Dislocation in the reverse total shoulder - the effect of baseplate inclination

The relationship between glenoid inclination and instability following primary reverse shoulder arthroplasty

These authors point out that despite advances in implant design and surgical technique, instability remains the most common early complication and reason for early revision after reverse shoulder arthroplasty (RSA) (see this link).

Their goal was to evaluate the glenoid implant inclination, as measured by the Beta-angle, as an independent risk factor for instability after primary RSA. According to the authors, "the Beta-angle represents the supplementary angle to the angle between the floor of the supraspinatus fossa and superior and inferior margins of the glenoid (or baseplate)." Increasing superior inclination is reflected by a smaller Beta angle.

Reverse shoulders with instability (34) were matched 1:3 with cases without instability (102) using age, sex, body mass index, and baseplate type.

The authors found no statistically significant differences betweenb the study group and controls with  regard to the proportion of shoulders with rotator cuff tear arthropathy, use of retentive liner, and rate of subscapularis repair.

In this series, despite attempting to impart an inferior tilt to the baseplate, 41% and 35% of the unstable and stable shoulders, respectively, had a slight net superior tilt compared with preoperative

radiographs. There was 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 (mean, 80.8 vs. 82.7 degrees or the change in b-angle (mean 1.7 vs. 3.4 degrees) between cases and controls, respectively. 

The authors concluded that neither the final prosthetic glenoid inclination or the change in glenoid inclination, as measured by the Beta-angle, significantly influenced the risk of prosthetic instability after primary RSA.

Comment: From this study one can see that a very wide range of Beta-angles (63-100 degrees) are found in stable shoulders. This may be because superior or inferior inclination of the baseplate does not change the fact that the humerus is articulating in the same way with the sphere known as the glenosphere (see below).

Another pertinent observation is that 4 of the 38 dislocations (>10%) in the standard Zimmer reverse total shoulders (excluding 2 RSAs from another arthroplasty system and one in a massive composite allograft arthroplasty) were found to be associated with polyethylene liner dissociation at the time of revision (see this link). Surgeons should be aware of this possibility when attempting close management of a dislocation. 

Achieving stability in a reverse total shoulder requires careful attention to surgical detail: assuring appropriate soft tissue tension and avoiding unwanted contact between the humeral component and soft tissue or bone in front of, below, and behind the glenosphere. 

How you can support research in shoulder surgery Click on this link.

Here are some videos that are of shoulder interest

Shoulder arthritis - what you need to know (see this link).

How to x-ray the shoulder (see this link).

The total shoulder arthroplasty (see this link).

The ream and run technique is shown in this link.

The cuff tear arthropathy arthroplasty (see this link).

The reverse total shoulder arthroplasty (see this link).

The smooth and move procedure for irreparable rotator cuff tears (see this link).

Shoulder rehabilitation exercises (see this link).

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.

To see our technique for reverse total shoulder, click on this link.

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How you can support research in shoulder surgery Click on this link.

To see our new series of youtube videos on important shoulder surgeries and how they are done, click here.

Be sure to visit "Ream and Run - the state of the art"  regarding this radically conservative approach to shoulder arthritis at this link and this link. Also see the essentials of the ream and run.

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You may be interested in some of our most visited web pages   arthritis, total shoulder, ream and run, reverse total shoulder, CTA arthroplasty, and rotator cuff surgery