"Mismatch" refers to the difference in curvature between the humeral head articular surface and the prosthetic glenoid articular surface.
In describing this difference, one must be careful to state if the difference is described in terms of the radius of curvature or the diameter of curvature.
It is reported that "The optimal amount of glenohumeral radial mismatch in anatomicshoulder arthroplasty remains undefined, but the consensus among multiple studies suggests between 4 and 8 mm. Current implant designs offer mismatch of the radii of curvature between the humeral head and the glenoid ranging from 1 to 38 mm"
However, implant companies commonly characterize their component combinations not in terms of radial mismatch, but in terms of diametral mismatch, with a range from 1-24 mm, as shown in the examples below.
This review suggests that conforming components (i.e. those with no or minimal diametral mismatch) may be more prone to loosening because the translations that occur with glenohumeral motion exert greater rocking moments when the humerus is constrained by conformity. These observations echo those made in the 1994 book, Practical Evaluation and Management of the Shoulder (available at this link). Illustrations from that book drawn by Steve Lippitt are reproduced below.
Translation can also result in rim loading of the glenoid component, leading to deformation through cold flow of the polyethylene.
Perhaps the key point here is that in total shoulder arthroplasty, glenohumeral stability should be achieved primarily on balancing the forces across the joint so that the humeral head remains centered, without depending on constraint from conforming joint surfaces.
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