Investigation of surface mechanical environment as an optimization criterion for improved tissue engineering scaffolds

Abstract

Trabecular bone, the porous bone found predominately in the spine and ends of long bones, is a mechanically regulated tissue. The hierarchy of bone consists of several levels of structure such as raw collagen and calcium phosphate on the microscale to trabecular packets, which are constantly being remodeled by bone cells on the tissue level. The remodeling of bone is believed to be explained through the concept of functional adaptation-where bone is a maximum strength yet minimum weight material. In functional adaptation, phenomenological models are able to predict the density distributions and bone shapes that are witnessed in vivo to a certain degree. Functional adaptation assumes there is an equilibrium state in which no changes in bone mass or structure will occur at the bony surface. Topological and mass changes are incurred on a local level when equilibrium is not achieved. The combination of these local changes produces a self-organized structure -meaning that the global bone shape is explained by simple local rules. Unfortunately, neither tissue engineering nor medical device design has incorporated the knowledge base of functional adaptation of bone into their orthopedic designs.

The objective of this dissertation work was to examine how the concept of functional adaptation could be applied to tissue engineering of bone in so much as it leads to the development of a computer-aided tissue engineering (CA TE) framework. The idea was to increase the specificity in which implant/scaffold architectural shape can be matched to tissue mechanical properties of the spine (or other locations), as well as matched to an individual patient who has experienced fracture. Because a variety of mechanical stimuli have been proposed in the functional adaptation literature, the first step of this work was to categorize the most probable variables that explain mechanical loading of trabecular bone in the spine. This was accomplished through reverse engineering cadaver specimens into J..t-finite element models. Two algorithms were developed for scaffold design, which makes use of the mechano-transductive principles specifically designed for the pre-determined mechanical variables. Finally, a framework for assembling scaffolds from local building blocks, which are derived from bone was proposed.