Biomedical Engineering Reference
In-Depth Information
of osteoblasts and osteoclasts that act together in refilling and eliminating bone
in a coupled way [10]. In situations of underloading or disuse, bone resorption
occurs, whereas under overloading bone formation is the net resulting process. In
the latter case, bone is likely to develop microcracks that eventually may collapse
and form a macroscopic crack, resulting in fatigue failure or stress bone fracture.
In contrast, modeling can be resorptive or formative, always happening through
an uncoupled process between osteoblasts and osteoclasts [2]. The role of bone
remodeling is multiple and complex [11]. From the biochemical point of view,
bone remodeling regulates minerals and hormones, influencing pathologies as
important as osteoporosis. From the mechanical point of view, bone remodeling
is responsible for the mechanical adaptation of bone tissue, determining its
mechanical capacity during the entire life. In fact, there are many experiments
that corroborate the effect of mechanical loads on bone adaptation [12-15]. For
example, it is widely assumed that remodeling is the mechanism that allows bone
to repair damage, reducing the risk of fracture [16-21]. On the other hand, both
modeling and remodeling are hypothesized to optimize the stiffness and strength
with minimum weight [22-24]. Indeed, it has been determined that peak periosteal
strains are similar across species during vigorous activities and rarely exceed
2000-3000
in long bones, which suggests that animal skeleton is continuously
''redesigned'' to control strain [25, 26].
µε
4.2.2
Bone Fracture Healing
A bone fracture involves the disruption of the resistant properties of bone. However,
in contrast to other nonliving materials, bone as a living tissue is able to regenerate
without scar formation. This regeneration process consists of the formation and
evolution of different tissue types in the fracture site with different mechanical
properties and spatial distribution, which are able to restore the original stiffness,
strength, and shape of the fractured bone. This process is really complex since the
bone callus forms at different stages that overlap over time [27]. Nevertheless, it is
important to mention that not all fractures heal correctly: around 5-10% result in
a delayed union or a nonunion [28].
Many factors influence the bone healing process, such as genetic, cellular, and
biochemical factors, age, type of fracture, interfragmentary motion, oxygen tension,
electrical environment, and fracture geometry [29-31]. Biochemical factors, such as,
soft tissue coverage and blood supply [31], are important to provide the biological
environment for the bone healing process. Several growth factors, such as the
transforming growth factor beta (TGF-
), fibroblastic growth factors (FGFs), and
platelet-derived growth factors (PDGF) [32], appear and interact in the fracture
site. Much interest in this research field has focused on mechanical factors. For
example, rigid fixation and moderate gap size can result in primary fracture repair
[33], while secondary healing is more likely to occur under flexible fixations [34].
It is generally accepted that some amount of axial movement between fracture
fragments stimulates callus formation and favors the quality and quantity of the
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