Sunday, April 23, 2017

Glenohumeral pathoanatomy - are we making its evaluation too complicated?

Interest in the glenoid hull method for analyzing humeral subluxation in primary glenohumeral osteoarthritis

These authors opine that "humeral subluxation is the main cause of failure of total shoulder arthroplasty". They set out to compare humeral head subluxation in various reference planes and to search for a correlation with retroversion, inclination, and glenoid wear using reconstructions of 109 computed tomography scans of primary glenohumeral osteoarthritis and 97 of shoulder problems unrelated to shoulder osteoarthritis (controls) in (a) the scapular plane and (b) the glenoid hull plane
(a line perpendicular to the line joining the center of the glenoid and the tip of the hull (where the anterior and posterior cortical regions meet)).

They concluded that posterior subluxation of the humeral head may be defined in terms of the  glenohumeral offset or the scapulohumeral offset. They prefer measuring subluxation in the glenoid hull plane because it is less affected by the shape of the scapular body.

Comment: It is of interest that many new and increasingly complex methods for assessing glenohumeral pathoanatomy continue to surface monthly. To reduce the resulting confusion, we need to consider several points:

(1) As stated in this link, we need to ask "what information is needed" before a shoulder arthroplasty is performed. Do we need CT scans at all (see this link and this link) in most cases?

(3) While the term "static posterior subluxation of the humeral head" is in common use, the anteroposterior position of the humeral head on the glenoid is not 'static', but rather changes from when the arm is at the side to when the arm is flexed - a phenomenon known as functional decentering. Of course what is most important is not the preoperative posterior subluxation, but rather that noted at surgery as reported recently (see this link).

(3) We need to be clear on the definition of 'subluxation' (see this link). Some authors use the term subluxation to refer to the position of the humeral head in relation to all or part of the the body of the scapula. However, "subluxation" actually means separation of the joint surfaces as in incomplete or partial dislocation of the joint.

Basically, there are three important pieces of information we can obtain from preoperative imaging, each of which can be assessed by an axillary view taken with the arm in the functional position of elevation in the plane of the scapula (rather than with the arm adducted as is the case for CT scans).

First is the shape of the glenoid face. This film shows that the posterior 25% of the glenoid has a pathological concavity.

Second is the retroversion of the glenoid face as shown by the angle between G and S (S is a line drawn along the 'hull' as the authors of this article describe.

Third is the amount of posterior decentering of the head on the face of the glenoid when the arm is elevated in the plane of the scapua, which can be characterized as the ratio of C (the distance from the anterior lip of the glenoid) to G (the distance from the anterior to the posterior lip of the glenoid). 
In over 98% of our cases, these key elements of glenohumeral pathoanatomy can be gained from a single axillary view, avoiding the expense and radiation of a CT scan.