DOI

10.17077/etd.a27xuf51

Document Type

Thesis

Date of Degree

Fall 2016

Degree Name

MS (Master of Science)

Degree In

Biomedical Engineering

First Advisor

Jessica Goetz

First Committee Member

David Wilder

Second Committee Member

Nicole Grosland

Abstract

Total ankle arthroplasties (TAAs) are mechanical devices used to replace the articular surfaces of the ankle joint in order to relieve pain for patients with osteoarthritis. Since most osteoarthritis is post-traumatic, and due to the highly variable individual foot geometry, TAAs are rarely inserted into normal geometry. This leads to serious problems with stresses and contact pressures in TAA components. This study uses finite element (FE) modeling to determine how hindfoot alignment, or how far in varus or valgus the most distal part of the calcaneus is perpendicularly from the axis of the tibia, affects the stresses and contact pressures in the articulating surfaces of two different TAA models.

To investigate the effects of foot alignment on hardware stresses after TAA, FE models were generated. Models of the mobile bearing, three component Scandinavian Total Ankle Replacement (STAR) and the fixed bearing, two component Zimmer Trabecular Metal Total Ankle (Zimmer) were generated from laser scans of the hardware and virtually implanted into 3D models of the tibia and talus. Ligaments were modeled as linear springs to impart physiologically realistic flexibility in the model. The stance phase of a walking gait cycle was applied and stresses and contact pressures at the articulation between model components were recorded for various degrees of hindfoot alignment [1].

Data analyzed shows that both models have areas of high concentrations of stress and contact pressure. The Zimmer TAA seems to favor a valgus alignment due to the lower stresses and contact pressures in valgus alignments compared to varus. Though the STAR does not generally favor one alignment over the other, it does have significantly lower stresses and contact pressures than the Zimmer. These differences may be due to the geometric congruency of the STAR versus the anatomical articulation of the Zimmer.

Public Abstract

Total ankle replacements are devices used to help treat patients with pain in their ankles associated with arthritis. These devices replace damaged bone and cartilage on the tibia and talus with metal and plastic implants. Unfortunately, abnormally directed loads through the ankle joint can cause problems, and the replacement may wear out faster or even break depending on the severity of these abnormal loads. Patients with flat feet or high arches are at high risk for developing these abnormal loading patterns in their ankle replacements.

To determine how foot alignment may affect a total ankle, computer models of two different ankle replacements (the STAR and the Zimmer) were created. Normal walking was modeled and the way these models were supported was varied to simulate different degrees of flat and high-arch feet. The mechanical behavior of the total ankle replacement was studied to determine the effects of foot alignment.

From these models, it was found that the total ankle replacements are more tolerant of a flat-foot alignment, and that models simulating high had much higher stresses and could have a higher occurrence of breaking. Knowledge about the effects of foot alignment on total ankle replacement is important in order for surgeons to know how best to correct an ankle during replacement surgery and give the patient a better chance of a positive outcome.

Keywords

Ankle Replacement, Hindfoot Alignment, Orthopedic, Total Ankle Arthroplasty

Pages

ix, 69 pages

Bibliography

Includes bibliographical references (pages 68-69).

Copyright

Copyright © 2016 Heidi Kirsten Johanna Bingenheimer

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