Biomedical Engineering Reference
In-Depth Information
These cells populate the central NP at birth but quickly disappear during early
childhood [
25
]. The function of these cells is still under investigation, but it is
believed that they produce matrix and serve to coordinate matrix production of the
NP cells in the vicinity.
In the normal healthy IVD, the cells not only produce matrix macromolecules
and growth factors, they also produce a myriad of proteases [
26
,
27
]. Included in
this list of proteases are the matrix metalloproteinases (MMPs) and aggrecanolytic
members of the disintegrin and metalloproteinase with thrombospondin motif
family (ADAMTS) as well as their respective inhibitors. It is the maintenance of
this critical balance that results in a healthy IVD ECM that is subsequently well
adapted for its physiologic and biomechanical function.
4
Intervertebral Disc Function and Mechanical Loading
IVDs are subjected to complex loads generated during compression,
flexion-extension, lateral bending, and axial rotations. The mechanical function
of the disc relies largely upon the synergistic relationship between NP and AF to
absorb and distribute spinal loads. Nearly 75% of the compressive load observed by
the IVD is borne by the NP itself, with the remaining 25% supported by the AF.
Load management in the AF and NP clearly relates to specific biochemical
components, their spatial relationships, and the resulting biphasic viscoelastic
properties. Under equilibrium conditions, compressive loads on the IVD are
borne predominantly by the swelling pressure (Donnan osmotic pressure) and
solid matrix of the NP. More specifically, the fixed charge density of the aggrecan
molecules found in the NP generate a swelling pressure of about 0.1-0.3 MPa [
28
].
When loads on the spine increase, for instance when bending to pick up a heavy
box, the low hydraulic permeability of the NP tissue causes fluid pressurization. As
a result of this, the NP not only supports compressive loading, but can convert some
of the compressive stresses into tensile stresses in the AF, which aids in stabilizing
the spine. Compressive loads typically observed within the human lumbar spine
have been estimated to be in the range of 150-250 N when lying prone, 500-800 N
during relaxed standing, and 1,900 N when lifting 10 kg with a rounded back [
29
].
Corresponding intradiscal pressures within the lumbar NP have been measured to
be 0.1-0.24 MPa lying prone, 0.5 MPa standing relaxed, and 2.3 MPa lifting 20 kg
weight with flexed back [
30
]. Application of such loads result in the exudation of
nearly 20-25% of disc water, resulting in a fluctuation in spine length of approxi-
mately 1-2 cm throughout the course of a day. As the volume fraction of water in
the NP decreases concomitant with an increase in fixed charge density, osmotic
pressure rises, allowing for the influx of fluid back into the IVD when the spine is
unloaded during diurnal rest periods.
