Biomedical Engineering Reference
In-Depth Information
6.
Horizontal Flexion - Motion in the horizontal plane anterior to the
coronal plane (Fig. 7.7). The normal range is from 0° to about 135°.
7.
Horizontal Extension - Motion in the horizontal plane posterior to the
coronal plane (Fig. 7.7). The normal range is from 0° to about -45°.
Any restriction of these motions is undesirable as it adversely affects function,
can produce undesirable loading on the prosthesis, ligaments, and the bone
fixation interface and can result in dislocation.
7.2.1.1 Axial Rotation
The rotation of the arm about the humeral or radial axis is difficult to define since
it varies with the position of the arm. Usually it is measured at 90° of forward
flexion or at 90° of abduction.
7.2.1.2 Glenohumeral Motion [3]
Since shoulder replacement generally involves the replacement of the glenohumeral
joint the motion of the joint is of special interest. Active glenohumeral motion is the
result of rhythmic motion between the scapula and the humerus, such that for each
15° of motion, either in forward flexion or abduction, 10°
is glenohumeral motion and
5° is scapular rotation
with precise ratios varying during the cycle. [4]
This 2:1
relationship allows 120° motion of the glenohumeral joint in both forward flexion
and abduction with 60°
of scapular rotation. Combined, this "scapulohumeral" rhythm
allows 180°
elevation or complete overhead positioning of the arm. With the arm at
the side in neutral position, internal rotation of 90°
and external rotation of about 90°
can be reached before the soft tissue tension of the rotator cuff restricts further
motion.
7.2.2 Stability and Secondary Motion [3]
Conventional thinking has converted the glenohumeral joint into a simple ball and
socket or enarthroidal joint. A closer look at the rotation mechanism, however,
discloses its true nature.
The role of the rotator cuff has been extensively described. Essentially, in
addition to individual specific functions of each rotator insertion, all the com-
ponents of the cuff act together as a tendinous socket which grasps the nearly
spherical humeral head, bringing it tightly into the shallow disk of the glenoid,
upon which articulation may proceed without dislocation. In active elevation, the
humeral head, while firmly anchored by the cuff, is depressed by the combined
action of the short rotators to provide better fixation and to increase the lever arm
for the deltoid in elevation. That this mechanism is important to elevation is
readily observed in rotator cuff tears of the supraspinatus insertion, where the
humeral head is not depressed, but rather rides high on the glenoid due to an
unopposed pull from the deltoid. This loss of stable fixation and elevated rotation
center decreases the lever arm for the deltoid and permits impingement of the
