Biomedical Engineering Reference
In-Depth Information
through microvessels that conform closely to the contour of the cancellous bone
surface.
18
a specialized vascular structure, the bone remodelling compartment
(BrC), has been characterized
19
and shown to be composed of flattened cells
with all the characteristics of bone lining cells. immunoreactivity for all major
osteotropic growth factors and cytokines has been shown in the cells lining
the BrC, which makes it a candidate structure for resorption and formation
coupling. The secretion of these factors inside a confined space separated
from the bone marrow would facilitate local regulation of the remodelling
process without interference from growth factors secreted by blood cells in
the marrow space. the BrC could constitute an environment where cells
inside the structure are exposed to denuded bone and where osteoclast/
osteoblast activity could be directly regulated by integrins and other matrix
factors.
as clearly pointed out by davies,
20
when considering bone healing and
regeneration around endosseous implants, one has to take into account both
the texture of the implant surface and the type of bone macroarchitecture.
in fact, the relatively slow regeneration of peri-implant cortical bone relies
exclusively on lamellar remodelling, whereas the generation of peri-implant
trabecular bone may involve, not only remodelling of existing lamellar
trabeculae, but can also include the rapid formation of new trabeculae by
the recruitment of new populations of osteogenic cells within the healing
compartment. in fact, cancellous bone has a very high surface area which
is contiguous with the marrow compartment.
Since marrow contains not only mesenchymal osteoprogenitor cells but also
a rich vasculature that can supply both the circulating mononuclear precursors
to osteoclasts and the endothelial population needed for angiogenesis, it is
not surprising that trabecular bone can remodel far more quickly than cortical
bone. around endosseous implants, osteoblasts may lay down bone on the
old bone surface or on the implant itself.
21
The first occurs in cortical bone
as a result of osteogenic activity that does not create bone on the implant,
but around it, starting from damaged tissue toward the implant. in contrast,
in contact osteogenesis, new bone forms first on the implant surface that
is colonized by bone cells before bone matrix formation begins, in a very
similar way as at sites of remodelling where a resorption surface of old bone
is populated by osteogenic cells that are separated from the old bone by a
proteoglycan-rich collagen-free cement line matrix (Fig. 3.3).
22
in fact, in
this process of
de novo
bone formation, the collagen compartment of bone
is also separated from the underlying substratum by a collagen-free layer
containing proteoglycans and non-collagenous bone proteins (osteopontin
and bone sialoprotein).
23
