ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

Hence, the most important role of the cement line is inhabitation of the fracture at
micro-scale level because the crack that penetrates into the osteon continues to grow
with increasing load while the crack that deflects into the cement line with increasing
load approaches a halt on the cement line boundary or enters to the interstitial bone
tissue.
In another case, for probing the effect of fracture properties of cement line on crack
penetration into osteons or crack deflection into cement line, the cohesive properties of
cement line were varied with respect to the osteon and the interstitial bone. The
interstitial bone and osteon were assigned the same cohesive properties that were held
constant for all simulations (table 1). The results indicate that increasing the strength
and fracture toughness of cement line reduced the ability of cement line to deflect the
crack, that is, tougher cement lines facilitated crack penetration into the osteon.
Although both strength and toughness of cement line are effective in determining the
crack growth path, the strength played a far more fundamental role in crack deflecting.
Indeed, the low strength of it irrespective of its toughness grantees crack deflection.
These results are in accordance with what were reported by Mischinski & Ural (2011)
[3], Adel et al. (2011) [7], and Ural (2011) [4]. Also Burr et al. (1988) [12] proposed
that cement lines may promote crack deflection because they are poorly mineralized and
consequently have higher ductility compared to the osteonal and interstitial bone.
In one more part of this research, we focused on the effect of crack length on
microcrack propagation path. So, two meshfree models with two different inititial crack
lengths, 202.5 [µm], and 384 [µm], were considered. For each crack length, normal
cohesive properties of the components were similar to model A. After analysis, long
crack entered to osteon while short crack deflected into the cement line. Consequently,
long cracks with enough energy have a greater chance for penetrating into the osteon
and bone fracture resistance will be less against them. Therefore, long crack require
cement lines with low strength and fracture toughness for deflection into the cement
line. Other researchers have also reported similar observations in experimental studies.
O’Brien et al. (2005) [2] reported that the longer cracks are more likely to penetrate
osteons than to stop growing when they encounter cement lines. Also the results of
current research are in line with Mischinski & Ural (2011) study [3].
In conclusion, the simulation results showed that the crack trajectory was influenced by
the strength and fracture toughness of the cement line relative to the bone matrix and
crack length. One of the advantage of this study is using meshfree fracture method
without needing remeshing process and EFG simulations performed in this study
utilized cohesive zone modeling, which incorporated both strength and fracture
toughness of the bone and the cement line to determine the crack growth trajectory.
However, it is necessary to consider the concepts of cement line debonding and osteonal
pullout to improve the current model.
5. REFERENCES
1. Hogan, H.A., 1992, Micromechanics modelling of Haversian cortical bone
properties, Journal of Biomechanics, 25 (5), 549–556.
2. O'Brien, F.J., D. Taylor, and T.C. Lee, 2005, The effect of bone microstructure on
the initiation and growth of microcracks, J Orthop Res, 23: 475-80.