Shear fatigue micromechanics of the cement-bone interface: An in vitro study using digital image correlation techniques

Abstract

Loss of fixation at the cement-bone interface is known to contribute to aseptic loosening, but little is known about the mechanical damage response of this interface. An in vitro study using cement-bone specimens subjected to shear fatigue loading was performed, and the progression of stiffness changes and creep damage at the interface was measured using digital image correlation techniques. Stiffness changes and creep damage were localized to the contact interface between cement and bone. Interface creep damage followed a three-phase response with an initial rapid increase in creep, followed by a steady-state increase, concluding in a final rapid increase in creep. The initial creep phase was accompanied by an increase in interface stiffness, suggesting an initial locking-in effect at the interface. Interface stiffness decreased as creep damage progressed. Power law models were reasonably successful in describing the creep and stiffness damage response and were a function of loading magnitude, number of loading cycles, and contact area at the interface. More microcrack damage occurred to the cement when compared to the bone, and the damage was localized along the interface. These findings indicate that damage to the cement-bone interface could be minimized by improving cement-bone contact and by strengthening the fatigue resistance of the cement.

Figure 1

Experimental apparatus used for loading of the shear fatigue specimens. Digital image correlation measurements were made at discrete points on each side of the interface, indicated here as white squares.

Figure 2

Definitions used to describe shear fatigue response of the cement-bone interface.

Figure 3

Digital image correlation (DIC) measure of vertical displacement at three sampling lines across the cement-bone interface (A, B, and C) for a representative sample. The large jump in displacement occurs at the interface between cement and bone in each case. The number of loading cycles for each line is indicated and data are shown for the beginning of the loading cycle (10% of maximum load) in each instance.

Figure 4

Example of interface creep and stiffness response as a function of loading cycles measured by DIC for shear fatigue loaded specimens.

Figure 5

Interface creep as a function of loading cycles for the shear fatigue loaded specimens (A). Straight lines are drawn between individual DIC data points. Interface stiffness as a function of interface creep for the fatigue loaded shear specimens (B). Mean (vertical line) and standard deviation limits (bounds of box) for the interface creep that corresponds to the stiffness maxima point is included. Solid data lines represent specimens with clear reductions in stiffness during the later stages of loading. Dashed data lines represent specimens that did not have a clear reduction in stiffness during the later stages of fatigue loading.

Figure 6

Examples of post-fatigue loaded specimens reveal micro-damage to the cement (small arrows) for a specimen with minimal damage (left) and extensive damage (right). The large arrows indicate shear loading direction.