Mechanical Factors in Initial Long Bone Ossification and Later Functional Adaptation

Book excerpt: Our bones constantly adapt to their mechanical environment through a biological response from the womb to the tomb. Mechanobiology, the biological response to mechanical loading, is important for determining various properties of bone such as size and shape. During embryonic development, rapid growth generates significant tension in the periosteum, and tension has previously been shown to lead to bone apposition. In adults, intracortical stresses dominate and increased loading leads to elevated rates in bone apposition. Periosteal tension and intracortical stresses, then, are both experienced by bones, but their influences on bone apposition rates vary over time. This dissertation analyzes how embryonic bone growth rates and adult bone adaptation rates in long bones are related to their respective mechanical environments. The hypothesis that bones grow and adapt at rates corresponding to changes in the mechanical environment is investigated. In the first study, I investigated the mechanical environment of the periosteum during embryonic growth and its relationship to bone growth rates. The specific growth rate, or percent growth per day, was calculated using microCT images taken over embryonic days 11-20. Bones grew faster in length than in circumference during this time. Finite element techniques were then used to analyze the opening dimensions of incisions through the periosteum. Longitudinal and circumferential residual strains decreased from 46.2% to 29.3%, and 10.6% to 3.9%, respectively, during embryonic days 14-20. Residual strains were positively correlated to specific growth rates (p