Glenoid augmentation for subcritical bone loss: a narrative review

Introduction

Anterior shoulder instability typically presents in young and active individuals, with those participating in contact sports or high-risk activities being particularly susceptible. Although anterior labral tears have been referred to as the “essential lesion” in the pathogenesis of anterior shoulder instability (1), severe initial trauma and subsequent instability events precipitate further instability and worsening pathology (2-4). Among the sequelae of recurrent anterior shoulder instability, increasing magnitudes of anterior glenoid bone loss (GBL) have been identified as perhaps the most significant risk factor for recurrent instability (5-7).

Although glenoid bone defects were originally felt to be uncommon (1), Sugaya et al. demonstrated that 50% of shoulders with recurrent instability had an osseous Bankart lesion, while another 40% had loss of the normal anterior glenoid contour, without an identifiable osseous fragment (8). Griffith et al. similarly showed a high prevalence of bone loss (86%) in patients with unilateral recurrent dislocations and further stratified patients quantitatively, demonstrating a small majority of patients (51%) had less than 10% GBL and 37% had between 10% and 20% GBL (9).

The success of surgical repair is predicated upon the identification of GBL deficiency. Many analyses have defined thresholds of GBL, termed “critical defects”, above which GBL was deemed too large to overcome with soft tissue procedures alone. Initially, thresholds around 25% GBL (relative to glenoid width) were often cited (10-14), but Shin et al. demonstrated that 17% GBL was an inflection point for failure of isolated labral repair (6). However, more recent clinical analyses have honed in on lesser magnitudes of GBL, raising concerns that this threshold should actually be considerably lower. These lesser magnitudes of GBL have been termed subcritical bone loss and are typically classified as greater than 13.5% (15).

Although the upper limit of subcritical GBL remains somewhat controversial, opposite the classically defined critical bone loss thresholds, in each patient with some appreciable degree of GBL, concerns persist regarding the effectiveness of arthroscopic Bankart repair (ABR) alone in restoring shoulder stability. This concern is evident from the number of recently published studies designed to assess other procedures, such as glenoid augmentation and ABR with remplissage, for patients with subcritical bone loss (16-19). However, as evidenced by two ongoing randomized controlled trials (20,21) a great deal of uncertainty remains as to the ideal surgical treatment for this population. Accordingly, this review article focuses on the current evidence and controversies associated with the treatment of anterior shoulder instability in the setting of subcritical GBL, highlighting ongoing debate between open augmentation procedures and ABR with remplissage for this population. We present this article in accordance with the Narrative Review reporting checklist (available at https://aoj.amegroups.com/article/view/10.21037/aoj-23-36/rc).

Methods

A narrative review of the current existing literature was conducted with a focus on subcritical bone loss in anterior glenoid stabilization procedures, including review of forward citation and reference lists of selected articles. PubMed, MEDLINE, SPORT Discus, and Cochrane Database of Systematic Reviews databases were searched using the terms “subcritical bone loss” and “shoulder stabilization” from January 2000 through March 2023 (Table 1). Articles written in the English language were considered with peer-reviewed publication only, excluding case reports, technique guides, animal studies and conference abstracts.

Pathoanatomy

The glenohumeral joint is an inherently unstable joint given its tremendous mobility and relatively limited bony constraint. The shallow articulation of the glenoid makes the shoulder dependent on soft tissue stabilizers such as the labrum, rotator cuff, glenohumeral ligaments and capsule, but insults to the bony architecture during initial or recurrent instability events have devastating effects on shoulder stability (2,3).

Most clearly, increasing magnitudes of GBL reduce the amount of force required to translate the humeral head, but anterior GBL also results in a loss of concavity/depth of the glenoid, further reducing glenohumeral constraint (22). Other glenoid parameters, such as glenoid version, have also been implicated as risk factors for recurrent instability (23-25). However, the extent to which they play a role may be more variable. For example, decreased retroversion may only serve as a clinically evident risk factor in individuals with concomitant subcritical bone loss (23). Similarly, sex-based differences in glenoid morphology in patients with and without instability also suggest that the pathophysiology and anatomic risk factors for instability may be different for males and females requiring distinct treatment considerations (26).

To discuss only GBL in anterior shoulder instability would be an incomplete characterization of the pathoanatomy. Although this review focuses on GBL as a key variable, when formulating a treatment plan, surgeons must consider other factors such as humeral-sided bone loss, ligamentous laxity, age, mechanism of injury, number of prior instability events and goals for future performance/activity, as these are all risk factors for continued instability and treatment failure (5,27,28).

Regarding the importance of soft tissue stabilizers, it is important to assess for combined labral injuries, extending posteriorly and/or superiorly (29-31). Failure to address the full extent of labral pathology will result in an inadequate recreation of the hammock suspension of the labral, capsular and inferior ligamentous complex, predisposing patients to an incomplete resolution of symptoms and recurrence. This also often serves as the rationale by some surgeons to perform diagnostic arthroscopy prior to open anterior glenoid augmentation procedures (32).

Next, similar to GBL, humeral-sided bone loss has important implications on treatment strategy. During a dislocation event the posterolateral humeral head often collides with the posterior rim of the glenoid causing an impaction type injury, or defect to bone and cartilage, termed a Hill-Sachs lesion. Afterward, the Hill-Sachs lesion decreases the arc of concentric glenohumeral joint motion and may engage on the anteroinferior glenoid, translating the humeral head and predisposing to further instability. Recognition of humeral bone loss (alongside GBL) helped establish the bipolar bone loss paradigm, which ultimately transformed the way surgeons think about shoulder instability. This enhanced understanding of the pathoanatomy has led to the utilization of surgical procedures that have been proposed to directly address deranged anatomy. Accordingly, a procedure such as remplissage, can directly address a Hill-Sachs lesion, whereas glenoid augmentation was designed to restore glenoid architecture in the setting of critical bone defects. However, more recently, both remplissage and glenoid augmentation procedures have seen increasingly utilization outside of these classic indications, especially in patient with subcritical bone loss, given their greater magnitude of effect on stabilization as compared to the ABR alone.

Finally, demographic factors such as age, participation in contact/overhead sports and ligamentous laxity factor into management decisions. While they are often thought of more subjectively than thresholds of bone loss, they undoubtedly influence treatment decisions. The Nonoperative Injury Severity Index Score (NSIS) includes six risk factors for failure of non-operative management of anterior shoulder instability including age >15 years, bone loss, type of instability, contact sport, male sex and arm dominance (33). Tokish et al. found low risk patients (NSIS <7) had an overall high return to sport rate (97%) when treated non-operatively (33). In order to encourage their inclusion in decision making and more objectively guide management, scoring systems such as the Instability Severity Index Score (ISIS) (34) and Glenoid Track Instability Management Score (GTIMS) (35) have been described. However, the validity of these scoring systems is still in question (35-38), in part due to their own designs, which utilize imprecise radiographic determinations of bipolar bone loss in order to maintain their simplicity and utility. The limitations of these scoring systems notwithstanding, what is truly important is for surgeons is to understand the risk factors for recurrence and to consider each of them when formulating a treatment plan.

Imaging

When evaluating patients with anterior shoulder instability, a standard radiographic series includes a Grashey, axillary (or axillary equivalent) and scapular Y-view. Magnetic resonance imaging (MRI) is the advanced imaging modality of choice for shoulder instability. In the days to weeks following a shoulder instability event, hematoma from the injury may act similarly to contrast in an MR arthrogram in delineating injuries of the chondrolabral junction and/or glenohumeral ligaments. However, in subacute or chronic injuries magnetic resonance arthrography enhances the sensitivity and specificity for diagnosing intraarticular pathology (39) While several reports have demonstrated the utility of MRI in accurately measuring GBL (40), computed tomography (CT) is perhaps the modality of choice for obtaining precise measurements of GBL. More recently, 3D CT reconstructions with and without humeral head subtraction have also been increasingly utilized to characterize the extent of bone loss. As an alternative, 3D MRI measurements for bone loss in glenohumeral instability using isotropic volumetric interpolated breath-hold examination (VIBE) sequencing has been shown to be equivalent to 3D CT models and nearly two times cheaper than MRI and CT with 3D reconstructions (41,42).

Assessment of GBL is performed via sagittal advanced imaging, wherein the inferior glenoid is modeled as a circle and the extent of bone missing from the circle is bone loss. There are several techniques to characterize bone loss this way, including as a percentage of the glenoid diameter, wherein the width of the bone loss (in mm) is divided by the diameter of the circle (Figure 1). Other techniques include utilizing the area (mm²) of bone loss and the area of the circle. Also relevant to a discussion of subcritical bone loss is the study by Lansdown et al. which demonstrated that identification of a flat anterior glenoid corresponds to 12.8%±3% GBL (43).

Figure 1 Left shoulder with 3D CT scan with humeral head subtraction demonstrating 15% GBL, with en face measurement with best fit circle, % GBL = d/D ×100. Arthroscopic view of left shoulder viewing from anterior superolateral portal in the lateral decubitus position, demonstrating intraoperative measurement of glenoid (G) width with a calibrated probe from the posterior portal, with flat anterior glenoid contour and deficient labrum. Red dotted circle, best fit circle; *, bare area. 3D CT, three-dimensional computed tomography; GBL, glenoid bone loss; H, humerus; PL, posterior labrum.

Finally, an assessment of Hill-Sachs lesion size, location and on-track/off-track status may help inform surgical management. A Hill-Sachs lesion is termed off-track when the Hill-Sachs lesion interval is greater than the glenoid track, with the glenoid track being defined as 0.83(D)-d, where D is the diameter of the circle modeled on the inferior glenoid and d is the extent of GBL (Figure 1). Although on-track and off-track have previously been defined as binary, more recent analyses suggest that it may be a spectrum of risk, with lesions that are closer to off-track status, termed “near-track lesions”, still being at higher risk of failure (44).

Surgical management of subcritical bone loss

In 2000, Burkhart and De Beer (13) proposed the concept of the inverted pear glenoid, an arthroscopic indicator of a substantial glenoid bone defect which is at high risk of failure with ABR. Later, Lo et al. (10) quantified the GBL associated with an inverted pear glenoid, demonstrating in cadavers that this geometry was the result of a mean GBL of 28.8% (Figure 2). Yamamoto et al. then performed a biomechanical cadaveric study demonstrating that 26% GBL (relative to the glenoid width) significantly diminished shoulder stability (11). The authors concluded that patients with this level of bony deficiency were not candidates for arthroscopic soft tissue repairs (11). Boileau et al. also noted recurrent instability after ABR was associated with GBL >25% (5). Altogether these studies created the basis for so called “critical bone loss”, deemed so severe that an ABR would not suffice.

Figure 2 Jeny3D CT scans with humeral head subtraction demonstrating varying degrees of GBL, including a flattened anterior contour corresponding to approximately 14% GBL and an inverted pear glenoid, corresponding to 28% GBL, with GBL in line with the supraglenoid tubercle. 3D CT, three-dimensional computed tomography; GBL, glenoid bone loss.

Subsequently, the durability of soft tissue repairs in patients with even less bone loss was questioned. Shin et al. first raised concerns that GBL of 15% or more was a critical defect, wherein soft tissue repair could not restore glenohumeral translation (7). However, clinically the same group (6) found that 17% GBL was a more accurate critical threshold for shoulder function and surgical failure, as have others (45). Later, Shaha et al. defined “subcritical bone loss” in a young, military population to be 13.5%, wherein patients had lower WOSI scores above this threshold (15). However, currently subcritical bone loss corresponds to >13.5% GBL, with no clear upper threshold, making comparison of subsequent and recent studies difficult. Nevertheless, we will highlight the available evidence in this population of patients, demonstrating the growing body of literature in support of operations aside from isolated ABR.

It should be noted that primary ABR remains the predominate surgery performed for primary anterior shoulder instability, while glenoid augmentation and open Bankart repair continue to be used sparingly in the United States (31). Rightfully so, Dickens et al. (46) showed that intercollegiate football players with <13.5% GBL went on to successful outcomes with ABR alone, with low rates of recurrent instability. However, when Dekker et al. analyzed more than 400 patients undergoing ABR at a minimum of four years follow-up, >15% GBL was a risk factor for recurrent instability and inferior clinical outcomes (47).

Historically, the Latarjet procedure was considered for substantial glenoid defects, but the illumination of subcritical bone loss, with and without Hill-Sachs lesions (i.e., bipolar bone loss), has expanded its indications. Min et al. (48) retrospectively compared ABR to open Latarjet procedures for subcritical bone loss, defined as 13.5–24% GBL. They found no significant difference in WOSI scores, but the Latarjet cohort had higher SANE scores (48). Similarly, Rossi et al. analyzed a population of rugby players with GBL <20% undergoing Latarjet, against a historical treatment group of ABRs, finding higher rates of recurrent instability (20% vs. 4%) and rates of reoperation (16% vs. 4%) for the ABR group as compared to the Latarjet group (49). Importantly, this study was born out of the authors transitioning their instability practice in Rugby players to Latarjet, given conspicuously high rates of recurrent instability with ABR. While the results clearly favor the Latarjet procedure in this specific subpopulation with <20% GBL, it is difficult to assess the exact role of subcritical bone loss given the majority of patients did not have any appreciable bone loss and these contact athletes are obviously exposed to unique risks for recurrence. Jeon et al. (50) similarly retrospectively reviewed patients undergoing ABR or Latarjet, for what the authors termed “borderline” bone defects from 15–20% GBL. While they concluded that both provide satisfactory clinic outcomes, the Latarjet procedure resulted in a lower rate of recurrent instability and less external rotation loss and thus they concluded it may be a more reliable option in this cohort. While Jeon et al. (50) excluded patients with off-track Hill-Sachs lesions, Yang et al. evaluated ABR plus remplissage versus Latarjet in a population of patients with off-track lesions and subcritical bone loss, which they defined as <25% (51). Notably, their mean GBL was 10.4%±6.8% and 12.3%±8.79% for ABR plus remplissage and Latarjet, respectively. Multivariate analysis revealed that Latarjet procedures had lower odds of recurrent instability in the revision setting than ABR plus remplissage and that the odds of recurrence were also lower in collision athletes and those with 10–15% bone loss and >15% bone loss.

Contrary to the results suggesting the supremacy of Latarjet over arthroscopic, soft tissue-only repairs, Horinek et al. recently demonstrated that ABR and remplissage yields similar 2 year outcomes to Latarjet in patients with >15% GBL (52). This lead the authors to conclude that remplissage is a consideration in patients with GBL >15% and it is notable that their cohort includes patients well beyond subcritical bone loss thresholds, with mean GBL in both groups of 25%. The same authors also published a similar comparative study on a wider range of GBL, wherein remplissage resulted in similar or better outcomes as compared to Latarjet, despite marginally higher preoperative GBL and a higher rate of off-track lesions in the remplissage group (53). Their work is also consistent with that from Pathak et al. reporting on ABR with remplissage in patients with GBL <20% and large Hill-Sachs lesions (54). While it appears that remplissage may be robust enough to rival the Latarjet procedure mid-term and long-term follow-up of these populations, with subcritical bone loss, undergoing these procedures are still need to provide a definitive answer as to the most appropriate treatment option.

Aside from the Latarjet procedure, utilization of distal tibial allograft (DTA) and local autograft is becoming increasingly prevalent. However, there has been limited study in a subcritical bone loss population. A recent cadaveric and CT scan based study demonstrated that Latarjet and distal clavicle grafts were able to completely or near completely restore the glenoid at 20% and 30% GBL, however scapular spine grafts could only restore the glenoid in two-thirds of 20% defects (55). However, in a population of subcritical bone loss, scapular spine autograft has already been shown to result in satisfactory outcomes at 2-year follow-up, with no episodes of recurrent instability (18), suggesting the other grafts also represent viable solutions for subcritical bone loss.

Finally, transfer of the long head of the biceps to the glenoid, termed dynamic anterior stabilization (DAS), has been shown to reduce anterior translation in a 15% glenoid defect model when combined with ABR as compared with ABR alone (56). However, Latarjet remained the most effective procedure at reducing anterior translation. Similarly, in another cadaveric study of 20% GBL, Latarjet also demonstrated significantly higher loads to dislocate than the long head of biceps tendon and finally a series of patients with GBL <20% undergoing DAS with ABR demonstrated a 13.6% (3/23) recurrence rate at 3.2 years follow-up leaving questions as to the role and utility of this procedure (17,57).

To a lesser extent, arthroscopic conjoint tendon transfer has also been described to produce a dynamic-sling effect using suspensory and interference fixation as an option for patients with a failed labral repair with subcritical bone loss <15% and an on-track Hill-Sachs lesion (58). In this small case series of eight patients, median WOSI scores improved with 100% return to sport and no further dislocation event at median follow-up of 31 months. Similar results have been shown in a case series for treatment of traumatic shoulder instability with 25% or greater GBL and an engaging Hill-Sachs lesion with follow-up of 31 months (59). However, outcomes from this procedure have only been described in case series with limited follow-up.

In an effort to answer many of the questions posed already, two prospective efforts, the STABLE (20) and OASIS (21) trials are ongoing. The STABLE Trial is a randomized controlled trial (RCT) comparing ABR with remplissage to Latarjet, and the OASIS Trial is a RCT comparing ABR with remplissage to open Bankart repair to Latarjet in patients with subcritical bone loss (Tables 2,3). The successful execution of these efforts will assuredly offer critical information as to the optimal treatment approach to recurrent shoulder instability observed in the setting of subcritical GBL.

Conclusions

There is no consensus regarding the optimal treatment approach to recurrent shoulder instability in the setting of subcritical GBL. Conversely, there is a growing agreement that isolated ABR is likely inadequate and subcritical GBL should prompt consideration of a more robust operation. To this end, interest in glenoid augmentation continues to grow as a reliable technique for recreating the native architecture and restoring glenohumeral stability.

Acknowledgments

Funding: None.

Footnote

Provenance and Peer Review: This article was commissioned by the Guest Editors (Jonathan D. Hughes and Albert Lin) for the series “Bone Loss in Shoulder Instability and Shoulder Arthroplasty” published in Annals of Joint. The article has undergone external peer review.

Reporting Checklist: The authors have completed the Narrative Review reporting checklist. Available at https://aoj.amegroups.com/article/view/10.21037/aoj-23-36/rc

Peer Review File: Available at https://aoj.amegroups.com/article/view/10.21037/aoj-23-36/prf

Conflicts of Interest: All authors have completed the ICMJE uniform disclosure form (available at https://aoj.amegroups.com/article/view/10.21037/aoj-23-36/coif). The series “Bone Loss in Shoulder Instability and Shoulder Arthroplasty” was commissioned by the editorial office without any funding or sponsorship. D.J.C. reports being an editorial board member for Arthroscopy and receiving research support from AANA unrelated to the current work. M.T.D. reports being a board or committee member AAOS Resident Assembly and an editorial board member for Arthroscopy. R.U.H. reports receiving personal fees from Stryker, Arthrex, Lippincott Williams & Wilkins, and Arthroscopy, being a board member for AANA and America Shoulder and Elbow Surgeons, receiving research support from AANA unrelated to the current work. A.J.S. reports receiving personal fees from Arthroscopy and research support from Embody Inc., unrelated to the current work. The authors have no other conflicts of interest to declare.

Ethical Statement: The authors are accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Open Access Statement: This is an Open Access article distributed in accordance with the Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International License (CC BY-NC-ND 4.0), which permits the non-commercial replication and distribution of the article with the strict proviso that no changes or edits are made and the original work is properly cited (including links to both the formal publication through the relevant DOI and the license). See: https://creativecommons.org/licenses/by-nc-nd/4.0/.

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