Biomechanics and Modeling in Mechanobiology
Material heterogeneity, microstructure, and microcracks demonstrate
differential inﬂuence on crack initiation and propagation in cortical
· Ani Ural
Received: 9 December 2017 / Accepted: 16 May 2018
© Springer-Verlag GmbH Germany, part of Springer Nature 2018
The recent studies have shown that long-term bisphosphonate use may result in a number of mechanical alterations in the
bone tissue including a reduction in compositional heterogeneity and an increase in microcrack density. There are limited
number of experimental and computational studies in the literature that evaluated how these modiﬁcations affect crack
initiation and propagation in cortical bone. Therefore, in this study, the entire crack growth process including initiation and
propagation was simulated at the microscale by using the cohesive extended ﬁnite element method. Models with homogeneous
and heterogeneous material properties (represented at the microscale capturing the variability in material property values and
their distribution) as well as different microcrack density and microstructure were compared. The results showed that initiation
fracture resistance was higher in models with homogeneous material properties compared to heterogeneous ones, whereas
an opposite trend was observed in propagation fracture resistance. The increase in material heterogeneity level up to 10
different material property sets increased the propagation fracture resistance beyond which a decrease was observed while
still remaining higher than the homogeneous material distribution. The simulation results also showed that the total osteonal
area inﬂuenced crack propagation and the local osteonal area near the initial crack affected the crack initiation behavior.
In addition, the initiation fracture resistance was higher in models representing bisphosphonate treated bone (low material
heterogeneity, high microcrack density) compared to untreated bone models (high material heterogeneity, low microcrack
density), whereas an opposite trend was observed at later stages of crack growth. In summary, the results demonstrated that
tissue material heterogeneity, microstructure, and microcrack density inﬂuenced crack initiation and propagation differently.
The ﬁndings also elucidate how possible modiﬁcations in material heterogeneity and microcrack density due to bisphosphonate
treatment may inﬂuence the initiation and propagation fracture resistance of cortical bone.
Keywords Material property heterogeneity · Microcrack · Cortical bone · Cohesive ﬁnite element modeling · Bisphosphonate ·
Atypical femoral fracture
The effect of bisphosphonate (BP) treatment on the tissue
level material properties of cortical bone has attracted sig-
niﬁcant interest due to the possible link between long-term
BP treatment and atypical femoral fractures (AFF). Although
AFF is a rare catastrophic fracture in the subtrochanteric
region and diaphysis of the femur, it has high morbidity and
mortality outcomes (Shane et al. 2010). In addition, it has
Department of Mechanical Engineering, Villanova
University, 800 Lancaster Avenue, Villanova, PA 19085, USA
led to a reduction in BP use as some patients decline tak-
ing BPs because of fear of side effects (Khosla and Shane
2016) which has the potential to increase fracture rates in the
population. As a result, understanding how BP related tissue
level modiﬁcations inﬂuence mechanical response of bone is
critical to address the causes of possible, even though rare,
side effects of BP treatment.
The recent studies have shown that the long-term use of BP
treatment may result in a number of mechanical alterations in
the bone tissue including a reduction in compositional hetero-
geneity and an increase in microcrack density (Donnelly et al.
2012; Mashiba et al. 2000). There is a lack of consensus on the
effects of compositional heterogeneity on bone fracture risk.
A group of studies conducted on bone biopsies from patients