The reverse shoulder arthroplasty angle may contribute to overt inferior inclination: comparison with alternative parameters.

PURPOSE: Radiographic measurements have gained wide acceptance in anticipating the risk of inappropriate glenoid component positioning in reverse shoulder arthroplasty (RSA). This study aims to investigate whether the RSA angle overcorrects the base plate inclination, tilting it inferiorly, compared to the base plate correction angle (BCA), which provides neutral inclination and lateralization.

METHODS

One hundred normal anteroposterior shoulder radiographs were evaluated to determine the average values of the RSA angle, BCA, base plate orientation angle (BOA), and the base plate shoulder angle (BSA), which represents the amount of overcorrection accomplished via the RSA angle beyond the BCA's scope. The interobserver reliability among two independent testers was assessed by the intraclass correlation coefficient. A t-test was applied to compare the mean values of the BCA and RSA angle.

RESULTS

The mean BOA and BCA values were 118° ± 5.7° and 17° ± 4.6°, respectively. The RSA angle value was 23° ± 6°. Compared to the BCA, the RSA angle overcorrected the base plate inclination by 15° ± 5° (BSA). The Student's t-test showed a statistically significant difference between the values of the BCA and RSA angle (p-value = 0.000). The interobserver reliability was excellent for all angle measurements among two independent testers.

CONCLUSION

The RSA angle and the BOA/BCA are reliable radiographic measurements to determine the inclination of the inferior glenoid segment prior to RSA. However, surgeons need to consider the inherent inferior inclination associated with positioning the base plate according to the RSA angle (15° ± 5°).