Didier Poncet

Prosthetic overhang is the most effective way to prevent scapular conflict in a reverse total shoulder prosthesis

Background and purpose Despite good clinical results with the reverse total shoulder arthroplasty, inferior scapular notching remains a concern. We evaluated 6 different solutions to overcome the problem of scapular notching. Methods An average and a “worst case scenario” shape in A-P view in a 2-D computer model of a scapula was created, using data from 200 “normal” scapulae, so that the position of the glenoid and humeral component could be changed as well as design features such as depth of the polyethylene insert, the size of glenosphere, the position of the center of rotation, and downward glenoid inclination. The model calculated the maximum adduction (notch angle) in the scapular plane when the cup of the humeral component was in conflict with the scapula. Results A change in humeral neck shaft inclination from 155° to 145° gave a 10° gain in notch angle. A change in cup depth from 8 mm to 5 mm gave a gain of 12°. With no inferior prosthetic overhang, a lateralization of the center of rotation from 0 mm to 5 mm gained 16°. With an inferior overhang of only 1 mm, no effect of lateralizing the center of rotation was noted. Downward glenoid inclination of 0º to 10º gained 10°. A change in glenosphere radius from 18 mm to 21 mm gained 31° due to the inferior overhang created by the increase in glenosphere. A prosthetic overhang to the bone from 0 mm to 5 mm gained 39°. Interpretation Of all 6 solutions tested, the prosthetic overhang created the biggest gain in notch angle and this should be considered when designing the reverse arthroplasty and defining optimal surgical technique.

Rocking-horse phenomenon of the glenoid component: the importance of inclination

BACKGROUND Abnormal glenoid version positioning has been recognized as a cause of glenoid component failure caused by the rocking horse phenomenon. In contrast, the importance of the glenoid inclination has not been investigated. MATERIALS AND METHODS The computed tomography scans of 152 healthy shoulders were evaluated. A virtual glenoid component was positioned in 2 different planes: the maximum circular plane (MCP) and the inferior circle plane (ICP). The MCP was defined by the best fitting circle of the most superior point of the glenoid and 2 points at the lower glenoid rim. The ICP was defined by the best fitting circle on the rim of the inferior quadrants. The inclination of both planes was measured as the intersection with the scapular plane. We defined the force vector of the rotator force couple and calculated the magnitude of the shear force vector on a virtual glenoid component in both planes during glenohumeral abduction. RESULTS The inclination of the component positioned in the MCP averaged 95° (range, 84°-108°) and for the ICP averaged 111° (range, 94°-126°). A significant reduction in shear forces was calculated for the glenoid component in the ICP vs the MCP: 98% reduction in 60° of abduction to 49% reduction in 90° of abduction. CONCLUSION Shear forces are significantly higher when the glenoid component is positioned in the MCP compared with the ICP, and this is more pronounced in early abduction. Positioning the glenoid component in the inferior circle might reduce the risk of a rocking horse phenomenon.

Depuy Synthes DELTA XTEND

The Delta XTEND design is an improvement of the original Delta CTA design. All the principals of Professor Grammont, which are proven to be clinical effective by many medical centers with a follow-up of more than 15 years, are included. The DELTA XTEND has been designed to improve the range of motion and to reduce the scapular notching by creating an inferior glenosphere overlap, with the center of rotation still located at the glenoid plane, thereby eliminating any rocking horse effects. Additionally, despite the fact that the metaglene is substantially smaller, the stability has been improved by adding a curved back glenoid with variable angle, locking, and compression screws. The introduction of a two-piece stem design allows for version adjustment and an offset epiphyseal component so that more anterior cortical bone can be preserved. The polyethylene cups are offered in multiple thicknesses with a standard, high mobility, or retentive option allowing the surgeon to achieve the optimal joint stability for each patient. It is also possible to mix the any size humeral cup diameter with any size glenosphere. At last but not at least the surgical technique to implant this reverse prosthesis is simple and made more easier because of the smaller (2/3 of a circle) reamers and the reduced volume of the glenosphere (less than have a sphere).