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.
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