Age-related and sex-specific effects on architectural properties and biomechanical response of the C57BL/6N mouse femur, tibia and ulna

Aging is known to reduce bone quality and bone strength. We sought to determine how aging affects the bio-mechanical and architectural properties of various long bones, and if sex influences age related differences/changes. While researchers have extensively studied these changes in individual bones of mice, there is no comprehensive study of the changes in the bones from the same mice to study the changes with aging. We performed three point bending tests and microcomputed tomography (microCT) analysis on femurs, tibiae and ulnae. Three point bending tests were utilized to calculate biomechanical parameters and imaging was also performed using high resolution microCT to reveal both cortical and trabecular microarchitecture C57BL/6N mice were divided into three age groups: 6, 12 and 22 months. Each age and sex group consisted of 6-7 mice. The ultimate load to failure (UL), elastic stiffness (ES), modulus of elasticity (E) and the moment of inertia about bending axis (MOI) for each bone was calculated using three point bending test. MicroCT scans of all the bones were analyzed to determine cortical bone volume per tissue volume (C.BV/TV), trabecular bone volume per tissue volume (Tb.BV/TV), cortical bone area (B.Ar) using CTAn's microCT analysis and tested for correlation with the biomechanical parameters. Mean (standard error) values of UL in femur decreased from 19.8(0.6) N to 12.8(1.1) N (p <.01) and 17.9(0.6) N to 14.6(1.0) N (p =.02) from 6 to 22 months groups in males and females respectively. Similarly, UL in tibia decreased from 19.8(0.5) N to 14.3(0.2) N (p <.01) and 14.4(0.6) N to 9.5(1.0) N (p <.01) from 6 to 22 months group in males and females respectively. ES in femur decreased from 113.2(7) N/mm to 69.6(6.7) N/mm (p <.01) from 6 to 22 months in males only. ES in tibia decreased from 78.6(3.2) N/mm to 65.0(2.3) N/mm (p=.01) and 53.1(2.9) N/mm to 44.0(1.7) N/mm (p=.02) from 6 to 22 months in males and females respectively. Interestingly, ES in ulna increased from 8.2(0.8) N/mm to 10.9(1.0) N/mm (p =.051) from 6 to 22 months of age in females only. E in femur decreased from 4.0(0.4) GPa to 2.8(0.2) GPa (p =.01) and 6.7(0.5) GPa to 4.5(0.4) GPa (p =.01) from 6 to 22 months of age in males and females respectively while tibia showed no change. However, E in ulna increased from 7.0(0.8) GPa to 11.0(1.1) GPa (p =.01) from 6 to 22 months of age in females only. Changes in age and sex-related bone properties were more pronounced in the femur and tibia, while the ulna showed fewer overall differences. Most of the changes were observed in biomechanical compared to architectural properties and female bones are more severely affected by aging. In conclusion, our data demonstrate that care must be taken to describe bone site and sexspecific, rather than making broad generalizations when describing age-related changes on the biomechanical and architectural properties of the skeleton.