Glenohumeral Balance.
Last updated Thursday, February 10, 2005
Figure 1 - The force is contained within the fossa Figure 2 - The angular range of stability resulting from glenohumeral balance Figure 3 - Hypoplasia, erosion, or fracture of the glenoid rim can diminish the arc available for balance stability Figure 4 - The vector sum of the deltoid and cuff muscle forces lies close to the axis of the humerus Figure 5 - Balance at the glenohumeral joint is determined by humeroscapular position Figure 6 - The balance stability mechanism may fail in cases of severe muscle imbalance About glenohumeral balance
Glenohumeral balance is a stabilizing mechanism in which the glenoid is
positioned so that the net humeral joint reaction force passes through
the glenoid fossa.
Joint positioning The joint can be positioned so that the force is contained within
the fossa. No other stabilizing mechanism is necessary as long as the
humeroscapular position is such that the glenoid supports the net
humeral joint reaction force. When the joint is in balance, its
stability is independent of the magnitude of net humeral joint reaction
force.
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Directions and angles Balance is sensitive to the direction of the net humeral joint
reaction force vector with respect to the glenoid fossa. The larger the
arc subtended by glenoid concavity, the larger the range of directions
of the net humeral joint reaction force vector that will be stabilized
by it. This range of direction can be estimated from simple geometric
calculations. A radian is the central angle of a circle which subtends
an arc equal in length to the radius of the circle. There are 2 pi
radians in a circle, thus one radian equals 360�/2 pi or almost 60�.
The angular range of stability resulting from glenohumeral balance can
be predicted by dividing the length of the glenoid arc by the radius of
the humeral head and multiplying the quotient by 360/2 pi. As an
example, if the anteroposterior arc length of the glenoid were equal to
the radius of the humeral head, the glenoid would balance net joint
reaction force vectors through a range of directions of approximately
60 degrees (from about 30 degrees anterior to 30 degrees posterior to
the glenoid center line). Hypoplasia, erosion, or fracture of the
glenoid rim can diminish the arc available for balance stability.
In an experiment with a series of cadaver shoulders, we demonstrated
this balancing effect in a manner similar to the golf ball and tee
demonstration. We found that the net humeral joint reaction force
vector was balanced through a wide range of angles. The combined
anteroposterior balance stability angle averaged 36 degrees and that
for the superior-inferior directions averaged 57 degrees. In these
cadaver shoulders, balance stability was relatively symmetrical around
the glenoid center line (the anterior and posterior stability angles
were approximately equal, as were the superior and inferior angles).
Owing to the increased vertical extent of the glenoid, the
superior-inferior stability angle was greater than the
anterior-posterior stability angle. The anterior stability angle was
reduced by an average of over 25 percent by the presence of a
relatively small glenoid rim defect.
The large number of component forces operating on the humerus makes
it difficult to calculate the exact direction of the net humeral joint
reaction force vector in vivo. A rough approximation is that in
midrange humeroscapular positions, the vector sum of the deltoid and
cuff muscle forces lies close to the axis of the humerus. In many
vigorous shoulder activities the scapula is positioned so that the
glenoid center line is closely aligned with the humerus. During the
critical moments of the boxer's knockout punch, the bench press, or the
tennis stroke, for example, the humeroscapular position appears to be
such that the glenoid center line and the humerus are aligned. Under
these circumstances the glenohumeral joint is stabilized by balance so
that muscle energy is preserved for power. This observation emphasizes
two special features of balance stability:
- as long as the net joint reaction force vector is relatively
aligned with the glenoid center line, the resulting stability is
unaffected by increasing the magnitude of this force; and
- the only muscular effort required to achieve balance is that for
positioning the glenoid in relation to the net humeral joint reaction
force.
Stability from balance is of particular interest because of its
relationship to humeroscapular position. It is apparent that
essentially identical humerothoracic positions can be achieved using
different humeroscapular positions. Some of these positions will favor
glenohumeral balance, while others will not. Consider the arm elevated
90 degrees in the plus 90 degree thoracic plane. If the scapula is
protracted, the humerus is closely aligned with the glenoid center
line. Alternatively, if the scapula is retracted, the humerus is almost
at right angles to the glenoid center line. The essential point is that
balance at the glenohumeral joint is determined by humeroscapular
position rather than the more easily observed humerothoracic position.
Which humeroscapular position is used to achieve a given humerothoracic
position is a question of neuromuscular control, habit, and training.
In analyzing patients with glenohumeral instability, it is important to
document the humeroscapular positions in which the instability occurs.
Neuromuscular retraining may help the patient regain stabilizing
balance.
The precision of neuromuscular control required for balance is
inversely related to the size of the glenoid fossa. The angle through
which the balancing mechanism can function is less in the anterior and
posterior directions, where the glenoid arc is relatively small, than
in the superior and inferior directions, where it is larger. The
balance stability angle is diminished in glenoid hypoplasia, fracture,
or degenerative erosion. In prosthetic shoulder arthroplasty,
components with a large articular surface offer a greater balance
stability angle.
The balance stability mechanism may fail in cases of severe muscle
imbalance where the net humeral joint reaction force is not aligned
with the glenoid, even though the humerus is close to the glenoid
center line. The balance stability mechanism may fail in the presence
of abnormal glenoid version where the glenoid center line deviates
substantially from the plane of the scapula and from the net.
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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