M.A. Theses: Richard Allan Lazenby, 1986

Interaction of the Bone Porosity and Cross-Sectional Geometry For the Conservation of Bone Strength

The study of bone strength is important in the study of aging populations. This fact is attested to by the great expenditure of time and energy devoted to understanding osteoporosis. Two aspects of morphology underlie the structural strength of bone: its physical properties (especially porosity), and its geometric properties. Few studies have attempted to examine interrelationships between these properties. This study examines whether or not the degree and distribution of cortical bone porosity reflects geometric parameters of femoral cross-sections.

A model stating that strength-reducing porosity should occur to a greater degree in the direction of maximum bending strength is proposed. In keeping with the underlying principles of functional adaptation (implicit in all analyses of bone morphology) it is argued that the interaction between porosity and geometry should result in a more uniform porosity distribution as cross-sections become more circular. The study also examines the contention that continuous periosteal apposition mechanically compensates for endosteal resorption.

Data collected from a series of human femoral cross-sections obtained from subjects of known sex and age are used to test the model. Geometric parameters are estimated using stereological methods and appropriate formulae derived from the literature. Coritical porosity is quantified from radiographs of the same sections using automated image analysis, and is evaluated in the anterior and lateral cortices, relative to the axes of maximum and minimum geometric bending strength. The data are analyzed using bivariate and multivariate statistical techniques.

The results of this study support a model that emphasizes conservation of bone strength. The analysis suggest that the overall degree of cortical porosity is independent of cross-sectional geometry, but that porosity distribution is not. Greater porosity occurs along the axis of maximum geometric resistance to bending. As cross-section geometry approaches circularity, porosity becomes more equally distributed between axes. Finally, mechanical compensation occurs only in the axial direction of least bending strength, achieved through endosteal resorption and not periosteal apposition as generally accepted.

The results of this study suggest that the processes of internal and external remodeling of human bone do not proceed independently. It is likely that the same ultimate stimulus mediates both processes. The nature of this stimulus is thought to be aspects of the strain environment, in keeping with current theoretical models based on in vivo studies of non-human bone.