Age-related differences in the morphology of microdamage propagation in trabecular bone
Introduction
Fatigue microdamage forms after repeated loading in daily activities (Burr et al., 1985). Damage formation is not necessarily pathologic; microdamage dissipates strain energy and limits the development of more serious fractures at sites subjected to high stress (Vashishth et al., 1997). Damaged sites are repaired using targeted remodeling mechanisms, preserving the structural integrity of the trabecular lattice (Burr et al., 1985). With aging, however, targeted remodeling processes diminish and allow fatigue microdamage to accumulate (Schaffler and Choi, 1995, Allen and Burr, 2008). Also, changes in trabecular matrix properties and morphology with age, including increased local mineralization levels, decreased thickness, increased spacing, loss of connectivity, and increased anisotropy, contribute to the deterioration of bone strength and stiffness and can promote microdamage formation and propagation (Burr et al., 1997). Significant damage accumulation lowers stiffness and increases the probability of fracture (Burr and Turner, 1998, Sobelman and Gibeling, 2004).
Much of what is known about age-related differences in microdamage and fatigue properties is derived from cortical (Schaffler and Choi, 1995, Vashishth and Tanner, 2000, Norman and Yeni, 1998, Zioupos, 2001a, Zioupos, 2001b) rather than trabecular bone studies (Seeman, 2003; Cook and Zioupos, 2009). Previous studies have reported that bone from younger individuals has a longer fatigue life due to increased formation of diffuse damage, whereas bone from older donors form fewer but longer linear microcracks (Diab et al., 2006). Linear microdamage, associated with a quadratic loss of modulus, propagates more easily than diffuse damage, which can dissipate energy while limiting propagation (Burr and Turner, 1998, Diab and Condon, 2006). However it is unclear whether these findings are translatable to trabecular bone.
Furthermore, it is unclear whether sex differences influence the fatigue behavior of trabecular bone. Histomorphometric studies have found few bone volume fraction or compressive strength differences attributable to sex in vertebral trabecular bone in age-matched groups (Bergot and Laval-Jeantet, 1988, Thomsen and Ebbesen, 2000, Keaveny and Yeh, 2002). One study reported an increased tendency for perforation of horizontal trabecular struts and increased trabecular spacing between horizontal trabeculae in females compared with males, but found no other histomophometric sex differences (Mosekilde, 1989). Given that 3 out of 4 fragility fractures occur in women, further investigation into sex differences in fatigue behavior is warranted.
In this study, trabecular bone cores from the femoral head of men and women aged 55–81 years were tested in a two-step mechanical testing protocol in order to investigate age and sex differences in crack propagation. We hypothesize that bone from older individuals will demonstrate decreased resistance to crack propagation, though we do not expect to see differences due to sex.
Section snippets
Methods
Femoral heads from 10 males and 10 females aged 55–81 years were obtained using the National Disease Research Interchange (NDRI). Donors had no history of bone disease, cancer, or use of medication altering bone morphology. Tissue was removed at autopsy within 12 h of death and fresh frozen at –80 °C. Donors were initially divided into four groups based on sex and age: young males (60.2±3.7 years; mean±s.d.), old males (77.2±1.5 years), young females (63.0±2.8 years), and old females (72.8±4.8
Results
No differences due to sex were noted for any comparisons, so sex groups were combined and comparisons were made between younger and older groups (younger: 61.3±3.1 years; older: 75.0±3.9 years). Gross morphological properties are reported for all mechanically tested samples (two cores/donor, n=20 per age group) and microdamage-assessed samples (one core/donor, n=10 per age group). An assessment of the global architectural properties of the trabecular bone samples revealed no significant
Discussion
In this study, trabecular bone cores from the femoral head of 10 younger and 10 older individuals were subjected to a two-step mechanical testing protocol. Microdamage was first induced with static uniaxial compressive loading to 0.8% then propagated with cyclic uniaxial compressive loading under a modest normalized stress range. It was found that samples from older donors reached the fatigue test endpoint with substantially fewer cycles than younger individuals. This was associated with
Conflict of interest statement
The authors have no conflicts of interest to disclose.
Acknowledgments
This research study was supported by NIH grant R01 AG027249. The micro-CT system was provided by NSF Major Research Instrumentation Award 9977551.
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