Bone tissue aging affects mineralization of cement lines

Cement lines are known as thin peripheral boundaries of the osteons. With a thickness below 5 μm their composition of inorganic and organic compounds has been a matter of debate. Here, we hypothesized that cement lines become hypermineralized and their degree of mineralization is not constant but related to the tissue age of the osteon. Therefore, we analyzed the calcium content of osteons and their corresponding cement lines in a range of different tissue ages reflected by osteonal mineralization levels in femoral cortical bone of both postmenopausal women with osteoporosis and bisphosphonate-treated cases. Quantitative backscattered electron imaging (qBEI) showed that cement lines are hypermineralized entities with consistently higher calcium content than their corresponding osteons (mean calcium content: 29.46 ± 0.80 vs. 26.62 ± 1.11 wt%; p < 0.001). Micro-Raman spectroscopy complemented the qBEI data by showing a significantly higher phosphate/amide I ratio in the cement lines compared to the osteonal bone (8.78 ± 0.66 vs. 6.33 ± 0.58, p < 0.001), which was both due to an increased phosphate peak and a reduced amide I peak in cement lines. A clear positive correlation of cement line mineralization and the mineralization of the osteon was observed (r = 0.839, p = 0.003). However, the magnitude of the difference between cement line and osteonal calcium content decreased with increased osteonal calcium content (r = −0.709, p < 0.001), suggesting diverging mineralization dynamics in these osseous entities. The number of mineralized osteocyte lacunae per osteon bone area correlated positively with both osteonal and cement line calcium content (p < 0.01). The degree of mineralization of cement lines may represent another tissue-age related phenomenon, given that it strongly relates to the osteonal mineralization level. Understanding of the cement lines' mineralization and their changes in aging and disease states is important for predicting crack propagation pathways and fracture resistance mechanisms in human cortical bone.