Reverse shoulder arthroplasty design: inlay vs. onlay does it really make a difference?

Background The design of reverse shoulder arthroplasty (RSA) implants has evolved significantly over the past 50 years. Today there are many options available that differ in design of the glenoid and humeral components, fixation methods, sizes and modularity. With respect to the humeral component the literature has generally focused on the differences between inlay and onlay designs and the potential impact on outcomes. However, inlay and onlay design represents only one factor of many. Methods It is our hypothesis that separating onlay and inlay designs into two distinct entities is an oversimplification as there can be wide overlap of the two designs depending upon surgical technique and the implant selected. As such, the differences between inlay and onlay designs should be measured in absolute terms—meaning combined distalization and lateralization. Results By reviewing the many factors that can contribute to the glenosphere – humerus realtiuonship the role of inlay and onlay humeral designs as an important distinguishing feature is shown to be limited Preliminary studies suggest that the amount of distalization and lateralization of the construct may be the most accurate method of describing the differences in the constructs. Conclusions Inlay and onlay humeral component design represents only one factor of many that may impact outcomes. A more accurate method of defining specific design and technique factors in RSA is the degree of lateralization and distalization.