CORS_Micromotion2022
Evaluating the Efficacy of Measuring Implant-bone Fixation Methods in the Evaluation of Orthopaedic Implants
Presenting Author: David E. Cunningham
Contributing Authors: Cole Fleet, Dr. G. Athwal, Dr. J. Johnson
Purpose
In the evaluation of orthopaedic implants, fixation to bone is an important outcome. There have been numerous studies that have focused on the fixation of orthopaedic implants. Additionally, the American Society for Testing and Materials (ASTM) has published standardized evaluation methodologies for the quantification of implant fixation. However, recent literature has suggested that the usage of the ASTM standardized testing methods may have some limitations, and it is therefore essential to re-evaluate the methods by which orthopaedic implants are currently being assessed. For the glenohumeral joint as an example, the ASTM F2028 standard for the dynamic evaluation of glenoid loosening and disassociation suggests the usage of linear variable differential transducers (LDVTs) or other direct displacement measurement techniques for the evaluation of glenoid baseplate fixation. Optical tracking methods using a camera directly focused on the interface have also been employed and as recent studies suggest, may present an accurate quantification of the bone-implant interface relative motion (micromotion). The purpose of this work was to compare the difference between micromotion measurements via LDVTs and an optical digital tracking method.
Methods
An LDVT (Model 0236-0000, Trans-Tek Incorporated, Ellington, CT, USA) was compared against an optical machine vision USB3 camera (acA4096-30uc, Basler AG, Ahrensburg, SH, Germany) outfitted with a c-mount premium lens (FL-BC3518-9M, Ricoh Canada, ON, Canada). We employed the model of a stainless-steel implant beam (35.00 mm x 19.02 x 2.32 mm) as an implant surrogate. The implant beam was positioned on bovine tibial bone that was milled to a flat surface. An incrementally increasing eccentric load of 10-160 [N] was applied at one end of the implant causing lift-off at the opposite end, and this was quantified simultaneously using the LDVT and optical tracking techniques (analogous to the “rocking horse” phenomena). Loading was increased in 10 [N] increments in a step-wise manner. Differences in micromotion measurement were evaluated over five trials via testing at different locations across the cancellous surface.
Results
Average percent difference between LDVT and Digital Tracking micromotion measurements were calculated at all loading increments (Figure 1). Over the entire dataset, the average micromotion via LDVTs was 158 ± 65% higher than the average micromotion via digital tracking. This trend was more noticeable at higher loads as the percent difference between these two methods reached 190 ± 57% at maximum load.
Figure 1: Applied Load vs Percent Difference between LDVT and Digital Tracking Micromotion Measurements. This is an interactive plot so feel free to investigate further. Please note that this plot is better investigated in landscape view on a phone.
Conclusion
The use of LVDTs during the evaluation of orthopaedic implant fixation are able to quantify overall displacement of the implant relative to bone but are not isolated to a specific location at the interface. They are thus applicable for studies where a comparison of variables such as implant design and/or fixation techniques are addressed. However, to quantify the absolute magnitude of micromotion at the interface, direct measurements such as those achieved using digital tracking methods may be more efficacious.