  Bone becomes more fragile with ageing. Among many structural changes, a thin
layer of highly mineralized and brittle tissue covers part of the external
surface of the thin femoral neck cortex in older people and has been proposed
to increase hip fragility. However, there have been very limited reports on
this hypermineralized tissue in the femoral neck, especially on its
ultrastructure. Such information is critical to understanding both the
mineralization process and its contributions to hip fracture. Here, we use
multiple advanced techniques to characterize the ultrastructure of the
hypermineralized tissue in the neck across various length scales. Synchrotron
radiation micro-CT found larger but less densely distributed cellular lacunae
in hypermineralized tissue than in lamellar bone. When examined under FIB-SEM,
the hypermineralized tissue was mainly composed of mineral globules with sizes
varying from submicron to a few microns. Nano-sized channels were present
within the mineral globules and oriented with the surrounding organic matrix.
Transmission electron microscopy showed the apatite inside globules were poorly
crystalline, while those at the boundaries between the globules had
well-defined lattice structure with crystallinity similar to the apatite
mineral in lamellar bone. No preferred mineral orientation was observed both
inside each globule and at the boundaries. Collectively, we conclude based on
these new observations that the hypermineralized tissue is non-lamellar and has
less organized mineral, which may contribute to the high brittleness of the
tissue.
