A computational model for cortical endosteal surface remodeling induced by mechanical disuse

In mechanical disuse conditions associated with immobilization and microgravity in spaceflight, cortical endosteal surface moved outward with periosteal surface moving slightly or unchanged, resulting in reduction of cortical thickness. Reduced thickness of the shaft cortex of long bone can be considered as an independent predictor of fractures. Accordingly, it is important to study the remodeling process at cortical endosteal surface. This paper presents a computer simulation of cortical endosteal remodeling induced by mechanical disuse at the Basic Multicellular Units level with cortical thickness as controlling variables. The remodeling analysis was performed on a representative rectangular slice of the cross section of cortical bone volume. The pQCT data showing the relationship between the duration of paralysis and bone structure of spinal cord injured patients by Eser et al. (2004) were used as an example of mechanical disuse to validate the model. Cortical thickness, BMU activation frequency, mechanical load and principal compressive strain for tibia and femur cortical models were simulated. The effects of varying the mechanical load and maximum BMU activation frequency were also investigated. The cortical thicknesses of femur and tibia models were both consistent with the clinical data. Varying the decreasing coefficient in mechanical load equation had little effect on the steady state values of cortical thickness and BMU activation frequency. However, it had much effect on the time to reach steady state. The maximum BMU activation frequency had effects on both the steady state value and the time to reach steady state for cortical thickness and BMU activation frequency. The computational model for cortical endosteal surface remodeling developed in this paper can be further used to quantify and predict the effects of mechanical factors and biological factors on cortical thickness and help us to better understand the relationship between bone morphology and mechanical as well as biological environment.