This Issue's Rounds Question - " Describe Scheuermann’s Disease"
(responses to be published in next issue)

In the last issue of Body Cast, Rounds asked:
“Tell us about Concavity Compression ”

The following responses were received:

From Mark Florian:

Concavity compression is the primary mechanism by which the head of the humerus is centred and stabilized in the glenoid fossa to resist the upward pull of the deltoid – head and rotator are held down away from the coracoacromial arch.

Concavity compression is an important mechanism of stability in shoulder arthroplasty. In the design of the shoulder anthroplasty components, the depth of the prosthetic glenoid fossa is a function of the radius of curvature of the joint surface and the height and width of the glenoid component. For a given radius of curvature, higher and broader glenoid components provide more depth. Oblong components have less stability anteroposteriorly than superoinferiorly. Components that narrow at the superior aspect are less stable in the anterosuperior and posterosuperior directions.

From Mary Perkins:

This response was a bit of a challenge. The word “concavity” means a shape that curves or bends inward, or a hollow or depression, with more or less evenly curved sides. The word “compression” means a process of becoming smaller, pressed together, or applied pressure. Therefore, the term concavity compression can be used in many areas of the body. In orthopaedics, the term concavity compression usually refers to the mechanism of the glenohumeral joint stability. The glenohumeral joint is the most mobile articulation of the human body and exhibits a ball and socket motion. Concavity compression is the primary mechanism by which the head of the humerus is centred and stabilized in the glenoid fossa, to resist the upward pull of the deltoid. Compression into the glenoid labral concavity keeps the humeral head centred. If the glenoid concavity is lost, repair of the rotator cuff tendons or complex capsular reconstruction, cannot completely restore the glenohumeral stability provided by compression into an intact concavity.

From Tom Yorke:

Concave: Rounded depressed surface.
Convex: Rounded elevated surface.

Concavity compression is a stabilizing mechanism in which compression of the convex humeral head into the concave glenoid fossa stabilizes it against translating forces. The stability is related to the depth of the concavity and the magnitude of the compression force.

The concept of concavity compression refers to the stabilizing effect of the depth of the concave glenoid fossa on translation of the convex humeral head. This is augmented by:

1. the increased thickness of the glenoid articular cartilage at the periphery of the glenoid relative to is centre
2. the glenoid labrum, and
3. the compressive force of an appropriately functioning rotator cuff.

Factors that affect this component include deficiencies of glenoid concavity (congenital flatness), labral hypoplasia, attrition or tearing, and rotator cuff dysfunction.

Superior stability refers specifically to the superior-inferior component of glenoid concavity, which resists proximal migration of the humeral head within the glenoid. Coupled with the compressive function of the cuff, even with a torn supraspinatus, this component can resist the upward pull of the deltoid. Factors that affect such superior stability include a deficient superior glenoid and the biceps-labral anchorage.

Responses were also received from Livain Arseneau, Lynn Arseneau, Lhea Burk, Adrian Crossman, Richard Grenier, Suzanne Groulx, Edd Hayes, Leo Helfer, Susan Ko, Mary Ann Lash, Brian Lavallee, Cam Longphee, Lori MacDonald, Gary Marshall, Blair Matheson, Joe Maulucci, Javad Movasseli, Earl Oborowsky, ThelmaOgden, James Punwassie, Cheryl Rivers, Vital Robichaud, Bert Sheppard, Angela Wentzell, Joan Wilson, Heather Wong, Susan Woodrow and Neauville Yao.