**2. Basic science and clinical support**

580 Recent Advances in Arthroplasty

high likelihood that revision surgery will be necessary to address glenoid component failure. In many of these cases, reimplantation of a glenoid component is not possible due to structural deficiency of the remaining glenoid vault, and functional outcomes are often uncertain.19,20 Studies on structural grafting of bone deficits have shown a high rate of shortterm reconstitution but long-term subsidence indicating that durable and reliable solutions

To address the need for surgical options to treat shoulder arthritis in younger patients, biological resurfacing procedures using a number of different interposition materials have been evaluated with largely varying short and mid-term success. Autogenous fascia lata, Achilles tendon23-24 and glenohumeral capsule,25,26 allograft lateral meniscus27,28 and dermal scaffolds,29 and xenographic tissues patches30 have all been used to resurface the worn glenoid. Both arthroscopic and open techniques have been reported and techniques have included humeral chondroplasty, humeral prosthetic surface replacement versus stemmed humeral hemiarthroplasty.23 While some of these reports demonstrate initial improvement in pain and function, progressive joint space narrowing and glenoid erosion are common and consequent eventual revision to definitive arthroplasty. Gerber has nicely summarized the literature on this field, stating "Biologic resurfacing of the glenoid has hitherto failed to adequately restore the geometry and biology of the glenoid."31 It is fair to say that lesser invasive approaches such as the arthroscopic techniques can be used as an interval step to delay arthroplasty but durable long-term results from these procedures seem to be the

Hemiarthroplasty without glenoid resurfacing or reshaping is yet another alternative to total shoulder replacement which avoids the risk of glenoid failure. There has been extensive comparison between hemiarthroplasty and total shoulder arthroplasty in the literature looking at comparative outcomes. It is fairly well-established based on these studies including a meta-analysis of the existing literature, that total shoulder arthroplasty provides superior pain relief and range of motion over time.32-34 Nevertheless, some series do show comparable outcomes recognizing that some patients with progressive glenoid erosion do require conversion to total shoulder arthroplasty.35 Levine et al have shown that results of hemiarthroplasty are inferior if preoperative glenoid erosion or posterior wear exists.36 This highlights the importance of recentering the humeral head and restoring proper load-bearing mechanics at the glenohumeral joint after prosthetic reconstruction. Sperling and colleagues studied long-term results of hemiarthroplasty versus total shoulder in patients 50 years or younger.33 While glenoid wear after hemiarthroplasty was present in 72% of cases, radiolucencies around the glenoid prosthesis were present in 76% of patients. The risk of painful glenoid erosion necessitating revision glenoid replacement lead to the conclusion that patients with total shoulder replacement have superior clinical outcomes. The authors also concluded however based on survivorship of total shoulders in this cohort that "great care must be exercised, and alternative methods of treatment considered before either hemiarthroplasty

or total shoulder arthroplasty is offered to patients aged 50 years or younger."

Based on this background, a definitive treatment option for young and physically demanding patients with end-stage shoulder arthritis remains both a need and a challenge. Experience with the failure modes of both hemiarthroplasty and total shoulder arthroplasty, along with a better understanding of glenohumeral biomechanics have laid a foundation on which such a treatment must be based to provided a lasting solution that withstands the

for the failed glenoid are not yet available for this growing cohort of patients.21,22

exception rather than the rule.

According to Matsen, "glenoid components fail as a result of their inability to replicate essential properties of the normal glenoid articular surface to achieve durable fixation to the underlying bone, to withstand repeated eccentric loads and glenohumeral translation, and to resist wear and deformation."37 The Ream and Run seeks to address these deficiencies by stimulating a biological response at the glenoid surface that can adapt to the applied stress through the process of healing and remodeling. Interest in this potential came from observations on retrieval studies of mold arthroplasty of the hip. Observations on this historical technique showed that the reamed acetabular bone was often covered with a smooth fibrous tissue layer that amounted to a biological resurfacing.38 In addition, histologic studies demonstrated a relatively normal subchondral bony architecture that had remodeled according to the loads born on the surface.39, 40 Failure of this technique was often due to loosening and bone resorption on the femoral side.41 These results suggest that reamed bone has a regenerative potential to yield a durable joint surface when articulating with a convex metallic prosthesis.

Laboratory studies were then undertaken to determine if the reamed glenoid concavity was comparatively stable to either the native or a prosthetic glenoid. Weldon et al, using a cadaveric model, demonstrated that the intrinsic glenoid stability was compromised by loss of articular cartilage and that this stability could be restored to levels comparable to a prosthetic glenoid through spherical reaming.42 In other words, the surface geometry of the bone predicts its influence on glenohumeral kinematics. To further characterize the healing process that occurs at the reamed glenoid surface, Matsen and colleagues performed histologic analysis of retrieved glenoids at serial follow-up intervals in a canine model of the Ream and Run.43 At 24 weeks post-surgery specimens consistently showed growth of a thick fibrocartilaginous tissue covering and firmly attached to the glenoid surface.(Figure 3) This progressive maturation between 3 and 6 months and remained congruent with the articulating humeral hemiarthroplasty.

The implication of these findings are: 1) healing and remodeling of the reamed bone is a progressive process demonstrating continued biological activity in response to the mechanical environment; and 2) progressive maturation of the regenerative surface suggests its ability to withstand its mechanical environment.

Based on this background, the Ream and Run procedure been in clinical application now for well over a decade, and its indications and techniques have been refined with increasing experience. As the length of follow-up continues for these patients, the foundations for sustained positive outcomes and the modes of failure have become clearer. These foundations are anchored in the principles behind this technique all of which relate to replicating the anatomical relationships of the glenohumeral joint and the biomechanical properties these relationships engender.

Humeral Hemiarthroplasty with Spherical Glenoid

**3.2 Conformity** 

and load transfer.

**3.3 Centerline** 

Reaming: Theory and Technique of The Ream and Run Procedure 583

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

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

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

concentrating loads over a smaller surface area on the glenoid surface.

achieve the other principles and goals of the Ream and Run.

Fig. 3. Fibrocartilage growth between reamed boney trabeculae.
