Biomedical Engineering Reference
In-Depth Information
of a mystery. Tissue differentiation does not always
proceed in a timely manner according to the stages
described above. Tissue differentiation is a result of
gene expression and biosynthetic activity in cells,
and the resulting tissue depends on the amount of
vascularization and the mechanical environment of
the fracture
[4,5]
. More vascularization in the region
is desired, but the amount of stability to achieve
a quick and successful fracture union has been
debated. Some mechanical stability is required, yet
a small amount of micromotion at the fracture site is
thought to be beneficial to the healing process.
The past 20 years have seen the development of
differing theories guiding the mechanical stability
discussion. Perren
[2]
introduced a simple theory
based on interfragmentary strain that predicted the
course of fracture healing based on the amount of
movement tolerated at the fracture site. According to
this theory, strain between 2% and 10% will lead to
the indirect healing process described above. Strains
below 2% are theorized to lead to a direct bone
formation without the aid of the callus formation, that
is, primary bone healing. Strain between 10% and
100% leads to a sustained environment of initial
connective tissue, while strain above 100% creates an
environment susceptible to nonunion.
Prior to this, Carter et al.
[4]
presented a view that
decomposed strain into dilational and distortional
components. Their hypothesis relies on the consid-
eration of the effect that strain has on the non-
distorted, low-energy spherical shape that a cell
naturally maintains. According to the theory, distor-
tional strain alters the cytoskeleton and affects the
gene expression (genes are activated and signal cells
to synthesize and secrete the chemical constituents of
tissue) and biosynthetic activity of mesenchymal
cells, fibroblasts, and chondrocytes. Similarly, dila-
tional strain in the form of hydrostatic pressure can
affect gene expression and also inhibit capillary
blood flow by decreasing tissue oxygen tension. In
early healing, intermittent, moderate shear stress
stimulates the proliferation of tissues and the
formation of callus, the size of which is dependent on
the degree of shear stress. In the case that distortional
stress or tensile dilational stress is too large, either
fibrocartilage or fibrous tissue is formed due to
fibroblasts increasing the production of collagen
oriented in the tensile direction. In the case of cyclic
compressive hydrostatic stresses, cartilage will form.
A compressive environment can result in poor
vascularity and low oxygen tension, possibly from
moderate hydrostatic pressure inhibiting capillary
flow to the area. In this environment, mesenchymal
cells are “shunted” to a chondrogenic pathway, as
opposed to an osteogenic pathway. This is due to the
lesser metabolic demands of cartilage and fibro-
cartilage, in comparison to bone and fibrous tissue. In
the case that cartilage forms, cyclical shear will
promote ossification while hydrostatic compression
will increase proteoglycan synthesis as well as other
biologic processes supporting chondrogenesis and
cartilage maintenance. This will prevent the forma-
tion of bone. In extreme cases, the fractures may
form cartilaginous caps
in a pseudarthrosis
to
accommodate the high compressive strains.
In an attempt to corroborate Carter's theory,
Gardner et al.
[5]
linked stress fields from a 2D finite
element model to clinical data from a tibial diaphy-
seal fracture in a 20-year-old male. The study results
support Carter's proposition that high octahedral
shear stress promotes tissue proliferation and
increases the size of the callus in all stages of healing.
They could not corroborate the theory that bone
formation is shunted to cartilage in the presence of
compressive hydrostatic stresses. They proposed that
as bone matures, it is able to withstand a higher
degree of compressive dilational stress while still
undergoing endochondral ossification. Recently,
Epari et al.
[6]
studied the components of fixation
that are beneficial toward timely fracture healing. As
opposed to focusing on the strain environment of the
fracture, the authors studied the directional move-
ment of the fragments that produced better healing in
osteotomized ovine tibiae. After 9 weeks of healing,
measurements for the torsional stiffness and failure
moment of the explanted specimens were taken and
relationships were determined in relation to the
degree of axial compression, torsion, bending, and
shear stability provided during healing. The general
conclusion of the work was that high shear stability
and moderate, not maximal, axial stability are asso-
ciated with high callus strength and stiffness and
a better healing outcome. The authors also deduced
that some degree of torsional stability is also helpful
for healing, although no direct relationship was
determined. A simple summary of their results given
in
Fig. 15.2
is a map of the appropriate fixation
stiffness for fast and uneventful healing.
In response to these and other studies, some
trauma applications have seen the “softening” of
hardware used to fix fractures with good results.
Many applications have exchanged stainless steel
