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.

Purpose: Despite good clinical results of the reverse total shoulder arthroplasty inferior scapular notching remains a concern. The aim of this study was to evaluate the effect of 6 different parameters on notching.

Materials and Methods: An average shape A-P view 2-D computer model of scapula was created, using data from 200 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, size of glenosphere and centre of rotation. The model calculates the maximum adduction (notch angle).

Results: A change in humeral neck shaft inclination from 155° to 145° resulted in a gain of 10° in notch angle. A change in cup depth from 8mm to 5mm resulted in a maximum gain of 12°. With no inferior prosthetic overhang a lateralisation of the centre of rotation from 0 to 5mm resulted in a maximum gain of 15° on notch angle. More lateralization resulted in increased gain in notch angle. With an inferior overhang of only 1 mm no effect of lateralizing the centre of rotation was calculated. Glenoid varus of 0 to 10°, without inferior overhang, results in a gain of 10° on notch angle. A change in glenosphere radius from 18 to 21mm resulted in no gain of notch angle without prosthetic overhang. A prosthetic overhang to the bone from 0 to 5mm results in a maximum gain on notch angle of 39°.

Conclusion: To prevent an inferior scapular conflict in reverse total shoulder arthroplasty the change in neck-shaft angle or depth of the polyethylene insert had a modest gain in notch angle. The effect of lateralization of the centre of rotation and putting the glenosphere in more varus was completely eliminated by adding a small inferior overhang. The main effect of increasing the size of the glenosphere was if it created a prosthetic overhang. Of all 6 tested parameters the prosthetic overhang resulted in the biggest gain in notch angle and this should be considered when designing the reverse arthroplasty and defining optimal surgical technique.