Management of the B2 Glenoid with Anatomic TSA: Ream, Step, or Wedge?
The authors review a historical treatment of posterior bone loss in advanced glenohumeral osteoarthritis – reaming the high side to correct pathologic version – and then discuss the more recent introduction of augmented glenoid components used to correct moderate to severe pathologic glenoid deformity and the B2 glenoid in anatomic total shoulder arthroplasty.
Jason C. Ho, MD; Joseph P. Iannotti, MD, PhD; and Eric T. Ricchetti, MD
Posterior bone loss in advanced glenohumeral osteoarthritis (OA) causes increased retroversion and often a biconcave (B2) glenoid, presenting a particularly difficult scenario for treating glenohumeral OA with anatomic total shoulder arthroplasty (TSA). The goals for treatment of these patients include:
- Correction of glenoid bony deformity to restore the patient’s native anatomy, particularly native version
- Restoration of the patient’s native joint line
- Balancing of the soft tissues
- Centering of the humeral head
Implant selection and design can significantly influence the achievement of these goals. Historically, B2 glenoid pathology has been addressed with use of a standard glenoid component and either asymmetric reaming for correction of version and the biconcavity or a posterior glenoid bone graft.
More recently, however, use of an augmented glenoid component – stepped, full wedge, or posterior half-wedge –has provided another surgical treatment option for the B2 glenoid.
In patients with moderate to severe asymmetric posterior glenoid bone loss and a B2 glenoid, it is difficult to: [1,2]
- Determine the patient’s pre-morbid or native glenoid version
- Effectively correct the pathologic deformity
Determining the native or pre-morbid glenoid version can be done with the use of 3-dimensional (3D) computed tomography (CT) modeling and the vault model. The vault model was developed by defining the shape of the glenoid vault in the non-arthritic shoulder and then virtually placing the model into the pathologic glenoid. [1,3-5] Once positioned, the vault model defines the area of posterior glenoid bone loss and has been shown to be predictive of the location of the native glenoid joint line, version, and inclination (Figure 1). [1,3-6] Therefore, this tool can be used to define patient-specific correction of the pathologic glenoid to its pre-morbid condition. [7,8]
Figure 1. Axial cut of 3D reconstructed CT scan depicting the vault model (blue) and the posterior bone loss in a patient with a B2 glenoid.
The authors currently use 3D CT imaging for preoperative planning in B2 glenoids. This allows more precise determination of the optimal implant choice based on the degree of pathologic correction and the joint line restoration that is possible with augmented or standard glenoid components. Prior studies have shown that using 3D CT preoperative planning can help predict the amount of correction achievable using standard and augmented glenoids, which can improve glenoid implant positioning postoperatively. [7,9,10]
The B2 glenoid is the most common pattern of moderate to severe bone loss in which the authors utilize an augmented glenoid component. However, there may be limitations to the use of augmented glenoids if bone loss is very severe or if the glenoid is dysplastic (B3 or C glenoids). In these cases, reverse TSA with or without bone grafting may be a more reliable option for implant stability and longevity.
The Evidence on Reaming
Before modern imaging techniques and augmented implants were available, the preferred method to manage mild to moderate glenoid retroversion was reaming the high side to correct pathologic version. [11-15] Yet reaming the high side to match native version as retroversion increases can cause significant medialization of the joint line, narrow the anteroposterior dimensions of the glenoid, and/or lead to possible peg perforation with standard glenoid components in anatomic TSA. Cadaveric and clinical studies have shown that these problems commonly occur when asymmetric reaming is used to correct 15° to 20° or more of retroversion. [2,16-18]
Residual glenoid retroversion of 15° or more in anatomic TSA can lead to increased rates of osteolysis of a pegged polyethylene glenoid component.  However, in a recent study with a minimum 2 years of follow-up, 15° or more of retroversion did not affect clinical outcomes when offsetting the humeral head to create a more concentric glenohumeral relationship; B2 glenoids were not specifically evaluated.  Walch et al  reviewed outcomes of B2 glenoids treated with standard glenoid components and primarily corrective reaming (85 cases of corrective reaming, 7 cases with posterior bone graft). They noted a 12% revision rate for glenoid loosening or posterior instability and a 20% radiographic loosening rate. 
It is likely that early osteolysis in standard pegged components will result in glenoid component loosening, as demonstrated in keeled glenoid designs in which there was a relationship between radiolucencies and pain.  Early radiolucent lines around keeled glenoids have been shown to be predictive of progressive radiolucent lines and worse patient-reported outcomes.  In addition to clinical studies using standard glenoid components, biomechanical and cadaveric studies have shown an increased risk of glenoid component loosening when the component is placed in more than 15° of retroversion. [13, 16, 24-31]
The Evidence on Stepped and Wedge Augments
As described above, reaming the high side allows for limited pathologic correction. In addition, the long-term durability of a retroverted glenoid implant for moderate to severe glenoid bone loss or a standard glenoid component for a B2 glenoid is worrisome. More recently the augmented glenoid component has been introduced as an alternative for correcting moderate to severe pathologic glenoid deformity and the B2 glenoid in anatomic TSA.
Prior to introduction of these augmented components, a posterior glenoid bone graft was the only technique available to correct glenoid version while also maintaining or correcting the joint line. However, mixed results with only small case series have been reported in the literature. [32-37] In addition, the procedure for using posterior glenoid bone graft is technically challenging, and the size and quality of the bone graft can be variable.
Numerous augmented polyethylene glenoid designs have been evaluated biomechanically in the recent literature; 3 are commercially available in the US in a variety of augmented sizes:
- Stepped design (Steptech; Depuy Synthes, Warsaw, Indiana; Figure 2a)
- Wedge-shaped design (Equinoxe Posterior Augment; Exactech, Gainesville, Florida; Figure 2b)
- Posterior wedge-shaped design (Aequalis Perform+, Wright Medical Group N.V., Memphis, Tennessee) (Figure 2c)
The stepped design has been in use the longest, since 2010.
Figures 2a-c. Steptech +7, +5 and +3 mm augmented stepped glenoids from Depuy Synthes (Figure 2a). Equinoxe Posterior Augmented all polyethylene wedge glenoid (left) and with metal-coated pegs (right) from Exactech (Figure 2b) Aequalis Perform+ posterior step wedged augmented glenoid from Wright Medical Group (Figure 2c).
Biomechanical and modeling studies have been conducted to determine which augmented glenoid design is the most biomechanically advantageous in glenoids with severe posterior wear or retroversion. One study compared the commercially available wedge augmented glenoid to a stepped augmented glenoid, standard glenoid placed in neutral version after reaming the high side, and standard glenoid placed in retroversion using finite element analysis (FEA).  The researchers found that the wedge design had more backside contact and less volume of bone at risk for strain damage compared with the standard glenoid after reaming the high side; however, they did not find differences when the wedge design was compared with the stepped design. 
Another study compared the wedged augment with a standard component in retroversion and found that the wedged augment required a smaller cement mantle and had greater bone fatigue life 
In a biomechanical study modeling early and late fixation states, the stepped augmented component was compared with 4 other augment designs – spherical asymmetric, spherical symmetric, flat angled, standard pegged – for resistance to anterior lift-off when posteriorly loaded. This study demonstrated that the stepped augmented component had decreased anterior glenoid liftoff compared with the other designs, which may lead to improved long-term durability. 
Another study tested a non-commercial version of a stepped augmented glenoid in cadaveric specimens modeling a B2 glenoid.  This study demonstrated that strains were not significantly different with a stepped component in a B2 glenoid versus a standard component in a normal glenoid. These strains were tested in a variety of arm positions in their cadaveric model, with all arm positions showing similar strains. 
The stepped augmented glenoid has also been compared with standard glenoids in a virtual simulation study using 3D CT scans of patients with posterior glenoid bone loss.  The results showed that compared with standard glenoid components, the +3mm, +5mm, and +7mm stepped augmented glenoids were able to correct larger amounts of pathologic retroversion – an average of 9.5°, 17.5°, and 27.9° of retroversion, respectively – to 0° and 6° of retroversion with significantly less joint line medialization. 
Finally, a computational modeling study compared the commercially available wedge, stepped, and posterior wedge components placed in B2 glenoids to assess the amount of bone removal and bone quality remaining. This study demonstrated that when correcting to neutral version, the posterior wedge design resulted in less bone removal compared with the wedge and stepped designs, and that the remaining posterior bone was of higher density when compared with the stepped design. [42,43]
Only a few clinical studies have evaluated augmented glenoid components; no clinical data are available on the posterior wedge augment. The wedge augmented glenoid was studied in patients with posterior glenoid wear and compared with standard glenoids without wear in an age- and sex-matched cohort study of 48 total patients (24 per group) with an average of 29.4 months of follow-up. This study showed significant improvement in pain and functional outcome scores in all patients, with no difference between groups in patient-reported outcomes at final follow-up. There were no complications in either group. No quantitative assessments of preoperative bone loss, retroversion, or degree of postoperative pathologic correction were reported. Seventeen of 20 augmented glenoids had a centered humeral head at final follow-up, 3 had an anteriorly translated humeral head, and none had a posteriorly translated humeral head. 
In a short-term clinical series with 6 to 15 months of follow-up, the stepped augmented glenoid was used in 24 patients; all showed significant improvement in range of motion and patient-reported outcomes. Eight patients had postoperative CT scans that showed stepped glenoids improved retroversion (16.7° vs. 11.3° of correction) and joint line correction (within 0.45mm vs. 3.56mm) compared with standard glenoids in patients with similar retroversion. 
Favorito et al  reported on 22 shoulders (20 B2 glenoids, 2 C glenoids) that underwent TSA with a stepped augmented glenoid at an average of 36 months of follow-up. This study demonstrated significant improvements in VAS, WOOS, SF-36, and range of motion postoperatively. The researchers also showed low rates of glenoid radiolucency, with Lazarus scores of 0 to 2; 18 of 22 shoulders had a score of 0 or 1. Only 1 glenoid demonstrated central peg osteolysis, and there were 2 cases of implant instability (1 anterior, 1 posterior). 
Stephens et al  also showed significant improvements in ASES score, SST, range of motion, pain, glenoid version, and humeral head alignment in 21 patients (19 B2 glenoids, 2 C glenoids) at an average of 35 months of follow-up with a stepped augment. Only 1 of 21 patients did not have central peg ingrowth, and no failures were reported. 
At the annual meeting of the American Academy of Orthopaedic Surgeons in 2016, we reported on our group’s study using CT scans (preoperative and with 3 months of surgery) to evaluate 80 patients with advanced glenohumeral osteoarthritis and Walch B2 or B3 glenoids who underwent TSA with a standard (n=32) or stepped augmented (n=48) glenoid component (unpublished data). All patients underwent preoperative planning with 3D CT imaging software prior to surgery. We found the following:
- Glenoid version correction (pre- to postoperative improvement in retroversion) was significantly greater with stepped than with standard implants (11.1±6.7° vs. 9±4.8°; P<0.001).
- Correction of pathologic joint line was significantly greater with stepped than with standard implants (1.8±2.4 mm vs. 0.7±1.6 mm lateralization from the preoperative joint line; P=0.02).
- Correction of postoperative humeral head subluxation relative to the scapular plane was significantly greater with stepped than with standard implants (9.5±7.3% vs. 5.0±4.1%; P<0.001).
Our group also reviewed 71 patients with B2 and B3 glenoids and a minimum 2-year follow-up who were treated with a posterior augmented step glenoid (unpublished data). We found significant clinical improvement and correction of the preoperative deformity:
- The Penn shoulder score improved from 30±17 (range 0 to 66) to a median of 94 (IQR 88-98, range 51 to 100; P<0.0001).
- Glenoid version improved from -24°±7° (range -42° to -9°) to -11°±6° (range -28° to 1°).
- Posterior humeral head subluxation was present in 60 of 71 shoulders (85%) preoperatively and the prosthetic humeral head was centered in 60 of 71 shoulders (85%) (P<0.0001).
- Sixty-four of 71 shoulders (90%) had a Penn shoulder score of 80 or more, 52 of 71 (73%) had a postoperative glenoid retroversion less than 15°, and only 11 of 71 glenoids (15%) had center peg osteolysis.
- Range of motion also improved from a median forward elevation of 110° (IQR 90° to 140°, range 60° to 170°) to a median of 160° (IQR 150° to 170°, range 90° to 180°) and median external rotation of 20° (IQR 10° to 30°, range 0° to 60°) to an average of 51°±13° (range 10° to 80°; P<0.0001).
In a case series, 5 patients with anterior glenoid deficiencies who were treated with the stepped augmented glenoid for posterior wear.  At an average of 33.2 months of follow-up (range 21.9 to 43.2 months), no patient had undergone revision surgery, and the average postoperative Penn shoulder score was 84.4 (range 58-100). 
Biomechanical and computational data support the use of wedge and stepped augmented polyethylene glenoid components when addressing significant glenoid wear and deformity, including the B2 glenoid, although information on clinical outcomes is still sparse in the literature. The stepped design may be more biomechanically stable; the wedge and posterior half wedge may preserve more bone.
Prior literature has shown a high revision rate (16%) in biconcave B2 glenoids treated with standard glenoid components. Recent literature with augmented glenoids shows much lower rates. 
We believe, therefore, that an augmented glenoid component is a reliable option for joint line restoration, version correction, and centering of the humeral head for B2 glenoids, and there is early literature supporting this conclusion (Figure 3). [44-47,48] Longer-term clinical follow-up will be needed to assess the longevity of the augmented glenoids, including maintenance of the pathologic glenoid correction and centering of the humeral head, patient-reported outcomes, component failure rates, and variables associated with these failures.
Figure 3. Preoperative Grashey view radiograph (a), axillary radiograph (b), and 2D CT scan of a B2 glenoid (c). Postoperative Grashey radiograph showing a centered head with bone ingrowth within the flanges of the central anchor peg (d). Axillary radiograph showing centering of the humeral head and correction of pathologic version with an augmented glenoid (e).
Jason C. Ho, MD, is a resident in the Department of Orthopaedic Surgery, Orthopaedic & Rheumatologic Institute, at the Cleveland Clinic, Cleveland, Ohio. Joseph P. Iannotti, MD, PhD, is chairman of the Orthopaedic and Rheumatologic Institute at the Cleveland Clinic, Cleveland, Ohio. Eric T. Ricchetti, MD, is director of the Shoulder Service of the Orthopaedic and Rheumatologic Institute at the Cleveland Clinic Cleveland, Ohio
Dr. Ho has no disclosures relevant to this article. Dr. Iannotti has disclosed that he receives royalties from Depuy Synthes and has stock or stock options in Custom Orthopaedic Solutions. Dr. Ricchetti has no disclosures relevant to this article.
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