Full Length ArticleBone tissue aging affects mineralization of cement lines
Introduction
In human cortical bone, bone remodeling gives rise to cylindrical structural elements termed Haversian systems or secondary osteons [1]. They are constituted of a central canal surrounded by concentric osseous lamellae, including interlamellarly located osteocyte lacunae and the outermost covering sheath often designated as “cement line” (CL) or “cement sheath”. The cement line is considered the boundary of the osteon [2,3], separating more freshly deposited osteonal bone packets from predominantly older surrounding bone (interstitial bone). Apart from representing a clear boundary, the cement line also serves as a material bond between the osteon and its' surrounding [4,5]. Although cement lines are often considered as impermeable barriers, we recently observed that in younger individuals osteocyte canaliculi often penetrate the cement line and anchor in the surrounding bone, thus establishing communication between bone segments of different tissue ages [6]. This is essential for a successful adaptation of the whole bone to mechanical demands [6]. In contrast, aged individuals show significantly fewer connections between osteocyte lacunae from neighboring osteons or interstitial bone, reflecting impaired communication between tissue structural units [6].
The composition of the cement line was often subject for debate [7], and it was for years unclear if it is mineral-deficient [8,9] or mineral-rich [10,11]. Lack of agreement even about the basic composition of cement lines likely precluded in-depth analyses focusing on their relationship with mineralization processes. However, in a comprehensive study using backscattered electron microscopy Skedros et al. [12] suggested the mineral-rich (and/or collagen deficient) nature of the cement lines. In an early study, Philipson compared the content of sulfur and the arrangement of mineral fraction between the cement line and the adjacent osteon in orangutans and whales but found no significant differences [13]. Several studies have shown that non-collagenous proteins (i.e. osteopontin, glycosaminoglycans, osteocalcin, and bone sialoprotein) are a component of the cement line's matrix [12,14,15].
In general, new bone matrix during osteonal remodeling is produced within a few months [1,16]. Yet, the mineral is deposited at a much slower rate and the secondary mineralization takes place over longer time periods [17,18]. Hence, it was shown that the overall degree of mineralization in the osteon increases with maturation and tissue age [[19], [20], [21]]. Fuchs et al. have shown a linear relationship between osteonal age and secondary mineralization (measured by Fourier-transform infrared imaging - FTIR) in rabbits for osteons up to an age of 350 days [22]. Unfortunately comparative data cannot be obtained for humans easily, since it would require manifold administration of labeling agents. Akkus et al. measured the mineral content of primary bone packets in men with Raman microspectroscopy and found it to increase continuously over two decades in non-remodeled bone packets after which a plateau was reached [23]. However, it is unknown if the cement line mineralization also changes with age and whether a relationship between osteon and cement line mineralization processes exists.
Indeed, bone material is a mosaic of packets with different degrees of mineralization, and the interface between areas with differing material properties has been considered as an important site for crack deflection [12,[24], [25], [26], [27]]. The characteristics of the cement lines are therefore of great importance for fully understanding the paths of crack propagation and deflection through bone [8,12,28,29], as well as general mechanisms of fracture resistance. Understanding the role of cement lines will help to create a more complete view of the complex fracture resistance properties of bone, which can be further influenced by porosity [30]. This is particularly relevant in osteoporotic individuals that often sustain a catastrophic fracture due to extensive crack growth, as well as in bisphosphonate-treated osteoporotic patients where remodeling and mineralization are likely changed [5,31].
Considering that increasing mineralization, e.g. through crystal growth occurs frequently in many biological tissues including bone [32,33], we hypothesized that cement lines are not always equally mineralized, but their composition is related to the osteon's mineralization level – a surrogate marker of osteon's tissue age. To test this hypothesis, we have evaluated the mineralization of osteons and their corresponding cement lines in osteoporotic bone from untreated and bisphosphonate-treated elderly individuals. The understanding of cement line mineralization in relation to osteonal mineralization is important for interpreting and determining crack propagation and fracture resistance mechanisms in human cortical bone.
Section snippets
Specimen collection and preparation
The specimens used in this study were obtained from ten elderly postmenopausal women with osteoporosis, half of which were treatment-naïve (age: 81 ± 5 years) and the other half received antiresorptive treatment with alendronate (age: 86 ± 8 years, treatment duration: 6 ± 1.6 years, dosage: 10 mg/day or 70 mg/week). Osteoporosis was diagnosed by dual-energy X-ray absorptiometry (DXA) measurements at autopsy, and all patients' characteristics were available from medical records and autopsy
Results
The composition of the osteons and their cement lines in the femoral cortical bone in elderly osteoporotic individuals was analyzed (Fig. 1). Quantitative backscattered electron imaging (Fig. 2) confirmed that cement lines are hypermineralized, showing a mean calcium content of 29.46 ± 0.80 Ca wt% (Fig. 2A,D). Moreover, cement lines were constantly higher mineralized than their corresponding osteons in all examined tissue ages (p < 0.001, Table 1, Fig. 2A).
However, there was a clear positive
Discussion
Assessing the calcium content of cement lines belonging to osteons of variable mineralization, we show that the cement lines are indeed hypermineralized structures and that their degree of mineralization may represent another tissue-age related factor in the bone quality framework. Namely, although cement lines had a consistently higher calcium content than the osteon that they are surrounding, they showed a range of mineralization levels that were strongly dependent on the osteonal
Conclusions
Backscattered electron imaging and micro-Raman spectroscopy revealed that cement lines are hypermineralized structures. Moreover, cement lines' calcium content may represent another tissue-age related factor, which should be considered in the bone quality framework, given that it is strongly correlated to the osteonal calcium content, a surrogate marker of tissue age. Further investigation of the nature of cement line mineralization will help to understand fracture resistance mechanisms in
Acknowledgements
The authors acknowledge the support from the German Research Foundation (DFG, BU 2562/2-1/3-1), GRK 1896, cluster of excellence “Engineering of Advanced Materials” at the Friedrich-Alexander-Universität Erlangen-Nürnberg, the European Union (H2020, npSCOPE, 720964), Alexander von Humboldt Foundation (1162414 - HFST-P/2015), Southeast-European Cooperation of the University Medical Center Hamburg-Eppendorf, and the Serbian Ministry of Science (III45005).
References (58)
- et al.
Sub-lamellar microcracking and roles of canaliculi in human cortical bone
Acta Biomater.
(2012) - et al.
Osteon interfacial strength and histomorphometry of equine cortical bone
J. Biomech.
(2006) - et al.
Chapter 1 - bone morphology and organization
- et al.
Time sequence of secondary mineralization and microhardness in cortical and cancellous bone from ewes
Bone
(2010) - et al.
In situ examination of the time-course for secondary mineralization of Haversian bone using synchrotron Fourier transform infrared microspectroscopy
Matrix Biol.
(2008) - et al.
Fracture resistance of human cortical bone across multiple length-scales at physiological strain rates
Biomaterials
(2014) - et al.
Multi-level characterization of human femoral cortices and their underlying osteocyte network reveal trends in quality of young, aged, osteoporotic and antiresorptive-treated bone
Biomaterials
(2015) - et al.
Nanostructure and mineral composition of trabecular bone in the lateral femoral neck: implications for bone fragility in elderly women
Acta Biomater.
(2011) - et al.
Nano-structural, compositional and micro-architectural signs of cortical bone fragility at the superolateral femoral neck in elderly hip fracture patients vs. healthy aged controls
Exp. Gerontol.
(2014) - et al.
Osteopetrosis, osteopetrorickets and hypophosphatemic rickets differentially affect dentin and enamel mineralization
Bone
(2013)
Increased proportion of hypermineralized osteocyte lacunae in osteoporotic and osteoarthritic human trabecular bone: implications for bone remodeling
Bone
Validation of quantitative backscattered electron imaging for the measurement of mineral density distribution in human bone biopsies
Bone
Trends in trabecular architecture and bone mineral density distribution in 152 individuals aged 30–90 years
Bone
Raman imaging of two orthogonal planes within cortical bone
Bone
The bone mineralization density distribution as a fingerprint of the mineralization process
Bone
Osteocytes remove and replace perilacunar mineral during reproductive cycles
Bone
Multimodal correlative investigation of the interplaying micro-architecture, chemical composition and mechanical properties of human cortical bone tissue reveals predominant role of fibrillar organization in determining microelastic tissue properties
Acta Biomater.
The effect of bone microstructure on the initiation and growth of microcracks
J. Orthop. Res.
Crack deflection at an interface between dissimilar elastic materials
Int. J. Solids Struct.
Osteonal and hemi-osteonal remodeling: the spatial and temporal framework for signal traffic in adult human bone
J. Cell. Biochem.
1997 Whitaker distinguished lecture: models to solve mysteries in biomechanics at the cellular level; a new view of fiber matrix layers
Ann. Biomed. Eng.
Micro-morphological properties of osteons reveal changes in cortical bone stability during aging, osteoporosis, and bisphosphonate treatment in women
Osteoporos. Int.
Osteocytic canalicular networks: morphological implications for altered mechanosensitivity
ACS Nano
Bone's material constituents and their contribution to bone strength in health, disease, and treatment
Calcif. Tissue Int.
Composition of the cement line and its possible mechanical role as a local interface in human compact bone
J. Biomech.
Morphology of the osteonal cement line in human bone
Anat. Rec.
Human cranial bone structure and the healing of cranial bone grafts: a study using backscattered electron imaging and confocal microscopy
Anat. Embryol.
Mineralisation density of human mandibular bone: quantitative backscattered electron image analysis
J. Anat.
Cement lines of secondary osteons in human bone are not mineral-deficient: new data in a historical perspective
Anat. Rec. A
Cited by (45)
A numerical study of dehydration induced fracture toughness degradation in human cortical bone
2024, Journal of the Mechanical Behavior of Biomedical MaterialsMultiscale theoretical model shows that aging-related mechanical degradation of cortical bone is driven by microstructural changes in addition to porosity
2023, Journal of the Mechanical Behavior of Biomedical MaterialsOn the fracture behavior of cortical bone microstructure: The effects of morphology and material characteristics of bone structural components
2023, Journal of the Mechanical Behavior of Biomedical MaterialsPhase field models of interface failure for bone application - evaluation of open-source implementations
2022, Theoretical and Applied Fracture MechanicsCitation Excerpt :To date, there is limited experimental data regarding the mechanical properties of the cement line interface [21]. In this study, we assumed the interface stiffness to be equal to the matrix stiffness, however, future studies should investigate the effect of lower and higher interface stiffness to contribute to the ongoing debate concerning this topic [7,53,55]. Furthermore, even though the local fracture toughness of osteons and matrix cannot be accurately measured directly [56] there is experimental evidence that osteons typically have higher fracture toughness than the surrounding matrix [51,57].
Ionic liquid treatment for efficient sample preparation of hydrated bone for scanning electron microscopy
2022, MicronCitation Excerpt :An osteon is shown in the healthy bone in Fig. 8 (a), which was selected as a region of interest with known differences in bone mineral density for EDS analysis. Given that bone is a natural composite material composed primarily of calcium phosphate hydroxyapatite crystals, organic components, and unbound water, it was expected that C, Ca, P, and O would be key detectable elements in the structure and that higher density mineralized regions would have higher amounts of Ca and P (Clarke, 2008; Milovanovic et al., 2018; Shah et al., 2019; Skedros et al., 2005). A typical EDS spectrum collected during analysis is shown in Fig. 8 (a), where distinct peaks are noted for C, Ca, P, O, Na, F, and Mg; trace amounts of N, Si, Cl and S were also noted.
- 1
These authors contributed equally.