Mechanics of Glenohumeral Instability.
Last updated Friday, February 04, 2005
The balance stability angle and the stability rati
The balance stability angle is the maximal angle that the net humeral
joint reaction force can make with the glenoid center line before
glenohumeral dislocation occurs. The tangent of this balance stability
angle is the ratio between its displacing component (perpendicular to
the glenoid center line) and its compressive component (parallel to the
glenoid center line), which is known as the stability ratio.Displacing forces The stability ratio is the maximal displacing force in a given
direction that can be stabilized by specified compressive load,
assuming frictional effects to be minimal (footnote 1). The effective
glenoid arc, the balance stability angle and stability ratios vary
around the perimeter of the glenoid (see figure 15). It is handy to
note that for small angles the stability ratio can be estimated by
dividing the balance stability angle by 57 degrees (footnote 2).Stability ratio The stability ratio is frequently used in the laboratory because it
is relatively easy to measure: a compressive load is applied and the
displacing force is progressively increased until dislocation occurs.
For example, Lippitt et al (Lippitt et al, 1993) found that a
compressive load of 50N resisted displacing loads up to 30N and that
the effectiveness of this stabilization mechanism varied with the depth
of the glenoid.
Investigation of these parameters provides important information on
stability mechanics, for example, resection of the labrum has been
shown to reduce the stability ratio by 20 per cent. (Lippitt,
Vanderhooft, Harris et al, 1993) Furthermore, a three millimeter
anterior glenoid defect has been shown to reduce the balance stability
angle over 25 per cent from 18 to 13 degrees. (Matsen, Lippitt, Sidles
et al, 1994)
Clinically, the stability ratio can be sensed using the "load and
shift" test wherein the examiner applies a compressive load pressing
the humeral head into the glenoid while noting the amount of
translating force necessary to move the humeral head from its centered
position. (Silliman and Hawkins, 1993) This test gives the examiner an
indication of the adequacy of the glenoid concavity and is one of the
most practical ways to detect deficiencies of the glenoid rim. Footnotes Footnote 1: Measured stability ratios may be influenced by the
friction of the joint surfaces and by other stabilizing mechanisms such
as adhesion/cohesion and the glenoid suction cup (which will be
discussed later). These effects will tend to increase the displacing
force necessary to dislocate the humeral head for a given compressive
load. It is essential to control for these effects in the laboratory.
Specifically, the under-lubricated, aged cadaver joints available to
the lab may have substantially greater coefficients of friction in
vitro than the exquisitely lubricated and smooth joint of the young
person in vivo.
Footnote 2: At small angles the tangent of an angle is approximately
equal to the angle expressed in radians. Thus the stability ratio
(tangent of the balance stability angle) is approximately the balance
stability angle divided by 57 degrees per radian.
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