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Cortical Bone Failure Mechanisms during Screw Pullout
E.M. Feerick, J.P. McGarry
Department of Mechanical and Biomedical Engineering, NUI Galway
e.feerick1@nuigalway.ie
Abstract
Many orthopedic devices use screws as their primary
mode of fixation. Screw pullout at the screw-cortical
bone interface is a common mode of failure for such
devices [1] . In this study, a computational model is
developed to investigate the failure mechanisms of
cortical bone which lead to screw pullout. Simulations
are compared to experimental results in order to
validate predicted failure mechanisms and failure loads
during screw pullout.
1. Introduction
The objective of this study is to develop a complete
understanding of the mechanisms of failure that occur
between orthopaedic screws and the surrounding
cortical bone.
2. Materials & Methods
Experimental monotonic tensile pullout tests were
conducted using commercially available orthopaedic
screws anchored in bovine cortical bone. Rectangular
mid-diaphyseal sections were extracted from the
metacarpus. A novel test rig was built to facilitate the
monitoring of failure modes in real time.
A 2D axisymmetric model was created based on the
geometry of 3.5mm cortical bone screws. The Drucker-
Prager constitutive formulation was used to model postyield
material behaviour of bovine cortical bone [2] .
Damage initiation and evolution based on effective
plastic strain was incorporated into the material
behavior. Crack propagation was simulated using an
element removal technique for fully damaged elements.
Predicted patterns of crack propagation during pullout
were compared to experimental tests that recorded the
failure mode of screw pullout in real time.
3. Results
The experimental failure load recorded was 2.5 ±
0.3kN. The experimentally observed failure mode for a
section of bovine cortical bone with osteons vertically
aligned is shown in Fig. 1(A-C). Cracks propagate
upwards from the tips of the screw threads. The
separated material remains between the screw threads
and is subsequently removed as the test progresses. The
final fracture surface following screw removal is shown
in Fig. 1(C).
Our computed failure mechanism leading to screw
pullout is illustrated in Fig. 1(D-E). Following initial
screw movement localised plastic deformation is
computed at the screw tips, leading to plastic zones that
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initially extend vertically upwards and then propagate at
45 o (Fig. 1(D)). The highest concentration of plastic
strain at the screw tips leads to a crack initiation. As the
crack propagates vertically upwards a vertical plastic
zone develops ahead of the crack tip. Further crack
propagation leads to full separation of the bone between
the screw threads (Fig. 1(E)) leading to complete
pullout and exposure of the fractured surface. A
maximum screw pullout force of 2.46 kN is computed
during the pullout simulation.
Max
(A) (B) (C)
(D) (E)
Figure 1 (A-C) Experimentally recorded failure mode
progression; (D-E) Computational equivalent plastic
strain for two time points
4. Discussion
The live imaging technique developed in this study
provides novel insight into failure mechanisms during
screw pullout. While earlier studies have reported a
[3]
similar final fracture surface , the evolution of
material failure during screw pullout has not been
previously uncovered. The process of element deletion
to replicate crack propagation is a predictive method
and does not predefine the crack pathway. Predicted
crack evolution correlates closely with experimental
observation. Future work will involve developing a
cohesive zone formulation to incorporate screw pullout
in macro scale 3D models of PHF repair to replicate
clinical failure modes.
5. References
[1] Owsley et al., JBJS 90:233-240, 2008. [2] Mercer et al.,
Acta Mater 2:59-68, 2006. [3] Chapman et al., J Biomech
Eng 118:391-398, 1996.
6. Acknowledgements
IRCSET, ICHEC.