Limited Joint Volume.
Last updated Thursday, February 10, 2005
Figure 2 - Plugged syringe: When the plunger is distracted, a "sulcus" is formed as the dam invaginates into the barrel About limited joint volume
The limited joint volume is a stabilizing mechanism in which the
humeral head is held to the socket by the relative vacuum created when
they are distracted.Strong enough to support the weight of the arm 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 lack 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. 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. 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.
This mechanism is defined as the limited joint volume effect. Our
cadaver experiments demonstrated that this mechanism is sufficiently
strong enough to support the weight of the arm. The limited joint
volume effect is reduced if the joint is vented (opened to the
atmosphere). These studies indicated 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 (28 to 13 Newtons anteriorly, 25 to 14 Newtons posteriorly,
and 33 to 14 Newtons inferiorly). The limited joint volume effect is
also compromised if the joint contains free fluid or if the capsule is
very compliant.
This mechanism of glenohumeral stability is, therefore, compromised
with arthrography, arthroscopy, articular effusions, hemarthrosis, and
in other situations in which free fluid is present within the
glenohumeral joint. The limited joint volume effect is also compromised
when the capsular boundaries of the joint are very compliant. Under
these circumstances, attempted distraction draws the flexible capsule
into the joint, producing a "sulcus". This may be one of the factors
that contributes to midrange glenohumeral instability in patients with
generalized ligamentous laxity.
One can observe the stabilizing effect of limited joint volume,
attempt to distract the plunger from the barrel of a plugged syringe.
The destabilizing effect of capsular venting is seen on distracting the
plunger after the hole in the end of the barrel is opened to the air.
The destabilizing effect of compliant capsular walls is seen when the
end of the syringe is replaced with a compliant material such as a
rubber dam. When the plunger is distracted, a "sulcus" is formed as the
dam invaginates into the barrel.
Adhesion-cohesion, the glenoid suction cup,
and limited joint volume provide a family of stabilizing mechanisms
that function throughout the range of glenohumeral motion, including
the midrange where the glenohumeral ligaments and capsule are not under
tension. Midrange stability is critical in that most of the activities
of daily living such as dressing, eating, working, and writing are
performed in midrange positions.
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Disclaimer
This resource has been provided by the University of Washington Department of Orthopaedics and Sports Medicine as general information only. This information may not apply to a specific patient. Additional information may be found at http://www.orthop.washington.edu or by contacting the UW Department of Orthopaedics and Sports Medicine.
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