Biomedical Engineering Reference
In-Depth Information
continuously throughout life such that bone is constantly being remodeled
in the growing, adult and senescent skeleton. In contrast to modeling, which
involves activation and isolated bone resorption (aÆR) or formation (aÆF)
at a bone locus, remodeling involves the temporally and spatially coordinated
actions of osteoclasts and osteoblasts. These cells form teams collectively
known as basic multicellular units (BMus) which always remodel bone in
a activationÆresorptionÆformation sequence (aÆRÆF) (Fig. 2.9).
during activation, osteoclast precursors differentiate as previously described
into multinucleate osteoclasts (Fig. 2.5a). although the precise pathways
directing osteoclast precursor recruitment are not fully understood, known
triggers include mechanical forces, microscopic bone damage (microdamage)
and systemic hormones.
121
Remodeling that occurs in response to mechanical
forces and microdamage replaces specific packets of bone and is referred to
as targeted remodeling.
122
This functions to maintain skeletal mechanical
integrity. In contrast, remodeling in response to systemic hormones is
apparently random in terms of location and, thus, is referred to as non-
targeted remodeling.
122
This type of remodeling enables bone to fulfill its
metabolic requirements, which includes the storage and release of minerals
such as calcium and phosphorus. It has been estimated that roughly 30% of
remodeling is targeted and that 70% in non-targeted.
122
Resorption follows activation, and consists of an advancing front of
actively resorbing osteoclasts (Fig. 2.9). during intracortical remodeling,
Blood vessel
New lining
cells
Local factors
Lining cells
ACTIVATION
Osteoblasts
Osteoclasts
Osteoid
FORMATION
New bone
RESORPTION
Cement line
Old bone
Direction of remodelling
Formation
2.9
Bone remodeling by a basic multicellular unit (BMU). A
stimulus activates osteoclast precursors to differentiate and form
an advancing front of actively resorbing osteoclasts. The resorptive
bay created by osteoclastic bone resorption is lined by mononuclear
cells (not shown) prior to the formation of osteoid (unmineralized
bone matrix) by osteoblasts (reproduced with permission of the
Massachusetts Medical Society [©2007; all rights reserved] from
Canalis
et al
.
234
).
