Micro-mechanical modeling of the cement–bone interface: The effect of friction, morphology and material properties on the micromechanical response
Introduction
In cemented total hip arthroplasty, fixation of the implants in the bone is achieved by bone cement inserted in a doughy form at the time of operation. The cement subsequently penetrates into bone lacunar and trabecular spaces, forming a complex interlock between the cement and bone, ensuring fixation of the cement mantle within the bone.
Much research has been conducted to determine the strength of the cement–bone interface (Arola et al., 2006; Bean et al., 1987; Dohmae et al., 1988; Funk and Litsky, 1998; Kim et al., 2004; Krause et al., 1982; Mann et al., 1997). However, these studies simplify the cement–bone interface to an apparent level, whereas in reality this interface is a morphologically complex cement–bone composite.
Recently, experiments have been performed to determine the micro-mechanical behavior of the cement–bone interface (Mann et al., 2008). Interface specimens were subjected to non-destructive fully reversible tension–compression loads, while the local micromotions of the cement, bone and cement–bone interface were monitored. The results showed that the interface is more compliant than the cement and bone. Substantial hysteresis occurred during one tension–compression cycle, attributed to sliding contact at the interface. It remains, however, unclear how loads are transferred across the interface, as this could not be assessed.
Cement–bone adhesion may play a role in the mechanical response observed experimentally, although it may be compromised by fat, blood and other fluids that are present in the bone during cement insertion. On the other hand, the micro-mechanical behavior of the cement–bone interface may also be attributed to the shape-closed interlock of cement penetrated into the bone trabecular and lacunar spaces, combined with frictional phenomena.
In addition to this, variations in the cement–bone interface morphology may affect the micromechanical response of the shape-closed interlock. For instance, more cement penetration may enhance the mechanical properties of the interface. On the other hand, cement is known to shrink during polymerization (Davies and Harris, 1995; Hamilton et al., 1988), which may cause gaps to occur at the cement–bone interface, causing inferior mechanical properties at the interface.
A third possible factor affecting the micro-mechanical behavior of the cement–bone interface is the variability of cement material properties. Lower modulus cement has been considered as an approach to reduce interface stresses (Funk and Litsky, 1998). The stiffness of commercially available bone cements varies between roughly 2.0 and 3.0 GPa (Lewis, 1997). The effect of this variation on the actual micro-mechanical response of the cement–bone interface is unclear.
In order to gain insight in the micro-mechanical behavior of the cement–bone interface, we developed a micro-mechanical finite element analysis (FEA) model based on an experimental cement–bone interface specimen (Mann et al., 2008) and analyzed the effect of parametric variations of frictional, morphological and material properties on the mechanical response. We asked the following questions: (1) are the mechanical properties of the cement–bone interface caused by frictional phenomena via shape-closed mechanical interlock or by adhesive properties of the cement?; (2) how do interface morphological variations affect the micro-mechanical response of the cement–bone interface? and (3) how do variations in cement stiffness affect the micro-mechanical response of the cement–bone interface?
Section snippets
Methods
The FEA models used for the parametric analyses were created from micro-computed tomography (μCT) scans of a cement–bone interface specimen (Fig. 1a) that was previously tested experimentally (Mann et al., 2008). The specimen (5×5×10 mm3) was sectioned from a cemented total hip arthroplasty in a fresh-frozen proximal human cadaver femur. The specimen was μCT-scanned at a 12 μm isotropic resolution (Scanco μCT 40, Scanco Medical AG, Basserdorf, Switzerland; Fig. 1b).
FEA meshes were created using
Results
In cases where the cement–bone interface was assumed to be unbonded, the cement–bone interface deformed in a non-homogeneous manner. For example, during application of the tensile load, the cement and bone remained in contact at some locations, while at other locations gaps occurred (Fig. 3). The normal morphology model with a friction coefficient of 0.3 resulted in a micro-mechanical response at the cement–bone interface that was similar to the experiment (Fig. 4). In all models, the majority
Discussion
In the current study, we analyzed the effect of parametric variations of frictional, morphological and material properties on the mechanical response of a cement–bone interface.
Our results show that when an ideally bonded contact interface was assumed, the deformation at the interface was underestimated with respect to the experimental values. Furthermore, interface stiffness was overestimated in tension and compression, and hysteresis was underestimated. These results suggest the
Conflict of interest statement
None of the authors have financial or personal prelationships with other peolpe or organizations that could inappropriately influence or bias the currently presented work.
Acknowledgement
This work was funded by NIH-NIAMS AR42017.
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