Mechanics of Glenohumeral Instability.
Last updated Friday, February 04, 2005
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Adhesion/cohesion
This is a stabilizing mechanism by which joint surfaces wet with joint
fluid are held together by the molecular attraction of the fluid to
itself and to the joint surfaces.Cohesion and adhesion Fluids such as water and joint fluid demonstrate the property of
cohesion; that is, they tend to stick together. Some surfaces, such as
clean glass or articular cartilage, can be wet with water or synovial
fluid, meaning that the fluid adheres to them. When two surfaces with
adherent fluid are brought in contact, the adhesion of the fluid to the
surfaces and the cohesion of the fluids tend to hold the two surfaces
together (like two wetted microscope slides). The amount of stability
generated by adhesion-cohesion is related to the adhesive and cohesive
properties of the joint fluid, the "wetability" of the joint surfaces,
and the area of contact between the glenoid socket and the humerus.
Joint fluid has the highly desirable properties of
- having high tensile strength (difficult to pull apart), and
- having little shear strength (allows easy sliding of the two joint surfaces on each other with low resistance). (Simkin, 1988)
The adhesion/cohesion effect is reduced by any factor that lowers
the cohesion of joint fluid (such as in inflammatory joint disease),
reduces wetability of the joint surfaces (as may occur in degenerative
joint disease), or diminishes the glenohumeral contact area (such as in
a displaced articular surface fracture or a congenitally small
glenoid). It is also noteworthy that adhesion/cohesion forces do not
stabilize a prosthetic shoulder replacement, because metal and
polyethylene are insufficiently compliant to provide the necessary
near-perfect congruence and because water does not adhere to their
surfaces.
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