Subluxation - have we forgotten how to define and measure it?

In their classic article, Primary glenohumeral osteoarthritis: clinical and radiographic classification. The Aequalis Group , the authors use the standard definition of "subluxation": a partial dislocation, i.e. a separation of the joint surfaces." They provide a diagram showing  non-subluxated joint with the humeral head centered on the glenoid articular surface.

 They also provide a diagram of a posteriorly subluxated joint, with the joint surface of humeral head displaced posteriorly with respect to the glenoid articular surface.

Importantly, the authors of the classic article point out that there was no statistical correlation between the amount of posterior decentering and glenoid version. They also observed that "when one corrects the posterior glenoid wear (using a glenoid component with our without a graft) the subluxation is not corrected. This therefore leaves the risk of recurrence and may be responsible for glenoid loosening due to the "rocking-horse" mechanism (J. Arthroplasty. 1988. 3, 39-46.)"''

This seems to make perfect sense.

But more recently, some authors have defined "subluxation" as posterior displacement of the humeral head relative to the plane of the scapula (i.e. the long line in the diagram below). This is confusing because in this image the humeral head is centered in the glenoid concavity, so this shoulder does not meet the conventional definition of "subluxation" because the joint surfaces are not separated.

Surgeons define "subluxation" of the hip as partial separation of the joint surfaces, not in terms of the position of the femoral head in relation to a line drawn through the pelvis.

To avoid this confusion, it may be clearer to observe that some arthritic humeral heads are centered on the glenoid fossa

while others have varying degrees of posterior decentering of the humeral head on the glenoid fossa

The meaning of the degree of posterior decentering (below left) and glenoid retroversion (below right) are clear.

These terms can be applied whether the images are standardized axillary views, as shown above, or CT scans. They avoid the current confusion over the term "subluxation".

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