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The contribution of the organic matrix to bone’s material properties
Bone Vol. 31, No. 1 July 2002:8 –11
PERSPECTIVE
The Contribution of the Organic Matrix to Bone’s Material Properties D. B. BURR1,2 1 Departments of Anatomy and Cell Biology Indianapolis, IN, USA and 2Orthopaedic Surgery, Indiana University School of Medicine, Indianapolis, IN, USA
The overwhelming majority of work done on the mechanical properties of bone has concentrated only on the role that mineral plays in the strength and stiffness of bone.11,14,17 This concentration on mineral stems from the positive proportionality between mineral content and strength/stiffness demonstrated repeatedly over the past 30 years.11,14,17,18 The emphasis on bone mineral is reflected in our current propensity to correlate the strength of bone — and by extension its risk of fracture — almost solely with bone mineral density, even though it has been demonstrated that decreased bone mineral density can be associated, through compensatory structural and geometric adaptations, with maintained or increased strength.8,27 There has been much less emphasis on the role that collagen plays in the mechanical behavior of bone than on the role that mineral plays. The study by Wang et al. focuses on studying the role that collagen contributes to the mechanical properties of bone tissue, and how this might impact risk of fracture. They showed that a 35% decrease in strength, 30% decrease in modulus, and 50% decrease in toughness of the collagen network are associated with age-related declines in the strength and failure energy of whole bone.
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. (Bone 31:8 –11; 2002) © 2002 by Elsevier Science Inc. All rights reserved. Key Words: Collagen; Biomechanics; Osteoporosis; Fragility; Fracture.
Introduction Changes in Collagen Alter Bone’s Mechanical Properties and Increase Fracture Risk
The strength of bone, and its ability to resist fracture, is dependent on its mass and geometry, but also on intrinsic (material) properties of the bone tissue itself. Bone tissue is considered to be a two-component composite material composed primarily of collagen and mineral, each of which changes with age to alter the material properties of bone tissue (Table 1). The mineral component confers strength and stiffness to the tissue,14,18 but at increasing levels of mineralization, the tissue can become brittle, reducing the energy required for fracture.17,19 The collagen phase is more ductile; it may confer a degree of tensile stiffness, but probably plays a greater role in affecting the postyield properties of bone and the overall toughness of the bone tissue.52–55 Burstein et al.11 attempted to experimentally define the properties of each of the two phases in bone, but in actuality the combination of the two confer properties unlike each constituent separately. This has made it extremely challenging to identify the contribution of each, as an alteration in either the collagen or the mineral could have a substantial and unexpected effect on the overall mechanical and material properties of bone.
It is generally recognized, based on clinical observation, that pathologies that affect the material properties of bone tissue by changing its mineralization, such as osteomalacia or osteopetrosis, increase the risk of fracture. It is also true, however, that diseases that are primarily the result of a collagen defect without any inherent alteration of tissue mineralization can likewise increase the risk of fracture. For example, collagen defects in a mouse model of osteogenesis imperfecta (OI) reduced the postyield deformation of bone by 60%,29,30 and consequently reduced the work to fracture.45 There is ample evidence that changes to the collagen molecule alone can alter fracture risk. There have been several reports showing a direct link between a polymorphism in the COL1A1 gene, which encodes the ␣1(I) protein chain of type I collagen, and increased risk of fracture independent of low bone mineral density (BMD).34,35 Using a meta-analysis, Mann et al.35 showed a stronger association between the COL1A1 Sp1 binding site polymorphism and osteoporotic fracture than between these and BMD or body mass index (BMI) at both the lumbar spine and the femoral neck. In a separate study, Bernad et al.6 also showed the COL1A1 (Sp1) TT genotype to be associated with a 5.9-fold increase in fracture risk in postmenopausal women, and 4.8-fold when prevalence was adjusted for age, BMI, and BMD.
Address for correspondence and reprints: Dr. David B. Burr, Department of Anatomy and Cell Biology, MS 5035, Indiana University School of Medicine, 635 Barnhill Drive, Indianapolis IN 46202. E-mail: [email protected] © 2002 by Elsevier Science Inc. All rights reserved.
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Bone Vol. 31, No. 1 July 2002:8 –11
D. B. Burr Organic matrix and material properties
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Table 1. Features of bone tissue that reduce strength and fracture toughnessa Effect Feature
Modulus
Ultimate strain
Ultimate stress
Toughness
Cause/example
Poorly mineralized bone Hypermineralized bone
s a
a s
s a
s s
Increased crystallinity: ⌬ morphology of apatite crystal
a
s
?
s
Denaturation of collagen moleculeb Debonding of mineral/collagenb
s s
s a
s s
s s
Osteomalacia Reduced turnover Increased mean tissue age Reduced turnover Increased mean tissue age Fluoride accumulationc Unclear Fluoride accumulationc,d
a
From ref. 9. From refs. 12, 43, 44, and 51, 52, 54. From ref. 49. d From ref. 51. b c
This polymorphism increases the collagen content of the tissue, and alters the ratio of the collagen ␣1(I) chain to the ␣2(I) chain. Although the effect on fracture risk could be partly explained by the lower mineral content in heterozygotes,31,50 the fact that the effects are at least partly independent of bone mass6 suggests a potential direct effect of the alteration in collagen on bone quality. Wang et al.53 recently attempted to address the difficulty of assessing the direct effect of collagen, independent of mineral, on the biomechanical properties of bone using a model of heatinduced collagen denaturation in human cadaver femurs. Collagen denaturation without a change in bone mineral significantly decreases bone’s toughness and overall strength, while having minimal effects on elastic modulus. This underscores the positive contribution that collagen makes to increase the energy required for bone failure.
cracks were initiated in specimens from older women (mean age 72 ⫾ 6 years), but not bone from younger women (mean age 26 ⫾ 5 years), even with an equivalent 34% decline in elastic modulus in both groups.13 This suggests that microdamage accumulation in bone from elderly women results from some inherent fragility in the tissue. In studies using a baboon model, the percentage of denatured collagen as compared with total collagen content was significantly related to failure energy and to the fracture toughness of the tissue.52,54 This means that collagen in bone is a primary arrestor of cracks, inhibiting their growth to critical dimension. This may be one explanation for the observation that aging has a more profound effect on the plastic deformation of bone than it has on elastic deformation.38 It is generally thought that one component of the increased tissue fragility in older people is the older mean tissue age that occurs as a result of the slowing of bone turnover as one
Collagen’s Role in Matrix Failure There is a natural tendency to focus on the importance of bone’s strength and stiffness, both as a structure and as a tissue. These are properties that are easily defined mechanically (Figure 1), and for which most of us have an intuitive understanding. Collagen may not be an influential contributor to the strength and stiffness of the whole bone, or bone matrix. This is evident from the respective elastic moduli of collagen (ⵒ1.5 GPa) and hydroxyapatite (114 GPa).58 Collagen is, however, crucial to how much energy is required to cause matrix failure.53 This mechanical property — failure energy of the material independent of its size or geometry — is defined by the area under the stress-strain curve and is called the modulus of toughness. (Toughness can also be defined as the level of stress required at a crack tip to cause the crack to propagate. This fracture mechanics property is called the stress intensity factor [Kc]. It is related to the modulus of toughness, but is a different property of bone, measured experimentally in a different way. The modulus of toughness is reported as N/m2 or J/m3, whereas Kc is reported in units of N/m or J/m2).36,41,48 Collagen may be the primary toughening mechanism in bone, having greater effects on bone toughness52–55 than on strength or stiffness.56 Indeed, when bone is subjected to ionizing radiation that specifically damages the collagen, bone toughness declines.20 As one ages, there is an inherent fragility in the bone tissue that allows it to be damaged more easily from cyclic loading.13 It is possible, in fact likely, that changes in the bone collagen with age, and in osteoporosis, are at least partly responsible for this increased fragility. In cyclic load tests performed ex vivo,
Figure 1. A typical mechanical test generates a load-deformation curve from which strength, stiffness and work (energy) to failure can be measured. Strength can be defined either by the ultimate force at failure, or by the load at yield. Stiffness is defined by the slope of the linear portion of the load-deformation curve, and work to failure is calculated as the area under the curve prior to the point of ultimate failure (shaded area). These properties are extrinsic properties of the structure; they are determined both by the properties of the tissue and by the size and geometry of the bone. When these measurements are normalized by cross-sectional area or moment of inertia, this curve can be converted to a stress-strain curve from which the properties of the tissue (intrinsic properties) itself can be estimated. In this case, the ultimate stress is a measure of the material strength. The slope of the stress-strain curve defines the elastic (Young’s) modulus of the tissue, whereas the area under the stress-strain curve is the modulus of toughness, which is the energy required to cause failure of the bone matrix itself, independent of the bone’s size or geometry. These concepts have been described more fully in references 17, 36, and 48.
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D. B. Burr Organic matrix and material properties
ages.7,10,15,16,19,24 It could also be related to molecular changes in either the inorganic26,43,44,47 or the organic3,23,39,42 fractions of the matrix.4 The age-related reduction in postyield deformation (i.e., less energy in the postyield region required for fracture; Figure 1) that occurs in the collagen moiety, demonstrated in the companion study by Wang et al.,55 may contribute equally to age-related fragility by decreasing overall fracture toughness,18,40,56,57 and increasing risk of fracture, in older men and women. The age-related reduction in the ability of bone to absorb energy prior to failure is clinically important in making osteoporotic bone more prone to failure from any impact load, such as one resulting from a fall. Collagen changes that reduce bone’s energy absorption capability, therefore, may be a primary factor increasing the risk of fracture in older women with low bone mass. Both Collagen Content and Collagen Cross-linking Affect Bone’s Mechanical Properties The precise reasons for the age-related declines in strength and failure energy of whole bone are not clear, and may be different in the processes of normal aging compared with those of an osteoporotic condition. The role of collagen may be related either to the amount of collagen or to its molecular stability and cross-linking. Bailey et al.2 found that the age-related decline in collagen content in bone from the human iliac crest was nonlinearly correlated to maximum stress at failure (a strength criterion, r2 ⫽ 0.46 – 0.50 in men and women, respectively) and to modulus of elasticity (r2 ⫽ 0.36 – 0.48 in men and women, respectively). This does not mean, of course, that changes in collagen content were necessarily responsible for the changes, only that they are associated with them. Other changes with age, for example, the rate of bone turnover or the degree of mineralization, could affect both the collagen content and the mechanical properties of bone independently. In other locations, such as the femoral head and neck, no changes in collagen content were detected,3 so the importance of such changes is not clear. It has been proposed that the interfibrillar pyrrole cross-links have a greater influence on the bending strength of bone than the intrafibrillar pyridinoline cross-links.2 This is consistent with the decline in pyrrole cross-links, and the constant level of pyridinoline cross-links, in osteoporosis. It has also been speculated, however, that the intramolecular cross-links are important for enhancement of bone toughness, whereas the intermolecular bonds may be less important to toughness. 58 The maturity of the cross-links may not be very important to bone’s mechanical properties,2,58 although there is a decrease in the number of immature collagen cross-links in newly formed collagen from osteoporotic subjects.1 Wang et al.55 suggested that the agerelated degradation of collagen’s mechanical properties is due to the increased concentration of pentosidine, a marker of nonenzymatic glycation of the collagen. Their work demonstrates that normal changes in collagen with age can have a significant effect on the properties of the bone. Studies on rat femora suggest that the decline in bone’s mechanical properties with age may be dependent on the stability of the collagen.21,22 With age, there is a decrease in collagen content,2 which is associated with an increased mean tissue mineralization, but there is no difference in cross-link levels compared with younger adult bone.3,58 However, the stability of the cross-links may change with age,21,22 and this can have an effect on the fragility of the bone tissue. In humans, the declines tend to be more marked and more uniform in men than in women.23 In osteoporosis, there is a decrease in the reducible collagen cross-links without an alteration in collagen concentration42; this
Bone Vol. 31, No. 1 July 2002:8 –11
would tend to increase bone fragility.3 Both animal33 and human1 studies suggest that newly synthesized collagen in osteoporotic bone matrix has increased lysine hydroxylation by 35%– 40%. This appears to be correlated to decreased strength in three-point bending.1,32,33 Because of the interactions between matrix collagens, integrin expression, and cell function,5 this may have implications for the regulation of the osteoblast phenotype37,46 and cell-signaling processes25 that underlie bone remodeling in osteoporosis. Conclusions Changes in collagen structure underlie an age-associated reduction in bone toughness, independent of mineral density or mean tissue age. This reduction in toughness increases fracture risk independent of bone mineral density.34,35 Apart from changes in mineral density, the inherent risk of fracture can change if the quality of the organic component of the bone is altered. This may contribute to the portion of the fracture risk that is currently unexplained.28 The contribution of Wang et al.55 regarding this issue is important in showing that changes in the mechanical properties in bone collagen occur with age, and that these are correlated to age-related changes in whole bone strength and toughness. They have gone beyond studies of simple association between biochemical changes in collagen and mechanical properties to identify specific changes in the mechanical properties of collagen with age. This means that it is not just changes in mineral content or mineral density that we must concern ourselves with in evaluating the propensity of bone to fracture, but also changes in the organic nature of the bone matrix. Attention must be paid to this component of fracture risk when risk assessments are made based on bone mineral density and geometry. References 1. Bailey, A. J. and Knott, L. Molecular changes in bone collagen in osteoporosis and osteoarthritis in the elderly. Exper Gerontol 34:337–351; 1999. 2. Bailey, A. J., Sims, T. J., Ebbesen, E. N., Mansell, J. P., Thomsen, J. S., and Mosekilde, Li. Age-related changes in the biochemical properties of human cancellous bone collagen: Relationship to bone strength. Calcif Tissue Int 65:203–210; 1999. 3. Bailey, A. J., Wotton, S. F., Sims, T. J., and Thompson, P. W. Biochemical changes in the collagen of human osteoporotic bone matrix. Connect Tissue Res 29:119 –132; 1993. 4. Batge, B., Diebold, J., Bodo, M., Fehm, H. L., Muller, P. K. 1992 Evidence for matrix alterations in osteoporosis. In: Ring, E. F. J., Ed. Current Research in Osteoporosis and Bone Mineral Measurement II. London: British Institute of Radiology. 5. Bennett, J. H., Moffatt, S., and Horton, M. Cell adhesion molecules in human osteoblasts: Structure and function. Histol Histopathol 16:603–611; 2001. 6. Bernad, M., Martinex, M. W., Escalona, M., Gonza´ lez, M. L., Gonza´ lez, C., Garce´ s, M. V., Del Campo, M. T., Martı´n Mola, E., Madero`, R., and Carren˜o´, L. Polymorphism in the type I collagen (COL1A1) gene and risk of fractures in postmenopausal women. Bone 30:223–228; 2002. 7. Birkenhager-Frenkel, D. H., and Nigg, A. L. Age-related bone loss as reflected by changes of interstitial bone thickness. Calcif Tissue Int 52(Suppl. 1):S60; 1993 [abstract]. 8. Burr, D. B., Hirano, T., Turner, C. H., Hotchkiss, C., Brommage, R., and Hock, J. M. Intermittently administered human parathyroid hormone (1-34) treatment increases intracortical bone turnover and porosity without reducing bone strength in the humerus of ovariectomized cynomolgus monkeys. J Bone Miner Res 16:157–165; 2001. 9. Burr, D. B. and Turner, C. H. Biomechanical measurements in age-related bone loss. In: Rosen, C. J., Glowacki, J., and Bilezikian, J. P., eds. The Aging Skeleton. San Diego, CA: Academic; 301–311; 1999. 10. Burstein, A. H., Reilly, D. T., and Martens, M. Aging of bone tissue: Mechanical properties. J Bone Jt Surg 58A:82–86; 1976.
Bone Vol. 31, No. 1 July 2002:8 –11 11. Burstein, A. H., Zika, J. M., Heiple, K. G., and Klein, L. Contribution of collagen and mineral to the elastic-plastic properties of bone. J Bone Jt Surg 57-A:956 –961; 1975. 12. Catanese, J. III and Keaveny, T. M. Role of collagen and hydroxyapatite in the mechanical behavior of bone tissue. J Bone Miner Res 11(Suppl. 1):S295; 1996. 13. Courtney, A. C., Hayes, W. C., and Gibson, L. J. Age-related differences in post-yield damage in human cortical bone: Experiment and model. J Biomech 29:1463–1471; 1996. 14. Currey, J. D. The mechanical consequences of variation in the mineral content of bone. J Biomech 2:1–11; 1969. 15. Currey, J. D. The mechanical properties of bone. Clin Orthop Rel Res 73:210 –231; 1970. 16. Currey, J. D. Changes in impact energy absorption of bone with age. J Biomech 12:459 –469; 1979. 17. Currey, J. D. The Mechanical Adaptations of Bones. Princeton, NJ: Princeton University; 1984. 18. Currey, J. D. The effect of porosity and mineral content on the Young’s modulus of elasticity of compact bone. J Biomech 21:131–139; 1988. 19. Currey, J. D., Brear, K., and Zioupos, P. The effects of aging and changes in mineral content in degrading the toughness of human femora. J Biomech 29:257–260; 1996. 20. Currey, J. D., Foreman, J., Laketic, I., Mitchel, J., Pegg, D. E., and Reilly, G. C. Effects of ionizing radiation on the mechanical properties of human bone. J Orthop Res 15:111–117; 1997. 21. Danielsen, C. C. Age-related thermal stability and susceptibility to proteolysis of rat bone collagen. Biochem J 272:697–701; 1990. 22. Danielsen, C. C., Andreassen, T. T., and Mosekilde, L. Mechanical properties of collagen from decalcified rat femur in relation to age and in vitro maturation. Calcif Tissue Int 39:69 –73; 1986. 23. Danielsen, C. C., Mosekilde, Li., Bollerslev, J., and Mosekilde, Le. Thermal stability of cortical bone collagen in relation to age in normal individuals and in individuals with osteoporosis. Bone 15:91–96; 1994. 24. Dickenson, R. P., Hutton, W. C., and Stott, J. R. R. The mechanical properties of bone in osteoporosis. J Bone Jt Surg 63-B:233–238; 1981. 25. Green, J., Schotland, S., Stauber, D. J., Kleeman, C. R., and Clemens, T. L. Cell-matrix interaction in bone: Type I collagen modulates signal transduction in osteoblast-like cells. Am J Physiol 37:C1090 –C1103; 1995. 26. Grynpas, M. Age and disease-related changes in the mineral of bone. Calcif Tissue Int 53(Suppl):S57–S64; 1993. 27. Hirano, T., Burr, D. B., Cain, R. L., and Hock, J. M. Changes in geometry and cortical porosity in adult, ovary-intact rabbits after 5 months treatment with LY333334 (hPTH 1-34). Calcif Tissue Int 66:456 –460; 2000. 28. Hui, S., Slemenda, C. W., and Johnston, C. C. Age and bone mass as predictors of fracture in a prospective study. J Clin Invest 81:1804 –1809; 1988. 29. Jepsen, K., Goldstein, S. A., Kuhn, J. L., Schaffler, M. B., and Bonadio, J. Type-I collagen mutation compromises the post-yield behavior of Mov13 long bone. J Orthop Res 14:493–499; 1996. 30. Jepsen, K. J., Schaffler, M. B., Kuhn, J. L., Goulet, R. W., Bonadio, J., and Goldstein, S. A. Type I collagen mutation alters the strength and fatigue behavior of Mov13 cortical tissue. J Biomech 30:1141–1147; 1997. 31. Keen, R. W., Woodford-Richens, K. L., Grant, S. F., Ralston, S. H., Lanchbury, J. S., and Spector, T. D. Association of polymorphism at the type I collagen (COL1A1) locus with reduced bone mineral density, increased fracture risk, and increased collagen turnover. Arthr Rheumat 42:285–290; 1999. 32. Knott, L. and Bailey, A. J. Collagen cross-links in mineralizing tissues: A review of their chemistry, function, and clinical relevance. Bone 22:181–187; 1998. 33. Knott, L., Whitehead, C. C., Fleming, R. H., and Vailey, A. J. Biochemical changes in the collagenous matrix of osteoporotic avian bone. Biochem J 310:1045–1051; 1995. 34. Langdahl, B. L., Ralston, S. H., Grant, S. F. A., and Eriksen, R. F. An Sp1 binding site polymorphism in the COL1A1 gene predicts osteoporotic fractures in both men and women. J Bone Miner Res 13:1384 –1389; 1998. 35. Mann, V., Hobson, E. E., Li, B., Stewart, T. L., Grant, S. F. A., Robins, S. P., Aspden, R. M., and Ralston, S. H. A COL1A1 Sp1 binding site polymorphism predisposes to osteoporotic fracture by affecting bone density and quality. J Clin Invest 197:899 –907; 2001. 36. Martin, R. B., Burr, D. B., and Sharkey, N. A. Skeletal Tissue Mechanics. New York: Springer; 1998 127–141. 37. Masi, L., Franchi, A., Santucci, M., Danielli, D., Arganini, L., Giannone, V., Formigli, L., Benvenuti, S., Tanini, A., Beghe, F., Mian, M., and Brandi, M. L.
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Date Received: November 7, 2001 Date Revised: March 11, 2002 Date Accepted: March 12, 2002
PERSPECTIVE
The Contribution of the Organic Matrix to Bone’s Material Properties D. B. BURR1,2 1 Departments of Anatomy and Cell Biology Indianapolis, IN, USA and 2Orthopaedic Surgery, Indiana University School of Medicine, Indianapolis, IN, USA
The overwhelming majority of work done on the mechanical properties of bone has concentrated only on the role that mineral plays in the strength and stiffness of bone.11,14,17 This concentration on mineral stems from the positive proportionality between mineral content and strength/stiffness demonstrated repeatedly over the past 30 years.11,14,17,18 The emphasis on bone mineral is reflected in our current propensity to correlate the strength of bone — and by extension its risk of fracture — almost solely with bone mineral density, even though it has been demonstrated that decreased bone mineral density can be associated, through compensatory structural and geometric adaptations, with maintained or increased strength.8,27 There has been much less emphasis on the role that collagen plays in the mechanical behavior of bone than on the role that mineral plays. The study by Wang et al. focuses on studying the role that collagen contributes to the mechanical properties of bone tissue, and how this might impact risk of fracture. They showed that a 35% decrease in strength, 30% decrease in modulus, and 50% decrease in toughness of the collagen network are associated with age-related declines in the strength and failure energy of whole bone.
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. (Bone 31:8 –11; 2002) © 2002 by Elsevier Science Inc. All rights reserved. Key Words: Collagen; Biomechanics; Osteoporosis; Fragility; Fracture.
Introduction Changes in Collagen Alter Bone’s Mechanical Properties and Increase Fracture Risk
The strength of bone, and its ability to resist fracture, is dependent on its mass and geometry, but also on intrinsic (material) properties of the bone tissue itself. Bone tissue is considered to be a two-component composite material composed primarily of collagen and mineral, each of which changes with age to alter the material properties of bone tissue (Table 1). The mineral component confers strength and stiffness to the tissue,14,18 but at increasing levels of mineralization, the tissue can become brittle, reducing the energy required for fracture.17,19 The collagen phase is more ductile; it may confer a degree of tensile stiffness, but probably plays a greater role in affecting the postyield properties of bone and the overall toughness of the bone tissue.52–55 Burstein et al.11 attempted to experimentally define the properties of each of the two phases in bone, but in actuality the combination of the two confer properties unlike each constituent separately. This has made it extremely challenging to identify the contribution of each, as an alteration in either the collagen or the mineral could have a substantial and unexpected effect on the overall mechanical and material properties of bone.
It is generally recognized, based on clinical observation, that pathologies that affect the material properties of bone tissue by changing its mineralization, such as osteomalacia or osteopetrosis, increase the risk of fracture. It is also true, however, that diseases that are primarily the result of a collagen defect without any inherent alteration of tissue mineralization can likewise increase the risk of fracture. For example, collagen defects in a mouse model of osteogenesis imperfecta (OI) reduced the postyield deformation of bone by 60%,29,30 and consequently reduced the work to fracture.45 There is ample evidence that changes to the collagen molecule alone can alter fracture risk. There have been several reports showing a direct link between a polymorphism in the COL1A1 gene, which encodes the ␣1(I) protein chain of type I collagen, and increased risk of fracture independent of low bone mineral density (BMD).34,35 Using a meta-analysis, Mann et al.35 showed a stronger association between the COL1A1 Sp1 binding site polymorphism and osteoporotic fracture than between these and BMD or body mass index (BMI) at both the lumbar spine and the femoral neck. In a separate study, Bernad et al.6 also showed the COL1A1 (Sp1) TT genotype to be associated with a 5.9-fold increase in fracture risk in postmenopausal women, and 4.8-fold when prevalence was adjusted for age, BMI, and BMD.
Address for correspondence and reprints: Dr. David B. Burr, Department of Anatomy and Cell Biology, MS 5035, Indiana University School of Medicine, 635 Barnhill Drive, Indianapolis IN 46202. E-mail: [email protected] © 2002 by Elsevier Science Inc. All rights reserved.
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D. B. Burr Organic matrix and material properties
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Table 1. Features of bone tissue that reduce strength and fracture toughnessa Effect Feature
Modulus
Ultimate strain
Ultimate stress
Toughness
Cause/example
Poorly mineralized bone Hypermineralized bone
s a
a s
s a
s s
Increased crystallinity: ⌬ morphology of apatite crystal
a
s
?
s
Denaturation of collagen moleculeb Debonding of mineral/collagenb
s s
s a
s s
s s
Osteomalacia Reduced turnover Increased mean tissue age Reduced turnover Increased mean tissue age Fluoride accumulationc Unclear Fluoride accumulationc,d
a
From ref. 9. From refs. 12, 43, 44, and 51, 52, 54. From ref. 49. d From ref. 51. b c
This polymorphism increases the collagen content of the tissue, and alters the ratio of the collagen ␣1(I) chain to the ␣2(I) chain. Although the effect on fracture risk could be partly explained by the lower mineral content in heterozygotes,31,50 the fact that the effects are at least partly independent of bone mass6 suggests a potential direct effect of the alteration in collagen on bone quality. Wang et al.53 recently attempted to address the difficulty of assessing the direct effect of collagen, independent of mineral, on the biomechanical properties of bone using a model of heatinduced collagen denaturation in human cadaver femurs. Collagen denaturation without a change in bone mineral significantly decreases bone’s toughness and overall strength, while having minimal effects on elastic modulus. This underscores the positive contribution that collagen makes to increase the energy required for bone failure.
cracks were initiated in specimens from older women (mean age 72 ⫾ 6 years), but not bone from younger women (mean age 26 ⫾ 5 years), even with an equivalent 34% decline in elastic modulus in both groups.13 This suggests that microdamage accumulation in bone from elderly women results from some inherent fragility in the tissue. In studies using a baboon model, the percentage of denatured collagen as compared with total collagen content was significantly related to failure energy and to the fracture toughness of the tissue.52,54 This means that collagen in bone is a primary arrestor of cracks, inhibiting their growth to critical dimension. This may be one explanation for the observation that aging has a more profound effect on the plastic deformation of bone than it has on elastic deformation.38 It is generally thought that one component of the increased tissue fragility in older people is the older mean tissue age that occurs as a result of the slowing of bone turnover as one
Collagen’s Role in Matrix Failure There is a natural tendency to focus on the importance of bone’s strength and stiffness, both as a structure and as a tissue. These are properties that are easily defined mechanically (Figure 1), and for which most of us have an intuitive understanding. Collagen may not be an influential contributor to the strength and stiffness of the whole bone, or bone matrix. This is evident from the respective elastic moduli of collagen (ⵒ1.5 GPa) and hydroxyapatite (114 GPa).58 Collagen is, however, crucial to how much energy is required to cause matrix failure.53 This mechanical property — failure energy of the material independent of its size or geometry — is defined by the area under the stress-strain curve and is called the modulus of toughness. (Toughness can also be defined as the level of stress required at a crack tip to cause the crack to propagate. This fracture mechanics property is called the stress intensity factor [Kc]. It is related to the modulus of toughness, but is a different property of bone, measured experimentally in a different way. The modulus of toughness is reported as N/m2 or J/m3, whereas Kc is reported in units of N/m or J/m2).36,41,48 Collagen may be the primary toughening mechanism in bone, having greater effects on bone toughness52–55 than on strength or stiffness.56 Indeed, when bone is subjected to ionizing radiation that specifically damages the collagen, bone toughness declines.20 As one ages, there is an inherent fragility in the bone tissue that allows it to be damaged more easily from cyclic loading.13 It is possible, in fact likely, that changes in the bone collagen with age, and in osteoporosis, are at least partly responsible for this increased fragility. In cyclic load tests performed ex vivo,
Figure 1. A typical mechanical test generates a load-deformation curve from which strength, stiffness and work (energy) to failure can be measured. Strength can be defined either by the ultimate force at failure, or by the load at yield. Stiffness is defined by the slope of the linear portion of the load-deformation curve, and work to failure is calculated as the area under the curve prior to the point of ultimate failure (shaded area). These properties are extrinsic properties of the structure; they are determined both by the properties of the tissue and by the size and geometry of the bone. When these measurements are normalized by cross-sectional area or moment of inertia, this curve can be converted to a stress-strain curve from which the properties of the tissue (intrinsic properties) itself can be estimated. In this case, the ultimate stress is a measure of the material strength. The slope of the stress-strain curve defines the elastic (Young’s) modulus of the tissue, whereas the area under the stress-strain curve is the modulus of toughness, which is the energy required to cause failure of the bone matrix itself, independent of the bone’s size or geometry. These concepts have been described more fully in references 17, 36, and 48.
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D. B. Burr Organic matrix and material properties
ages.7,10,15,16,19,24 It could also be related to molecular changes in either the inorganic26,43,44,47 or the organic3,23,39,42 fractions of the matrix.4 The age-related reduction in postyield deformation (i.e., less energy in the postyield region required for fracture; Figure 1) that occurs in the collagen moiety, demonstrated in the companion study by Wang et al.,55 may contribute equally to age-related fragility by decreasing overall fracture toughness,18,40,56,57 and increasing risk of fracture, in older men and women. The age-related reduction in the ability of bone to absorb energy prior to failure is clinically important in making osteoporotic bone more prone to failure from any impact load, such as one resulting from a fall. Collagen changes that reduce bone’s energy absorption capability, therefore, may be a primary factor increasing the risk of fracture in older women with low bone mass. Both Collagen Content and Collagen Cross-linking Affect Bone’s Mechanical Properties The precise reasons for the age-related declines in strength and failure energy of whole bone are not clear, and may be different in the processes of normal aging compared with those of an osteoporotic condition. The role of collagen may be related either to the amount of collagen or to its molecular stability and cross-linking. Bailey et al.2 found that the age-related decline in collagen content in bone from the human iliac crest was nonlinearly correlated to maximum stress at failure (a strength criterion, r2 ⫽ 0.46 – 0.50 in men and women, respectively) and to modulus of elasticity (r2 ⫽ 0.36 – 0.48 in men and women, respectively). This does not mean, of course, that changes in collagen content were necessarily responsible for the changes, only that they are associated with them. Other changes with age, for example, the rate of bone turnover or the degree of mineralization, could affect both the collagen content and the mechanical properties of bone independently. In other locations, such as the femoral head and neck, no changes in collagen content were detected,3 so the importance of such changes is not clear. It has been proposed that the interfibrillar pyrrole cross-links have a greater influence on the bending strength of bone than the intrafibrillar pyridinoline cross-links.2 This is consistent with the decline in pyrrole cross-links, and the constant level of pyridinoline cross-links, in osteoporosis. It has also been speculated, however, that the intramolecular cross-links are important for enhancement of bone toughness, whereas the intermolecular bonds may be less important to toughness. 58 The maturity of the cross-links may not be very important to bone’s mechanical properties,2,58 although there is a decrease in the number of immature collagen cross-links in newly formed collagen from osteoporotic subjects.1 Wang et al.55 suggested that the agerelated degradation of collagen’s mechanical properties is due to the increased concentration of pentosidine, a marker of nonenzymatic glycation of the collagen. Their work demonstrates that normal changes in collagen with age can have a significant effect on the properties of the bone. Studies on rat femora suggest that the decline in bone’s mechanical properties with age may be dependent on the stability of the collagen.21,22 With age, there is a decrease in collagen content,2 which is associated with an increased mean tissue mineralization, but there is no difference in cross-link levels compared with younger adult bone.3,58 However, the stability of the cross-links may change with age,21,22 and this can have an effect on the fragility of the bone tissue. In humans, the declines tend to be more marked and more uniform in men than in women.23 In osteoporosis, there is a decrease in the reducible collagen cross-links without an alteration in collagen concentration42; this
Bone Vol. 31, No. 1 July 2002:8 –11
would tend to increase bone fragility.3 Both animal33 and human1 studies suggest that newly synthesized collagen in osteoporotic bone matrix has increased lysine hydroxylation by 35%– 40%. This appears to be correlated to decreased strength in three-point bending.1,32,33 Because of the interactions between matrix collagens, integrin expression, and cell function,5 this may have implications for the regulation of the osteoblast phenotype37,46 and cell-signaling processes25 that underlie bone remodeling in osteoporosis. Conclusions Changes in collagen structure underlie an age-associated reduction in bone toughness, independent of mineral density or mean tissue age. This reduction in toughness increases fracture risk independent of bone mineral density.34,35 Apart from changes in mineral density, the inherent risk of fracture can change if the quality of the organic component of the bone is altered. This may contribute to the portion of the fracture risk that is currently unexplained.28 The contribution of Wang et al.55 regarding this issue is important in showing that changes in the mechanical properties in bone collagen occur with age, and that these are correlated to age-related changes in whole bone strength and toughness. They have gone beyond studies of simple association between biochemical changes in collagen and mechanical properties to identify specific changes in the mechanical properties of collagen with age. This means that it is not just changes in mineral content or mineral density that we must concern ourselves with in evaluating the propensity of bone to fracture, but also changes in the organic nature of the bone matrix. Attention must be paid to this component of fracture risk when risk assessments are made based on bone mineral density and geometry. References 1. Bailey, A. J. and Knott, L. Molecular changes in bone collagen in osteoporosis and osteoarthritis in the elderly. Exper Gerontol 34:337–351; 1999. 2. Bailey, A. J., Sims, T. J., Ebbesen, E. N., Mansell, J. P., Thomsen, J. S., and Mosekilde, Li. Age-related changes in the biochemical properties of human cancellous bone collagen: Relationship to bone strength. Calcif Tissue Int 65:203–210; 1999. 3. Bailey, A. J., Wotton, S. F., Sims, T. J., and Thompson, P. W. Biochemical changes in the collagen of human osteoporotic bone matrix. Connect Tissue Res 29:119 –132; 1993. 4. Batge, B., Diebold, J., Bodo, M., Fehm, H. L., Muller, P. K. 1992 Evidence for matrix alterations in osteoporosis. In: Ring, E. F. J., Ed. Current Research in Osteoporosis and Bone Mineral Measurement II. London: British Institute of Radiology. 5. Bennett, J. H., Moffatt, S., and Horton, M. Cell adhesion molecules in human osteoblasts: Structure and function. Histol Histopathol 16:603–611; 2001. 6. Bernad, M., Martinex, M. W., Escalona, M., Gonza´ lez, M. L., Gonza´ lez, C., Garce´ s, M. V., Del Campo, M. T., Martı´n Mola, E., Madero`, R., and Carren˜o´, L. Polymorphism in the type I collagen (COL1A1) gene and risk of fractures in postmenopausal women. Bone 30:223–228; 2002. 7. Birkenhager-Frenkel, D. H., and Nigg, A. L. Age-related bone loss as reflected by changes of interstitial bone thickness. Calcif Tissue Int 52(Suppl. 1):S60; 1993 [abstract]. 8. Burr, D. B., Hirano, T., Turner, C. H., Hotchkiss, C., Brommage, R., and Hock, J. M. Intermittently administered human parathyroid hormone (1-34) treatment increases intracortical bone turnover and porosity without reducing bone strength in the humerus of ovariectomized cynomolgus monkeys. J Bone Miner Res 16:157–165; 2001. 9. Burr, D. B. and Turner, C. H. Biomechanical measurements in age-related bone loss. In: Rosen, C. J., Glowacki, J., and Bilezikian, J. P., eds. The Aging Skeleton. San Diego, CA: Academic; 301–311; 1999. 10. Burstein, A. H., Reilly, D. T., and Martens, M. Aging of bone tissue: Mechanical properties. J Bone Jt Surg 58A:82–86; 1976.
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Date Received: November 7, 2001 Date Revised: March 11, 2002 Date Accepted: March 12, 2002