**3.3 Centerline**

In the normal shoulder, the glenoid is retroverted on average 10 degrees. The glenoid centerline (the line perpendicular to the glenoid face) thus normally points 10 degrees posterior to the axis of the scapular body. This line exits the scapula anteriorly at the base of the glenoid vault between the superior and inferior subscapularis crurae. Recentering the humeral head within the glenoid concavity is essential for shoulder mobility, stability and load transfer. The pathomechanics of primary and secondary arthritis often lead to posterior subluxation of the humerus and consequent posterior glenoid wear and erosion. These pathologic changes must be corrected to optimize load-bearing mechanics at the joint after prosthetic reconstruction. As will be discussed later, careful planning is necessary to determine the amount of correction that will restore the orientation of the glenoid.(Figure 4) Reorientation of the glenoid concavity through corrective reaming can diminish the surface area of the articulation because the glenoid vault narrows as one moves medially. In addition, corrective reaming to "bring down the high side" may result in significant penetration of the subchondral bone, which is softer and less tolerant of bearing significant loads when articulating with a convex metal prosthesis. Medialization of the glenoid may also increase the reaction force at the glenohumeral joint by reducing the lever arm of the rotator cuff muscles and may result in secondary impingement by bringing the tuberosity underneath the lateral acromion. Thus, preoperative planning must assess the degree of necessary correction and whether this will exceed the anatomical parameters necessary to achieve the other principles and goals of the Ream and Run.

Humeral Hemiarthroplasty with Spherical Glenoid

superior and inferior margins of the glenoid.

fornix humeri.

of pain relief and function.

Reaming: Theory and Technique of The Ream and Run Procedure 585

Fig. 5. The centering point on the glenoid is defined by the larger sphere created by the coracoacromial arch. The point is slightly inferior to the center of a line connecting the

Anatomical reconstruction of the proximal humerus seeks to reestablish the head to tuberosity and head to shaft relationships and to replace a head of similar diameter and thickness. In a perfect world, this should restore the proper center of rotation though prior studies have shown that there can be significant displacement of the rotation center depending on how the chosen system fits relative to a given patient's anatomy. 45, 46 (Figure 6) This displacement has the potential to affect the location of the slippage point on the glenoid face and how the motion of the glenohumeral joint is defined by its location with

When the glenoid is spherically reamed, the surgeon is effectively choosing a centering point for slippage of the humeral prosthesis. Much of the time in conventional shoulder arthroplasty, this point is chosen by looking at what appears to be the deepest point of the existing concavity. Current techniques and technology do not allow the reconstructed center of rotation of the humerus to chose it's proper slippage point that the surgeon can then use as the centering point for the reaming. The finding on failed glenoid retrievals that wear to conformity occurs at a point other than the geometric center of the prosthetic glenoid concavity, suggests that the humeral head is seeking ball and socket kinematics as defined by placement of the reconstructed humeral rotation center within the fornix humeri. Thus, one can presume that optimal registration between the rotation center of the humeral arthroplasty and the center of the reamed glenoid concavity would result in a blend of kinematics, stability and load distribution that would lend itself to long-term maintenance

Fig. 4. Axillary lateral radiograph showing double concavity, posterior glenoid erosion and posterior humeral subluxation. Preoperative planning can determine the degree of corrective reaming needed to restore proper glenoid orientation relative to the axis of the scapula

In cases where significant correction is required, precontouring the glenoid with a burr and then using a relatively flatter conventional reamer to start may allow reorientation without as much medialization and subchondral penetration. Some surgeons have advocated under correction of the posterior erosion to maximize the articular surface area of the reamed glenoid.44 While undercorrection risks recurrent posterior instability and consequent wear, excessive reaming risks reduced surface area for load distribution. Ultimately, the surgeon needs to make a judgment call intraoperatively after corrective reaming as to whether the concavity can sufficiently replicate the mechanical properties of a normal glenoid to provide lasting pain relief, stability and unrestricted function.
