Date of Award

5-2016

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Mechanical Engineering

Committee Member

Dr. John DesJardines, Co-Chair

Committee Member

Dr. Lonny Thompson, Co-Chair

Committee Member

Dr. Jeffrey Anker

Committee Member

Dr. Rodrigo Martinez-Duarte

Committee Member

Dr. Joshua Summers

Abstract

This thesis describes the development of a series of finite element models that simulate a fractured human tibia treated with internal fixation, and a strain sensor prototype capable of objectively monitoring fracture healing with standard radiography. The models provide otherwise inaccessible insight on the mechanics of the fracture callus and fixation implant as the bone heals. Lower extremity injuries with fracture are common in the United States population with 28 million musculoskeletal injuries treated annually. Approximately two million fracture fixation surgeries are performed in the USA each year, and this is expected to exceed three million by 2025. Current clinical methods do not provide an objective measure of fracture healing or weight bearing for lower extremity fractures treated with fracture fixation. Access to the mechanical environment of bones treated with internal fixation is limited to invasive procedures where testing a variety of loading conditions is impractical. The first goal of the models is to approximate strain distributions of the internal fixation implant and the fracture callus against simulated bone healing progress. The second goal is to support the development of a passive mechanical strain sensor that is capable of monitoring bone healing through the load sharing relationship between implant and bone. The models showed rapid reductions in strain within the fracture callus, orthopedic screws and plate with increasing callus stiffness. The results also indicate significant shifts in load sharing from the implant to the healing bone between callus stiffness values of 0-10% of intact bone, when the bone begins to support the vast majority of any applied load. The computational models also confirmed orthopedic plate bending to be an effective indicator of bone healing progress. A mechanical strain sensor was developed that is capable of monitoring orthopedic plate bending with standard radiography. The sensor uses a plate-mounted cantilevered indicator pin that spans the fracture with an internal radiopaque scale that tracks the relative motion of the pin to the plate. The sensor appropriately responded to compressive loading of the tibia in cadaveric trials and provided objective measurements of changes in plate bending with a resolution of ~250 µm. Finite element analysis results also predict a positive linear relationship between bone healing (percent callus stiffness) and relative pin displacement. Both the computational models and the cadaveric experiments indicate that the plate strain sensor is an effective indicator of bone healing for internal fixation applications and will provide an objective assessment of safe initiation of weight bearing.

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