Abstract
Introduction
Periprosthetic bone remodelling after Total Knee Arthroplasty (TKA) may be attributed to local changes in the mechanical strain field of the bone as a result of the stiffness mismatch between high modulus metallic implant materials and the supporting bone. This can lead to significant loss of periprosthetic bone density, which may promote implant loosening, and complicate revision surgery. A novel polyetheretherketone (PEEK) implant with a modulus similar to bone has the potential to reduce stress shielding whilst eliminating metal ion release. Numerical modelling can estimate the remodelling stimulus but rigorous validation is required for use as a predictive tool. In this study, a finite element (FE) model investigating the local biomechanical changes with different TKA materials was verified experimentally using Digital Image Correlation (DIC). DIC is increasingly used in biomechanics for strain measurement on complex, heterogeneous anisotropic material structures.
Methodology
DIC was used following a previously validated technique [1] to compare bone surface strain distribution after implantation with a novel PEEK implant, to that induced by a contemporary metallic implant. Two distal Sawbone® femora models were implanted with a cemented cobalt-chromium (CoCr) and PEEK-OPTIMA® femoral component of the same size and geometry. A third, unimplanted, intact model was used as a reference. All models were subjected to standing loads on the corresponding UHMWPE tibial component, and resultant strain data was acquired in six repeated tests. An FE model of each case, using a CT-derived bone model, was solved using ANSYS software.
Results and Discussion
The sensitivity of DIC strain measurements was <+130με and experimental error was +230με, or 8.5% of the peak magnitude in the region of interest. High bone strain adjacent to the CoCr implant and low bone strain in the central metaphyseal region compared to the intact case (Fig.1) indicated that stress shielding may lead to resorption, a theory corroborated by bone density scans of implanted metallic TKRs [2]. Quantitatively, wider scatter and greater deviation was observed between the intact-vs-CoCr datasets (R2: 0.425, slope = 0.508). A closer agreement was shown between the intact-vs-PEEK datasets (R2: 0.771, slope = 1.270) (Fig.2). These strain distributions corroborated the predictions of the FE analysis (Fig.1). High bone strain in regions close to the CoCr implant can be attributed to the high stiffness mismatch between implant and bone, where the bone is constrained to the implant with cement. High strain gradients near the stiff CoCr could potentially compromise implant fixation, leading to loosening. The compressive strains in the PEEK implanted model were similar to those in the intact case, suggesting that bone would be maintained in these regions, and high strain gradients were not observed.
Conclusion
Digital image correlation and FE analysis have been successfully employed for evaluation of a novel PEEK-OPTIMA® TKA implant in comparison to a metallic implant. The polymeric implant produced a strain distribution closer to that of the intact bone, and therefore would be expected to have less of a stress shielding effect, improving long term bone preservation.
