Opinion editorial—concavity compression







ABOUT THE EXPERTS


Frederick A. Matsen III, MD, is a Professor of Orthopaedics and Sports Medicine at the University of Washington. His life in shoulder began with a fellowship in 1975 with Charles Neer at Columbia Presbyterian Hospital in New York City, and he was a founding member and past president of the American Shoulder and Elbow Surgeons. Dr. Matsen’s clinical, research, and teaching interests focus on primary and revision shoulder arthroplasty.


Steve Lippitt, MD, is a Professor of Orthopaedic Surgery at Northeast Ohio Medical School in practice at Cleveland Clinic Akron General in Akron, Ohio. His early interest in shoulder specialty began in his residency with PGY4 3-week electives with Dr. Richard Hawkins, Dr. Robert Cofield, Dr. Charles Rockwood, and Dr. Richard Caspari in 1989. He had his ultimate shoulder training with a 2-year fellowship with Dr. Rick Matsen, University of Washington, in Seattle in 1990–92. He is currently Orthopaedic Residency teaching faculty and Research Director with principal interest in shoulder arthroplasty as well as medical illustration.



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 throwing a baseball over 100 miles/h or bench-pressing over 700 lb.


  • •

    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 that had a deep socket as found in the hip ( Fig. 41.1 ), or by a joint with isometric ligaments as found in the knee ( Fig. 41.2 ). How, then, is the precise centering of the glenohumeral joint accomplished?




Fig. 41.1


While the deep socket of the hip (A) provides substantial stability, the shallow socket of the shoulder (B) provides little constraint.



Fig. 41.2


While the isometric ligaments of the knee (A) constrain its motion to flexion and extension, the ligaments of the shoulder (B) are lax and nonconstraining in most functional positions.


In the early 1990s, the mechanism for this robust centering of the large humeral ball on the small glenoid socket was given the name “concavity compression.” As the name implies, centering is achieved by a force pressing the humeral head into the glenoid concavity ( Fig. 41.3 ).




Fig. 41.3


The primary mechanism by which the humeral head is centered in the glenoid is known as concavity compression. The net force on the humeral head (i.e., the vector sum of all forces acting on it) must be contained within the glenoid concavity.


Note that the rotator cuff muscles act as humeral head “compressors” ( Fig. 41.4 ).




Fig. 41.4


The small inferiorly directed (“depressor”) component of the supraspinatus force is insufficient to resist the upward force applied by the deltoid (top) . Instead, the humeral head is stabilized by the cuff muscles compressing the humeral head into the glenoid (bottom) .


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 ( Fig. 41.5 ).


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Aug 21, 2021 | Posted by in ORTHOPEDIC | Comments Off on Opinion editorial—concavity compression

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