Date of Award

4-2014

Degree Name

Doctor of Philosophy

Department

Mechanical and Aerospace Engineering

First Advisor

Dr. Peter A. Gustafson

Second Advisor

Dr. Judah Ari-Gur

Third Advisor

Dr. Daniel Kujawski

Fourth Advisor

Dr. James R. Jastifer

Abstract

Poor bone fracture fixation leads to malunion, delayed union, non-union, or infection. These malunited fractures affect a bone’s ability to carry loads. Patient outcomes regarding fixation quality can be affected by the healing environment and human factors such as bone quality and surgeons’ perception. Furthermore, the stiffness and strength of the screw-plate construct affect the healing environment. Therefore, this dissertation investigates the stiffness and strength of the non-locking (conventional) and locking (fixed angle) type screw-plate constructs and the factors that contribute to them, such as screw-plate interface, screw design, bone density, cortical bone thickness and load orientation. Additionally, the surgeon’s ability to perceive stripping of the bone while driving screws is evaluated. Finite element analyses and experiments are performed for these investigations.

The type of construct is found to have a minimal effect on the stiffness of the construct, whereas the plate thickness has a larger influence. Moreover, it is observed that the uniformity of force distribution at the bone-screw interface and the bone plastic strain distribution determine the construct strength behavior. The locking screw construct provides the greater strength under shear load and the conventional screw construct offers greater strength under pullout loads for the analyzed cortex thicknesses, cancellous bone densities and screw diameters. The load-displacement plot from the finite element analysis was compared to the experimental data. The correlation validates the finite element model.

In non-locking plates, construct stability relies on the friction between the plate and bone. This friction is controlled by the compressive force produced through applied screw torque. The finite element analysis demonstrates that an over-tightened (higher pre-tension) screw deteriorates the load carrying ability of the bone. In addition, the surgeons’ perception was found to be unrelated to the likelihood of bone stripping. Furthermore, the maximum torque achieved before stripping is surgeon dependent and surgeons stripped bone more frequently than they perceived.

Access Setting

Dissertation-Open Access

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