**3.4 Center of rotation**

Because the humeral head is nearly spherical, it has a center of rotation, which is slightly medially and posteriorly offset from the axis of the humeral shaft. In the normal shoulder this rotation center is aligned with center of the glenoid concavity. This phenomenon has been termed glenohumeral register.44 This center of rotation actually exists within a larger arc of a sphere created by the coracoacromial arch and coracoid process – a boundary which partially constrains glenohumeral motion and helps define the path of rotation.(Figure 5) The motion of the humeral articular surface on the glenoid face can actually be described as slippage of the surfaces relative to one another. The centering point on the glenoid can thus be thought of as a slippage point. In the normal shoulder, this point is slightly inferior and anterior to the midpoint of Saller's line which bisects the glenoid along its superior to inferior axis.

Fig. 4. Axillary lateral radiograph showing double concavity, posterior glenoid erosion and

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

Because the humeral head is nearly spherical, it has a center of rotation, which is slightly medially and posteriorly offset from the axis of the humeral shaft. In the normal shoulder this rotation center is aligned with center of the glenoid concavity. This phenomenon has been termed glenohumeral register.44 This center of rotation actually exists within a larger arc of a sphere created by the coracoacromial arch and coracoid process – a boundary which partially constrains glenohumeral motion and helps define the path of rotation.(Figure 5) The motion of the humeral articular surface on the glenoid face can actually be described as slippage of the surfaces relative to one another. The centering point on the glenoid can thus be thought of as a slippage point. In the normal shoulder, this point is slightly inferior and anterior to the

midpoint of Saller's line which bisects the glenoid along its superior to inferior axis.

posterior humeral subluxation. Preoperative planning can determine the degree of corrective reaming needed to restore proper glenoid orientation relative to the axis of the

lasting pain relief, stability and unrestricted function.

scapula

**3.4 Center of rotation** 

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 superior and inferior margins of the glenoid.

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 fornix humeri.

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 of pain relief and function.

Humeral Hemiarthroplasty with Spherical Glenoid

option if other criteria are met.

perioperative period

4. Social History:

a method to ensure smoking cessation.

from narcotics as the healing progresses.

other surgical alternatives should be considered.

Reaming: Theory and Technique of The Ream and Run Procedure 587

2. *General Health Status*: optimal health is critical to good results after any procedure but particularly the Ream and Run. Patients with multiple comorbidities, especially those that may impair tissue healing are likely not suitable candidates for this procedure. Poorly controlled diabetes, poor nutritional status and medical problems that require the use of immunosuppressive drugs are contraindications. If there is concern about general health and nutritional status, blood work including absolute lymphocyte count (<1500 cells/mm3), serum transferrin level (<200 mg/dL) and albumin level (<3.5 g/dL) can be used to screen patients who may benefit from further optimization or consultation with a nutritionist prior to the procedure. For patients with diabetes the glycosylated hemoglobin (HgA1C) can be used to screen for glycemic control, which correlates with risk for infection and healing potential. Ideally this value should be below 7.0 for optimal healing potential. If patients are well above this mark, surgery should be delayed until their diabetes can be better controlled throughout the

3. *Expectations*: experience has shown that recovery after this procedure generally takes longer than for a standard shoulder arthroplasty as healing of the reamed bone may progress and mature for 6 months or more and range of motion can improve for up to a year in motivated individuals. Achieving good range of motion is critical to outcomes for the Ream and Run as residual stiffness can lead to altered glenohumeral kinematics due to obligate capsular mediated translations – similar to the pathomechanics of primary shoulder osteoarthritis. Thus, patients who wish to pursue this alternative must be willing to accept a more prolonged recovery and must be sufficiently motivated to comply with the rehabilitation program including maintenance home exercises. Patients should also have exhausted all conservative measures and demonstrate sufficient pain, disability and joint degeneration to justify arthroplasty.

a. *Smoking*: the dystrophic effect of smoking on tissue healing makes this a contraindication to the Ream and Run. Patients who wish to undergo this operation must be in optimal physical and nutritional health. Serum cotinine levels can be used as

b. *History of narcotic habituation or chronic pain*: patients with a history of heavy regular use of narcotic medications are less likely to have a desirable outcome after the Ream and Run due to the potential for a more prolonged recovery process. These patients need to be appropriately counseled and advance and provisions should be made in advance of surgery to enlist a pain management specialist who can help steadily wean patients

c. *Worker's Compensation claim or litigation*: as has been documented with many other conditions and surgeries, patients involved in a compensation claim or those litigating an injury invariably have worse outcomes after a surgical procedure. To the extent that the salvage of a failed Ream and Run requires prosthetic glenoid resurfacing, the results which are itself inferior in the revision setting, primary total shoulder arthroplasty or

unless patients specifically request to forgo glenoid resurfacing after discussion of the options. For the occasional older patient in this category who remains physically very active and engaged in "at risk" activities, non-prosthetic glenoid arthroplasty may be an

Fig. 6. Anatomical prosthetic reconstruction of the proximal humerus should seeks to restore the center of rotation relative to the axis of the humeral shaft and the transition between the superior articular surface of the head with the insertion of the articular side of the rotator cuff on the greater tuberosity

In the absence of a scientific method to register the humeral reconstruction with the slippage point on the reamed glenoid, the shoulder arthroplasty surgeon must resort to preoperative planning, intraoperative observation, precise surgical technique and perhaps a bit of good fortune. While the reamed glenoid bone is not as compliant as articular cartilage, it does have the capacity to adapt and remodel according to its mechanical environment and thus, over time there may be some forgiveness to a small margin of error through a process of bedding-in wear.
