Biomedical Engineering Reference
In-Depth Information
reduction of the fracture fragments and the stabilization of the fracture combined
with enough strength to support the loads during the period when they are acting.
In both cases, the surgical implantation of orthopedic implants can result in a
strong modification of the load transfer mechanism, over- and underloading the
bone in specific sites. This effect causes long-term alterations in bone structure
around the implant. These changes can include increased bone formation in some
regions and bone loss in others. Increased bone formation can be caused by
biological bone induction mechanisms associated with the surgical trauma. Bone
resorption can occur by mechanobiological adaptation to the altered loading pattern
transmission produced when an implant is incorporated (''stress shielding''). Other
causes may also produce bone resorption, such as osteolysis due to wear particles,
micromotions in the interface, or poor vascularization. To analyze the functional
adaptation that can occur around implants, computer simulations based on finite
element analysis (FEA) are normally used. In this chapter, dedicated to the
application of mechanobiological models to bone implant design, we shall show
some examples related to the analysis of this effect after implantation of different
types of prostheses and implants.
4.2
Biological and Mechanobiological Factors in Bone Remodeling and Bone Fracture
Healing
4.2.1
Bone Remodeling
Bone is one of the most active tissues in the body, being constantly rebuilt in a
process named as
bone adaptation
or
bone remodeling
. Depending on the surface
where bone is added or removed, it is usual to distinguish between two different
adaptative processes: internal remodeling (or simply remodeling) on trabecular
surfaces in the cancellous bone and Haversian systems in the cortical bone,
and external remodeling (or modeling) in the periosteum and endosteum. Bone
remodeling only occurs at bone surfaces where the effector bone cells are located [2].
The main bone cells responsible for bone remodeling are osteoblasts, osteoclasts,
and bone lining cells located on bone surfaces and osteocytes inside the bone
matrix [2]. Osteoblasts produce bone. They differentiate from mesenchymal stem
cells, which can come from the periosteum layer or from the stromal tissue of
the bone marrow. Osteoclasts remove the bone, demineralizing it with acid and
dissolving collagen with enzymes. The origin of these cells is the bone marrow.
Bone lining cells are inactive osteoblasts that escaped from being buried in the new
bone and remained on the bone surface when bone formation stopped. They can be
reactivated to osteoblasts in response to various chemical and mechanical stimuli
[3]. Osteocytes are, like bone lining cells, former osteoblasts that became buried in
the newly formed bone. Many authors [4-9] have proposed that osteocytes are the
mechanosensor cells that control bone remodeling. Bone remodeling is the final
result of the activity of cellular packets (named as basic multicellular units, BMU)
