Limited Joint Volume

Last Updated: Thursday, February 10 2005

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.