**3.1 Concavity**

Stability of the glenohumeral joint in the midrange relies on dynamic centering of the humeral head into the glenoid concavity by the force couples generated by the rotator cuff. Coordination between the rotator cuff and periscapular stabilizing muscles ensures that the net reaction force at the glenohumeral joint is directed within the confines of the glenoid concavity. The depth and shape of this concavity can affect stability by altering the glenoids ability to contain the resultant force from shoulder motion. This can be measured as the balance stability angle (BSA) – the maximal angle the net humeral reaction force vector can make with the glenoid centerline before the head dislocates.42 Both the width and the depth of the glenoid factor into the intrinsic stability and these, in turn, can be affected both by pathologic changes from arthritis as well as by corrective reaming.

Reestablishing a smooth concavity with sufficient depth and surface area is a central goal of the successful Ream and Run. Because reaming affects both depth and width, depending on the degree of necessary correction to recenter the humeral head, careful attention must be paid through preoperative planning and precise surgical technique to restore a sufficient concavity without compromising other principles as will be subsequently discussed. In cases where there is severe posterior glenoid wear, as can occur from capsulorraphy arthropathy, corrective reaming to restore glenoid version may sacrifice too much surface area in order to restore a sufficient concavity thus obviating the benefit of this procedure. Prosthetic glenoid resurfacing may be necessary in such cases despite the inherent risks of eventual failure.
