Computed tomography evaluation of shoulder prosthesis retroversion

Computed tomography (CT) allows calculation of anatomic and prosthetic humeral head retroversion. The purpose of this study was to demonstrate how the retroversion angle measured by CT scan varied with changes in arm position in the CT scan reference system. A trigonometric analysis shows that the measured retroversion angle decreases when the arm is in extension and increases when it is in flexion, compared with the true retroversion angle determined perioperatively. For the same degree of flexion, or extension, the error of measurement is greater when the initial true retroversion is low. A shoulder prosthesis with 20 degrees of true head retroversion was radiologically scanned with different degrees of flexion, extension, and abduction. The results validate the theoretical analysis. Trigonometric formulas are proposed to correct the retroversion angle measured by CT scan.