Most effective parameter to prevent the scapular notching with a reverse shoulder

Didier Poncet (DePuy Synthes)

Introduction

Despite good clinical results with the reverse total shoulder arthroplasty, scapular notching remains a concern. Five implant design parameters have been analyzed to understand their impact on the scapular notching and to provide some guidelines.

Methods

A 2-D computer model of a scapula was created allowing to modify the dimensions and angulation of the scapular pillar as well as design parameters such as humeral neck shaft angle, lateralization of the center of rotation, the tilt of the glenoid implant, depth of the polyethylene insert, inferior overlap of the glenoid component. The model calculates for any kind of scapula dimension the maximum adduction (notch angle) until having a conflict between the humeral cup and the scapula. It provides the minimum and maximum adduction improvement for each design parameter.

Results

10° of neck shaft angle reduction improves the adduction by 10°. The cup depth reduction also improves the adduction, 2mm of depth reduction improves the adduction by 8.06°. Depending on the shape of the scapula, a 10° inferior tilt result in an adduction improvement of 2.36° to an adduction reduction of 8.26°. A 5mm glenoid lateralization can improve the adduction between 0° to 12.9°, but more interesting, there is a “plateau” effect, after some millimeters of glenoid lateralization, it doesn’t improve the adduction on some scapula shapes. 5mm of glenosphere inferior overlap improves the adduction by 18.25° to 39.16° depending on the scapula shape.

Conclusion

On the 5 parameters analyzed, the glenosphere inferior overlap is the most effective parameter to decrease the scapular notching. This inferior overlap is more efficient than the glenoid lateralization. The inferior tilt of the glenoid component has mainly a negative impact on the adduction and is not recommended.

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