Distribution of tissue modulus in bone matrix changes during maturation

Objectives: Mineralization of bone matrix is a long term process lasting several years. It is likely that the magnitude of tissue modulus varies depending on degree of mineral maturation in bone matrix. The objective of this study was to examine how mean and variability of tissue modulus are altered during maturation of bone matrix.

Methods: Mandibular condyle bone specimens from 4 premature (5 months) and 6 fully mature (>1 year) male beagle dogs were utilized. Condyles were dissected in the SI direction and polished. Nanoindentation was performed to measure tissue modulus at core trabecular packets. Because new bone remodeling occurs at the surface region of trabeculae, bone matrix from the core region likely maintains the gradual mineral maturation. There were 9 specimens in the premature group and 24 in the fully mature group. A total of 2703 indentations were performed. Mean and variability (COV=standard deviation/mean) of tissue modulus for each specimen were computed. The t-test and regression were used to compare the two groups (sig. p<0.05).

Results: Mean value of tissue modulus was lower in the premature group (5.55±0.96 GPa) than in the fully mature group (9.87±1.09 GPa) (p<0.001). However, the variability was higher in the premature group (0.46±0.12) than in the fully mature group (0.32±0.07) (p<0.008). There was a significant negative correlation between COV and mean for the premature group (COV=-0.094*mean+0.98, r²=0.60, p<0.001) and a moderate negative correlation for the fully mature group (COV=-0.026*mean+0.58, r²=0.16, p<0.057).

Conclusion: Premature bone matrix is characterized by low mean and high variability of tissue modulus, giving rise to its low mechanical stability. The higher sensitivity (higher slope) of COV to mean values is observed for the premature group under active mineralization. The finding that the variability decreases with increasing mean suggests that increasing homogeneity of mechanical properties helps support loads increase during maturation.