Joints.
Last updated Wednesday, January 19, 2005
Joint lubricationHow are joints lubricated? Synovial joints act as mechanical bearings that facilitate the work
of the musculoskeletal machine. As such, normal joints are remarkably
effective with coefficients of friction lower than those obtainable
with manufactured journal bearings. Furthermore, the constant process
of renewal and restoration ensures that living articular tissues have a
durability far superior to that of any artificial bearing. No
artificial joint can equal the performance of a normal human joint.
The mechanics of joint lubrication have provided a focus of
investigation beginning with the unique structure of the bearing
surface. Articular cartilage is elastic, fluid-filled, and backed by a
relatively impervious layer of calcified cartilage and bone. This means
that load-induced compression of cartilage will force interstitial
fluid to flow laterally within the tissue and to surface through
adjacent cartilage. As that area, in turn, becomes load bearing, it is
partially protected by the newly expressed fluid above it. This is a
special form of hydrodynamic lubrication, so-called because the dynamic
motion of the bearing areas produces an aqueous layer that separates
and protects the contact points.
Boundary layer lubrication is the second major low-friction
characteristic of normal joints. Here, the critical factor is proposed
to be a small glycoprotein called lubricin. The lubricating properties
of this synovium-derived molecule are highly specific and depend on its
ability to bind to articular cartilage where it retains a protective
layer of water molecules. Lubricin is not effective in artificial
systems and thus does not lubricate artificial joints.
Other lubricating mechanisms have been proposed; some remain under
investigation. Interestingly, hyaluronic acid, the molecule that makes
synovial fluid viscous (synovia means "like egg white"), has largely
been excluded as a lubricant of the cartilage-on-cartilage bearing.
Instead, hyaluronate lubricates a quite different site of surface
contact-that of synovium on cartilage. The well-vascularized,
well-innervated synovium must alternately contract and then expand to
cover non-loaded cartilage surfaces as each joint moves through its
normal range of motion. This process must proceed freely. Were synovial
tissue to be pinched, there would be immediate pain, intraarticular
bleeding, and inevitable functional compromise. The rarity of these
problems testifies to the effectiveness of hyaluronate-mediated
synovial lubrication.
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