit a high sensitivity to pH dysregulation. As a result, acid exposure of bone, both clinically and in the laboratory is expected to cause compositional and mechanical changes to the tissue. Clinically, Metabolic acidosis (MA), a condition characterized by a reduced physiological pH, has been shown to have negative implications on bone health, including a decrease in bone mineral density and volume as well as increased fracture risk. The addition of bone-like apatite to ionic solutions such as phosphate buffered saline (PBS) and media has been shown to acidify the solution leading to bone acid exposure. Therefore, is it essential to understand how reduced pH physiochemically affects bone composition and in turn its mechanical properties. This study investigates the specific changes in bone due to physiochemical dissolution in acid. Excised murine bones were placed in PBS solutions at different pHs: a homeostatic pH level (pH 7.4), an acidosis equivalent (pH 7.0), and an extreme acidic solution (pH 5.5). After 5 days, the bones were removed from the solutions and characterized to determine compositional and material changes. We found that bones, without cells, were able to regulate pH via buffering, leading to a decrease in bone mineral content and an increase in collagen denaturation. Both of these compositional changes contributed to an increase in bone toughness by creating a more ductile bone surface and preventing crack propagation. Therefore, we conclude that the skeletal systems' physiochemical response to acid exposure includes multifaceted and spatially variable compositional changes that affect bone mechanics.

Effect of gamma irradiation and supercritical carbon dioxide sterilization with Novakill™ or ethanol on the fracture toughness of cortical bone

Sterilization of structural bone allografts is a critical process prior to their clinical use in large cortical bone defects. Gamma irradiation protocols are known to affect tissue integrity in a dose dependent manner. Alternative sterilization treatments, such as supercritical carbon dioxide (SCCO2 ), are gaining popularity due to advantages such as minimal exposure to denaturants, the lack of toxic residues, superior tissue penetration, and minor impacts on mechanical properties including strength and stiffness. The impact of SCCO2 on the fracture toughness of bone tissue, however, remains unknown. Here, we evaluate crack initiation and growth toughness after 2, 6, and 24 h SCCO2 -treatment using Novakill™ and ethanol as additives on ~11 samples per group obtained from a pair of femur diaphyses of a canine. All mechanical testing was performed at ambient air after 24 h soaking in Hanks' balanced salt solution (HBSS). Results show no statistically significant difference in the failure characteristics of the Novakill™-treated groups whereas crack growth toughness after 6 and 24 h of treatment with ethanol significantly increases by 37% (p = .010) and 34% (p = .038), respectively, compared to an untreated control group. In contrast, standard 25 kGy gamma irradiation causes significantly reduced crack growth resistance by 40% (p = .007) compared to untreated bone. FTIR vibrational spectroscopy, conducted after testing, reveals a consistent trend of statistically significant differences (p < .001) with fracture toughness. These trends align with variations in the ratios of enzymatic mature to immature crosslinks in the collagen structure, suggesting a potential association with fracture toughness. Additional Raman spectroscopy after testing shows a similar trend with statistically significant differences (p < .005), which further supports that collagen structural changes occur in the SCF-treated groups with ethanol after 6 and 24 h. Our work reveals the benefits of SCCO2 sterilization compared to gamma irradiation.

Acidosis induces significant changes to the murine supraspinatus enthesis organic matrix

Rotator cuff pathology is a common musculoskeletal condition that disproportionately affects older adults, as well as patients with diabetes mellitus and chronic kidney disease. It is known that increased age and kidney dysfunction have been correlated to acidotic states, which may be related to the increased incidence of rotator cuff injury. In order to investigate the potential relationship between acidosis and rotator cuff composition and mechanics, this study utilizes a 14-day murine model of metabolic acidosis and examines the effects on the supraspinatus tendon-humeral head attachment complex. The elastic matrix in the enthesis exhibited significant changes beginning at day 3 of acidosis exposure. At day 3 and day 7 timepoints, there was a decrease in collagen content seen in both mineralized and unmineralized tissue as well as a decrease in mineral:matrix ratio. There is also evidence of both mineral dissolution and reprecipitation as buffering ions continually promote pH homeostasis. Mechanical properties of the tendon-to-bone attachment were studied; however, no significant changes were elicited in this 14-day model of acidosis. These findings suggest that acidosis can result in significant changes in enthesis composition over the course of 14 days; however, enthesis mechanics may be more structurally mediated rather than affected by compositional changes.

Relative Effects of Radiation-Induced Changes in Bone Mass, Structure, and Tissue Material on Vertebral Strength in a Rat Model

The observed increased risk of fracture after cancer radiation therapy is presumably due to a radiation-induced reduction in whole-bone strength. However, the mechanisms for impaired strength remain unclear, as the increased fracture risk is not fully explained by changes in bone mass. To provide insight, a small animal model was used to determine how much of this whole-bone weakening effect for the spine is attributable to changes in bone mass, structure, and material properties of the bone tissue and their relative effects. Further, because women have a greater risk of fracture after radiation therapy than men, we investigated if sex had a significant influence on bone's response to irradiation. Fractionated in vivo irradiation (10 × 3 Gy) or sham irradiation (0 Gy) was administered daily to the lumbar spine in twenty-seven 17-week-old Sprague-Dawley rats (n = 6-7/sex/group). Twelve weeks after final treatment, animals were euthanized, and lumbar vertebrae (L4 and L5 ) were isolated. Using a combination of biomechanical testing, micro-CT-based finite element analysis, and statistical regression analysis, we separated out the effect of mass, structural, and tissue material changes on vertebral strength. Compared with the sham group (mean ± SD strength = 420 ± 88 N), the mean strength of the irradiated group was lower by 28% (117 N/420 N, p < 0.0001). Overall, the response of treatment did not differ with sex. By combining results from both general linear regression and finite element analyses, we calculated that mean changes in bone mass, structure, and material properties of the bone tissue accounted for 56% (66 N/117 N), 20% (23 N/117 N), and 24% (28 N/117 N), respectively, of the overall change in strength. As such, these results provide insight into why an elevated clinical fracture risk for patients undergoing radiation therapy is not well explained by changes in bone mass alone. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Effect of ribose incubation on physical, chemical, and mechanical properties of human cortical bone

Raman spectroscopy (RS) is sensitive to the accumulation of advanced glycation end-products (AGEs), and it measures matrix-sensitive properties that correlate with the fracture toughness of human cortical bone. However, it is unclear whether sugar-mediated accumulation of AGEs affects the fracture toughness of human cortical bone in a manner that is consistent with the negative correlations between amide I sub-peak ratios and fracture toughness. Upon machining 64 single-edge notched beam (SENB) specimens from cadaveric femurs (8 male and 7 female donors between 46 years and 61 years of age), pairs of SENB specimens were incubated in 15 mL of phosphate buffered saline with or without 0.1 M ribose for 4 weeks at 37 °C. After acquiring 10 Raman spectra per bone specimen (n = 32 per incubation group), paired SENB specimens were loaded in three-point bending at a quasi-static or a high loading rate approximating 10-4 s-1 or 10-2 s-1, respectively (n = 16 per incubation group per loading rate). While 2 amide I sub-peak ratios, I1670/I1640 and I1670/I1610, decreased by 3-5% with a 100% increase in AGE content, as confirmed by fluorescence measurements, the ribose incubation to accumulate AGEs in bone did not affect linear elastic (KIc) nor non-linear elastic (KJc) measurements of bone's ability to resist crack growth. Moreover, AGE accumulation did not affect the change in these properties when the loading rate changed. Increasing the loading rate increased KIc but decreased KJc. Ribose incubation did not affect mineral-related RS properties such as mineral-to-matrix ratios, Type B carbonate substitutions, and crystallinity. It did however increase the thermal stability of demineralized bone (differential scanning calorimetry), without affecting the network connectivity of the organic matrix (i.e., maximum slope during a hydrothermal isometric tension test of demineralized bone). In conclusion, RS is sensitive to AGE accumulation via the amide I band (plus the hydroxyproline-to-proline ratio), but the increase in AGE content due to ribose incubation was not sufficient to affect the fracture toughness of human cortical bone.

Fatigue crack propagation and fracture toughness of cortical bone are radiation dose-dependent

Cortical bone allograft sterilized with a standard γ-radiation dose of 25-35kGy has demonstrated reduced static and cyclic fracture resistance compared with unirradiated bone. To mitigate radiation damage, we recently observed a dose-dependent response of high-cycle fatigue behavior of human cortical bone from 0 to 25 kGy, with lower doses exhibiting logarithmically longer fatigue lives. The objectives of this study were as follows: (1) to determine whether fracture toughness, work-to-fracture, and fatigue crack propagation resistance of human cortical bone are also radiation dose-dependent, and (2) to determine the associations of radiation dose and a Raman biomarker for collagen disorder with fracture properties. Compact tension specimens were machined from two donor femoral pairs and allocated to four treatment groups: 0 (unirradiated control), 10, 17.5, and 25 kGy. Fracture toughness specimens were monotonically loaded to failure and the critical stress intensity factor (KC ) was determined. Work-to-fracture was calculated from the load versus displacement integral up to fracture. Fatigue crack propagation specimens were cyclically loaded under constant room-temperature irrigation and fatigue crack growth rate (da/dN) and cyclic stress intensity (∆K) were calculated. Fracture toughness, work-to-fracture, and fatigue crack propagation resistance decreased 18%, 33%, and 15-fold from 0 to 25 kGy, respectively (p < 0.05). Radiation dose was more predictive of fracture properties than collagen disorder. These findings support that quasi-static and fatigue fracture properties of cortical bone are radiation dose-dependent within this dose range. The structural alterations arising from irradiation that cause these losses in fracture resistance remain to be elucidated.

The High-cycle Fatigue Life of Cortical Bone Allografts Is Radiation Sterilization Dose-dependent: An In Vitro Study

BACKGROUND

Structural cortical bone allografts are a reasonable treatment option for patients with large cortical bone defects caused by trauma, tumors, or complications of arthroplasty. Although structural cortical bone allografts provide the benefit of an osteoconductive material, they are susceptible to fatigue failure (fracture) and carry a risk of disease transmission. Radiation-sterilization at the recommended dose of 25 kGy decreases the risk of disease transmission. However, previous studies demonstrated that radiation sterilization at this dose can negatively impact the high cycle-fatigue life of cortical bone. Although the effects of higher doses of radiation on cortical bone allografts are well described, the effects of lower doses of radiation on a high-cycle fatigue life of cortical bone are poorly understood.

QUESTIONS/PURPOSES

(1) Does the cycle-fatigue life of human cortical allograft bone vary with gamma radiation dose levels of 0 (control), 10 kGy, 17.5 kGy, and 25 kGy? (2) What differences in Raman spectral biomarkers are observed following varying doses of gamma radiation exposure?

METHODS

The high-cycle fatigue behavior of human cortical bone specimens was examined at different radiation sterilization doses under physiologic stress levels (35 MPa) and in a 37° C phosphate-buffered saline bath using a custom-designed rotating-bending fatigue device. Six human femora from three donors were obtained for this study (two male, 63 and 61 years old, respectively, and one female, 48 years old). Test specimens were allocated among four treatment groups (0 kGy [control], 10 kGy, 17.5 kGy, and 25 kGy) based on donor and anatomic location of harvest site (both length and cross-sectional quadrant of femoral diaphysis) to ensure equal variation (n = 13 per group). Specimens underwent high-cycle fatigue testing to failure. The number of cycles to failure was recorded. Raman spectroscopy (a noninvasive vibrational spectroscopy used to qualitatively assess bone quality) was used to detect whether any changes in Raman spectral biomarkers occurred after varying doses of gamma radiation exposure.

RESULTS

There was a decrease in the log-transformed mean high-cycle fatigue life in specimens irradiated at 25 kGy (5.39 ± 0.32) compared with all other groups (0 kGy: 6.20 ± 0.50; 10k Gy: 6.35 ± 0.79; 17.5 kGy: 6.01 ± 0.53; p = 0.001). Specimens irradiated at 25 kGy were also more likely to exhibit a more brittle fracture surface pattern than specimens with more ductile fracture surface patterns irradiated at 0 kGy, 10 kGy, and 17.5 kGy (p = 0.04). The Raman biomarker for the ratio of the relative amount of disordered collagen to ordered collagen showed a decrease at the 10 kGy radiation level from 1.522 ± 0.025 preirradiation to 1.489 ± 0.024 postirradiation (p = 0.01); no other detectable changes in Raman biomarkers were observed.

CONCLUSION

The high-cycle fatigue life of cortical bone undergoes a nonlinear, dose-dependent decrease with an increase in gamma radiation sterilization in a clinically relevant dose range (0-25 kGy). Importantly, a notable drop-off in the high-cycle fatigue life of cortical bone appeared to occur between 17.5 kGy and 25 kGy, correlating to a sixfold decrease in mean cycles to failure. We speculate that the decrease in the Raman biomarker for disordered collagen at 10 kGy with no loss in high-cycle fatigue life may be caused by an increased amount of nonenzymatic crosslinking of the collagen backbone relative to collagen chain-scission (whereas the benefits of crosslinking may be outweighed by excess scission of the collagen backbone at higher radiation doses), but future studies will need to ascertain whether this in fact is the case.

CLINICAL RELEVANCE

Radiation sterilization at the industry standard of 25 kGy has a substantial negative impact on the high-cycle fatigue life of cortical bone. Given these findings, it is possible to provide a meaningful increase in the high-cycle fatigue life and improve the overall functional lifetime of cortical bone allografts by lowering the radiation-sterilization dose below 25 kGy. Future work on radiation-sterilization methods at these clinically relevant doses is warranted to aid in preserving the high cycle fatigue life of cortical bone allografts while maintaining sterility.

Compositional assessment of bone by Raman spectroscopy

Raman spectroscopy (RS) is used to analyze the physiochemical properties of bone because it is non-destructive and requires minimal sample preparation. With over two decades of research involving measurements of mineral-to-matrix ratio, type-B carbonate substitution, crystallinity, and other compositional characteristics of the bone matrix by RS, there are multiple methods to acquire Raman signals from bone, to process those signals, and to determine peak ratios including sub-peak ratios as well as the full-width at half maximum of the most prominent Raman peak, which is nu1 phosphate (ν1PO4). Selecting which methods to use is not always clear. Herein, we describe the components of RS instruments and how they influence the quality of Raman spectra acquired from bone because signal-to-noise of the acquisition and the accompanying background fluorescence dictate the pre-processing of the Raman spectra. We also describe common methods and challenges in preparing acquired spectra for the determination of matrix properties of bone. This article also serves to provide guidance for the analysis of bone by RS with examples of how methods for pre-processing the Raman signals and for determining properties of bone composition affect RS sensitivity to potential differences between experimental groups. Attention is also given to deconvolution methods that are used to ascertain sub-peak ratios of the amide I band as a way to assess characteristics of collagen type I. We provide suggestions and recommendations on the application of RS to bone with the goal of improving reproducibility across studies and solidify RS as a valuable technique in the field of bone research.

Altered mechanical behavior of demineralized bone following therapeutic radiation

Post-radiotherapy (RTx) bone fragility fractures are a late-onset complication occurring in bone within or underlying the radiation field. These fractures are difficult to predict, as patients do not present with local osteopenia. Using a murine hindlimb RTx model, we previously documented decreased mineralized bone strength and fracture toughness, but alterations in material properties of the organic bone matrix are largely unknown. In this study, 4 days of fractionated hindlimb irradiation (4 × 5 Gy) or Sham irradiation was administered in a mouse model (BALB/cJ, end points: 0, 4, 8, and 12 weeks, n = 15/group/end point). Following demineralization, the viscoelastic stress relaxation, and monotonic tensile mechanical properties of tibiae were determined. Irradiated tibiae demonstrated an immediate (day after last radiation fraction) and sustained (4, 8, 12 weeks) increase in stress relaxation compared to the Sham group, with a 4.4% decrease in equilibrium stress (p < .017). While tensile strength was not different between groups, irradiated tibiae had a lower elastic modulus (-5%, p = .027) and energy to failure (-12.2%, p = .012) with monotonic loading. Gel electrophoresis showed that therapeutic irradiation (4 × 5 Gy) does not result in collagen fragmentation, while irradiation at a common sterilization dose (25 kGy) extensively fragmented collagen. These results suggest that altered collagen mechanical behavior has a role in postirradiation bone fragility, but this can occur without detectable collagen fragmentation. Statement of Clinical Significance: Therapeutic irradiation alters bone organic matrix mechanics and which contribute to diminished fatigue strength, but this does not occur via collagen fragmentation.

Raman spectroscopic determination of bone matrix quantity and quality augments prediction of human cortical bone mechanical properties

Being independent contributors to bone mechanical resistance at the apparent level, quality and quantity of bone primary constituents are essential factors in better fracture risk assessment. Raman spectroscopy (RS) holds great potential for being a clinical tool with providing quality and quantity measurements of the bone mineralized matrix. Beyond mineral quality and quantity, recent years have revealed newly developed RS-derived bone compositional measurements focusing on organic matrix and water though their associations with bone mechanics have not been fully established yet. Herein, the author reported first thorough characterization study investigating associations between twenty different RS-derived measurements and mechanical properties of human cortical bone (i.e., yield and ultimate strength, elastic modulus, toughness, post-yield toughness, and post-yield strain). Forty-five rectangular human cortical beams harvested from all four anatomical quadrants of two male donors were tested under three-point bending. Raman spectra of each specimen were collected at the spectral range of 800 to 4000 cm^-1. While correlations were tested among RS-derived measurements via Spearman's rank correlations, multivariate linear regression using mixed effects were used to determine the best RS-derived measurement or the combination of RS-derived measurements in predicting various mechanical properties of human cortical bone. Most of the RS-derived measurements were associated with the mechanical properties (Rm² ranges from 8.9 to 68.3%, p < 0.05). The various linear combinations of six RS-derived measurements focusing on different aspects of bone matrix (i.e., ν₁PO₄/Amide I, ν₁PO₄/Amide III, Carbonate/ν₁PO₄, ~I₁₆₇₀/I₁₆₄₀, ~I₃₄₅₃/I₂₉₄₉, ~I₃₅₈₄/I₂₉₄₉) improved the prediction (Rm² = 43.5 to 70.2%, p < 0.05). While a causal relationship still needs to be investigated, RS has a great potential to establish a robust patient-specific fracture risk prediction with the latest advances in technologies.

Raman Spectroscopy Methods to Characterize the Mechanical Response of Soft Biomaterials

Raman spectroscopy has been used extensively to characterize the influence of mechanical deformation on microstructure changes in biomaterials. While traditional piezo-spectroscopy has been successful in assessing internal stresses of hard biomaterials by tracking prominent peak shifts, peak shifts due to applied loads are near or below the resolution limit of the spectrometer for soft biomaterials with moduli in the kilo- to mega-Pascal range. In this Review, in addition to peak shifts, other spectral features (e.g., polarized intensity and intensity ratio) that provide quantitative assessments of microstructural orientation and secondary structure in soft biomaterials and their strain dependence are discussed. We provide specific examples for each method and classify sensitive Raman characteristic bands common across natural (e.g., soft tissue) and synthetic (e.g., polymeric scaffolds) soft biomaterials upon mechanical deformation. This Review can provide guidance for researchers aiming to analyze micromechanics of soft tissues and engineered tissue constructs by Raman spectroscopy.

The age-related decrease in material properties of BALB/c mouse long bones involves alterations to the extracellular matrix

One possibility for the disproportionate increase in fracture risk with aging relative to the decrease in bone mass is an accumulation of changes to the bone matrix which deleteriously affect fracture resistance. In order to effectively develop new targets for osteoporosis, a preclinical model of the age-related loss in fracture resistance needs to be established beyond known age-related decreases in bone mineral density and bone volume fraction. To that end, we examined long bones of male and female BALB/c mice at 6-mo. and 20-mo. of age and assessed whether material and matrix properties of cortical bone significantly differed between the age groups. The second moment of area of the diaphysis (minimum and maximum principals for femur and radius, respectively) as measured by ex vivo micro-computed tomography (μCT) was higher at 20-mo. than at 6-mo. for both males and females, but ultimate moment as measured by three-point bending tests did not decrease with age. Cortical thickness was lower with age for males, but higher for old females. Partially accounting for differences in structure, material estimates of yield, ultimate stress, and toughness (left femur) were 12.6%, 11.1%, and 40.9% lower, respectively, with age for both sexes. The ability of the cortical bone to resist crack growth (right femur) was also 18.1% less for the old than for the young adult mice. These decreases in material properties were not due to changes in intracortical porosity as pore number decreased with age. Rather, age-related alterations in the matrix were observed for both sexes: enzymatic and non-enzymatic crosslinks by high performance liquid chromatography increased (femur), volume fraction of bound water by 1H-nuclear magnetic resonance relaxometry decreased (femur), cortical tissue mineral density by μCT increased (femur and radius), and an Amide I sub-peak ratio I1670/I1640 by Raman spectroscopy increased (tibia). Overall, there are multiple matrix changes to potentially target that could prevent the age-related decrease in fracture resistance observed in BALB/c mouse.

Raman Biomarkers Are Associated with Cyclic Fatigue Life of Human Allograft Cortical Bone

BACKGROUND

Structural bone allografts are an established treatment method for long-bone structural defects resulting from such conditions as traumatic injury and sarcoma. The functional lifetime of structural allografts depends on resistance to cyclic loading (cyclic fatigue life), which can lead to fracture at stress levels well below the yield strength. Raman spectroscopy biomarkers can be used to non-destructively assess the 3 primary components of bone (collagen, mineral, and water), and may aid in optimizing allograft selection to decrease fatigue fracture risk. We studied the association of Raman biomarkers with the cyclic fatigue life of human allograft cortical bone.

METHODS

Twenty-one cortical bone specimens were machined from the femoral diaphyses of 4 human donors (a 63-year old man, a 61-year-old man, a 51-year-old woman, and a 48-year-old woman) obtained from the Musculoskeletal Transplant Foundation. Six Raman biomarkers were analyzed: collagen disorganization, mineral maturation, matrix mineralization, and 3 water compartments. The specimens underwent cyclic fatigue testing under fully reversed conditions (35 and 45 MPa), during which they were tested to fracture or to 30 million cycles ("runout"), simulating 15 years of moderate activity. A tobit censored linear regression model for cyclic fatigue life was created.

RESULTS

The multivariate model explained 60% of the variance in the cyclic fatigue life (R = 0.604, p < 0.001). Increases in Raman biomarkers for disordered collagen (coefficient: -2.74×10, p < 0.001) and for loosely collagen-bound water compartments (coefficient: -2.11×10, p < 0.001) were associated with a decreased cyclic fatigue life. Increases in Raman biomarkers for mineral maturation (coefficient: 3.50×10, p < 0.001), matrix mineralization (coefficient: 2.32×10, p < 0.001), tightly collagen-bound water (coefficient: 1.19×10, p < 0.001), and mineral-bound water (coefficient: 3.27×10, p < 0.001) were associated with an increased cyclic fatigue life. Collagen disorder accounted for 44% of the variance in the cyclic fatigue life, mineral maturation accounted for 6%, and all bound water compartments accounted for 3%.

CONCLUSIONS

Increasing baseline collagen disorder was associated with a decreased cyclic fatigue life and had the strongest correlation with the cyclic fatigue life of human cortical donor bone. This model should be prospectively validated.

CLINICAL RELEVANCE

Raman analysis is a promising tool for the non-destructive evaluation of structural bone allograft quality for load-bearing applications.

Assessing matrix quality by Raman spectroscopy helps predict fracture toughness of human cortical bone

Developing clinical tools that assess bone matrix quality could improve the assessment of a person's fracture risk. To determine whether Raman spectroscopy (RS) has such potential, we acquired Raman spectra from human cortical bone using microscope- and fiber optic probe-based Raman systems and tested whether correlations between RS and fracture toughness properties were statistically significant. Calculated directly from intensities at wavenumbers identified by second derivative analysis, Amide I sub-peak ratio I1670/I1640, not I1670/I1690, was negatively correlated with Kinit (N = 58; R2 = 32.4%) and J-integral (R2 = 47.4%) when assessed by Raman micro-spectroscopy. Area ratios (A1670/A1690) determined from sub-band fitting did not correlate with fracture toughness. There were fewer correlations between RS and fracture toughness when spectra were acquired by probe RS. Nonetheless, the I1670/I1640 sub-peak ratio again negatively correlated with Kinit (N = 56; R2 = 25.6%) and J-integral (R2 = 39.0%). In best-fit general linear models, I1670/I1640, age, and volumetric bone mineral density explained 50.2% (microscope) and 49.4% (probe) of the variance in Kinit. I1670/I1640 and v1PO4/Amide I (microscope) or just I1670/I1640 (probe) were negative predictors of J-integral (adjusted-R2 = 54.9% or 37.9%, respectively). While Raman-derived matrix properties appear useful to the assessment of fracture resistance of bone, the acquisition strategy to resolve the Amide I band needs to be identified.

Raman spectroscopy-based water content is a negative predictor of articular human cartilage mechanical function

OBJECTIVE

Probing the change in water content is an emerging approach to assess early diagnosis of osteoarthritis (OA). We herein developed a new method to assess hydration status of cartilage nondestructively using Raman spectroscopy (RS), and showed association of Raman-based water and organic content measurement with mechanical properties of cartilage. We further compared Raman-based water measurement to gravimetric and magnetic resonance imaging (MRI)-based water measurement.

DESIGN

Eighteen cadaveric human articular cartilage plugs from 6 donors were evenly divided into two age groups: young (n = 9, mean age: 29.3 ± 6.6) and old (n = 9, mean age: 64.0 ± 1.5). Water content in cartilage was measured using RS, gravimetric, and MRI-based techniques. Using confined compression creep test, permeability and aggregate modulus were calculated. Regression analyses were performed among RS parameters, MRI parameter, permeability, aggregate modulus and gravimetrically measured water content.

RESULTS

Regardless of the method used to calculate water content (gravimetric, RS and MRI), older cartilage group consistently had higher water content compared to younger group. There was a stronger association between gravimetric and RS-based water measurement (R_g² = 0.912) than between gravimetric and MRI-based water measurement (R_c² = 0.530). Gravimetric and RS-based water contents were significantly correlated with permeability and aggregate modulus whereas MRI-based water measurement was not.

CONCLUSION

RS allows for quantification of different water compartments in cartilage nondestructively, and estimation of up to 82% of the variation observed in the permeability and aggregate modulus of articular cartilage. RS has the potential to be used clinically to monitor cartilage quality noninvasively or minimally invasively with Raman probe during arthroscopy procedures.

Manipulating the Amount and Structure of the Organic Matrix Affects the Water Compartments of Human Cortical Bone

Being predictors of the mechanical properties of human cortical bone, bound and pore water measurements by magnetic resonance (MR) imaging are being developed for the clinical assessment of fracture risk. While pore water is a surrogate of cortical bone porosity, the determinants of bound water are unknown. Manipulation of organic matrix properties by oxidative deproteinization, thermal denaturation, or nonenzymatic glycation lowers bone toughness. Because bound water contributes to bone toughness, we hypothesized that each of these matrix manipulations affect bound water fraction (V_bw/V_bone). Immersing cadaveric bone samples in sodium hypochlorite (NaClO) for 96 hours did not affect tissue mineral density or cortical porosity, but rather decreased V_bw/V_bone and increased short‐T₂ pore water signals as determined by ¹H nuclear MR relaxometry (¹H NMR). Moreover, the post treatment V_bw/V_bone linearly correlated with the remaining weight fraction of the organic matrix. Heating bone samples at 110°C, 120°C, 130°C, and then 140°C (∼24 hours per temperature and rehydration for ∼24 hours before ¹H NMR analysis) did not affect V_bw/V_bone. After subsequently heating them at 200°C, V_bw/V_bone increased. Boiling bone samples followed by heating at 110°C, 120°C, and then 130°C in water under pressure (8 hours per temperature) had a similar effect on V_bw/V_bone. Raman spectroscopy analysis confirmed that the increase in V_bw/V_bone coincided with an increase in an Amide I subpeak ratio that is sensitive to changes in the helical structure of collagen I. Glycation of bone by ribose for 4 weeks, but not in glucose for 16 weeks, decreased V_bw/V_bone, although the effect was less pronounced than that of oxidative deproteinization or thermal denaturation. We propose that MR measurements of bound water reflect the amount of bone organic matrix and can be modulated by collagen I helicity and by sugar‐derived post translational modifications of the matrix. © 2019 The Authors. JBMR Plus published by Wiley Periodicals, Inc. on behalf of American Society for Bone and Mineral Research.

Preservation of Bone Tissue Integrity with Temperature Control for In Situ SR-MicroCT Experiments

Digital volume correlation (DVC), combined with in situ synchrotron microcomputed tomography (SR-microCT) mechanics, allows for 3D full-field strain measurement in bone at the tissue level. However, long exposures to SR radiation are known to induce bone damage, and reliable experimental protocols able to preserve tissue properties are still lacking. This study aims to propose a proof-of-concept methodology to retain bone tissue integrity, based on residual strain determination using DVC, by decreasing the environmental temperature during in situ SR-microCT testing. Compact and trabecular bone specimens underwent five consecutive full tomographic data collections either at room temperature or 0 °C. Lowering the temperature seemed to reduce microdamage in trabecular bone but had minimal effect on compact bone. A consistent temperature gradient was measured at each exposure period, and its prolonged effect over time may induce localised collagen denaturation and subsequent damage. DVC provided useful information on irradiation-induced microcrack initiation and propagation. Future work is necessary to apply these findings to in situ SR-microCT mechanical tests, and to establish protocols aiming to minimise the SR irradiation-induced damage of bone.

Assessing glycation-mediated changes in human cortical bone with Raman spectroscopy

Establishing a non-destructive method for spatially assessing advanced glycation end-products (AGEs) is a potentially useful step toward investigating the mechanistic role of AGEs in bone quality. To test the hypothesis that the shape of the amide I in the Raman spectroscopy (RS) analysis of bone matrix changes upon AGE accumulation, we incubated paired cadaveric cortical bone in ribose or glucose solutions and in control solutions for 4 and 16 weeks, respectively, at 37°C. Acquiring 10 spectra per bone with a 20X objective and a 830 nm laser, RS was sensitive to AGE accumulation (confirmed by biochemical measurements of pentosidine and fluorescent AGEs). Hyp/Pro ratio increased upon glycation using either 0.1 M ribose, 0.5 M ribose or 0.5 M glucose. Glycation also decreased the amide I sub-peak ratios (cm-1 ) 1668/1638 and 1668/1610 when directly calculated using either second derivative spectrum or local maxima of difference spectrum, though the processing method (eg, averaged spectrum vs individual spectra) to minimize noise influenced detection of differences for the ribose-incubated bones. Glycation however did not affect these sub-peak ratios including the matrix maturity ratio (1668/1690) when calculated using indirect sub-band fitting. The amide I sub-peak ratios likely reflected changes in the collagen I structure.

Distribution of tightly and loosely bound water in biological macromolecules and age-related diseases

This mini-review article is focused on publications devoted to the changes in water binding energy and content of bound water in biological tissues during aging processes, when bound water lost from the hydration layer becomes free water. Bound water is released during cataractogenesis. In skin, water bound to proteins and other biomacromolecules becomes more mobile with increasing skin age. Extracellular to intracellular water ratio increases with age and was associated with muscle cell atrophy. Bound water concentration decreases with age in hydrated human bone and can be correlated with the strength and toughness of the bone. Higher fraction of free water in malignant tissues compared to normal tissues was observed. Hydration water mobility is enhanced around tau amyloid fibers. Water plays a decisive role in amyloid formation as entropic driving force. In the natural aging processes dehydration and glycation may be considered as subsequent steps.

The Role of Matrix Composition in the Mechanical Behavior of Bone

PURPOSE OF REVIEW

While thinning of the cortices or trabeculae weakens bone, age-related changes in matrix composition also lower fracture resistance. This review summarizes how the organic matrix, mineral phase, and water compartments influence the mechanical behavior of bone, thereby identifying characteristics important to fracture risk.

RECENT FINDINGS

In the synthesis of the organic matrix, tropocollagen experiences various post-translational modifications that facilitate a highly organized fibril of collagen I with a preferred orientation giving bone extensibility and several toughening mechanisms. Being a ceramic, mineral is brittle but increases the strength of bone as its content within the organic matrix increases. With time, hydroxyapatite-like crystals experience carbonate substitutions, the consequence of which remains to be understood. Water participates in hydrogen bonding with organic matrix and in electrostatic attractions with mineral phase, thereby providing stability to collagen-mineral interface and ductility to bone. Clinical tools sensitive to age- and disease-related changes in matrix composition that the affect mechanical behavior of bone could potentially improve fracture risk assessment.

Shortwave-infrared Raman spectroscopic classification of water fractions in articular cartilage ex vivo

Water loss is an early onset indicator of osteoarthritis. Although Raman spectroscopy (RS) holds the potential for measurement of cartilage hydration, the knowledge of Raman OH-stretch bands of biological tissue is very limited. We assesed here the sensitivity of RS to identify and classify water types in the cartilage. Raman spectrum measurements over the high wavenumber range were employed to identify different water fractions in articular cartilage. Raman spectra were collected from wet and sequentially dehydrated cartilage along with pure collagen type II and chondroitin sulfate standards. OH-stretch band of cartilage is dominated by mobile water, up to 95% of total intensities. We identified six peaks in cartilage spectrum using second-derivative analysis: peaks at 3200 and 3650  cm-1 are associated with organic matrix (both collagen and proteglycan) and matrix-bound water molecules. Peaks at 3250, 3453, and 3630  cm-1 are associated with collagen and collagen-related water molecules, whereas the peak at 3520  cm-1 is associated with proteoglycan (PG) and PG-related water molecules. The current work is the first thorough analysis of the Raman OH-stretch band of the cartilage and with the knowledge generated by this study, it may now be possible to study on cartilage hydration by RS.