Concavity Compression

Depth and stability

The stability is related to the depth of the concavity and the magnitude of the compressive force.

The anatomy of the glenohumeral joint is well adapted to facilitate stabilization through concavity compression. The rotator cuff is ideally situated to provide a compressive load throughout the range of motion of the glenohumeral joint. The concavity in the glenoid is provided by the shape of the glenoid bone by the increased thickness of the articular cartilage at the periphery of the glenoid fossa and by the glenoid labrum.

As the humeral head is translated from the center of the glenoid fossa over the glenoid lip it must displace laterally (i.e. in a direction parallel to the glenoid center line). The path of the humeral head center during this ascent from the center over the lip has a particular "gull wing" shape. The narrowness of this "gull wing" is a major contributor to the centering of the head in the glenoid: essentially no translation is possible without the head being lifted from the depths of the glenoid fossa. The effective depth of the glenoid in a specified direction of translation is the amount of displacement in the lateral direction required for the head of the humerus to translate from the center of the glenoid to the top of the lip of the glenoid.

We conducted a series of experiments to determine the degree to which concavity compression can stabilize the humeral head against translating forces parallel to the surface of the glenoid. For each shoulder we measured the effective glenoid depth in each of four directions of translation: superior inferior anterior and posterior. For all ten cadaver shoulders the average effective glenoid depths were greater superiorly (4.8 mm) and inferiorly (4.9 mm) than anteriorly (2.2 mm) and posteriorly (2.1 mm). The greater depth for translation in the superior and inferior directions is a direct consequence of the oblong shape of the fossa and its constant radius of curvature.

We measured the stability from concavity compression with compressive loads of 50 and 100 Newtons. Concavity compression proved to be an effective mechanism for stabilizing the humeral head against translating forces. For example a compressive load of 50 Newtons stabilized the humeral head against inferiorly directed translating forces averaging 32 Newtons. Doubling the compressive load to 100 Newtons increased the inferior force that could be stabilized to an average of 56 Newtons. The effectiveness of the concavity compression mechanism varied with different directions of translating force. For a given compressive load the stability was greater against superiorly and inferiorly directed forces than against forces directed anteriorly and posteriorly. Doubling the compressive load from 50 to 100 Newtons did not quite double the translating force that can be stabilized. This suggests that deformability of the lip of the glenoid fossa may provide less effective glenoid depth with greater applied loads.

To facilitate the comparison of the effectiveness of concavity compression under different conditions a "stability factor" was calculated as:

Stability Ratio = (Translation Force at Dislocation)/(Compressive Load).

The stability ratios for the different directions of translation can be shown in a glenoid map.

After characterizing the stability factors for the ten shoulders with the labrum intact the labrum was excised entirely and the tests repeated. Excision of the labrum diminished the stability factors for all directions of displacement and for both magnitudes of compressive loading. In the shoulder specimens from these older cadavers with relatively atrophic labra labral excision reduced the stability factor by an average of 20 percent. The contribution of the labrum to stability is likely to be even greater in younger shoulders.

The stability factors correlated with the effective depth of the glenoid concavity both when the labrum was present and after it was excised. A plot of the eight stability factors as a function of the respective values for the effective glenoid depth reveals a consistent relationship.

The strong relationship between depth and stability from concavity compression suggests that this stabilizing mechanism is compromised when the glenoid is developmentally small or flat or when the effective concavity of the glenoid has been lessened by injury or wear. Glenoids with flat posterior lips contribute to posterior glenohumeral subluxation and dislocation. Glenoid rim fractures involving significant loss of glenoid concavity are associated with glenohumeral instability. Avulsion of the glenoid labrum in traumatic instability lessens the effective depth of the glenoid concavity predisposing the joint to recurrent subluxation and dislocation. Anatomic reattachment of a detached labrum and glenohumeral ligament back to the glenoid rim helps restore the effective glenoid depth and stability.

Click to play Concavity compression Concavity compression

Concavity compression is the primary mechanism by which the head of the humerus is centered and stabilized in the glenoid fossa to resist the upward pull of the deltoid. By virtue of this stability the head and rotator cuff are held down away from the coracoacromial arch. Previously the rotator cuff muscles were viewed as head "depressors." However the net force vector of the supraspinatus muscle and other cuff muscles are not oriented optimally to depress the head of the humerus against the upward pull of the deltoid. Thus we suggest that the cuff muscles provide stability by functioning as "compressors" of the head into the glenoid concavity.

The coracoacromial arch provides a rigid backstop to upward displacement of the humeral head relative to the glenoid. Even when a substantial supraspinatus defect is present compression from the subscapularis and infraspinatus can hold the humeral head centered on the glenoid away from the coracoacromial arch.

More severe cases of chronic rotator cuff deficiency however may be associated with superior subluxation of the head of the humerus and wear on the superior lip of the glenoid fossa. This erosive wear flattens the superior glenoid concavity and thereby reduces the effective glenoid depth in that direction. Once the effective glenoid concavity is lost repair of the rotator cuff tendons or complex capsular reconstructions cannot completely restore the glenohumeral stability provided by compression into an intact concavity.

Concavity compression is a versatile mechanism for stabilizing the glenohumeral joint. When an effective glenoid concavity is present this mechanism can operate in any position in which a compressive force can be generated Furthermore concavity compression does not require intact capsule or glenohumeral ligaments.

Concavity compression is an important mechanism of stability in shoulder arthroplasty. In this situation the capsule and ligaments are routinely sectioned as a part of the soft tissue release. In the design of shoulder arthroplasty 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.