Evaluation of the accuracy and interobserver agreement of the reverse shoulder angle in the preoperative planning of reverse total shoulder arthroplasties

Abstract

Background

Differences in implant positioning between anatomical and reverse shoulder arthroplasties have raised concerns about the adequacy of assessing the global glenoid inclination (GGI) using the method described by Maurer to define the position of the metallic base in reverse shoulder arthroplasty. The reverse shoulder angle (RSA) has been proposed to measure the inclination of the lower half of the glenoid. This study aims to evaluate the interobserver agreement of manual measurements of the RSA using two-dimensional (2D) computed tomography (CT) images and its relationship with the automated measurement of the GGI.

Methods

This cross-sectional study evaluated 2D CT images of 38 CT scans of patients with degenerative shoulder diseases. Manual measurements of the RSA were conducted by five independent shoulder surgeons. GGI measured by automated software was determined.

Results

The interclass correlation coefficient was 0.72 for RSA. Mean RSA was 25.7° ± 7.1°, significantly higher than automated GGI measurements (11.2 ± 9.0; p < .0001). On average, RSA was 14.6 ± 6.3 greater than GGI, irrespective, diagnosis, Favard's glenoid subtype, and version angle.

Conclusion

Our study demonstrated that the RSA angle can be reproducibly used to assess glenoid inclination on 2D CT images presenting high interobserver agreement. RSA differs significantly from the GGI, indicating that measuring glenoid inclination for reverse or anatomical arthroplasty requires distinct methodologies that account for the inherent differences in these angles.

Introduction

In reverse total shoulder arthroplasties (RTSA), the preoperative evaluation of the glenoid version and inclination is essential for the implantation of the metallic base in a neutral position. ¹ Proper positioning of the implants directly impact outcomes, as several studies have shown that excessive retroversion and/or upward inclination predisposes patients to instability, loosening, or early failure.^2–7

The use of software programs to assist surgeons in the manual measurement of anatomical parameters, or the current automated platforms for preoperative planning, enables the identification of three-dimensional deformities and the necessary corrective actions. Although there is much debate about the best way to correct superior glenoid inclination, it is known the metallic base must be placed in neutral inclination.^8,9

Differences in implant positioning between anatomical and reverse shoulder arthroplasties led to the realization that the assessment of global glenoid inclination (GGI), using the method described by Maurer et al. ¹⁰ was appropriate for planning anatomic total shoulder arthroplasty (ATSA), as it evaluates the entire glenoid where the anatomical component must be implanted. ¹¹ Given this limitation, Boileau et al. (2019) described a new angle for measuring the inclination of the lower half of the glenoid, called the reverse shoulder angle (RSA), which corresponds to the area where the circular metal base must be implanted. ¹²

Since its description in 2019, few studies have evaluated the reproducibility of measuring this new angle and its relationship with the GGI. Therefore, the objective of this study was to evaluate the interobserver agreement of the RSA measured manually using two-dimensional (2D) computed tomography (CT) images compared to the GGI measured by an automated method. Besides, as has been shown that glenoid inclination may vary according to shoulder pathology, we evaluated if the severity of glenoid deformity or the pathology would affect the difference between GGI and RSA. ¹³

The hypothesis is that RSA and GGI values differ substantially and should be identified, particularly in cases where the circular metallic base must be positioned in a neutral inclination.

Materials and methods

The study was approved by the Institutional Research Ethics Committee (35243920.4.0000.5273).

Thirty-eight CT scans of patients with degenerative shoulder diseases were randomly selected from the exams performed on patients treated at the institution, from January 2015 to December 2019. All CT scans were performed using the same device, model Philips Brilliance^® (Philips, Amsterdam, Holland) with 64 channels, with the patient in the supine position. Patient's diagnosis was based on CT images evaluation.

The criteria for inclusion of the CT scans were a minimum slice thickness of 1 mm, inclusion of the entire scapula, absence of metal on the proximal end of the humerus or glenoid, and acceptance for segmentation by the selected automated planning program. Images of patients who had undergone previous shoulder surgeries and who had artifacts or other alterations on CT images that would make segmentation unfeasible using the selected automated program were excluded.

The glenoids were classified according to Sirveaux et al., who describe the patterns of glenoid morphology and erosion evaluated on CT images in the coronal plane. ¹⁴

Manual measurements were performed independently by five orthopedic surgeons specialized in shoulder surgery, with at least five years of practice, blinded to each other's results.

2D CT images, in coronal sections, were used for manual measurement of the RSA as described by Boileau et al. ¹² The Radiant^® Dicom Viewer program (Poznan, Poland) was utilized for this purpose. ¹⁵ The RSA was measured on 2D CT images in the oblique coronal plane at the mid-glenoid level, using the same program. The angle was formed by three lines: line (R) along the axis of the supraspinatus fossa floor, line (S) starting from the inferior border of the glenoid and extending to the intersection with line (R), and line (A) perpendicular to line (R). The angle between lines (S) and (A) represents the RSA ¹² (Figure 1).

Figure 1.

Reverse shoulder angle (RSA) measurement method. 2D tomography image with delimitation of line R on the axis of the floor of the supraspinatus fossa, line S, which extends from the inferior edge of the glenoid to line R and line A perpendicular to line R. In the image represented, the angle measured is 42°.

The GGI measurement was performed using the Blueprint^® automated measurement software (Stryker-Tornier Blueprint^TM, Memphis, USA). The program performs an automatic segmentation process, determining the scapula and glenoid planes. ¹⁶ Positive values of glenoid inclination and version angles indicated superior inclination and anteversion, while negative values indicated inferior inclination and retroversion, respectively.

Statistical analysis

The interclass correlation coefficient was calculated to determine variability in the measurement of the RSA among surgeons. For descriptive statistics, data were presented as mean, and standard deviation. Comparison between the different measures evaluated was performed using the Mann–Whitney test. The cases were categorized considering those with retroversion within the normal range (0 to −10°) and those with a retroversion > −10°, assessed using the automated method. All analyses were performed using MedCalc software (IBM, Armonk, NY, USA) and GraphPad Prism version 7.0.

Results

A total of 38 cases were included in the study, of which 17 had a diagnosis of cuff tear arthropathy (CTA) and 21 osteoarthritis.

The interclass correlation coefficient for the manual measurements of the RSA using 2D CT images was 0.72, 95% CI [0.463–0.857], indicating that there was significant agreement between the evaluators.

The mean RSA was significantly higher than GGI (25.7 ± 7.1 vs 11.2 ± 9.0, p < .0001). Individual values of RSA and GGI are presented in Figure 2.

Figure 2.

Comparison between global glenoid inclination (GGI) and reverse shoulder angle (RSA).

Cases were categorized based on whether glenoid retroversion values were within or outside the range considered acceptable in clinical practice. In both groups, retroversion ranged from 0° to −10°, and for those with retroversion greater than 10°, the mean RSA was significantly higher than the GGI (Table 1). Despite this difference, the mean difference between RSA and GGI in both groups was not statistically significant (RSA-GGI version 0–10°: 12.79 ± 10; RSA-GGI version >10°: 16.8 ± 5.1, p = .29).

Table 1.

Comparison between manual measurements of reverse shoulder angle (RSA) and automated measurement of global glenoid inclination (GGI) in patients categorized according to retroversion. Mann-Whitney test. Data presented as mean ± standard deviation.

	GGI	RSA	p-value
Version 0–10° (n = 21)	11.48 ± 11.46	24.27 ± 7.78	.0001
Version > 10° (n = 17)	10.82 ± 5.05	27.62 ± 6.03	.0001

The cases were categorized according to the glenoid morphology according to Sirveaux and Favard classification. Most of the samples evaluated in the study had type E0 glenoid, 20 cases, or type E1, 15 cases. The remaining three cases had type E3 glenoid morphology and were excluded from this analysis due to the small sample size. Once again, we observed that RSA measurements resulted in significantly higher values than GGI in both groups (p < .0001) (Table 2), but the mean difference between RSA and GGI was similar between groups (RSA-GGI EO: 15.9 ± 6.3; RSA-GGI E1: 12.4 ± 10.9, p = .15).

Table 2.

Measurement of global glenoid inclination (GGI) and reverse shoulder angle (RSA) considering glenoid types according to Favard's classification. Mann–Whitney test. Data presented as mean ± standard deviation.

	GGI	RSA	p-value
EO (n = 20)	8.2 ± 4.6	24.11 ± 6.3	.0001
E1 (n = 15)	13.07 ± 11.98	25.52 ± 6.3	.0001

Finally, the cases were grouped according to the diagnosis of CTA or osteoarthritis. The RSA and GGI values obtained in each group are shown in Figure 3. Although the RSA was significantly higher than GGI in both groups, the mean difference between the RSA and GGI was not significant in the two subgroups evaluated (RSA-GGI CTA: 12.7 ± 9.9; RSA-GGI osteoarthritis: 16.4 ± 6.9, p = .20).

Figure 3.

Comparison between global glenoid inclination (GGI) and reverse shoulder angle (RSA) in patients diagnosed with (a) cuff tear arthropathy or (b) osteoarthritis.

Discussion

Recently, there has been a significant increase in the incidence of reverse shoulder arthroplasties. The technical aspects to be considered for the correct positioning of the prosthetic components in this type of implant are different from those used in ATSA. Here we have evaluated the discrepancies between measurements of GGI and RSA, a novel angle proposed by Boileau and found that RSA result in higher values than GGI disregard to patient's disease, and glenoid characteristics (Favard's classification and degrees of retroversion).

The understanding of the differences between RSA and GGI is important due to the intrinsic differences between ATSA and RTSA. In the former, the glenoid component occupies the entire length of the glenoid, whereas in the latter, the circular metal base, used in most cases, is positioned in the lower half of the glenoid.

Particularly regarding the implantation of this type of metal base, Boileau et al. ¹² found that the inclination of the lower half of the glenoid, where the circumferential metal base must be placed, is on average 10° higher than the GGI of the fossa. Therefore, the authors concluded that the GGI, traditionally used in ATSA, does not accurately reflect the inclination of the glenoid, necessary to implant this type of metal base in neutral inclination. ¹²

In our study, the average RSA value, measured manually using 2D CT images, was 25.7° ± 7.1°, which is similar to the value found by Boileau in his original article using CT images, reported as 20° ± 6°. ¹² The interobserver agreement was 0.72, a value comparable to that reported by Boileau in the article describing the RSA (radiograph ICC: 0.75 [0.72–0.77]), indicating a moderate correlation between observers.

Our results show that the manual measurement of RSA yields values approximately 14.6° ± 6.3° higher than the automated GGI. This difference exceeds the 10° ± 5° reported by Boileau et al. Possible explanations for this discrepancy include the severity of deformities in the cases studied or variations in patient diagnoses. ¹²

Remarkably, the differences between RSA and GGI were maintained even when our samples were categorized according to retroversion values, the type of glenoid (Sirveaux–Favard classification), or the diagnosis (CTA or osteoarthritis). It is important to emphasize that the RSA angle determines the inclination of the lower half of the glenoid, and its clinical applicability may not be representative for some arthroplasty systems. Before adopting the RSA over the GGI, surgeons need to understand the relationship between implant design, implant placement, and options for correcting the glenoid deformity (Figure 4).

Figure 4.

Schematic figure showing the differences in baseplate positioning considering inclination measured through the β angle or reverse shoulder angle.

Under RSA guidance, most circumferential metal bases positioned in the lower half of the glenoid would require a bone graft or metal base with augmentation to correct the superior inclination. ¹⁷ It should be noted that, although this principle is true for circular metallic bases with flat and curved backs, this does not necessarily occur in metallic bases with other shapes.

On the other hand, the literature states that each implant has benefits depending on the glenoid's morphology. Implants with a curved back conform to the natural curvature of the glenoid and require less bone removal as they do not convert a curved surface into a flat one. In addition, some studies show that curved implants offer greater contact on the bone surface due to a better distribution in biconcave glenoids (Walch type B2) as well as removing a smaller amount of bone in concentric glenoids (Walch type A2).^18,19

Corrections of deformities by traditional reaming cause excessive medialization of the glenoid, and its well-known consequences such as a smaller free angle of movement and less tension in the remaining cuff muscles. ⁹ Thus, currently, corrections of severe deformities, identified by automated planning software or with measures such as the RSA, are surgical options that should be used to correct the superior inclination of the lower half of the glenoid and obtain the recommended neutral inclination of the metallic base without medialization. In this scenario, there would be a need to use a metallic base with an augment^20,21 or a bone graft, asymmetrical, inclined inferiorly, as described in the Bony Increased Offset Reversed Shoulder Arthroplasty™.^22,23

Werthel et al. demonstrated that the RSA angle is a reproducible measure of the tilt of the inferior glenoid. This angle is reliable in most cases for circular glenoid bases of 24–25 mm in diameter. However, for bases of different sizes or for patients of short or large stature, the RSA angle may amplify the upper inclination resulting in an overestimation of the necessary correction. In these cases, surgeons must be aware that the relative RSA is adapted to the selected size of the metal base and the characteristics of the patients. ²⁴

Recently, Roache et al. described a modification of the RSA which they called modified RSA (mRSA). This angle would make the inclination measurement specific to the patient and the selected implant, establishing the relationship between the dimensions of the glenoid fossa versus the size of the selected metallic base. In this way, he determined a ratio that he called the percentage of occupancy of the glenoid fossa. Therefore, the mRSA is a proposed modification to determine the glenoid inclination in relation to the dimensions of the selected metallic base, preventing the implant from being implanted in a superior inclination. Finally, the authors concluded that the measurement of the dimensions of the glenoid, the recognition of the dimensions of the metallic bases, and the evaluation of the glenoid inclination will allow for better positioning of the implants. ²⁵

This study has some limitations, including the small number of cases and the evaluation of only one automated software, meaning the results may not be generalizable to other commercially available software. A key strength of the study is that manual measurements were independently performed by five shoulder-specialized orthopedic surgeons, blinded to each other's results, allowing for the calculation of interobserver reliability. Additionally, the inclusion of patients with conditions other than CTA, such as osteoarthritis, is another strong point as it enabled the measurement of RSA in deformities associated with these diseases.

Conclusion

Our study demonstrated that the RSA angle can be reproducibly used to assess glenoid inclination on 2D CT images. RSA differs significantly from the GGI, indicating that measuring glenoid inclination for reverse or anatomical arthroplasty requires distinct methodologies that account for the inherent differences in these angles.