Abstract
INTRODUCTION
Joint replacement is one of the most common orthopaedic procedures, with over 2 million surgeries performed each year across the globe. Loss of implant fixation, or aseptic loosening, is the leading cause of revision following primary joint replacement, accounting for ∼25% of all revision cases [1]. However, diagnosis of aseptic loosening and its underlying causes remain challenging due to the low sensitivity and specificity of plain radiographs. To address this, we propose a novel approach inspired by [2] involving the use of a self-sensing bone cement (by imparting strain-dependent electrical conductivity or piezoresistivity) combined with electrical impedance tomography (EIT). Piezoresistivity is imparted to cement via incorporation of micro/nanoscale conductive fillers. Therefore mechanical effects such as loosening and cracks will manifest as a conductivity change of the cement. This work explores if EIT is able to detect strains and cracks within the bone cement volume.
METHODS
Experiments were designed to determine whether EIT combined with piezoresistive cement can be used to detect strains and cracks (Fig. 1). The setup consists of a tank filled with water, 16 electrodes, sample, a loading machine (MTS), and an EIT system. To develop the piezoresistive bone cement, microscale carbon fibers were used with varying CF/PMMA volumetric ratios (VR) from VR = 0.25% to 3.0%. Three conical samples were made to model a loading condition similar to knee implants (Fig. 1). The samples were compressed while the conductivity map of the tank was measured with the EIT system.
RESULTS
Figure 2 shows the conductivity of the piezoresistive bone cement with respect to the CF/PMMA VR, the percolation happens at VR = 1.0% and the maximum gradient occurs at VR = 1.5%. Three conical samples were built and experimented to examine the hypothesis. The samples were loaded from F = 0 to F = 4000 N for the strain measurement and then loaded until the first crack initiates. Figure 3 (a) and (b) show the conductivity difference map measured by EIT for strain measurement and crack detection respectively. It can be seen in Fig. 3(a) that due to the shear stresses within the bone cement the conductivity of the sample decreases under compression. At the crack initiation the conductivity of the samples increases significantly (Fig. 3(b)). Figure 3(c) shows evolution of sample conductivity difference measured by EIT as a function of the applied load, VR = 1.5% shows the largest sensitivity.
DISCUSSION
The results validate our hypothesis; both cracks and strains resulted in electrical conductivity changes measurable by EIT. While these initial results are encouraging, the approach must be validated via testing of surrogate and cadaver bones in an EIT phantom. If successful, this approach could for the first time provide means of in-vivo studying of aseptic loosening, leading to a paradigm shift in the understanding of this important clinical problem.
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