Cement Lines are Stiffer and Harder than Bone but Exhibit Different Mineral–Mechanics Relationships due to Thicker and Shorter Mineral Particles

The remarkable mechanical performance of bone arises from its complex hierarchical structure and from the presence of numerous internal interfaces joining the different components. Formed during bone remodeling, the cement line (CL) is a thin interface surrounding osteons. Although often neglected due to its small size, the CL has been suggested to play an important role in bone fracture toughness. However, there is an ongoing debate about its specific structure and mechanical behavior. Here, we investigate the composition–mechanics relationship at the CL and surrounding bone in human osteons using multiple methods in a correlative manner. We found that the CL is stiffer and harder than adjacent osteonal tissue. However, the CL requires more mineral than bone to attain the same stiffness and hardness. Analyzing the nanoscale properties of the mineral, we found thicker but shorter particles at the CL. Using a mechanical model, we interpreted the lower aspect ratio of the mineral particles as a less effective way to reinforce the collagen matrix. Casting our findings into a computational model, we questioned the possible protective role of the CL, which is not a soft interface as traditionally believed.