Mechanics of Glenohumeral Instability.
Last updated Friday, February 04, 2005
Limited joint volumeRelative vacuum This is a stabilizing mechanism in which the humeral head is held to
the glenoid by the relative vacuum created when they are distracted
(see figures 38 and 39). While it is common to speak of the
glenohumeral joint space, there is essentially no space and minimal
free fluid within the confines of the articular surfaces and the joint
capsule of the normal glenohumeral joint. The scarcity of fluid within
the joint can be confirmed on MRI scans of normal joints, on inspection
of normal joints, and on attempts to aspirate fluid from normal joints.
The appearance of the potential joint volume can only be demonstrated
after instilling fluids such as air, saline, or contrast materials into
the joint. Osmotic action by the synovium removes free fluid, keeping a
slightly negative pressure within the normal joint. (Levick, 1983;
Müller, 1929; Simkin, 1988) This negative intra-articular pressure
holds the joint together with a force proportional to the joint surface
area and the magnitude of the negative intra-articular pressure. For
example, if the colloid osmotic pressure of normal synovial fluid is 10
mm Hg and the colloid osmotic pressure of the synovial interstitium is
14 mm Hg, the equilibrium pressure in the joint fluid will be -4 mm Hg.
(Simkin, 1988) This negative intra-articular pressure adds a small
amount of resistance to distraction (about one ounce per square inch)
to the limited joint volume effect. Because the normal joint is sealed,
attempted distraction of the joint surfaces lowers the intra-articular
pressure even more, progressively adding substantial resistance to
greater displacement. (Harryman, Lazarus, Sidles et al, 1996; Itoi et
al, 1993)
The limited joint volume effect is reduced if the joint is vented
(opened to the atmosphere) or when the capsular boundaries of the joint
are very compliant. Under the latter circumstances, attempted
distraction draws the flexible capsule into the joint, producing a
"sulcus" (see figures 38 and 39). The decreased stability from venting
the joint was initially described by Humphry in 1858(Humphry, 1858) and
subsequently by others. (Cotton, 1921; Fairbank, 1948; Kumar and
Balasubramaniam, 1985b; Neer, 1970; Ovesen and Nielsen, 1985a; Ovesen
and Nielsen, 1985b; Sidles et al, 1989; Thompson and Winant, 1950;
Thompson and Winant, 1961; Wulker, Rossig, Korell et al, 1995) Gibb et
al (Gibb et al, 1991; Matsen, Lippitt, Sidles et al, 1994) found that
simply venting the capsule with an 18 gauge needle reduced the force
necessary to translate the head of the humerus halfway to the edge of
the glenoid by an average of 50 percent. Wulker(Wulker et al, 1993)
found that venting the joint increased the joint displacement with an
applied load of 50 N by 50 per cent in all directions.
From these results it is expected that glenohumeral stability from
limited joint volume is compromised by arthrography, arthroscopy,
articular effusions, hemarthrosis, and in other situations in which
free fluid is allowed to enter the glenohumeral joint. In a very
interesting study, Habermeyer et al (Habermeyer and Schuller, 1990;
Habermeyer et al, 1992) found the mean stabilizing force obtained by
atmospheric pressure was 146 N (32 lbs). In fifteen stable living
shoulders, traction on the arm caused negative intra-articular pressure
proportionate to the amount of force exerted. In contrast, unstable
shoulder joints with a Bankart lesion did not exhibit this phenomenon.
These stabilizing mechanisms may be overwhelmed by the application
of traction, as in the cracking of the metacarpophalangeal joint. A
"crack" is produced as the joint cavitates: subatmospheric pressure
within the joint releases gas (>80 per cent carbon dioxide) from
solution in the joint fluid. This is accompanied by a sudden increase
in the separation of the joint surfaces. Once a joint has cracked it
cannot be cracked again until about 20 minutes later when all the gas
has been reabsorbed. (Roston and Haines, 1947; Unsworth et al, 1971)
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