The measured mechanical properties of osteoporotic trabecular bone decline with the increment of deformation volume during micro-indentation.

PURPOSE: Osteoporosis is a critical global health issue. However, the biomechanical properties of osteoporotic trabecular bone have not been well understood due to its hierarchically complex structure mingled with accumulated microcracks. Previous studies indicated the mechanical behaviors of trabecular bone may differ with varying amounts of deformation. Therefore, this study aims to further reveal the relationship between the measured mechanical properties of osteoporotic trabecular bone and various amounts of deformation volume during micro-indentation. METHODS: Two trabecular specimens were dissected transversally and frontally from an osteoporotic lumbar vertebral (L5) cadaver and embedded into Methyl methacrylate. On each specimen, two orthogonal cuts were performed to make a right-angle, followed by five parallel slicing. On each slice, the region of interest was gridded into 16 (4 × 4) sub-regions with the size equal to the microscope field. Within each sub-region, indentations were made on a single trabecula with five different indentation depths (3, 4, 5, 6, 7 µm) to induce different deformation volume. Both the indentation hardness and modulus were computed from the indenting curve for each measurement. The results of the five slices are pooled together to represent the longitudinal and circumferential mechanical characteristics, respectively. Linear regression was performed to investigate the relationship between the measured mechanical properties and various deformation volumes. RESULTS: A total of 1055 indents were made. After eliminating outliers, 840 indents were left for data analysis with 490 indents from transversal slices and 350 indents from frontal slices. Both the hardness and modulus decreased with the increasement of indentation depths. The hardness decreased by slopes of -0.65 (R2 = 0.72, p = 0.044) and -0.869 (R2 = 0.95, p = 0.003) longitudinally and circumferentially while the modulus decreased by slopes of -0.39 (R2 = 0.82, p = 0.02) and -0.348 (R2 = 0.94, p = 0.004) longitudinally and circumferentially. CONCLUSIONS: Mechanical properties of trabecular bone measured by micro-indentation can alter with the variation of deformation volume, which reflects the nonlinear behavior of vertebra from the material perspective.