Reverse Total Shoulder Arthroplasty (R-TSA) Component Placement Affects Muscle Function

URL:

http://www3.aaos.org/education/anmeet/anmt2009/podium/podium.cfm?Pevent=565

Abstract:

Reducing the medialization of the glenosphere in R-TSA does not alter mechanical efficiency and may have other advantages, such as reduced notching and maintenance of infraspinatus tension. In significant rotator cuff deficiency, a reverse TSA (R-TSA) design is recommended, which converts the glenohumeral joint into a ball-and-socket articulation. Medialization of the glenosphere in R-TSA is recommended to keep the center of rotation within bone to minimize the risk of loosening. However, medialization of the glenosphere increases the potential for scapular notching and alters the external contour of the shoulder. An alternate method to medialization is bone grafting under the glenosphere. A computational model of the shoulder was used to simulate active dynamic abduction in a shoulder implanted with an R-TSA. The following conditions were simulated: R-TSA with the center of rotation unchanged, medialization of center of rotation by 10 and 16 mm, and inferior placement of R-TSA by 4 mm. Glenohumeral contact forces predicted by the model were within 5% of reported in vivo measurements. Inferior placement of the glenosphere component increased the mechanical advantage of deltoid muscle by increasing the muscle moment arm at 90° abduction. Medialization of the glenosphere had little effect on deltoid forces. Reducing the medialization (to 10 mm, by simulating the effect of a bone graft under the glenosphere) did not change the mechanical advantage relative to full medialization (16mm). The major mechanical advantage of the R-TSA is due to the inferior placement of the glenoid. These results support the use of reduced medialization and bone grafting in the presence of other advantages, such as reduced notching and maintenance of infraspinatus tension and improved shoulder contour.