UW Shoulder and Elbow Academy: Shoulder arthritis - the challenges of glenoid typing

Three-dimensional characterization of the anteverted glenoid (type D) in primary glenohumeral osteoarthritis

These authors analyzed the anatomic characteristics of 18 patients with type D glenoid, defined as an anteverted (≥5°) glenoid with or without anterior subluxation. These patients represented 3% of their osteoarthritis cohort).

The mean glenoid anteversion was 12° (range, 5°-24°), the mean inclination was 0°, and the mean anterior subluxation was 38% (range, 6%-56%). Eight patients (44%) had a biconcave glenoid with a posterosuperiorly positioned paleoglenoid and an anteroinferiorly positioned neoglenoid, and 10 patients had a monoconcave glenoid.

Comment: This study points out the continuous variability in the pathoanatomy of arthritic glenoids and the challenge of creating "dividing lines" between different glenoid types:

A1 is differentiated from A2 by the degree of glenoid erosion

B1 is differentiated from B2 by the degree of glenoid erosion

A2 is differentiated from B3 by the degree of version

A1 and A2 are differentiated from D by the degree of version

These author discuss the challenge of classification of glenoid types with a continuum of variability in version and subluxation, "The type D glenoid has been described as a glenoid with anteversion with or without anterior humeral head subluxation. Analyzing a population of 611 primary glenohumeral osteoarthritis patients in a database of CT scans performed before shoulder arthroplasty, we found 19 with an anteversion between + 1° and + 4°, which were initially classified as type A1...To avoid an overlap between types D and A1 we decided to modify the threshold and arbitrarily set the limit at + 5° anteversion to define the type D glenoid. In addition to version, humeral head subluxation of less than 40% was another criterion used to define the new type D.
The subluxation threshold has been modified to 45% instead of 40%..When assessing anterior humeral head subluxation on the CT based imaging software, interestingly, we found only 61% of our cases fell within the strict definition of anterior subluxation. The other cases in the type D cohort had subluxations between 45% and 56%. A careful analysis of each case demonstrated that how the patient was positioned for the CT scan might have influenced the subluxation percentage. In some cases, patients positioned supine autocorrected their anterior subluxation to neutral, because gravity lying supine brought the humeral head posterior. This indicates that in some cases, the anterior subluxation is dynamic rather than static, or at least not a fixed subluxation. ..Therefore, anterior subluxation of < 45% measured on a supine CT scan should not be considered as an absolute parameter for the definition of a type D. "

Even within the type D group in this series, there is variability that may suggest that there are subtypes of "D" (compare those in the upper left quadrant of the scatter plot of the authors' data with those in the lower right quadrant). It is apparent that the averages of these data (12 degrees of ante version and 38% subluxation) do not reflect the complexity.

It would be of great interest to see a plot like the one above of ALL glenoids in the authors' experience (without attempting to assign glenoid types). It seems likely that such a plot would show a continuous variation in version and subluxation. Perhaps it would be more straightforward to characterize each glenoid in terms of its version, subluxation, and degree of biconcavity.

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