The glenohumeral joint.
>The humeral head is precisely centered on the glenoid as the hand carries out precision activities, such as painting or surgery.
>The humeral head is precisely centered in the glenoid as the arm carries out forceful activities, such as the throwing a baseball over 100 miles per hour or bench-pressing over 700 pounds.
>The glenohumeral joint allows a greater range of motion than any other articulation in the body while enabling these activities.
These remarkable features could not be achieved by a joint constrained by a deep socket as is found in the hip
or by a joint constrained by isometric ligaments as are found in the knee
How, then, is the precise centering of the glenohumeral joint accomplished?
In the early 1990’s, the mechanism for this robust centering of the large humeral ball on the small glenoid socket was given the name “concavity compression” [1][2][3][4]. As the name implies, centering is achieved by a force pressing the humeral head into the glenoid concavity.
Note that the rotator cuff muscles act as humeral head “compressors” (not "humeral head depressors")
Even when a displacing force is applied, as long as the net vector of the forces acting on the humeral head is contained within the glenoid, the shoulder is stable
The maximal angle between the glenoid centerline and the net force vector that can be stabilized is referred to as the balance stability angle
If the net force vector does not lie within the balance stability angle, the shoulder is unstable
The balance stability angle is reduced by loss of the glenoid labrum and by glenoid rim fractures
When the symmetry of glenoid concavity is compromised by osteoarthritis, the forces across the joint cause the arthritic humeral head to sit in the posterior concavity
The superior articulation.
While the foregoing presents the concept of concavity compression at the glenohumeral joint, the same mechanism also provides stability at the articulation between the proximal humeral convexity and the coracoacromial arch
This articulation is particularly effective in centering the humeral head when superiorly directed loads are applied, such as when pushing up out of an armchair or when leaning on crutches. The effectiveness of the coracoacromial concavity is compromised by acromioplasty and by sectioning of the coracoacromial ligament, potentially leading to anterosuperior escape and pseudoparalysis
Concavity compression and shoulder arthroplasty
Just as in the normal shoulder, the concavity of the anatomic prosthetic glenoid stabilizes the prosthetic humeral head when a compressive load is applied. Many component designs provide a small degree of glenohumeral mismatch, sacrificing a bit of the centering effect to allow some translation of the humeral component
before rim loading and deformity of the plastic glenoid occurs
Just as in the normal shoulder, as long as the net force on the humeral head is contained within the balance stability angle, the articulation is stable. This explains how an anatomic shoulder arthroplasty can be stable, even if the glenoid component is placed in retroversion
If the glenoid is worn, the effectiveness of its concavity is compromised
In the case of the reverse total shoulder, the humeral concavity is stabilized on the glenosphere by concavity compression. As long as the force across the joint lies within the humeral cup, the joint remains stable.
If, however, the net force lies outside the humeral cup concavity, the joint dislocates
Loss of the concavity of the humeral cup can contribute to instability of the reverse total shoulder
In conclusion, the glenohumeral joint will remain centered as long as there is a concavity that captures the net force across the joint.
[1] "Mechanics of glenohumeral instability." Clin Sports Med 10(4): 783-788, 1991
[2] "Mechanisms of glenohumeral joint stability." Clin Orthop Relat Res(291): 20-28, 1993
[3] "Glenohumeral stability from concavity-compression: A quantitative analysis." J Shoulder Elbow Surg 2(1): 27-35, 1993
[4] Practical Evaluation and Management of the Shoulder. Saunders, Philadelphia. 1994. Now in the public domain. Click here for a copy.
To see a YouTube of our technique for a reverse total shoulder arthroplasty, click on this link.
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To see a YouTube video on how the ream and run is done, click on this link.
To see a YouTube of our technique for total shoulder arthroplasty, click on this link.To see a YouTube of our technique for a reverse total shoulder arthroplasty, click on this link.
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