Abstract
Hallux valgus, or bunion deformity, is one of the most common foot problems that patients present to foot and ankle specialists. Bunions occur in up to 35.7% of people over the age of 65 and 23% of people between the ages of 18 and 65, and in many cases, these deformities are corrected with an implant. Identification of potential failure mechanisms in implants is key to post-operative early weight bearing and patient recovery. However, robust, laboratory evaluations of these implants are rare in the literature because it is difficult to capture the complicated loading mechanisms, biological tissue interactions, and patient biometric factors, which all contribute to implant performance. Thus, the objective of this course of study was to demonstrate novel, laboratory test methodologies for implants and to evaluate the performance of selected implants in both bone mimics and cadaveric specimens.
Three-dimensional digital image correlation (3D-DIC) was used to capture displacement and strain fields across the bone structures and the implants. Two implant candidates were selected for evaluation: a locking plate and nitinol staple. The implants were surgically inserted into bone substitute or cadaveric specimens and subjected to cyclic loading. 3D-DIC measurements tracked deformation under simulated motion and the gap displacement of the 1st tarsometatarsal (TMT) joint to quantify the implant performance, and to reveal areas of high stain/stress across the bone and implant structure.
This test methodology is anticipated to provide an effective, laboratory scale technique to evaluate the performance of hallux valgus implants prior to conducting clinical trials. Already, the results of this study have revealed critical strain redistribution and failure mechanisms inherent in the different implant designs. It was observed that the locking plates were affected less by plantar implant location and provided more initial stability and resistance against gapping under load. Unlike the nitinol staples, the locking plates were unable to recover compression throughout repetitive loading, indicating a limited lifecycle of the implant. However, the nitinol staple also exhibited significant compromises; the ability of the staple to flex and accommodate load helped it to maintain its clamping capability but also created a concentration of high strain along the bridge of the staple, indicative that this location could be susceptible to fatigue and failure. Importantly, this study demonstrated that the innovative asymmetric bend tests coupled with DIC measurement techniques could inspire a paradigm shift in implant design, implant materials, and how implants are biomechanically tested for early weight bearing protocols.
