Date of Final Oral Examination (Defense)

8-2011

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Mechanical Engineering

Department

Mechanical and Biomechanical Engineering

Major Advisor

Kotaro Sasaki, Ph.D.

Abstract

With the prevalence of stress fractures in the military and athletes of all levels, research into the pathology of this injury has taken flight in recent years. One area of research has focused on the role bone strain, which is known to be a factor in bone remodeling, has on stress fracture development. It has been difficult to perform studies in this area of research due to the invasiveness of in vivo measurements of the bone strain. Recently, a methodology for approximating the bone strain using a computational model was proposed by Al Nazer et al. (Al Nazer et al., 2008b). This methodology employs the combination of a dynamic simulation with a flexible body (finite element model), replacing one of the rigid bodies in the musculoskeletal model. The use of a flexible body, generated from the deformation modes of the bone, sufficiently decreases the degrees of freedom of the finite element model so that it can be used in a fully dynamic simulation. This study used a similar methodology, with an improved methodology for generating the flexible tibia, to establish a normative range of strains seen in a homogenous population of young, healthy, male subjects. The flexible tibia was generated by first segmenting the CT scanned tibia to regenerate a 3D solid model of the tibia geometry, then applying the material properties developed from the CT scan Hounsfield Units (HU) values for each element in the finite element model, and finally performing a modal analysis on the finite element model to generate the deformation modes of the tibia model. Strain data from five reference locations around the tibial mid-shaft, and a simulated staple were obtained using subject-specific forward dynamics simulations. The results showed large variability in strain magnitude for a homogenous population. The mean peak and standard deviation for the maximum principal strain, minimum principal strain, and maximum shear strain for the anterior medial location were 488με (+175 με), -473με (+93με), and 814με (+177με), respectively. However, comparisons with previous in vivo research showed that nearly all in vivo data were within two standard deviations of the mean values. The ability to differentiate between normal and potentially harmful strain levels is key to determining their effect on stress fracture development.

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