Is the humeral head shown in the image below posteriorly subluxated?
These authors used 3D computed tomography imaging to define the baseline relationship between the center of the humeral head and glenoid and scapula in sixty patients with advanced osteoarthritis and fifteen controls with no osteoarthritis.
The center of rotation of the humeral head was defined by fitting a sphere that was positioned and sized with use of the intact lateral humeral osseous landmarks. The plane of the glenoid fossa was defined by three points, one on the superior aspect of the glenoid, one on the anterior-inferior aspect, and one on the posterior-inferior aspect. The scapular plane was defined with three points: one on the inferior tip of the scapula, one on the center of the glenoid vault, and one on the intersection of the spine and the medial border of the scapula.The relationship of the plane of the scapula to the plane of the glenoid defined the glenoid version and inclination. The center line of the scapula is defined by a line through the center of the glenoid fossa and the medial extent of the scapular spine. The humeral offset in relation to the scapular center line is measured between the center of rotation of the best-fit sphere and the scapular center line. The humeral offset in relation to the glenoid face is the distance from the humeral center of rotation to a line perpendicular to the glenoid plane from the center of the glenoid. The measurements are normalized to the humeral head diameter.
Comment: As expected, the amount of glenoid retroversion correlated strongly with the amount of posterior offset of the humeral head center in relation to the scapular center line. The amount of posterior humeral offset also correlated with the Walch type of glenoid pathoanatomy: A1: -.68, A2: -3.8, B1: -5.2, B2: -11.9 mm.
However, the humeral head alignment in relation to the glenoid plane was variable and not strongly correlated with the amount of glenoid retroversion. The average offset of the humeral head in relation to the glenoid for patients with osteoarthritis was 1 mm in the posterior direction while the average offset for the normal shoulders was 0.5 mm in the anterior direction. The suggestion from these data is that glenoid retroversion is not strongly associated with posterior humeral head subluxation on the face of the glenoid. For example, if we look at figure 4 in this paper, we see an axial view of a shoulder with a retroverted glenoid, the humeral head well posterior to the plane of the scapula, but well centered in the glenoid fossa. The question then becomes is this humeral head posteriorly sublimated? Since the term subluxation refers to the relative positions of the articular surfaces, the answer would be 'no'. And, of importance to the shoulder surgeon, would this shoulder require "correction of the version" at shoulder arthroplasty? In our practice the answer is also 'no'.
Since eccentric loading is an important factor in glenoid failure, It may be that the most important measurement is the centering of the contact point between the humeral head and the glenoid. This is easily measured on the axillary view and does not require the fitting of a sphere to the humeral head or scaling of the measurements to the head size as shown here.
So if we take this axillary image
We can obtain the clinically important measurements of glenoid version and the location of glenohumeral contact easily as shown here
There is one other caveat to the measurement of posterior humeral subluxation: the amount of posterior subluxation may be strongly influenced by the position of the arm when the image is obtained. Our standardized plain axillary view is obtained with the humerus abducted and flexed - a position of function in which the humeral head may translate posteriorly. By contrast, a CT scan is obtained with the arm at the side, a position of rest in which the humeral head may translate anteriorly as shown in the figure at the top of this post - one can imagine that if this arm were brought to a position of abduction and flexion the humeral head may drop into the posterior concavity. Thus it is important to know the position of the arm and to understand the effects of arm position on the degree of subluxation.
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