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    Glenoid Bone Loss in Primary RTSA: Management Options and Outcomes

    Reverse total shoulder arthroplasty has become a reliable, successful operation for treating rotator cuff arthropathy. Concomitant glenoid bone loss creates special challenges that surgeons need to be prepared to overcome.

     

    Authors

    Andrew R. McNamara, MD; Rowan J. Michael, MD; Thomas W. Wright, MD; Bradley S. Schoch, MD; and Joseph J. King III, MD

    Introduction

    Reverse total shoulder arthroplasty (RTSA) has provided a reliable reconstruction option for older patients with a deficient rotator cuff and glenohumeral osteoarthritis. The reverse prosthesis is a semi-constrained implant, and unlike implants used for an anatomic total shoulder, it does not rely on rotator cuff soft tissue balancing for success. 

    Despite this advantage, reverse prosthesis implantation remains challenging in patients with glenoid bone loss. This article reviews the treatment options for bone loss when performing RTSA.

    Radiographic Evaluation of Glenoid Bone Loss

    Initial evaluation includes plain radiographs of the affected shoulder. The Grashey view should be evaluated for joint space narrowing, acromio-humeral distance (<7mm considered evidence of chronic, irreparable rotator cuff deficiency), [1] and glenohumeral subluxation. The axillary view is particularly helpful to assess glenoid morphology. 

    A 3-dimensional (3D) CT scan allows for further characterization of glenoid morphology and has been shown to be more reliable than 2-dimensional CT scans, especially with regard to accuracy of glenoid version and inclination measurements. [2-5] Frankle et al [6] reported that 37.5% of glenoids demonstrate wear, with 3D CT models allowing further sub-classification of wear patterns in posterior (17.6%), superior (9.3%), global (6.5%), and anterior (4.2%) locations. Specific wear patterns can be seen with certain pathologies:

    • Posterior wear with primary osteoarthritis [7]
    • Superior wear with rotator cuff tear arthropathy [8]
    • Anterior bone loss with chronic instability [9]
    • Central/global loss with revision shoulder arthroplasty [10]

    Classification of Glenoid Wear

    Osteoarthritis is the most common reason for primary shoulder arthroplasty, with posterior wear being the most common bony deficiency. Walch et al [7] classified these glenoid defects in the axial plane as follows:

    • A1, concentric
    • A2, concentric and centrally eroded
    • B1, posteriorly subluxated
    • B2, posteriorly eroded and subluxated
    • C, retroverted (hypoplastic)

    This classification was recently modified based on 3D glenoid reconstructions to improve interobserver and intraobserver reliability: [11]

    • A B3 subtype was added representing a monoconcave deformity secondary to severe posterior wear and retroversion of at least 15°, subluxation of 70%, or both.
    • A type D subtype was added for glenoid anteversion or anterior humeral head subluxation.
    • In the A2 subtype, a line drawn from the anterior to the posterior native glenoid rim transects the humeral head, whereas in the A1 glenoid, it does not.

    Glenoid erosion has also been classified in the coronal plane by Sirveaux et al: [12]

    • E0, humeral head migration without glenoid erosion
    • E1, humeral head migration with concentric glenoid erosion
    • E2, humeral head migration with superior glenoid erosion
    • E3, humeral head migration with inferior glenoid erosion

    Glenoid Bone Loss in RTSA: Eccentric Reaming Technique and Outcomes

    Eccentric reaming involves reaming of the neoglenoid (worn side of the glenoid) down to version that’s similar to the paleoglenoid (native glenoid). This technique results in medialization of the joint line and removal of a significant amount of glenoid bone (Figure 1). The goal is to create a flat surface with adequate surface contact with the baseplate. In addition, surgeons should slightly inferiorly tilt the baseplate (most authors strive for 10°), which has been shown to minimize implant shear, maximize baseplate compression, and facilitate osteointegration. [13] 

    Figure 1.  In a posteriorly worn glenoid, the eccentric reaming technique involves greater bone removal to correct glenoid baseplate retroversion. If off-axis reaming is used, a posterior bony or metal augmented baseplate is required.

    The eccentric reaming technique can potentially compromise glenoid fixation secondary to removal of subchondral bone. The maximum amount of recommended correction in retroverted glenoids is 15°. [14] A posteriorly augmented glenoid component or bone graft is recommended for retroversion greater than 15-20°.

    A retrospective review of 42 patients by McFarland et al [15] reports outcomes following an average 36-month follow-up in RTSA patients with severe glenoid bone loss, including A2 bone loss (19), B2 bone loss (5), and C bone loss (18). The RTSA baseplate used in the study had a central screw integrated with the baseplate and 4 peripheral locking screws. A screw was not placed if no bony support was present under a hole. 

    McFarland et al [15] found that all outcome scores and range of motion had improved by final follow-up, with the exception of 1 failed glenoid baseplate requiring revision surgery. In this case, the patient had a type C glenoid and 2 baseplate screws and was subsequently revised to a repeat RTSA without bone grafting 38 months after the index procedure. This suggests that despite medialization of the baseplate, good results can be obtained with eccentric reaming without glenoid component failure. [15]

    Bone Grafting of the Glenoid: Technique and Outcomes

    A humeral head autograft is recommended when a graft is needed to correct significant glenoid bone deficiency. The graft can be contoured to match the glenoid defect and correct the deformity. Some companies have specific instrumentation to fashion the graft, but it can also be done with an oscillating saw and a rongeur. The goal of the graft is complete baseplate coverage with maximum graft to native bone contact (without excessive graft overhang to avoid humeral impingement on the final construct).  Alternatively, allograft bone can be used, but it is typically reserved for revisions.

    Following is the technique for bone grafting of the glenoid:

    • After the graft has been prepared, any remaining glenoid cartilage is removed using a Cobb elevator. The subchondral bony surface can be perforated with a small Kirschner wire multiple times to create a biologically active interface for graft incorporation.
    • The graft is placed into the defect and held in place provisionally with several Kirschner wires. These are placed outside the future path of the reamer.
    • The guide wire for the glenoid reamers is then positioned to achieve a slightly inferior tilt of approximately 10°, with between 0° and 10° of retroversion.
    • The glenoid is prepared according to the manufacturer’s directions, with sequential reaming and drilling for the central hole. Reaming must be done in a controlled fashion to avoid displacing or fracturing the graft. The reamers may be run in reverse if the graft is unstable during this step.
    • The baseplate is inserted and the screws are passed through the baseplate and graft into the native glenoid to stabilize the construct (Figures 2). Fixation of the baseplate can be performed in such a manner that compressive force is placed between the graft and native bone interface.
    • Alternatively, the graft can be fashioned on the back table to fit the defect and allow for placement on the baseplate prior to implantation (Figures 3). This allows optimal native glenoid visibility during central hole drilling.
    • Mallet impaction of the baseplate is then performed to maximize baseplate contact with the graft, as well as graft contact with the native glenoid given the relatively soft autograft bone.

    Figure 2. Axial CT scan (top) showing a B3-type glenoid with approximately 28° of glenoid retroversion. Two week postoperative AP view of the shoulder (bottom left) after bone grafting of the above case using humeral head autograft. Six-month postoperative AP (bottom right) demonstrating interval graft healing.

    Figure 3. The humeral head graft (top) is fashioned on the back table to fit the defect and allow for placement on the baseplate prior to implantation.  A posterior/superior augmented baseplate (bottom) is used in this example.

    Wagner et al [10] compared 40 shoulders that required glenoid bone grafting during revision from total shoulder to RTSA with 102 shoulders that did not require grafting. The 2- and 5-year implant survival rates free of revision were 88% and 76%, respectively for the grafted group, compared with 94% at both time points for patients who did not have bone grafting.

    Radiographic loosening was also worse in the bone graft group, with revision from previous total shoulder arthroplasty, smoking, increased body mass index, and implants with a lateral center of rotation identified as risk factors for glenoid loosening. Pain and functional scores improved at final follow-up. [10]

    Few studies have focused on structural grafting in RTSA. In a recent series by Jones et al, [16] 44 patients with large uncontained defects were treated with RTSA (Exactech Equinoxe; Gainesville, Florida) and humeral head autograft, iliac crest autograft, or femoral head allograft. No significant differences were found between the allograft and autograft groups in terms of postoperative outcome measures, pain scores, and range of motion at 2 years postoperatively. Graft incorporation rates, however, were higher in the autograft group (86% complete or partial incorporation for autografts vs 67% for allografts). Of the 6 radiographically loose glenoids, only 2 required revision. [16]

    In another recent series, 14 patients who had received humeral head allografts for defects up to 35° were followed for 2.6 years. The study authors reported a 93% chance of baseplate survival and a 100% graft incorporation rate. Functional and range of motion results were improved at final follow-up. [17]

    A newer technique known as bony increased-offset reverse shoulder arthroplasty (BIO-RSA), which involves a trapezoidal bone graft harvested from the humeral head and fixed with a long-post baseplate and screws, has been described by Boileau et al. [18] In their study of 54 patients, 51 (94%) underwent complete radiographic incorporation of the graft, with only 2 patients experiencing baseplate loosening. [18]

    RTSA Glenoid Augments: Indications, Technique, and Outcomes

    Metal augments, introduced for use with RTSA in 2011, are attractive alternatives to bone grafting. Augments are available in a number of configurations to address the varying posterior and superior bony wear patterns in RTSA patients. Augmented RTSA baseplates are indicated when adequate backside contact cannot be achieved with eccentric reaming such that potential fixation would be compromised. In our practice, we prefer to use augments when there would be less than 50% contact with the backside of the baseplate.

    Excess superior inclination is seen in patients with cuff tear arthropathy, and it is a common indication for the use of a superior augment. Severe primary osteoarthritis may have posterior bone loss leading to increased retroversion, making a posterior augment beneficial. A posterior superior augmented glenoid baseplate is used when the patient has a combined deformity or when the surgeon wishes to tension the posterior rotator cuff (Figures 4). [19]

    Figure 4. Anteroposterior (top) and axillary (bottom) views of a reverse total shoulder implant using a posterior/superior augmented glenoid baseplate.

    Three types of augmented baseplates are available:

    • 10° superior augment
    • 8° posterior augment
    • Combined 10° superior/8° posterior augment for the Exactech Equinox system

    At least 2 other companies have recently released metal baseplate augments as well: the Tornier Aequalis Perform and the Zimmer Biomet Comprehensive Augmented Baseplate.

    Following is the technique for use of glenoid augments:

    • The glenoid is exposed in the usual fashion.
    • Some surgeons using augments perform minimal or no reaming, electing to remove the remaining cartilage with a Cobb elevator. If reaming is done, it is most easily performed using navigation or cannulated reamers. Keep in mind that the reaming and drilling axes are different.
    • If the glenoid deformity is severe and computer assistance is not available, place a guide wire with inferior inclination based on preoperative planning. Penetration of this wire down the glenoid neck can be palpated and compared with the preoperative plan.
    • The appropriate augment guide is placed, along with a second guide wire using the guide aligning the reaming axis (Figure 5). The first wire is removed and the glenoid is reamed with a cannulated reamer.
    • Once the reaming is completed, the original wire is replaced in the same trajectory and the reaming wire is removed.
    • The cannulated drill is then used in the first wire trajectory for preparation of the central cage.
    • The augmented baseplate is impacted into place and screw fixation is obtained.

    Figure 5. Drawing demonstrates the guide used to facilitate placement of the off-axis K-wire used for off-axis reaming and subsequent placement of a metal-augmented glenoid baseplate.

    In our experience, glenoid augments for RTSA have performed well at short- and mid-term follow-up. [20] Between October 2011 and July 2016, 139 patients undergoing RTSA received either a posterior, superior, or posterior-superior augmented glenoid baseplate. All groups demonstrated improvements in functional outcome measures compared with baseline. The posterior-superior augment group saw the greatest improvement in active forward flexion and external rotation, while the posterior augment group experienced the largest improvement in ASES and Constant scores. [20]

    In the superior augment group (n=22), 3 patients (13%) sustained a total of 5 complications. One of these patients sustained 3 episodes of instability, eventually requiring revision. In the posterior augment group (n=50), 4 patients (8%) had complications. Two of these were postoperative humeral fractures related to traumatic events, while another patient sustained an intraoperative tuberosity fracture. The last complication in this group involved a draining wound, treated with oral antibiotics. [20]

    A total of 8 implant-related complications occurred in the posterior-superior augment group (13%, n=67), including aseptic baseplate loosening (5), glenoid fracture (1), humeral fracture (1), and acromial stress fracture (1). [20]

    Jones et al [21] performed a retrospective review of 80 patients who underwent RTSA with either a structural bone graft or an augmented glenoid baseplate. Although all patients had improvements in pain, range of motion, and functional scores, the structural bone graft group had a 14.6% complication rate compared with 0% in the augment group. In addition, the augmented baseplate group had a significantly lower rate of scapular notching compared compared with the bone graft group – 10% vs 18.5% – at similar follow-up intervals. [20]

    A study by Wright et al [19] compared posterior versus superior augmented baseplates in RTSA using the Exactech Equinoxe system. The posterior augment group demonstrated lower rates of scapular notching – 6.3% vs 14.3% – and showed greater improvements in ASES and Constant scores and active forward elevation. The study authors hypothesized that this may be due to the posterior augment baseplate being better able to tension the external rotators, leading to better range of motion. [19]

    Conclusion

    Glenoid bone loss remains a difficult problem in primary RTSA. Reaming techniques, bone grafting, and use of metal augments are all viable options, depending on the extent of the defect and surgeon familiarity. All these techniques have advantages and disadvantages. As preoperative planning and computer navigation technology continue to evolve and become more widely used, surgeons will be better equipped to evaluate and appropriately manage these challenging cases.

    Author Information

    Andrew R. McNamara, MD; Rowan J. Michael, MD; Thomas W. Wright, MD; Bradley S. Schoch, MD; and Joseph J. King III, MD, are from the Department of Orthopaedics and Rehabilitation at the University of Florida in Gainesville.

    Disclosures

    Dr. McNamara, Dr. Michael, and Dr. King have no disclosures relevant to this article. Dr. Wright has disclosed that he receives royalties from and is a consultant for Exactech. Dr. Schoch has disclosed that he is a paid speaker for DJO Surgical.

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