Biomedical Engineering Reference
In-Depth Information
TABLE 11.1
Material Parameters Used in the Model
Parameter
Value
References
10
−
11
m
2
/sec
IGF-I diffusion coecient (
D
I
)
4.1
×
[29]
10
−
15
m
4
/Ns
Hydraulic permeability (
κ
)
1.6
×
[9]
Aggregate elastic modulus (
H
+
A
)
13.2 MPa
[30]
Fluid phase volumetric fraction (
φ
f
)
0.8
[30,60]
the solution for any size of incremental time step were avoided. The mate-
rial parameters used in the model prediction are shown in Table 11.1. These
parameters are typical of the bovine calf cartilage typically used in explant
experiments [9,29,30,60].
11.3 The Effect of Cyclic Loading and IGF-I Binding
on IGF-I Transport in Cartilage
11.3.1 Introduction
IGFs are key regulators of cellular proliferation, survival, and differentia-
tion in mammals [61] and are implicated in many human diseases (including
osteoarthritis [OA] and atherosclerosis) [62]. As chondrocytes synthesize little
or no IGF-I under normal physiological conditions [63], the primary source of
IGF-I is the liver. IGF-I must penetrate into cartilage from synovial fluid to
reach chondrocytes, where it exerts its biological effects. However, because of
its avascular nature and the thickness of cartilage at diarthroidal joints, IGF-I
is unable to diffuse into the cartilage easily.
Cyclic loading has been shown experimentally to enhance the transport of
solutes depending on the size of the solute. Bonassar et al. [37] showed exper-
imentally that fluid flow induced by dynamic compression may alter the peri-
cellular concentrations of growth factors, cytokines, newly synthesized matrix
macromolecules, or other nutrients. This observation may potentially explain
some of the increase in matrix synthesis observed in cartilage undergoing
dynamic loading compared with cartilage not undergoing dynamic loading.
From a transport experiment involving radiolabeled bovine serum albumin,
IGF-I, urea, and sodium. O'Hara et al. [39] concluded that dynamic compres-
sion enhanced the desorption of large solutes much more than that of small
solutes. This might be expected based on increase in the Peclet number for
larger molecules.
Recent theoretical studies generally support the idea that cyclic loading
enhances large molecule transport through advective transport. Mauck et al.
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