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

The cement line (CL) is a thin layer, 1-3 μm in width, separating secondary osteons from interstitial bone and other osteons. Despite the possible role for bone quality, the CL is still one of the least understood features of bone. This study aims to investigate how the mineral content of the CL varies not only with osteon age but also with the surrounding environment. Using quantitative backscattered electron imaging to measure the mineral content, we analyzed 35 osteons from femoral bone of 2 male individuals (40 and 81 yr old). We implemented a new approach to investigate the mineral content based on a spatially resolved analysis in layers along the CL and incorporating regions both inside the osteon (formed soon after CL deposition) and outside (already present at the time of CL deposition). We found that the CLs had always higher mineral content than the corresponding osteon ( < .001) and that not only the osteon, but also the CL increases its mineral content with time. Including areas outside the osteon in the analysis improved considerable our understanding of CL mineralization. After a rapid primary phase, where the CL incorporates more mineral than the osteon, secondary mineralization is about 60% lower in the CL than in the osteon. One key finding is that the mineralization of the CL is not universal but depends on the region in which the osteon is formed. This is supported by a strong correlation between the mineral content of the CL and outside the osteon ( = 0.75,  < .001), but not inside. One possible explanation is that mineral released during bone resorption may contribute to the mineralization of the CL, as higher mineral content in resorbed bone was associated with greater mineralization in the CL.