Adaptation is a complex feature of living tissues, including bone. Compared with engineering materials, the structural and material properties of bone keep changing to fulfil the requirements of physiological functions throughout a lifetime. However, a disorder of this process could lead to serious bone diseases such as osteoporosis. Understanding the adaptation associated with architecture and mechanical properties of trabecular bone are key factors in diagnosis, prediction and treatment of osteoporosis. There are well-established theories that mechanical loads such as exercise can stimulate structural adaptation of trabecular bone. Previous research focused mostly on the effect of in-vivo loading on trabecular microstructure at the whole-bone level in both animal or human studies, while understanding of mechanically-induced local adaptation of human trabecular bone is rather limited.

The study was based on a hypothesis that trabecular-bone adaptation, including microstructural and mechanical competence, is inherently heterogeneous, hence, may not be captured adequately by the global analysis based on averaging. By implementing a local trabecular analysis, the PhD project aims to advance our understanding on the effects of in-vivo exercise on local realisation of the trabecular adaptation process in human distal tibia employing a combination of experimental and computational studies.

Assessment of ten white European-origin postmenopausal women before and after a prescribed six-month physical exercise was accomplished, with high-resolution peripheral quantitative computed tomographic (HR-pQCT) scans taken before and after the intervention to characterise regional variations of microstructural and mechanical properties of trabecular bone in different anatomical regions (anterior, lateral, posterior, medial). Region-specific trabecular parameters such as trabecular volume fraction, trabecular thickness, trabecular number, trabecular surface area, trabecular separation, plate-like structure fraction and trabecular stiffness predicted with finite-element analysis were determined from in-vivo (HR-pQCT) images. A 3D image-registration method was used to ensure that the microstructure and finite-element analysis were performed in the same region between baseline and follow-up images to quantify the trabecular-bone formation, resorption rate and microstructural changes.

Significant differences were found across anatomical regions before exercise, with the anterior region exhibiting the loosest and softest microstructure. The exercise intervention could significantly supress the bone-resorption rate, resulting in an improved microstructural distribution in the anterior region.

Information obtained from the experiments was coupled with a modified adaptation theory to develop region-specific finite-element models of trabecular-bone microstructure. To compare the effect of different mechanical signals on trabecular adaptation, both strain- and strain-gradient-based adaptation algorithms were implemented in the developed FE models. The simulated trabecular-bone microstructures and their mechanical properties for both proposed mechanical stimuli were in good agreement with the experimental observation.

In conclusion, local variations in trabecular microstructure and mechanical competence of human distal tibia were characterised in this study. Variations in trabecular-bone adaptation was quantified for the first time in postmenopausal women following a sixmonth regular hopping exercise. This study is important for advancing our knowledge on local adaptation of trabecular bone to exercise at its microarchitectural level.