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

Abstract

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.

Figure 1.

Delta CTA prosthesis build in the average anatomical model based on 200 measured scapulae, with specific interest in the shape of the glenoid cavity, the infraglenoid tubercle, and the lateral border of the scapula. The prosthesis has a flat baseplate situated as distally as possible to ensure full bony coverage of this baseplate with the inferior screw surrounded by minimum 2 mm bone, and a glenosphere 36 mm with a standard polyethylene cup. The notch angand le is the maximum adduction angle before a conflict arises between the PE cup and scapular pillar.

Figure 2.

The parameters studied: 1. change in the angle of the humeral neck shaft inclination; 2. change in the depth of the polyethylene cup; 3. lateralization of the center of rotation; 4. downward glenoid inclination; 5. increase in glenosphere radius; 6. creation of an inferior prosthetic overlap with the glenoid bone.

Figure 3.

Influence of neck-shaft inclination on the notch angle. (Simulation of maximal adduction in average scapular morphology and in worse-case scapular anatomy: no horizontal pillar. Images of average scapular morphology).

Figure 4.

Influence of reduction of cup depth on the notch angle. (Simulation of maximal adduction in average scapular morphology and in worse-case scapular anatomy: no horizontal pillar. Images of average scapular morphology).

Figure 5.

Influence of lateralization on the notch angle. 2 cases with or without inferior overhang. (Simulation of maximal adduction in average scapular morphology and in worse-case scapular anatomy: no horizontal pillar. Images of average scapular morphology).

Figure 6.

Influence of downward glenoid inclination on the notch angle (called “glenoid varus” in the spreadsheet). (Simulation of maximal adduction in average scapular morphology and in worse-case scapular anatomy: no horizontal pillar. Images of average scapular morphology).

Figure 7.

Influence of glenosphere radius on notch angle. (Simulation of maximal adduction in average scapular morphology and in worse-case scapular anatomy: no horizontal pillar. Images of average scapular morphology).

Figure 8.

influence of inferior prosthetic overhang on the notch angle. (Simulation of maximal adduction in average scapular morphology and in worse-case scapular anatomy: no horizontal pillar. Images of average scapular morphology).

Figure 9.

Exponential relationship between the inferior prosthetic overhang and the notch angle.

Figure 10.

Explanation of the dilemma of Gutiérrez: a prosthetic overhang can also be created by the prosthesis if the center of rotation is lateralized and if the glenosphere is more than half a sphere.