The mineralization of osteonal cement line depends on where the osteon is formed

The cement line (CL) is a thin layer separating secondary osteons from interstitial bone and other osteons. It is assumed to play a significant role in bone fracture resistance, owing to its ability to deflect or arrest microcracks. Despite the possible role for bone quality, the CL is still one of the least understood microstructural features of bones, with unknowns on CL composition, mineralization, and mechanical properties. This study, focusing on CL mineralization, aims to elucidate the interplay between the mineral content of the CL and of adjacent bone tissue. Using quantitative backscattered electron imaging, osteons with different degrees of mineralization coming from human femoral samples were analyzed. We implemented a spatially resolved analysis of the mineral content in layers along the CL, considering both regions inside the osteon (i.e., formed soon after CL deposition) and outside (i.e., already present at the time of CL deposition). We found that the mineral content of the CL correlates strongly with the mineral content outside of the osteon, but not inside. Assuming the mineral content of the osteon as a proxy of its age, we demonstrate that not only the osteon, but also the CL increases its mineral content with time. However, the rate of increase is lower in the CL. Importantly, the specific value of the high initial mineral content of the CL depends on the mineral content of the local surrounding, in which the osteon was formed. Our findings highlight the central role of the local degree of mineralization of the bone around the osteon for building the CL.