These authors used a 3-dimensional model to explore the effects of lateralizing the center of rotation (CoR) on the deltoid muscle moment arm and glenohumeral joint contact forces. This model was virtually implanted with 5 progressively lateralized reverse shoulder prostheses. The joint contact loads and deltoid moment arms were calculated for each lateralization over the course of 3 simulated standard humerothoracic motions.
In this model, lateralizing the CoR led to an increase in the overall joint contact forces across the glenosphere. Most of this increased loading occurred through compression, although increases in anterior/posterior and superior/inferior shear were also observed. Moment arms of the deltoid consistently decreased with lateralization. Bending moments at the implant interface increased with lateralization. Progressive lateralization resulted in improved stability ratios.
Comment: These observations are important, but they are also predictable from from a free body diagram (as suggested by the illustration by Steve Lippitt below). When the center of rotation is moved laterally (as in the diagram to the right), the deltoid force becomes increasingly effective in pressing the glenosphere and the humeral cup together, increasing the contact force and the stability by the concavity compression mechanism. The deltoid moment arm is decreased by lateralization (note the change in the distance between the dot at the CoR and the red deltoid muscle). The bending moment at the implant interface (the distance between the dot and the face of the glenoid bone) is increased by lateralization.
The additional benefit of lateralization - distancing the medial humeral component from the lateral glenoid neck - is also seen on these diagrams.
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