Mapping the imprint of bone remodeling by atomic force microscopy

Understanding bone remodeling is essential for understanding bone as a whole. Bone remodeling takes place through a stepwise cellular process, with osteoclasts carving small cavities, also known as resorption pits, into the surface of trabecular bone, followed by osteoblasts, which refill the pits with new soft bone collagen matrix tissue (osteoid). The detailed structure of the surface in the frontier of a resorption pit before, during, and after the osteoid is being laid down is not well known. We present detailed atomic force microscope (AFM) images from the edge, the front end, and the bottom of a resorption pit in a human trabecular bone sample that showed signs of incomplete remodeling. The images reveal a scalloped surface left behind by the osteoclasts and the surface morphology of preexisting bone tissue and new bone tissue. In addition, we display the bone formation front in the pit showing the anchor points between the new bone and the existing bone. We also found evidence of microcracking in the front end of the pit, suggesting that microcracking was the initiator of this particular resorption pit. We anticipate that AFM may initiate a more detailed understanding of the influence of the remodeling process on the structure of bone, as well as a better understanding of the surface on which new bone tissue can be anchored.