UW Shoulder and Elbow Academy: Arthritic subluxation of the shoulder

Posterior subluxation of the humeral head on the glenoid as shown by an axillary view taken with the arm in a position of functional elevation.

It is important to use correct and consistent terms in the analysis of preoperative glenohumeral pathoanatomy and of postoperative glenohumeral relationships. Arthritic shoulder subluxation is obviously of great interest in that the number of articles relating to it is increasing each year.

The authors critically reviewed the use and misuse of the term "subluxation" in the characterization of arthritic shoulders. Their bullet points are listed below:

» The term “subluxation” means partial separation of the joint surfaces. In the arthritic shoulder, “arthritic glenohumeral subluxation” refers to displacement of the humeral head on the surface of the glenoid.

» The degree of arthritic glenohumeral subluxation can be measured using radiography with standardized axillary views or computed tomography (CT).

» Shoulders with a type-B1 or B2 glenoid may show more posterior subluxation on an axillary radiograph that is made with the arm in an elevated position than on a CT scan that is made with the arm at the side.

» The degree of arthritic glenohumeral subluxation is not closely related to glenoid retroversion.

» The position of the humeral head with respect to the plane of the scapula is related to glenoid retroversion and is not a measure of glenohumeral subluxation.

» Studies measuring glenohumeral subluxation before and after arthroplasty should clarify its importance to the clinical outcomes of shoulder reconstruction.

Applying the standard definition, "subluxation" refers to the displacement of the humeral head relative to the glenoid articular surface. In the diagram below, posterior displacement of the center of a circle that is fit to the humeral head articular surface (decentering) is shown with respect to the perpendicular bisector (red line) of a line segment connecting the anterior and posterior edges of the glenoid. The amount of displacement (the length of the line segment with double arrows) can be expressed as a percentage of the diameter of the circle.

The figure below shows the posterior displacement of the center of a circle fit to the humeral head articular surface with respect to the scapular body reference (long blue line). The amount of displacement (the length of the line segment with double arrows) can be expressed as a percentage of the diameter of the circle. Displacement of the humeral head relative to the plane of the scapular body is not the same as subluxation in that it does not indicate the degree of displacement of the joint surfaces.

As shown in the diagram below, the degree of posterior subluxation ( decentering) of the humeral head realative to the glenoid (red symbols) for the different glenoid types is not related to the amount of glenoid retroversion. On the other hand the degree of posterior displacement of the humeral head relative to the scapular body (blue symbols) for the different glenoid types is essentially linearly related to the amount of glenoid retroversion.

The amount of posterior subluxation of the humeral head relative to the glenoid is affected by the position of the arm when the image is made. In the graph below, note that the amount of posterior decentering for shoulders with B1 and B2 glenoids is greater for axillary views taken with the arm in a position of functional elevation (blue symbols, see x-ray at the top of this post) in comparison to that for CT scans taken with the arm positioned at the side (red symbols). Note also that the A1, A2 and B3 glenoids remain centered in both arm positions.

Comment: The measurement of subluxation of the humeral head in relation to the glenoid is an important element in understanding and managing the arthritic shoulder. The different glenoid types show characteristic patterns of glenohumeral subluxation that are not closely related to the degree of glenoid retroversion. The relationship of the humeral head to the body of the scapula is mostly related to glenoid retroversion and is not a measure of glenohumeral subluxation.

Consistency and appropriate use of the term “subluxation” will enhance our understanding of arthritic glenohumeral pathoanatomy and its management.

Use of the same imaging technique and arm position before and after surgery enables the surgeon to evaluate the effectiveness of arthroplasty in re-centering the humeral head on the glenoid