One of the many remarkable features of the normal shoulder is that the glenohumeral joint (the ball and socket joint between the upper arm bone and shoulder blade) is very stable in spite of the vast range of motion available to the joint (see figure 1).
The glenohumeral joint does not have a deep socket or ligaments that are always under tension (see figures 2 and 3). Thus the glenohumeral joint must be stabilized by mechanisms quite different than those which stabilize the hip or knee. The mechanisms of shoulder stability are different but effective: the humeral head which is slightly smaller than a billiard ball is held precisely centered on the glenoid which is about the size of a tea spoon. It is amazing that such an arrangement can allow the shoulder to throw pull up lift punch and do gymnastics without coming apart!
Using Dr. John Sidles' Orthokine Laboratory we have done a lot of research into how the normal shoulder is stabilized.
Sockets capsules ligaments
We have found that glenohumeral stability requires that there be a socket into which the ball can be pressed and that the muscles around the joint work in a balanced way to press the humeral head into the glenoid (see figures 4 and 5).
Glenohumeral stability also requires that the capsule and ligaments check the motion of the joint so that it does not rotate to positions where the forces become unbalanced. You can see more about how the ligaments help control shoulder stability in activities such as the baseball pitch. However it is interesting to note that in many important positions of the joint these ligaments do not stabilize the shoulder. The normal joint is in fact quite loose. In most positions the ball is held in the socket primarily by muscle action as described above.
Because glenohumeral stability is so important we discuss it in great detail. To keep it from being too dry we have included some very interesting videos prepared by Dr. Douglas Harryman of the UW Shoulder and Elbow Team.
List of movies
Below is a list of movies about the mechanics of shoulder stability.
Precise definitions are necessary in the discussion of glenohumeral stability.
Glenoid center line
The glenoid center line is the line perpendicular to the surface of the glenoid fossa at its midpoint (see figure 6).
Net humeral joint reaction force
The net humeral joint reaction force is the vector sum of all forces acting on the humeral head relative to the glenoid fossa. It is this force that needs to be stabilized at the glenohumeral joint. This force includes component forces applied to the humerus by muscles capsule and ligaments as well as by external factors such as gravity contact with objects and inertia (see figure 7).
Glenohumeral translation is movement of the center of the humeral head with respect to the face of the glenoid (see figure 8).
Glenohumeral translational laxity
Glenohumeral translational laxity is the translation observed on examination of the joint. A substantial amount of laxity is characteristic of normal glenohumeral joints.
Glenohumeral stability is the ability to maintain the humeral head centered in the glenoid fossa.
Glenohumeral instability is the inability to maintain the humeral head centered in the glenoid fossa.
Glenohumeral apprehension is the sense of impending instability in certain glenohumeral positions.
Traumatic glenohumeral instability
Traumatic instability is instability that arises from an injury of sufficient magnitude to tear the glenohumeral capsule ligaments or rotator cuff or to produce a fracture of the humerus or glenoid.
Atraumatic glenohumeral instability
Atraumatic instability is instability that arises without significant trauma.