On the Horizon From the ORS.

Bone is a fabric of organic material reinforced and stiffened by hydroxyapatite mineral crystals attached and interdigitated with proteins.1 When mineralized regions grow large enough, however, they could interfere with each other, conceivably in a manner analogous to rocks in a bag. At the 53rd Meeting of the Orthopaedic Research Society, a presentation by K. Tai, working with F. Ulm and Christine Ortiz from The Massachusetts Institute of Technology,2 showed fairly conclusively that “rocks in a bag” could be a better model for bone than one might think. Dr. Ortiz runs a laboratory where nanoindentation testing of bone is performed. In nanoindentation experiments, a diamond-tipped indenter is forced into a highly polished bone surface. The resulting forcedisplacement results can be used to calculate the stiffness and failure properties of the tissue. When the indentations are completed, atomic force microscopy (AFM) can be used to image the resulting indentations in the polished surface. It is when the nanometer resolution AFM images are examined that the story begins to become interesting. The nanoindenter tips are smooth, three-sided pyramids. Therefore, when the indentation is imaged by AFM, one expects to see a smooth, pyramidal dent in the surface. Generally, this is what is seen. However, at lengths smaller than a micrometer, the surface of the polished and indented surfaces is “cobbled” with a particulate structure <1 μm in diameter (about 0.25 μm) but much larger than the ordinarily observed hydroxyapatite crystal (tens of nanometers in length, but only 1 to 5 nm in thickness). The “cobbles” can be idealized as interacting particles with an interparticular friction coefficient. Tai et al2 measured the friction coefficient by preparing deorganified bone powder and measuring the “angle of repose” of the powder in vacuo. The angle of repose is simply the angle that the side of the pile of powder has in one gravity. Angle of repose is the key input for the Mohr-Coulomb strength theory, a simple strength theory widely used for soils and rock. Essentially, using the angle of repose and the soils failure theory, Tai et al2 were able to predict accurately the original indentation test results using nonlinear finite element analysis. The concept of rocks in a bag explains much of bone tissue mechanics that otherwise is fairly obscure. For example, the Mohr-Coulomb theory predicts different tensile and compressive strengths for a material, just as is observed for actual bone tissue. In addition, soils in compression tend to fail along slip lines, whereas soils in tension tend to separate into individual grains. This difference in failure modes is sufficiently like the difference between linear bone microcracks caused by compression3 and the microcomminuted (diffuse) damage4 caused by tension5,6 that Tai at al2 seem to have presented a key new observation of the mechanical organization of bone at the ultrastructural level.