Humerothoracic Positions and Motion.
Last updated Wednesday, January 26, 2005
Activities demonstrated on global diagram Indicating the rotational orientation of the arm at maximal elevation Series of points and arrows Codman's paradox movement sequence Humerothoracic global diagramDisplaying humerothoracic positions The global diagram is an effective method for displaying
humerothoracic positions because it allows presentation of both the
planes of elevation ("longitudes") and the angles of elevation
("latitudes"). The "South Pole" of the globe represents zero degrees of
elevation. The range of possible positions and the positions required
for various activities can be demonstrated on global diagrams. Note
that the maximal elevation in the different planes defines the envelope
of humerothoracic motion available to this shoulder. The positions used
for the Simple Shoulder Test functions lie within this envelope.
By using an arrow pointing in the direction of the anterior humerus
(the direction of the forearm if the elbow was flexed to 90¼), the
global diagram also provides a method for indicating the rotational
orientation of the arm at maximal elevation and for carrying out the
functions of the Simple Shoulder Test.
The details of sequential humeral motions, for example, during a
throw, can be indicated on a global diagram as a series of points and
arrows. Codman's paradox Codman proposed that the completely elevated humerus could be shown
to be in either extreme external rotation or in extreme internal
rotation by lowering it either in the coronal or sagittal plane,
respectively, without allowing rotation about the humeral shaft axis.
We can use the global diagram to examine Codman's paradox:
Carry out the movement sequence described below without allowing rotation about the humeral shaft axis:
- Place the arm at the side with the forearm internally rotated across the stomach.
- Elevate the arm 180 degrees in the plus 90 degree thoracic (sagittal) plane.
- Lower the arm 180 degrees to the side in the 0 degree (coronal) plane.
ote that the forearm now points 180 degrees from its original position.
This entire motion can be drawn on a global diagram. The fraction of
the surface area of the sphere that is enclosed by this path of motion
is 1/4. This result demonstrates the relationship between enclosed area
and rotation. The area of a unit sphere is 4 pi. One-fourth of this is
pi; 360 degrees of rotation is equal to 2pi; thus pi is equal to 180
degrees of rotation. Here we see that a humeral path without rotation
about the humeral shaft axis circumscribing one-fourth of a sphere
results in an induced rotation of 180 degrees.
This relationship between area and induced rotation holds true for
any sequence of motions in a closed path in which there is no rotation
about the humeral shaft axis. From this relationship, we can see that
the apparent paradox of induced rotations on Codman's motions is a
property of motion on the surface of a sphere and not a paradox at all! 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.
How useful was this page or article?
|
|