The reverse shoulder arthroplasty angle: a new measurement of glenoid inclination for reverse shoulder arthroplasty.
BACKGROUND
Avoiding superior inclination of the glenoid component in reverse shoulder arthroplasty (RSA) is crucial. We hypothesized that superior inclination was underestimated in RSA. Our purpose was to describe and assess a new measurement of inclination for the inferior portion of the glenoid (where the baseplate rests).


METHODS
The study included 47 shoulders with rotator cuff tear arthropathy (mean age, 76 years). The reverse shoulder arthroplasty angle (RSA angle), defined as the angle between the inferior part of the glenoid fossa and the perpendicular to the floor of the supraspinatus, was compared with the global glenoid inclination (β angle or total shoulder arthroplasty [TSA] angle). Measurements were made on plain anteroposterior radiographs and reformatted 2-dimensional (2D) computed tomography (CT) scans by 3 independent observers and compared with 3-dimensional (3D) software (Glenosys) measurements.


RESULTS
The mean RSA angle was 25° ± 8° on plain radiographs, 20° ± 6° on reformatted 2D CT scans, and 21° ± 5° via 3D reconstruction software. The mean TSA angle was on average 10° ± 5° lower than the mean RSA angle (P < .001); this difference was observed regardless of the method of measurement (radiographs, 2D CT, or 3D CT) and type of glenoid erosion according to Favard. In Favard type E1 glenoids with central concentric erosion, the difference between the 2 angles was 12° ± 4° (P < .001).


CONCLUSION
The same angle cannot be used to measure glenoid inclination in anatomic and reverse prostheses. The TSA (or β) angle underestimates the superior orientation of the reverse baseplate in RSA. The RSA angle (20° ± 5°) needs to be corrected to achieve neutral inclination of the baseplate (RSA angle = 0°). Surgeons should be aware that E1 glenoids (with central erosion) are at risk for baseplate superior tilt if the RSA angle is not corrected.
['Boileau|Pascal|P|', 'Gauci|Marc-Olivier|MO|', 'Wagner|Eric R|ER|', 'Clowez|Gilles|G|', 'Chaoui|Jean|J|', 'Chelli|Mikaël|M|', 'Walch|Gilles|G|']
D000368:Aged D000369:Aged, 80 and over D000072228:Arthroplasty, Replacement, Shoulder D005260:Female D061165:Glenoid Cavity D006801:Humans D021621:Imaging, Three-Dimensional D008297:Male D008875:Middle Aged D012785:Shoulder Joint D014057:Tomography, X-Ray Computed
reverse shoulder arthroplasty Glenoid inclination RSA angle β angle superior tilt BIO-RSA augmented baseplate