**3.2 Conformity**

582 Recent Advances in Arthroplasty

The key principles behind the Ream and Run which the surgeon must consider in reestablishing a lasting articulation that can withstand wear can be remembered as the 4 Cs: concavity, conformity, centerline and center of rotation. These are further discussed as follows.

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

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

Fig. 3. Fibrocartilage growth between reamed boney trabeculae.

pathologic changes from arthritis as well as by corrective reaming.

**3. Principles** 

**3.1 Concavity** 

eventual failure.

Although the boney anatomy is not conforming between the humeral head and glenoid concavity, the compliance afforded by the articular cartilage and glenoid labrum provide for conformity and congruency as the humeral head is centered and compressed. It is this conformity and compliance that permits load distribution over the glenoid face. Because bone and polyethylene are not as compliant as cartilage and labrum, some degree of mismatch between the diameter of curvature of the humeral head and glenoid has become a convention in total shoulder arthroplasty to avoid excessive constraint and allow physiologic translations. While historically mismatch seems to improve the longevity of prosthetic glenoid fixation, translations also allow eccentric glenoid loading, which contributes to eventual fixation failure. Mismatch also affects load distribution by concentrating loads over a smaller surface area on the glenoid surface.

In principle, the Ream and Run procedure must respect the biomechanical principles on which glenohumeral stability and load transfer are based while simultaneously reconciling the kinematic conflict that occurs between conformity and constraint as is seen with prosthetic glenoids. Stability is afforded by creation of a concavity into which the head can be centered after appropriate releases have been performed. Preservation of the labrum further deepens the socket and improves congruency between the ball and socket. Load distribution is optimized by choosing a mismatch of 2mm, which reduces point contact but allows some forgiveness in terms of constraint. It also provides some forgiveness in allowing the prosthetic humeral head to chose a centering point about which adaptive remodeling of the glenoid surface can define the final shape that optimizes joint kinematics and load transfer.
