In 1974, Neer 1 introduced the first

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1 Review Article The Glenoid Component in Anatomic Shoulder Arthroplasty Daphne Pinkas, MD Brett Wiater, MD J. Michael Wiater, MD From the Kayal Orthopaedic Center, Franklin Lakes, NJ (Dr. Pinkas) and the Department of Orthopaedic Surgery, William Beaumont Hospital, Royal Oak, MI (Dr. Brett Wiater and Dr. J. Michael Wiater). Dr. Pinkas or an immediate family member has stock or stock options held in Merck. Dr. J. Michael Wiater or an immediate family member is a member of a speakers bureau or has made paid presentations on behalf of DePuy and Zimmer; serves as a paid consultant to Biomet and Zimmer; has stock or stock options held in ArthroCare; has received research or institutional support from Synthes, Tornier, and Zimmer; and serves as a board member, owner, officer, or committee member of the American Academy of Orthopaedic Surgeons and the American Shoulder and Elbow Surgeons. Neither Dr. Brett Wiater nor any immediate family member has received anything of value from or has stock or stock options held in a commercial company or institution related directly or indirectly to the subject of this article. J Am Acad Orthop Surg 2015;23: JAAOS-D Copyright 2015 by the American Academy of Orthopaedic Surgeons. Abstract Ideal management of the glenoid in anatomic shoulder arthroplasty remains controversial. Glenoid component loosening remains a common source of clinical concern and, in young, active patients, implantation of a glenoid prosthesis is often avoided. Efforts to decrease glenoid loosening have resulted in changes to prosthetic design and implantation techniques. Currently, a wide variety of glenoid component options are available, including metal-backed or all-polyethylene, bone ingrowth or ongrowth, inset, and augmented designs. Additionally, several alternatives are available for the young, active patient, including hemiarthroplasty, nonprosthetic resurfacing, and tissue interposition. Many recent clinical and biomechanical studies have examined these implant options. A thorough knowledge of glenoid anatomy, pathology, implant options, indications, and principles of implantation is necessary to optimize the outcome following anatomic shoulder arthroplasty. In 1974, Neer 1 introduced the first modern total shoulder arthroplasty (TSA) that included a glenoid prosthesis, which consisted of a monoblock humeral stem and a cemented, all-polyethlene keeled component. In 1982, Neer et al 2 published a case series documenting the significant improvements in shoulder range of motion and function that were obtained with TSA. Since that time, the use of TSA has increased significantly in the United States. Day et al 3 reported a 319% increase in TSA procedures between 1993 and 2007, with an estimated 10.6% increase annually. TSA has proven to be a successful long-term solution for glenohumeral arthritis. One study reported that implant survival without revision was 93% at 10 years and 87% at 15 years, with only a mild decline in long-term patient satisfaction from 92% to 83%. 4 The most common long-term complication of TSA is glenoid loosening, which accounts for approximately 24% of all TSA complications. 5 A recent systematic review of the literature reported that asymptomatic radiolucent lines occurred at a rate of 7.3% per year after primary TSA, with symptomatic glenoid loosening and surgical revision occurring at rates of 1.2% and 0.8% annually, repectively. 6 Radiolucent lines around glenoid components are common; however, they do not necessarily precede symptomatic loosening or even a need for revision. Despite clinical concern, a metaanalysis showed that only approximately 28.5% of glenoids suspected of loosening required revision. 5 The etiology of glenoid loosening is likely multifactorial and may be related to implant design, surgical technique, patient characteristics, the integrity of the rotator cuff, or the presence of May 2015, Vol 23, No 5 317

2 The Glenoid Component in Anatomic Shoulder Arthroplasty Figure 1 Figure 2 A and B, Photographs of a cadaveric scapula demonstrating the acromion (A), scapular body (B), coracoid (C), glenoid height (D) and width (E), scapular spine (F), and the glenoid inclination angle (G). indolent infection. Knowledge of the native glenoid anatomy and pathology, indications and techniques for implantation, mechanisms of failure, and the rationale behind various implant designs allows the surgeon to minimize risks and maximize outcomes following TSA. The Native Glenoid The normal anatomy of the bony glenoid is highly variable (Figure 1). The glenoid articular surface is often described as pear-shaped; however, 29% are ovoid in shape. 7 Mean glenoid width is approximately 26.8 mm, with normal values ranging from 20 to 35 mm and an average difference of 4.2 mm between men and women. 7-9 Average glenoid height is 38 mm, with normal values ranging from 29.4 to 50.1 mm and an average difference of 4.7 mm between men and women. Inclination (or tilt) is the slope of the glenoid face in the superior-inferior direction relative to a line drawn perpendicular to the tangent of the medial scapular border. Average inclination ranges from 22.2 (inferior tilt) to 14.2 (superior tilt), with normal inclination ranging from 212 to ,10 Glenoid version is the anglebetweentheglenoidsurfaceand a line drawn perpendicular to the axis of the scapular spine. Average glenoid version is 2 of retroversion, but variability exists, with most shoulders ranging from 12 of anteversion to 14 of retroversion. 8,9 The glenoid center line is an important parameter in shoulder arthroplasty. It is defined asthelineperpendiculartothepresumed articular surface in the absence of deformity and is based on the mild average retroversion of the glenoid in the general population. This line is an important landmark for deformity correction and should exit the anterior scapular neck medially. Another important anatomic dimension is the glenoid vault. This structure lies between the articular surface of the glenoid and the body of the scapula and is made up primarily of cancellous bone outlined by a thin rim of cortical bone. When evaluated in both axial and coronal cross-sections, the glenoid vault is triangular in shape, becoming narrower from lateral to medial (Figure 2). This structure serves as the bony support for a glenoid component in TSA. Illustration of the glenoid vault crosssection. Note that the vault becomes narrower from lateral to medial. If medialization occurs from bony erosion or excessive reaming, the supporting bone for a glenoid implant is diminished. Glenoid Pathology Shoulder pathology can have effects on all structural dimensions of the glenoid. Glenoid bone loss is of particular importance when planning for TSA because inadequate bone support for the component can increase the risk of loosening. Glenohumeral instability can cause anterior bone loss and an inverted pear shape, with diminished glenoid width inferiorly. Glenohumeral arthritis may result in osteophyte formation and a relatively increased glenoid width. 11 Osteoarthritis has been shown to lead to significant posterior glenoid wear and increased retroversion. 9,11,12 Walch et al 12 reported an average retroversion of 17.3 (range, 0 to 26 ) in osteoarthritic shoulders and developed a classification system that is widely used to describe common wear patterns. Habermeyer et al 10 also reported that osteoarthritic shoulders have inferior tilt of the glenoid surface that increases with the advanced stages of disease. 318 Journal of the American Academy of Orthopaedic Surgeons

3 Daphne Pinkas, MD, et al Glenoid dysplasia is associated with posteroinferior hypoplasia, causing pathologically increased retroversion and inferior tilt. Increased superior inclination is associated with full-thickness rotator cuff tears. It is theorized to be a predisposing factor for tears and a result of superior wear after tears have occurred. 13,14 Central glenoid erosion and medialization can be seen in patients with osteoarthritis, rotator cuff tears, rheumatoid arthritis, and other forms of inflammatory arthritis. This particular wear pattern can significantly decrease the size of the glenoid vault. An in-depth understanding of glenoid anatomy and variability is important to accurately restore shoulder mechanics and function after arthroplasty and to recognize factors that may increase the risk of glenoid component failure. Glenoid Assessment A careful history and physical examination are important to determine factors that may influence arthroplasty outcomes. Assessment should include an account of the patient s shoulder pain or disability, activity level, functional requirements, and specific goals of treatment. Because glenoid component loosening remains an issue over time, patients may be at increased risk if they are younger than 60 years or have higher functional demands of the shoulder after arthroplasty. Any history of previous injuries or surgeries may have serious implications on shoulder anatomy, and other comorbidities that may affect the shoulder, such as osteoporosis, type of arthritis, dysplasia, ligamentous laxity, or instability, should be noted. When planning for TSA, the surgeon should be aware of pathology that may affect the bony support and alignment of the glenoid component. The physical examination should evaluate active and passive range of motion, sources of pain, and any potential neurologic deficits. Thorough evaluation for rotator cuff pathology with or without MRI is vital because this pathology has been shown to cause accelerated glenoid loosening and arthroplasty failure. 15 Basic radiographic evaluation should include three standard radiographic views of the shoulder: a true AP view (also referred to as a Grashey view), an axillary view, and a scapular outlet view. Advanced glenohumeral arthritis will be visible in one or more of these views. Erosive medialization of the glenohumeral joint, which can raise concerns about the remaining glenoid vault, can be assessed by referencing the position of the articulation relative to the coracoid or lateral acromion edge. Proximal humeral migration may also be noted on radiography as an indication of rotator cuff dysfunction. 14 Although glenoid inclination can be evaluated on the true AP view, version is evaluated on the axillary view of the shoulder. Glenoid wear, dysplasia, and humeral head subluxation can also be assessed on the axillary view. Although glenoid pathology may be evident on standard shoulder radiographs, the degree of retroversion and the severity of bone erosion may not be well delineated. Advanced imaging is often necessary. CT offers detailed evaluation of glenoid version, wear patterns, humeral head subluxation, and other degenerative pathology such as osteophytes or subchondral cysts. CT can also be used to assess for fatty degeneration and poor muscle quality of the rotator cuff. 16 Three-dimensional CT reconstruction may allow better visualization of glenoid deformities than traditional two-dimensional scans and can be useful in preoperative planning for component implantation. MRI also can be used to evaluate glenoid version and bone loss and is more sensitive than CT for evaluation of soft-tissue abnormalities. Indications and Procedures for Glenoid Implantation The decision to implant a glenoid component in TSA is influenced by confirmed glenoid involvement in degenerative disease as well as patient risk factors for implant loosening. Glenoid resurfacing is indicated in the setting of painful glenoid degeneration when adequate bone stock is present for implantation and the rotator cuff is intact. 17 Contraindications to arthroplasty include active shoulder infection, neuroarthropathy, and paralysis of the musculature about the shoulder. Age,50 years, high functional demand, significant bone loss, and rotator cuff dysfunction are relative contraindications to glenoid component implantation; patients with these contraindications should be evaluated on an individual basis. Reluctance to implant a glenoid component for fear of loosening, failure, and the need for revision has led many surgeons to seek alternative treatments for glenohumeral arthritis in young patients. Reverse total shoulder arthroplasty (RTSA) is an alternative option for elderly patients (age.75 years) with poor glenoid bone stock and has been shown to have some success in this population. 18 In patients without clear glenoid involvement, many surgeons prefer to perform some form of humeral head replacement; revision surgery may be required at a later date. Glenoid wear and arthrosis are expected outcomes following hemiarthroplasty performed in young, active patients and can lead to progressive pain and unsatisfactory results. 19,20 In a series of 62 patients aged #50 years who underwent hemiarthroplasty for osteoarthritis, May 2015, Vol 23, No 5 319

4 The Glenoid Component in Anatomic Shoulder Arthroplasty Figure 3 Illustrations demonstrating the rocking horse phenomenon. Edge loading occurs as the humeral head translates from side to side, resulting in compression at the ipsilateral side and distraction at the contralateral side of the glenoid implant. The arrows represent the direction of force and counterforce associated with edge loading of the glenoid. The amount of micromotion is exaggerated for illustration. Sperling et al 19 found that 60% were not satisfied with the surgical outcome at an average of 15 years. In a recent series of a similar group of patients aged #60 years, Dillon et al 21 reported the risk of revision after hemiarthroplasty to be greater than four times that associated with TSA. Several techniques have been used to address glenoid wear associated with hemiarthroplasty. One is to ream the glenoid without implanting a glenoid component. Not only does the reamand-run procedure have the advantage of correcting deformity while avoiding the risk of glenoid component loosening, but reaming may also lead to formation of a protective fibrocartilage layer. 22 Although this technique has been proposed as a good option for arthritic but active patients, patient comfort and function do not reach a steady state until approximately 20 months postoperatively, and the best results are seen in men older than 60 years. 23 Another option for nonprosthetic resurfacing is soft-tissue interposition arthroplasty. Tissues used for this procedure include capsular tissue, dermal scaffolds, fascia lata, Achilles tendon, and meniscal allografts; however, inconsistent results and eventual recurrence of shoulder pain have been reported with the use of all of these. 24 Osteochondral allograft may also be used for resurfacing the degenerative glenoid. Although short-term outcomes of these procedures are inferior to those of standard TSA, they can offer temporary relief for years while avoiding complications associated with implantation of a glenoid component. Painful arthritis in young patients lacks a definitive treatment. Anatomic TSA offers the best option for pain relief, but there is a significant risk of implant loosening over a long period of time. Hemiarthroplasty with or without nonprosthetic resurfacing may provide temporary improvement, but there is a high likelihood that the pain and dysfunction will return. The decision to perform any of these procedures in a young patient should be made carefully, and patients should be counseled on the possibility of future revision surgery. Revision surgery can be difficult, with a potentially increased risk of complications. Because of soft-tissue scarring and an altered surgical field, dissection and exposure can be more technically demanding and may increase the risk of neurovascular complications or infection. Glenoid Loosening Progressive radiolucent lines are often thought to be a harbinger of component loosening and are commonly seen with nearly all glenoid component designs. Although some studies have correlated this finding with shoulder pain, radiolucent lines have yet to be definitively associated with eventual clinical loosening or overall poorer outcomes. 25,26 Franklin et al 15 developed a system for classifying radiolucencies around keeled glenoid components that was later adapted by Lazarus et al 25 to classify loosening around pegged components. These classification schemes are frequently cited; however, the reliability of each has not been established. The classic explanation for the mechanism by which glenoid components loosen over time is the rocking horse phenomenon 27 (Figure 3). When the component is edge loaded, it is compressed at one side of the glenoid and distracted at the other. This micromotion or rocking 320 Journal of the American Academy of Orthopaedic Surgeons

5 Daphne Pinkas, MD, et al eventually causes a breakdown at the bone-implant interface. This theory has never been conclusively proven in clinical or laboratory studies, but it is supported by clinical knowledge of other pathologies that worsen component loosening. Edge loading can be worsened by glenohumeral instability and rotator cuff dysfunction and, in these settings, glenoid loosening may progress rapidly. 15 Similar edge-loading effects and excessive implant micromotion are thought to occur when glenoid components are implanted in retroversion or superior inclination. 28,29 Eccentric implant wear patterns and implant micromotion likely exacerbate the process of loosening through generation of polyethylene wear debris and particle-induced osteolysis. The influence of inflammation and biologic factors is likely of great importance, although it remains poorly understood. Cementation of the glenoid component exposes the bone to possible thermal necrosis, which can lead to radiolucency and possible weakening of the bony architecture. Stress-shielding and bone resorption adjacent to a rigid cement mantle or uncemented metal backing have also been implicated as a source of radiolucent lines and clinical loosening. 30 Indolent periprosthetic infection is also increasingly recognized as being associated with implant loosening in TSA. Inflammatory cytokines can be increased by the arthritic disease process initially, leading to shoulder arthritis or infection, or even increased as a response to foreignbody wear particles from metal or polyethylene. These cytokines may share a role in the process of glenoid component loosening over time. 31 When the glenoid component has failed or shoulder pain caused by component loosening is not tolerated, options for revision depend largely on the remaining glenoid bone stock and the integrity of the surrounding soft tissues. This scenario does not currently have a reliable solution. When considering revision surgery, the possibility of infection should be ruled out with evaluation of serum C-reactive protein level, erythrocyte sedimentation rate, serum white blood cell count, and possibly joint aspiration. Even in the absence of infection, the bone loss found with glenoid loosening at the time of revision can be difficult to manage. Options for revision are limited and include component reimplantation in one or two stages, glenoid removal without reimplantation, isolated glenoid bone grafting, and RTSA. In a recent study of 42 cases of glenoid loosening or failure after TSA, Bonnevialle et al 32 reported poor results after component reimplantation, with a 67% rate of recurrent loosening. RTSA has had some success for revision of shoulder arthroplasty in the setting of glenoid loosening and bone loss with or without rotator cuff failure; however, the risk of complications may be increased. 33 Glenoid Implantation Techniques Glenoid implantation principles can help to maximize the long-term success of the TSA procedure. Adequate glenoid exposure is the necessary first step to allow ease of preparation and instrumentation of the glenoid bone. This exposure is possible through a series of careful soft-tissue releases and optimal retractor placement. The next step involves recognition and correction of deformity, while avoiding excessive reaming and preserving bone stock. Preservation of the subchondral bone of the glenoid is important for rigid structural support of the implant. Reaming technique is also important; the goal is to prepare a perfectly congruent glenoid bone surface to fully support the prosthesis. Regardless of the type of implant used, glenoid preparation should avoid perforation of the glenoid vault. Proper implant sizing to minimize overhang can also aid component stability. Meticulous exposure, bone preparation, and proper implantation of the prosthesis, with cement pressurization and full seating of the component in the appropriate position, are all vital principles of glenoid implantation. Unobstructed visualization allows the surgeon to recognize glenoid deformity and ensure appropriate component position. In the setting of advanced deformity, particularly posterior wear and retroversion, assessment may be significantly compromised. In a series of 13 patients treated with TSA and evaluated with a three-dimensional surgical simulator, Iannotti et al 34 demonstrated diminished accuracy of component implantation as preoperative retroversion increased. Implant placement within 5 of the ideal placement occurred in only 7 of 13 patients. In those with.10 preoperative retroversion, 86% were implanted in.10 of final retroversion to the scapular plane. Recent efforts have been made to enhance implantation accuracy with intraoperative navigation and customized instrumentation. Computer-assisted glenoid implantation has been shown to significantly improve accuracy and precision, but is currently associated with a high rate of technical challenges and increased surgical time. 35 Hendel et al 36 showed that patient-specific instrumentation also significantly improved implant positioning; however, this method is expensive and time-consuming and requires three-dimensional CT planning software. When preparing the glenoid bone for the component, careful consideration is given to correction of deformity, and excessive medicalization May 2015, Vol 23, No 5 321

6 The Glenoid Component in Anatomic Shoulder Arthroplasty Figure 4 Photograph of the Global Steptech Anchor Peg Glenoid component (DePuy Synthes Orthopaedics), an all-polyethylene augmented component. The stepped design of the implant allows correction of retroversion caused by asymmetric posterior wear, which is seen in patients with osteoarthritis. should be avoided. Uniform reaming of the bone is important while attempting to preserve some subchondral bone support for the component. Correction of pathologic version to neutral orientation (ie, perpendicular to the glenoid center line) can be achieved by preferentially reaming the high side of the glenoid. This eccentric reaming is intended to minimize edge loading and subsequent loosening of the implant. However, the degree of deformity correction using this method may be limited by the structure of the glenoid vault. The tapered morphology of the vault causes the diameter of the bone surface to decrease as it is reamed more medially. At some point, bony support for the prosthesis becomes compromised. In advanced cases of deformity, complete correction may not be possible. In a cadaver study, Gillespie et al 37 showed that complete correction of deformity.15 results in an inability to successfully implant a prosthesis because of inadequate bone support, peg penetration, or Figure 5 Photograph of the HemiCAP glenoid replacement system (Arthrosurface), an inset glenoid component. The inlay design permits partial resurfacing of the glenoid, leaving a peripheral rim of bone that potentially improves liftoff resistance and stability. both.50% of the time. When penetration of the glenoid vault occurs, fixation is compromised, and the suprascapular nerve is at risk of penetration, compression, or thermal injury from extruded cement. Several additional surgical options exist to manage advanced deformity, including downsizing the glenoid component, slight undercorrection and implantation in retroversion, bone grafting, nonprosthetic resurfacing, augmented implants, and RTSA (Figure 4). The backing of augmented implants can be wedgeshaped or step-cut. Recent biomechanical studies have demonstrated superior fixation and liftoff resistance to eccentric loading of these implants 38 (Figure 5). Proper seating of the implant onto smoothly reamed bone is another important consideration because this has been shown to benefit stability. 25,27 Proper implantation of the prosthesis is the critical last step. Most implants require cementation. Cementing of glenoid components has evolved over time to include such techniques as burr curettage, lavage, meticulous glenoid drying, and cement pressurization. These techniques have been shown to decrease rates of radiolucent lines as well as improve implant survivorship. Modern cementing techniques have been shown to reduce the rate of early radiolucent lines and loosening with both keeled and pegged glenoid components. 39,40 A contemporary three-step cement pressurization technique provides rigid fixation of the implant, distributes stresses over a larger area, improves pullout strength, and leads to fewer radiolucent lines The desire to maximize these benefits has led to development of methods to increase the area and depth of cement penetration into the glenoid vault. However, excessive cementation may have detrimental effects. When the cement is curing, it reaches a maximum temperature well over the limit necessary for bone necrosis to occur. 42 In addition, removal of a large cement mantle during revision for loosening or infection may leave behind a large and unsupportive bony defect. Glenoid Designs A wide variety of options is currently available for the glenoid component. Many feature unique design characteristics developed with the goal of improving implant longevity. However, long-term outcome studies do not exist for most of these implants, and the surgeon is left to rely on an understanding of the implant design rationale when choosing the appropriate implant. Component Shape Modern glenoid components are available in several shapes and sizes. Some implants are pear-shaped to mimic the shape of the normal glenoid, whereas others are elliptical. An anatomic pear shape offers the potential for less implant overhang 322 Journal of the American Academy of Orthopaedic Surgeons

7 Daphne Pinkas, MD, et al Figure 6 Figure 7 Photographs of a failed metal-backed glenoid component demonstrating breakage of the tantalum keel (A) and resultant metal debris (B). Photograph of the Trabecular Metal Glenoid component (Zimmer), a reinforced tantalum-backed component, which is designed to allow bony ingrowth for improved implant stability. This component is typically implanted with peripheral cementation at the polyethylene backing. superiorly and less uncovered bone inferiorly, but this shape has not been shown to be superior to elliptical designs. This may be because the arthritic glenoid is not often pear-shaped, and properly sized elliptical implants often fit well after reaming. The back of the components may be flat or convex, and curved back designs may resist micromotion more effectively than flat designs. As the depth of the implantbone interface increases, shear stresses may be converted to compressive stresses that improve the stability of the implant. In a radiographic comparison of flat and convex components, Szabo et al 43 reported that, at 2 years, glenoid designs with a curved back had better seating and significantly better radiolucency scores than did designs with a flat back. In a follow-up study, however, the same patients showed no difference in progression of radiolucent lines at 10 years. 26 Recently, inset glenoid designs have been developed to aid implant stability (Figure 5). By leaving a peripheral rim of native bone, displacement and micromotion can be minimized by preventing edge loading. Radial Mismatch The ideal conformity between the humeral and glenoid implant is debatable. A fully conformed articulation, such as the original Neer prosthesis, may uniformly distribute stress at the implant-bone interface. However, glenohumeral translation can then occur only with articular separation and edge loading. Translation can occur more freely with less conformity between the implants, but then contact pressures are not uniform. Optimal radial mismatch in TSA remains unclear, despite multiple clinical and biomechanical studies. Glenohumeral translation and the effect of radial mismatch are also likely influenced by other confounding factors, such as implant position and rotator cuff function. Metal-backed Components Early efforts to improve the stability of glenoid components led to the development of metal-backed implants. Most of these implants were uncemented and fixed to the glenoid with screws. These designs had a high failure rate, with early loosening, screw breakage, polyethylene dissociation, shoulder pain, and the need for revision (Figure 6). Later efforts improved this design with the incorporation of a bony ingrowth material on the metal backing. Taunton et al 44 examined the results of TSA with one such implant, reporting a 31% revision rate associated with loosening and an implant survival rate of 52% at 10 years. Porous ingrowth was improved and screw fixation was eliminated from the design. The metal backing was changed to a monoblock polyethylene platform with a central bony ingrowth attachment. However, these components fractured at the keel-glenoid junction or through the bone ingrowth platform, resulting in an unacceptably high failure rate 45 (Figure 6). These implants were redesigned yet again, and the bony ingrowth platform and its connection to the polyethylene have been solidified to resist fracture (Figure 7). Modern metal-backed implants hold promise; however, because of the history of loosening and catastrophic failure of early components, judicious use and close monitoring of these implants are recommended. May 2015, Vol 23, No 5 323

8 The Glenoid Component in Anatomic Shoulder Arthroplasty Figure 8 Figure 9 Photograph of the Solar Total Shoulder component (Howmedica Osteonics), an all-polyethylene glenoid component featuring divergent pegs to enhance stability. Photographs of the Affiniti Shoulder System (Tornier; A) and the Global Advantage Anchor Peg Glenoid (DePuy; B), two glenoid implants that are currently available and use a deeply fluted central peg to allow for bone ingrowth. The additional fluting at the base of the central peg of the Affiniti system s glenoid component helps to lock the implant under the subchondral bone of the native glenoid. The peripheral pegs of both implants also have indentations to improve fixation with bone cement. Figure 10 All-Polyethylene Components In general, all-polyethylene implants have proved to be a more durable option than metal-backed implants (Figure 8). This may be related in part to the favorable mechanical properties of all-polyethylene implants that allow translation without imparting excessive stress at the implant-bone interface. Most glenoid implants in use today are composed entirely of polyethylene. Many of the advances in polyethylene processing and manufacturing associated with hip and knee arthroplasty have been applied to glenoid components. Cross-linked, ultrahigh-molecular-weight polyethylene has been shown to have favorable wear properties and a low osteolytic potential of wear particles. 46 Backside texture of all-polyethylene cemented implants has also been examined, and rough texture and A and B, Photographs of the Comprehensive Total Shoulder system with a Regenerex hybrid glenoid (Biomet), which is an all-polyethylene component with a modular central peg. This implant has a porous metal backing to allow for bony ingrowth, with a solidifying central peg. The component also uses a roughened backing and fluted peripheral pegs for cement interdigitation to increase pullout strength. threading of pegs has been shown to improve implant pullout strength. 41 Most all-polyethylene components are designed for cemented fixation around three or four parallel pegs or a central keel (Figures 8 to 10). Several comparative studies have examined pegged and keeled implants and most have shown no 324 Journal of the American Academy of Orthopaedic Surgeons

9 Daphne Pinkas, MD, et al difference in implant survival or clinical outcome despite a reportedly higher rate of radiolucent lines with keeled implants. 25,47 It has been theorized that this higher rate of radiolucency is caused by difficult implant seating associated with poor glenoid preparation and insufficient cement pressurization with early surgical techniques. In a prospective randomized trial comparing pegged and keeled components, Edwards et al 47 showed that, even with the careful application of modern cementing technique, pegged components remain radiographically superior to keeled glenoid components. In recent years, several variations to the traditional all-polyethylene pegged implants have been introduced, all with the goal of improving implant stability. Divergent pegs are used to provide additional stability against micromotion (Figure 8). All-polyethylene components that allow for bone ongrowth onto an interference-fit central peg provide the possibility of long-term biologic fixation (Figure 9). These implants have demonstrated promising clinical results particularly when radiographic density is observed between the flutes of the central peg, indicating bone ingrowth. 48 Other newer implants have been designed to take advantage of the bone ingrowth potential of porous coated metal and the favorable mechanical properties of polyethylene by adding an optional porous, coated metal central post to a polyethylene-pegged component (Figure 10). Summary Many options currently exist for managing the glenoid in the setting of shoulder arthritis, with no single option universally accepted for all patients. In TSA, glenoid component loosening remains a common clinical concern and, over the years, implants have evolved to minimize this risk. However, failure of the glenoid is likely multifactorial, with both patient and surgeon factors playing a role. A thorough understanding of surgical indications, principles of implantation, component options, and modes of failure is necessary to ensure an optimal outcome following TSA. References Evidence-based Medicine: Levels of evidence are described in the table of contents. In this article, 35, 36, 40, and 47 are level I studies. References 7, 13, 16, 23, 25-30, 34, 37, 38, 41, 43, and 46 are level II studies. References 3, 5, 6, 15, 18, 19, 21, 22, 25, and 33 are level III studies. References 1, 2, 4, 11, 12, 14, 20, 32, 39, 42, 44, 45, and 48 are level IV studies. References printed in bold type are those published within the past 5 years. 1. Neer CS II: Replacement Arthroplasty for Glenohumeral Osteoarthritis. J Bone Joint Surg Am 1974;56(1): Neer CS II, Watson KC, Stanton FJ: Recent experience in total shoulder replacement. 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Clin Orthop Relat Res 1992;277: Churchill RS, Brems JJ, Kotschi H: Glenoid size, inclination, and version: An anatomic study. J Shoulder Elbow Surg 2001;10(4): Habermeyer P, Magosch P, Luz V, Lichtenberg S: Three-dimensional glenoid deformity in patients with osteoarthritis: A radiographic analysis. J Bone Joint Surg Am 2006;88(6): Mullaji AB, Beddow FH, Lamb GH: CT measurement of glenoid erosion in arthritis. J Bone Joint Surg Br 1994;76(3): Walch G, Badet R, Boulahia A, Khoury A: Morphologic study of the glenoid in primary glenohumeral osteoarthritis. J Arthroplasty 1999;14(6): Hughes RE, Bryant CR, Hall JM, et al: Glenoid inclination is associated with fullthickness rotator cuff tears. Clin Orthop Relat Res 2003;407: Hamada K, Fukuda H, Mikasa M, Kobayashi Y: Roentgenographic findings in massive rotator cuff tears: A long-term observation. 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