The promotional materials for one of the glenoid components on the market states "The glenoid articular surface provides a central conforming zone surrounded by a non-conforming zone. This patented variable conformity helps ensure stability throughout the range of motion." This is a study from the center where this component originated that concludes "The stress at the transition region of the hybrid design was not higher than the other 2 designs. The hybrid design has favorable characteristics based on its low stress at the periphery and greater contact area with the humeral head at the center."
It is important to consider the effects of glenoid shape on load transfer, stability, rocking horse loosening, and component wear and to recognize that among these factors there are tradeoffs. It is also important to recognize that the structure and composition of the polyethylene may affect the way these characteristics change with in vivo use, so that the relationship of 'bench top" studies to clinical performance needs to be considered.
An important fact emphasized by this article is that, irrespective of glenoid component surface shape, rim loading is associated with small contact areas and large contact stresses (think of the humeral prosthesis sitting on the edge of the plastic socket). The stress was higher for conforming designs in which even a small amount of translation results in rim loading. These stresses no doubt account for the cold flow observed in retrieved glenoids. Rim loading is also a factor in what we have described as rocking horse loosening. The hybrid design had larger contact area when the humeral head was centered than the non-conforming design. However, it is important to note that the authors chose an extreme degree of non-conformity in the 'non-conforming design' : a 46 mm diameter of curvature for the humerus and a 72 mm diameter of curvature for the glenoid component. We know of no evidence that clinically available 'non-conforming designs' fail because of excessive stress on the center of the component.
It remains to be seen if the variable conforming of the design promoted in this study remains after in vivo use or if the edge of the conforming zone undergoes cold flow with repeated translation of the humeral head across the transition zone.
In the Introduction, the authors state "loosening and wear of all-polyethylene glenoid components remain a significant concern that often jeopardizes the longevity of total shoulder arthroplasties". We look forward to long term clinical studies to see if this design is associated with improvements in clinical outcomes.
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