Quantitative information about bone tissue‐level loading is essential for understanding bone mechanical behavior. We made microfinite element models of a healthy and osteoporotic human femur and found that tissue‐level strains in the osteoporotic femoral head were 70% higher on average and less uniformly distributed than those in the healthy one.Introduction: Bone tissue stresses and strains in healthy load‐adapted trabecular architectures should be distributed rather evenly, because no bone tissue is expected to be overloaded or unused. In this study, we evaluate this paradigm with the use of microfinite element (μFE) analyses to calculate tissue‐level stresses and strains for the human femur. Our objectives were to quantify the strain distribution in the healthy femur, to investigate to what extent this distribution is affected by osteoporosis, to determine if osteoporotic bone is simply bone adapted to lower load levels, and to determine the “safety factor” for trabecular bone.Materials and Methods: μFE models of a healthy and osteoporotic proximal femur were made from microcomputed tomography images. The models consisted of over 96 and 71 million elements for the healthy and osteoporotic femur, respectively, and represented their internal and external morphology in detail. Stresses and strains were calculated for each element and their distributions were calculated for a volume of interest (VOI) of trabecular bone in the femoral head.Results: The average tissue‐level principal strain magnitude in the healthy VOI was 304 ± 185 microstrains and that in the osteoporotic VOI was 520 ± 355 microstrains. Calculated safety factors were 8.6 for the healthy and 4.9 for the osteoporotic femurs. After reducing the force applied to the osteoporotic model to 59%, the average strain compared with that of the healthy femur, but the SD was larger (208 microstrains).Conclusions: Strain magnitudes in the osteoporotic bone were much higher and less uniformly distributed than those in the healthy one. After simulated joint‐load reduction, strain magnitudes in the osteoporotic femur were very similar to those in the healthy one, but their distribution is still wider and thus less favorable.

Bone adapts its morphology (density/micro- architecture) in response to the local loading conditions in such a way that a uniform tissue loading is achieved (‘Wolff’s law’). This paradigm has been used as a basis for bone remodeling simulations to predict the formation and adaptation of trabecular bone. However, in order to predict bone architectural changes in patients, the physiological external loading conditions, to which the bone was adapted, need to be determined. In the present study, we developed a novel bone loading estimation method to predict such external loading conditions by calculating the loading history that produces the most uniform bone tissue loading. We applied this method to murine caudal vertebrae of two groups that were in vivo loaded by either 0 or 8 N, respectively. Plausible load cases were sequentially applied to micro-finite element models of the mice vertebrae, and scaling factors were calculated for each load case to derive the most uniform tissue strain-energy density when all scaled load cases are applied simultaneously. The bone loading estimation method was able to predict the difference in loading history of the two groups and the correct load magnitude for the loaded group. This result suggests that the bone loading history can be estimated from its morphology and that such a method could be useful for predicting the loading history for bone remodeling studies or at sites where measurements are difficult, as in bone in vivo or fossil bones.

Recently developed in vivo animal high‐resolution micro‐CT scanners offer the possibility to monitor longitudinal changes in bone microstructure of small rodents, but may impose high radiation doses that could damage bone tissue. The goal of this study was to determine the effects on the bone of 8 weeks of in vivo scanning of the proximal tibia in female Wistar rats. Eight weekly CT scans were made of the right proximal tibia of nine female, 30‐week‐old, retired‐breeder, Wistar rats. Two weeks after the last weekly scan, a final scan was made. The left leg was only scanned during first and final measurements and served as a control. A two‐way ANOVA with repeated measures was performed on the first and last measurements of left and right tibiae for six bone structural parameters. Bone marrow cells were flushed out and tested for cell viability. No significant difference was found between left and right for any of the bone structural parameters (p > 0.05). Structure model index and trabecular separation significantly changed as a result of aging, while none of the other parameters did. No significant difference was found between left and right in absolute and percentage number of cell viability. We did not find any indication that the applied scanning regime, in combination with the particular settings used, would affect the results of in vivo bone structural measurements in long‐term studies using aged, female Wistar rats. However, careful consideration should be made when determining the number of scans, particularly when a different experimental design is used. © 2007 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 25:1325–1332, 2007