Background: Although the glenohumeral joint
is the most mobile articulation of the human body, it is known to
exhibit ball-and-socket kinematics. Compression into the glenoid
labral concavity keeps the humeral head centered. The purpose of
the present study was to determine the effects of joint position
on glenohumeral stability through concavity-compression.
Methods: Ten cadaveric shoulders were tested. The
glenoid was mounted horizontally onto a six-component load-cell
while the humerus was clamped to a vertically unconstrained slide.
An x-y stage translated the load-cell with the glenoid underneath
the humeral head in eight different directions. Compressive loads
of 20, 40, and 60 N were applied. The tests were repeated in 0°,
30°, 60°, and 90° of glenohumeral abduction with and without the labrum.
Relative translations between the glenoid and the humeral head and
the forces resisting translation were recorded. Then the stability
ratio, defined as the peak translational force divided by the applied
compressive force, was calculated.
Results: The average stability ratio was higher
in the hanging-arm position than it was in glenohumeral abduction.
The highest stability ratio was detected in the inferior direction
(59.8% 7.7%) when the labrum was intact and in
the superior direction (53.3% 7.9%) when the
labrum had been resected. Under both conditions, the anterior direction
was associated with the lowest stability ratio (32.0%
4.4% with the labrum and 30.4% 4.1% without
the labrum). Resection of the glenoid labrum resulted in an average
decrease in the stability ratio of 9.6% 1.7%.
With increasing compressive load, the average stability ratio slightly decreased.
Conclusions: Glenohumeral stability through concavity-compression
was greater in the hanging-arm position than it was in glenohumeral
abduction. The average contribution of the labrum to glenohumeral
stability through concavity-compression was approximately 10%,
about one-half of the value previously reported. With the labrum
intact, the glenohumeral joint was most stable in the inferior direction.
Without the labrum, it was most stable in the superior direction.
Under both conditions, it was least stable in the anterior direction.
Glenohumeral joint stability through concavity-compression decreases
with higher compressive loads.
Clinical Relevance: Anterior dislocation of the
shoulder may be facilitated by the lower stability demonstrated
in glenohumeral abduction. The labrum may not contribute to glenohumeral
stability as much as was previously assumed. However, even moderate
compressive forces are sufficient to provide stability through concavity-compression.