Thermal stability of cortical bone collagen in relation to age in normal individuals and in individuals with osteopetrosis

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.

Structural equation modelling of mercury intra-skeletal variability on archaeological human remains

Archaeological burial environments are useful archives to investigate the long-term trends and the behaviour of mercury. In order to understand the relationship between mercury, skeletons and soil, we applied Partial Least Squares - Structural Equation Modelling (PLS-SEM) to a detailed, multisampling (n = 73 bone samples +37 soil samples) design of two archaeological graves dating to the 6th to 7th centuries CE (A Lanzada site, NW Spain). Mercury content was assessed using a DMA-80, and data about bone structure and the grave soil/sediments were obtained using FTIR-ATR spectroscopy. The theoretical model is supported by proxies of bone structure, grave soil/sediments, and location of the bone within the skeleton. The general model explained 61 % of mercury variance. Additionally, Partial Least Square - Prediction Oriented Segmentation (PLS-POS) was also used to check for segmentation in the dataset. POS revealed two group of samples depending on the bone phase (hydroxyapatite or collagen) controlling the Hg content, and the corresponding models explained 86 % and 76 % of Hg variance, respectively. The results suggest that mercury behaviour in the graves is complex, and that mercury concentrations were influenced by i) the ante-mortem status of the bone matrix, related to the weight of each bone phase; ii) post-mortem evolution of bone crystallinity, where bone loses mercury with increasing alteration; and iii) the proximity of the skeletal pieces to mercury target organs, as decomposition and collapse of the thoracic and abdominal soft tissues causes a secondary mercury enrichment in bones from the body trunk during early post-mortem. Skeletons provide a source of mercury to the soil whereas soil/sediments contribute little to skeletal mercury content.

Post-translational modifications in collagen type I of bone in a mouse model of aging

The fracture resistance of cortical bone and matrix hydration are known to decline with advanced aging. However, the underlying mechanisms remain poorly understood, and so we investigated levels of matrix proteins and post-translational modifications (PTM) of collagen I in extracts from the tibia of 6-mo. and 20-mo. old BALB/c mice (female and male analysis done separately). Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis revealed that the levels of collagen I deamidation at specific asparagine (Asn) and glutamine (Gln) residues significantly increased with age. Other non-enzymatic PTMs such as carboxymethylation of lysine (CML) were detected as well, but the relative abundance did not vary with age. No significant age-related differences in the abundance of hydroxylysine glycosylation sites were found, but hydroxylation levels at a few of the numerous lysine and proline hydroxylation sites significantly changed by a small amount with age. We performed molecular modeling and dynamics (MD) simulations for three triple helical fragments representing collagen I regions with prominent age-dependent increases in deamidation as identified by LC-MS/MS of male extracts. These 3 fragments included deamidated Asn and Gln residues as follows: 1) an Asn⁴²⁸ site of the α2(I) chain in which deamidation levels increased from 4.4% at 6-mo. to 8.1% at 20-mo., 2) an Asn⁹⁸³ site of the α2(I) chain with a deamidation increase from 18.3% to 36.8% with age and an Asn¹⁰⁵² site of the α1(I) chain with consistent deamidation levels of ~60% across the age groups, and 3) a Gln⁴¹⁰ site of the α1(I) chain that went from no detectable deamidation at 6-mo. to 2.7% at 20-mo. and a neighboring Asn⁴²¹ site of the same chain with an age-related deamidation increase from 3.6% to 16.3%. The deamidation levels at these sites inversely correlated with an estimate of toughness determined from three-point bending tests of the femur mid-diaphysis. MD revealed that the sidechains become more negatively charged at deamidated sites and that deamidation alters hydrogen bonding with water along the collagen backbone while increasing water interactions with the aspartic and glutamic acid sidechains. Our findings suggest a new mechanism of the age-dependent reduction in the fracture resistance of cortical bone whereby deamidation of Asn and Glu residues redistributes bound water within collagen I triple helix.

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.

Bone embrittlement and collagen modifications due to high-dose gamma-irradiation sterilization

Bone allografts are often used in orthopedic reconstruction of skeletal defects resulting from trauma, bone cancer or revision of joint arthroplasty. γ-Irradiation sterilization is a widely-used biological safety measure; however it is known to embrittle bone. Irradiation has been shown to affect the post-yield properties, which are attributed to the collagen component of bone. In order to find a solution to the loss of toughness in irradiated bone allografts, it is important to fully understand the effects of irradiation on bone collagen. The objective of this study was to evaluate changes in the structure and integrity of bone collagen as a result of γ-irradiation, with the hypothesis that irradiation fragments collagen molecules leading to a loss of collagen network connectivity and therefore loss of toughness. Using cortical bone from bovine tibiae, sample beams irradiated at 33kGy on dry ice were compared to native bone beams (paired controls). All beams were subjected to three-point bend testing to failure followed by characterization of the decalcified bone collagen, using differential scanning calorimetry (DSC), hydrothermal isometric tension testing (HIT), high performance liquid chromatography (HPLC) and gel electrophoresis (SDS-PAGE). The carbonyl content of demineralized bone collagen was also measured chemically to assess oxidative damage. Barium sulfate staining after single edge notch bending (SEN(B)) fracture testing was also performed on bovine tibia bone beams with a machined and sharpened notch to evaluate the fracture toughness and ability of irradiated bone to form micro-damage during fracture. Irradiation resulted in a 62% loss of work-to-fracture (p≤0.001). There was significantly less micro-damage formed during fracture propagation in the irradiated bone. HPLC showed no significant effect on pentosidine, pyridinoline, or hydroxypyridinoline levels suggesting that the loss of toughness is not due to changes in these stable crosslinks. For DSC, there was a 20% decrease in thermal stability (p<0.001) with a 100% increase (p<0.001) in enthalpy of denaturation (melting). HIT testing also showed a decrease in thermal stability (20% lower denaturation temperature, p<0.001) and greatly reduced measures of collagen network connectivity (p<0.001). Interestingly, the increase in enthalpy of denaturation suggests that irradiated collagen requires more energy to denature (melt), perhaps a result of alterations in the hydrogen bonding sites (increased carbonyl content detected in the insoluble collagen) on the irradiated bone collagen. Altogether, this new data strongly indicates that a large loss of overall collagen connectivity due to collagen fragmentation resulting from γ-irradiation sterilization leads to inferior cortical bone toughness. In addition, notable changes in the thermal denaturation of the bone collagen along with chemical indicators of oxidative modification of the bone collagen indicate that the embrittlement may be a function not only of collagen fragmentation but also of changes in bonding.

A comparative study of young and mature bovine cortical bone

The mechanical properties and microstructure of young and mature bovine femur bone were investigated by optical microscopy and compression testing in the longitudinal and transverse directions for untreated, deproteinized and demineralized cases. Optical microscopy revealed that mature bone has a more established and less porous microstructure compared to young bone. Mature bone was found to be stronger in both directions for the untreated and deproteinized cases. Mature untreated bone was also found to be stiffer and less tough compared to young bone in both directions. These results are related to the increase in mineralization of mature bone and significant microstructural differences. Young bone was found to be stronger in both directions for the demineralized case, which is attributed to alterations in the collagen network with age.

Age-related factors affecting the postyield energy dissipation of human cortical bone

The risk of bone fracture depends in part on tissue quality, not just the size and mass. This study assessed the postyield energy dissipation of cortical bone in tension as a function of age and composition. Specimens were prepared from tibiae of human cadavers in which male and female donors were divided into two age groups: middle aged (51 to 56 years, n = 9) and elderly (72 to 90 years, n = 8). By loading, unloading, and reloading a specimen with rest periods inserted in between, tensile properties at incremental strain levels were assessed. In addition, postyield toughness was estimated and partitioned as plastic strain energy related to permanent deformation, released elastic strain energy related to stiffness loss, and hysteresis energy related to viscous behavior. Porosity, mineral and collagen content, and collagen crosslinks of each specimen were also measured to determine the micro- and ultrastructural properties of the tissue. Age affected all the energy terms plus strength but not elastic stiffness. The postyield energy terms were correlated with porosity, pentosidine (a marker of nonenzymatic crosslinks), and collagen content, all of which varied significantly with age. General linear models suggested that pentosidine concentration and collagen content provided the best explanation of the age-related decrease in the postyield energy dissipation. Among them, pentosidine concentration had the greatest contribution to plastic strain energy and was the best explanatory variable of damage accumulation.

Age-related effect on the concentration of collagen crosslinks in human osteonal and interstitial bone tissue

Collagen crosslinks are important to the quality of bone and may be contributors to the age-related increase in bone fracture. This study was performed to investigate whether age and gender effects on collagen crosslinks are similar in osteonal and interstitial bone tissues. Forty human cadaveric femurs were collected and divided into two age groups: middle-aged (42-63 years of age) and elderly (69-90 years of age) with ten males and ten females in each group (n = 10). Micro-cores of bone tissue from both secondary osteons and interstitial regions in the medial quadrant of the diaphysis were extracted using a custom-modified, computer-controlled milling machine. The bone specimens were then analyzed using high performance liquid chromatography to determine the effects of age and gender on the concentration of mature, enzymatic crosslinks (hydroxylysyl-pyridinoline-HP and lysyl-pyridinoline-LP) and a non-enzymatic crosslink (pentosidine-PE) at these two microstructural sites. The results indicate that age has a significant effect on the concentration of LP and PE, while gender has a significant effect on HP and LP. In addition, the concentration of the crosslinks in the secondary osteons is significantly different from that in the interstitial bone regions. These results suggest that the amount of non-enzymatic crosslinking may increase while that of mature enzymatic crosslinking may decrease with age. Such changes could potentially reduce the inherent quality of the bone tissue in the elderly skeleton.

Effect of ultrastructural changes on the toughness of bone

The ultrastructure of bone can be considered as a conjunction between the biology and the biomechanics of the tissue. It is the result of cellular and molecular activities of bone formation, and its organization dominates the mechanical behavior of bone. Following this perspective, the objective of this review is to provide a current understanding of bone ultrastructure and its relationships with the toughness of the tissue. Therefore, we first provide a discussion on the organization of bone constituents, namely collagen, mineral, and water. Then, we present evidence on how the toughness of bone relates to its ultrastructure through the formation of micro damage. In addition, attention is given to how damage accumulation serves as a toughening mechanism. Finally, we describe how changes in the ultrastructure-caused by osteogenesis imperfecta, gamma irradiation, fluoride treatment, and aging affect the toughness and competence of bone.

Mechanical properties of the human lens capsule

The human lens capsule has recently been the subject of much attention in an attempt to understand its physiological function in relation to the accommodative function, its functional reserve in the elderly population, and its potential in relation to cataract surgery. This overview presents our current knowledge of the mechanical properties of the human lens capsule, discussed on basis of its structure and its role in accommodation and cataract surgery.

Age-related changes in the collagen network and toughness of bone

The hypothesis of this study is that the mechanical integrity of the collagen network in bone deteriorates with age, and such adverse changes correlate with the decreased toughness of aged bone. To test the hypothesis, 30 human cadaveric femurs from donors ranging from 19 to 89 years of age were tested to determine the age-related changes in the mechanical properties of demineralized bone and fresh bone samples. Along with bone porosity, bone density, and weight fractions of the mineral and organic phases, collagen denaturation and concentrations of collagen cross-links (HP, hydroxylysylpyridinoline; LP, lysylpyridinoline; PE, pentosidine) were determined for these bone specimens as a function age. Analysis of variance (ANOVA) showed that age-dependent changes were reflected in the decreased strength, work to fracture, and fracture toughness of bone; in the decreased strength, elastic modulus, and work to fracture of the collagen network; as well as in the increased concentration of pentosidine (a marker of nonenzymatic glycation) and increased bone porosity. Regression analyses of the measured parameters showed that the age-related decrease in work to fracture of bone (especially its postyield portion) correlated significantly with deterioration in the mechanical integrity of the collagen network. The results of this study indicate that the adverse changes in the collagen network occur as people age and such changes may lead to the decreased toughness of bone. Also, the results suggest that nonenzymatic glycation may be an important contributing factor causing changes in collagen and, consequently, leading to the age-related deterioration of bone quality.

The contribution of the organic matrix to bone's material properties

Bone is a two-phase porous composite material comprised primarily of collagen and mineral, which together provide its mechanical properties. The contribution of the mineral phase to bone's mechanical properties has dominated scientific thinking. Collagen's role has been underappreciated and not very well studied. However, there is evidence that changes in collagen content, or changes to inter- and intrafibrillar collagen cross-linking, can reduce the energy required to cause bone failure (toughness), and increase fracture risk. Although collagen may have less effect on bone's strength and stiffness than does mineral, it may have a profound effect on bone fragility. Collagen changes that occur with age and reduce bone's toughness may be an important factor in the risk of fracture in older women with low bone mass.

Thermal stability of bone collagen as an indicator of bone turnover in gonadectomized and multiparous rats

Previous findings indicate that the thermal stability of bone collagen is related to age. In this study, collagen from rat bone with reported different turnover rates was investigated. Cortical and trabecular bone from femur were obtained from intact, ovariectomized, orchidectomized and multiparous breeder rats. Thermal stabilities of fibrillar collagen in decalcified bone matrix and molecular collagen obtained by pepsin treatment were measured as shrinkage (Ts) and 'melting' temperature (Tm), respectively. Both Ts and Tm of cortical collagen from intact female rats decreased in parallel with age as previously found in male rats indicating that Ts and Tm measurements are interchangeable techniques in characterizing the thermal stability of bone collagen. Tm of trabecular collagen from intact rats decreased with age, however, with a decay only one-third of that for cortical collagen. The different rates possibly reflect different ages of collagen due to remodeling activity present in trabecular and minimal in cortical bone. Compared with control rats the Tm of trabecular collagen from gonadectomized and multiparous rats with a reported increased trabecular turnover rate was elevated, whereas only minor variations in Tm of cortical collagen were found. In conclusion, the thermal stability of bone collagen decreases with the age of the collagen. Increased bone turnover implies elevated thermal stability of bone collagen. Thus, thermal stability of bone collagen appears to be an indicator of bone turnover.

Ageing human bone: factors affecting its biomechanical properties and the role of collagen

The incidence of fractures increases with age. This is partly due to extraosseous factors and partly to the increased fragility of the bone material itself. Ageing adversely affects the "quality" of human bone material, its elastic and ultimate properties. The hypothesis here is that these effects are caused by factors such as architectural changes, compositional changes, physicochemical changes, changes at the micromechanical level, and the degree of prior in vivo microdamage. Examination of the extent of the secondary osteonal area, the porosity level, the calcium content, the mineral/wet weight fraction, the dry density, the condition of the collagen and its content in mature x-links, the elasticity of osteonal and interstitial lamellae at the microscopic level and the numerical- and surface-density of the in vivo fatigue microcracks has been undertaken. The findings show that some factors simply affect the stiffness and the strength of bone, while others soley affect its toughness. We discuss the implications of these findings in the context of the composite nature of the ageing bone material matrix.

The role of collagen in the declining mechanical properties of aging human cortical bone

The importance of the mechanical role of collagen in bone is becoming increasingly more clear as evidence mounts on the detrimental effects of altered collagen on the mechanical properties of bone. We previously examined a set of mechanical properties (material stiffness, strength, and toughness) of human femoral bone (ages 35-92) and found that a gradual deterioration in these properties occurs with age. The present study examines the collagen of the same specimens and relates the collagen properties to the mechanical ones. In the collagen we measured the concentration of stable mature crosslinks, the shrinkage temperature, and the rate of contraction during isometric heating. The changes in the concentration of mature (pyridinium and deoxypyridinium) crosslinks showed no clear relationship to age nor did they correlate with the mechanical properties. The shrinkage temperature declined with age and correlated with a bone's toughness. The maximum rate of contraction was strongly correlated with three different measures of tissue toughness, but much less to stiffness and strength. Our results reinforce speculation regarding the toughening role of collagen in bone mechanics and suggest that the fragility of aging bone may be related to collagen changes.

Thermal and mechanical stability of the lens capsule

PURPOSE

Several procedures in cataract surgery carry the risk of high temperature increases in the capsular bag. The present study was undertaken to determine the shrinkage temperature of the human lens capsule and to investigate the effect of temperature on the mechanical behavior of the lens capsule.

METHOD

Thermal-shrinkage characteristics of the lens capsule were determined during gradual heating of circular specimens (2 mm in diameter) prepared from anterior lens capsules from 25 human donors, ranging in age from 20 to 98 years. Uniaxial mechanical testing was carried out at 22 degrees C, 36 degrees C and 61 degrees C on ring-shaped test specimens prepared from anterior lens capsules from 5- to 6-month-old pigs.

RESULTS

The mean shrinkage temperature (Ts) for the human lens capsule was 51.5 degrees C (range 49.3-54.3) and the mean shrinkage area in percent of the original area (AST) was 49% (36-66). Ts was significantly associated with the age of the donors and decreased 0.1 degree C per year until age 65 after which Ts was found to increase. AST showed no association with age. The mechanical effect of temperatures below the shrinkage temperature was modest. The capsule became slightly more extensible with increasing temperature. The effect of temperatures above the shrinkage temperature was an increased ultimate strain, a reduced ultimate stiffness and a slightly reduced ultimate stress.

CONCLUSION

Thermal stability of the human lens capsule (type IV collagen) seems to be considerably lower than that of fibrous connective tissue (type I collagen). A potential risk of capsular shrinking has to be taken into account when the capsule is exposed to thermal stress during cataract surgery.

Effect of estrogen suppression on the mineralization density of iliac crest biopsies in young women as assessed by backscattered electron imaging

The effects of estrogen suppression on bone mineralization in young women were studied by quantitative backscattered electron (BSE) imaging of transiliac biopsies taken before and after treatment for endometriosis. Treatment (6 months) was with analogs of gonadotrophin releasing hormone (GnRH) given either alone (six paired biopsies), which resulted in a marked reduction in the levels of circulating estrogen, or in conjunction with tibolone, a synthetic steroid with estrogenic, progestrogenic, and androgenic properties (four paired biopsies). Estrogen withdrawal increased (p < 0.01) and concomitant tibolone treatment decreased (p < 0.05) the overall mean bone density. Estrogen withdrawal increased the fraction of bone with a high mineralization density [pretreatment: 0.236+/-0.007; GnRH: 0.279+/-0.009, mean +/- standard error of the mean (SEM); p < 0.01]. The concomitant addition of tibolone reversed these effects and increased the proportion of bone with a low mineralization density (pretreatment: 0.198 +/- 0.005; tibolone: 0.230 +/-0.008, p < 0.01). Using previously published data, the mean bone density was inversely correlated with mean wall thickness in cancellous bone (p = 0.030) and with the percentage of active osteons (p = 0.023) in cortical bone. Although treatment had similar effects on the mean bone mineralization density of cortical and cancellous bone, there were different distributions of mineralization between the two sites, with cancellous bone having more skewed and kurtotic distributions both before and after estrogen withdrawal. This study indicates that a short-term estrogen suppression results in the accumulation of bone with a higher mineralization density. As bone with a high mineral content has a decreased impact resistance, this might increase fracture risk. Understanding the cellular and biochemical mechanisms responsible for the local distribution of bone mineral when estrogen is withdrawn may allow the development of new strategies for maintaining bone quality after menopause.

The effects of ageing and changes in mineral content in degrading the toughness of human femora

Static toughness and impact strength of human cortical bone decline with age. This decline is strongly associated with an increase in the ash content of the bones but other factors, associated with age, are probably also responsible for the degradation of the bones' mechanical properties.