Abstract
The survivorship of cementless orthopaedic implants may be related to their initial stability; insufficient press-fit can lead to excessive micromotion between the implant and bone, joint pain, and surgical revision. However, too much interference between implant and bone can produce excessive strains and damage the bone, which also compromises stability. An understanding of the nature and mechanisms of strain generation during implantation would therefore be valuable. Previous measurements of implantation strain have been limited to local discrete or surface measurements. In this work, we devise a Digital Volume Correlation (DVC) methodology to measure the implantation strain throughout the volume. A simplified implant model was implanted into analogue bone media using a customised loading rig, and a micro-CT protocol optimised to minimise artefacts due to the presence of the implant. The measured strains were interpreted by FE modelling of the displacement-controlled implantation, using a bilinear elastoplastic constitutive model for the analogue bone. The coefficient of friction between the implant and bone was determined using the experimental measurements of the reaction force. Large strains at the interface between the analogue bone and implant produced localised deterioration of the correlation coefficient, compromising the ability to measure strains in this region. Following correlation coefficient thresholding (removing strains with a coefficient less than 0.9), the observed strain patterns were similar between the DVC and FE. However, the magnitude of FE strains was approximately double those measured experimentally. This difference suggests the need for improvements in the interface failure model, for example, to account for localised buckling of the cellular analogue bone structure. A further recommendation from this work is that future DVC experiments involving similar geometries and structures should employ a subvolume size of 0.97 mm as a starting point.
Highlights
Long-term aseptic loosening is a primary failure mode of orthopaedic implants [1]; the quality of fixation plays a crucial role in arthroplasty outcomes
Strain accuracy of static and translated scans was less than 10 με for all subvolume sizes; the Digital Volume Correlation (DVC) calculated translation was 2% higher than the applied stage displacement
Precision error for the magnification change scans plateaued at the central subvolume size
Summary
Long-term aseptic loosening is a primary failure mode of orthopaedic implants [1]; the quality of fixation plays a crucial role in arthroplasty outcomes. Materials 2020, 13, 4050 attempt to meet the demands of such patients. These implants achieve long-term fixation by bone integration (osseointegration) with the implant’s surface. This takes time: retrieval of failed implants and animal studies have shown long-term osseointegration typically takes between. The relative difference in the size of the cavity to the dimensions of the implant generates stress in the bone, resulting in friction at the implant–bone interface, that generates the initial fixation. A better understanding of periprosthetic bone strain generated upon implantation would support implant development in several ways: reduce the incidence of post-implantation fractures, improve osseointegration via better control of stress-shielding effects, and reduce micromotion at the implant–bone interface by optimising the local deformation
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