Reverse total shoulder - does inferior baseplate inclination reduce the risk of dislocation?

The Relationship Between Glenoid Inclination and Instability Following PrimaryReverse Shoulder Arthroplasty

These authors point out that instability remains the most common early complication and reason for early revision following reverse shoulder arthroplasty (RSA).

They sought to determine whether the glenoid implant inclination, as measured by the Beta angle, was an independent risk factor for instability following primary RSA.

The Beta angle was measured on preoperative and postoperative true anteroposterior radiographs. It represents the supplementary angle to the angle between the floor of the supraspinatus fossa and the superior and inferior margins of the glenoid (or baseplate).

They identified 34 cases of  instability following primary RSA and matched to controls by age, sex, BMI and baseplate type (1:3 ratio of cases to controls). 

They found a wide range of postoperative (63 to 100 degrees) and pre to postoperative change (-16.5 to +30.5 degrees) in Beta angles collectively. 

There was no significant difference in the postoperative Beta angle or change in Beta angle between cases and controls, respectively. 

Regression analysis demonstrated no increased odds of instability with postoperative Beta angle. There was no significantly increased odds of instability,.

Finally, there was no difference in risk of instability in patients whose implant positioning resulted in a net superior increase in inclination.

Comment: This article suggests that achieving inferior angulation may be less important than previously thought in the prevention of dislocation.

Some authors advocate the "subchondral smile" technique in which the glenoid is reamed at a coronal inclination that would result in the appearance of a “smile” of reamed bone of at least 120°. One of the risks of this reaming to achieve inferior angulation is that this change may prevent support of the superior aspect of the baseplate by the bone of the glenoid as suggested in the case below (see thick red arrow)

The importance of contact between the superior baseplate and glenoid bone is emphasized in this article, "Factors affecting fixation of the glenoid component of a reverse total shoulder prothesis" and in this figure below.

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