While the bony glenoid is superiorly inclined, the glenoid surface is not

Abstract

Background

The labral complex plays a crucial role in shaping the glenoid fossa morphology, thereby enhancing passive joint stability. This study aimed to investigate the influence of the labrum on glenoid inclination, a key determinant for load distribution within the joint. In addition, the labral influence on glenoid concavity depth and radius of curvature in the supero-inferior plane was evaluated.

Methods

Forty-three patients (mean age: 42 years [range 21-64 years]; 35 males, 8 females) with acromioclavicular (AC)-joint dislocation and no glenohumeral pathologies, who received a full series of magnetic resonance imaging or magnetic resonance arthrography, were retrospectively included. For each patient, the glenoid surface inclination, concavity depth, and radius of curvature was measured and compared to their respective values of the bony glenoid. In addition, the bony humeral head radius was measured to evaluate the influence of the labrum on joint congruency. Paired t-tests were used to assess the differences between bony and glenoid surface inclination and concavity depth. Repeated measures analysis of variance and pairwise comparisons were made to compare radius of curvature measurements. The correlation between the bony glenoid inclination and its difference to glenoid surface inclination, as well as between the bony glenoid radius and its difference to glenoid surface radius, was analyzed.

Results

The bony glenoid inclination measured 7.1° ( ± 4.1° standard deviation [SD]), the glenoid surface inclination 1.6° ( ± 3.2° SD); the bony glenoid concavity depth measured 4.0 mm ( ± 0.8 mm SD), the glenoid surface concavity depth 7.1 mm ( ± 0.9 mm SD). The bony glenoid radius of curvature measured 33.4 mm ( ± 3.3 mm SD), the glenoid surface radius 25 mm ( ± 2.1 mm SD) and the humeral head radius 24.1 mm ( ± 1.7 mm SD). The labrum significantly decreased the glenoid fossa inclination by 5.5° (P < .001), significantly decreased the glenoid fossa radius by 8.4 mm (P < .001) and significantly increased the concavity depth by 3.2 mm (P < .001). There was a positive correlation between the bony glenoid inclination and its respective difference to the glenoid surface inclination (r = 0.71, P < .001). Also, there was a positive correlation between bony glenoid radius of curvature and its respective difference to the glenoid surface radius (r = 0.77, P < .001).

Conclusion

The labral complex decreases glenoid inclination and increases joint concavity and congruency in the supero-inferior plane. Differences between bony and surface glenoid measures were higher in individuals with increased superior bony inclination and larger bony radii, suggesting a compensatory role to the labrum for the underlying bony morphology.

The functional integrity of the highly mobile shoulder joint relies on the interplay between its passive and active stabilizers. Active stabilizers, including the rotator cuff muscles, exert compressive forces to centralize the humeral head, whereas passive stabilization is provided by the underlying bony morphology of the glenoid and the adjacent labral and capsuloligamentous complex, contributing to the joint's concavity and congruency.^1,6,43 The glenoid inclination is an important morphologic parameter, as it determines humeral head positioning and force load, thereby influencing the biomechanics of the glenohumeral joint.⁸ When the glenoid is tilted more upward (increased superior glenoid inclination), the force vector acting on the humeral head changes from a predominantly compressive load in neutral inclination to an increase in translational forces, favoring superior subluxation of the humeral head.^8,12,14,37 This increases the risk of superior shoulder instability and is associated with a higher likelihood of rotator cuff injuries.^8,11,20,29

Beyond its bony architecture, the effective contour of the glenoid fossa is shaped by surrounding soft tissue structures. The fibrocartilaginous labrum, as an extension of the bony glenoid, deepens the glenoid fossa and thus enhances articular surface area.¹⁸ In addition, the labrum has an important stabilizing function as it strengthens concavity-compression and acts as a passive constraint, which limits translational movements of the humeral head.^17,18,31

Although the labral complex has a relevant influence on the shape and stability of the glenoid fossa, conventional inclination measurements only consider the bony glenoid.^7,32 In this study, we establish a new method for a comprehensive measurement of glenoid inclination, which takes into account both the bony glenoid and the adjacent labral complex, defined as glenoid surface inclination. We hypothesized that by incorporating the labral complex into inclination measurements, thereby evaluating the effective contour of the glenoid fossa, the measured inclination is decreased in comparison to the inclination measured on the bony glenoid alone. Also, we aimed to investigate whether the influence of the labral complex on inclination differs depending on the bony inclination values. In addition, the influence of the labral complex on concavity and joint congruency was analyzed by evaluating its effect on the supero-inferior radius of curvature and concavity depth of the glenoid fossa.

Materials and methods

Patient selection

From our institutional patient registry, we retrospectively identified all patients with acromioclavicular (AC)-joint dislocation, who underwent a complete series of magnetic resonance imaging (MRI) or magnetic resonance (MR) arthrography within 48 hours after presentation. In total, 61 patients treated in the period 2016-2024 were initially included. All patients with concomitant glenohumeral pathologies or insufficient image quality were excluded. All MRI/MR arthrography scans were independently reviewed by 2 examiners (M.M., A.S.). The degree of AC-joint dislocation was classified according to the Rockwood (RW) classification system.⁴⁰ RW type III injury was subclassified into horizontally stable type IIIa and unstable type IIIb according to the International Society of Arthroscopy, Knee Surgery and Orthopaedic Sports Medicine (ISAKOS) recommendation.⁵ Classification was determined in consensus between 2 examiners (M.M., A.S.).

The final sample consisted of 43 patients (35 males, 8 females) with normal glenohumeral joint conditions. The mean patient age was 42 years (range 21-64 years). In total, 18 right shoulders and 25 left shoulders were evaluated. In 39 patients, standard shoulder MRI protocols were performed, while 4 patients underwent MR arthrography. Three patients presented with RW type II injury, 5 with type IIIa, 17 with type IIIb, and 18 with type V. Ethical approval was obtained by the competent ethics committee (KEK-ZH-Nr. 2021-00675).

MRI-based measurements

Multiplanar image reformatting of all MRI and MR arthrography scans was performed to obtain measurements in a standardized coronal plane aligned with the supero-inferior longitudinal axis of the glenoid. Starting in coronal view, the line defining the axial cross-section was aligned parallel to the supraspinatus fossa floor passing through the center of the glenoid. This adjustment facilitated the alignment of the mediolateral scapular axis in axial views. Next, the glenoid plane was aligned in both coronal and axial views to ensure en-face visualization of the glenoid fossa on sagittal views. In the en-face view, a best-fitting circle was drawn in the lower glenoid to define its center. Finally, the sagittal scans were aligned along the longitudinal axis of the glenoid, passing through the center of the best-fitting circle, which ensured a standardized coronal imaging plane (Fig. 1).

Figure 1.

Multiplanar image reformatting was performed to obtain a standardized coronal plane aligned to the scapular axis and the longitudinal axis of the glenoid (yellow line on sagittal view). The defined coronal plane passes through the center of the best fitting circle of the lower glenoid. The glenoid plane was defined (red line) to enable an en-face view of the glenoid in sagittal scans.

For inclination measurements, the sagittal image stack was followed medially in the longitudinal axis of the glenoid until the vertical scapular axis became visible. The sagittal image was then aligned parallel to the scapular axis, and in the corresponding coronal image, a reference line was drawn following the supraspinatus fossa. After defining this reference, the longitudinal axis of the glenoid was realigned in sagittal views to recreate the standardized coronal measurement plane. Bony inclination was defined as the inclination between a line connecting the superior and inferior rim of the bony glenoid and the defined reference line of the supraspinatus fossa floor (Fig. 2A). To measure the glenoid surface inclination, the labral borders were defined as follows: the superior border was defined as the transition between the triangular shaped superior labrum¹⁰ and the long head of the biceps tendon (LHBT); the inferior border was defined by the external end of the inferior labrum. Consequently, glenoid surface inclination was determined between the line connecting the labral borders and the supraspinatus fossa floor reference line (Fig. 2B). Glenoid inclination was measured according to the β-angle method as described by Maurer et al.³² The inclination was defined by calculating 90° – β-angle, with values > 0° describing superior inclination and values < 0° inferior inclination.

Figure 2.

Measurements for (a) bony glenoid inclination, (b) glenoid surface inclination, (c) bony glenoid concavity depth, (d) glenoid surface concavity depth, (e) radius of the bony glenoid, (f) radius of the glenoid surface.

The concavity depth of the bony glenoid was defined by the perpendicular distance from the interconnecting line of the superior and inferior glenoid rim to the deepest point of the bony glenoid (Fig. 2C). For the glenoid surface concavity depth, the distance was measured from the line connecting the defined labral borders to the deepest point of the articular surface of the glenoid fossa (Fig. 2D).

To define the radius of curvature in coronal plane, a best fit circle was drawn and aligned to the glenoid fossa. To define the radius of the bony glenoid, the circle was applied to the superior and inferior glenoid rim and the surface of the bony glenoid (Fig. 2E). For radius measurement of the glenoid surface, the circle was aligned to the surface of the labral borders and the articular surface of the glenoid, following the soft tissue contour of the glenoid fossa (Fig. 2F). The radius of the humeral head was also measured. The humerus was aligned to its longitudinal axis in sagittal plane, passing through the top of the humeral head. For radius measurements, the best fit circle was then applied to the cortical borders of the humeral head in the adapted coronal plane. The humeral head cartilage was not included into measurements as a clear and consistent differentiation between cartilage and subchondral bone was often not possible. To ensure measurement reproducibility, solely the bony humeral head radius was evaluated.

All measurements were conducted by 2 independent examiners (M.M., A.K.) using the Picture Archiving and Communication System (PACS) imaging software JiveX (VISUS Health IT GmbH, Bochum, Germany). All MRI and MR arthrography examinations were performed using clinical scanners with a minimum field strength of 1.5 Tesla. Proton density-weighted turbo spin echo (PD-TSE) or fat-suppressed PD-TSE sequences with a minimum slice thickness of 2 mm were used for all measurements.

Statistical analysis

For all measurements, the intraclass correlation coefficient (ICC) for absolute agreement with 95% confidence interval (CI) was calculated. ICC was interpretated according to Cicchetti et al,⁹ with poor reliability for values < 0.4, fair reliability for values between 0.4 and 0.59, good reliability for values between 0.6 and 0.74 and excellent reliability for values ≥ 0.75. In addition, Bland-Altmann plots were used to analyze interrater agreement. After reliability testing, mean values of both examiners were used for further statistical analysis. Data were tested for normality using the Shapiro-Wilk test. Results of all measured parameters were presented as mean with standard deviation and range.

The difference between glenoid inclination measurements and concavity depth measurements between the bony glenoid and glenoid surface was assessed using a paired t-test. Pearson correlation analysis was performed between bony glenoid inclination and its corresponding difference to the glenoid surface inclination. Radius of curvature measurements (humerus, bony glenoid, glenoid surface) were compared by means of repeated measures analysis of variance. Pairwise comparisons were made; for multiple comparisons, a Bonferroni correction was applied. Pearson correlation analysis was performed between the bony radius of curvature and its corresponding difference to glenoid surface radius of curvature. For all parameters, the influence of patient age was analyzed using correlation analysis. Data between male and female patients were compared using Mann-Whitney U test. A P value <.05 was considered statistically significant. Statistical analysis was performed using the packages NumPy, Matplotlib, Pingouin, SciPy, and Statsmodels in Python (version 3.13.0; Python Software Foundation, Wilmington, DE, USA).

Results

For all measured parameters, the calculated ICC values indicate excellent interrater reliability according to Cicchetti et al.⁹ The exact ICC values and the results of the Bland-Altmann analysis are presented in Table I. In our sample, patient age did not significantly influence any of the measured parameters (all P values >0.05). The mean values for each parameter are shown in Table II. In male patients, the radii of the humerus (24.6 ± 1.3 vs 21.7 ± 0.9, P < .001) and glenoid surface (25.6 ± 1.9 vs 22.6 ± 1.2, P < .001) as well as the concavity depth for both the bony glenoid (4.1 ± 0.7 vs 3.3 ± 0.9, P = .014) and glenoid surface (7.4 ± 0.7 vs 6.0 ± 0.8, P < .001), were higher compared to female patients.

Table I.

Interclass correlation coefficients for each parameter are presented with 95% confidence interval, along with the results of the Bland-Altmann analysis for interrater agreement.

	ICC [95% CI]	Mean interrater	95% limits of agreement of difference
		Difference ± SD
Glenoid inclination (°)
Bony inclination	0.87 [0.75, 0.93]	−0.77 ± 2.10	(−4.88, 3.33)
Surface inclination	0.76 [0.53, 0.88]	−1.12 ± 2.06	(−5.16, 2.93)
Concavity depth (mm)
Bony glenoid	0.93 [0.87, 0.96]	0.06 ± 0.30	(−0.53, 0.66)
Glenoid surface	0.80 [0.60, 0.90]	0.31 ± 0.53	(−0.73, 1.35)
Radius of curvature (mm)
Bony glenoid	0.88 [0.79, 0.93]	0.34 ± 1.63	(−2.85, 3.53)
Glenoid surface	0.86 [0.75, 0.92]	0.33 ± 1.13	(−1.89, 2.56)
Humeral head	0.91 [0.83, 0.95]	0.24 ± 0.69	(−1.11, 1.59)

SD, standard deviation; CI, confidence interval; ICC, intraclass correlation coefficient.

Table II.

Mean values for each measurement parameter are presented with standard deviation and range.

	Mean ± SD (range)	P value
Glenoid inclination (°)
Bony inclination	7.1 ± 4.1 (0.1-16.6)	<.001
Surface inclination	1.6 ± 3.2 (−3.6-9.8)
Concavity depth (mm)
Bony glenoid	4.0 ± 0.8 (2.0-5.6)	<.001
Glenoid surface	7.1 ± 0.9 (5.2-9.1)
Radius of curvature (mm)
Bony glenoid	33.4 ± 3.3 (26.2-41.7)	<.001
Glenoid surface	25.0 ± 2.1 (20.5-30.6)
Humeral head	24.1 ± 1.7 (20.3-27.9)

SD, standard deviation.

The corresponding P values indicate significant differences between all pairwise comparisons.

By incorporating the labral complex into inclination measurements, significantly lower glenoid surface inclination values were observed in comparison to conventional, bony glenoid inclination, with a mean difference of 5.5° (P < .001). The mean value for glenoid surface inclination was 1.6° +/− 3.2° standard deviation and for bony glenoid inclination 7.1° +/− 4.1° standard deviation. Superior bony glenoid inclination showed a positive correlation with its corresponding difference to glenoid surface inclination (r = 0.71, P < .001) (Fig. 3).

Figure 3.

In this figure, the Pearson correlation between bony glenoid inclination and its angle difference to glenoid surface inclination is depicted. An increase in superior bony glenoid inclination is associated with an increase in difference between bony and surface inclination angles.

The labral complex increased the concavity depth perpendicular to the supero-inferior axis of the glenoid fossa by a mean of 3.2 mm (P < .001). Results of repeated measures analysis of variance and pairwise comparisons showed a significant difference between all radius of curvature measurements (P values <0.001). Radius of curvature was greatest for the bony glenoid with a mean difference of 9.3 mm to the humeral head radius (P < .001). The labral complex reduced the radius of the bony glenoid by a mean of 8.4 mm (P < .001). The bony glenoid radius of curvature showed a positive correlation with its corresponding difference to the glenoid surface radius of curvature (r = 0.77, P < .001) (Fig. 4).

Figure 4.

This figure shows the Pearson correlation between bony glenoid radius of curvature and its radius difference to the glenoid surface radius. Similarly to bony inclination, an increase in bony glenoid radius of curvature is associated with an increase in difference between bony and surface radius of curvature.

Discussion

In the shoulder joint, active muscular compression of the humeral head against the glenoid fossa plays a key role in preserving joint stability.^27,31 This mechanism, termed concavity-compression, is significantly influenced by the morphology of the glenoid. In this study, we investigated the influence of the labral complex on glenoid fossa shape measurements compared to the bony geometry of the glenoid. The main findings of this study indicate that glenoid inclination, radius of curvature, and glenoid depth are significantly influenced by inclusion of the labral complex when compared to the bony glenoid morphology.

Including the labral complex into glenoid inclination measurements significantly decreased the inclination angle of the glenoid fossa when compared to the inclination of the bony glenoid. The influence of the labrum on glenoid version has already been investigated, indicating that the labral complex increases the retroversion of the bony glenoid.³ Interestingly, in contrast to its effect on version, we observed the opposite trend for inclination. The effectiveness of the concavity-compression mechanism is influenced by glenoid inclination, as it affects load distribution within the joint. In superiorly inclined glenoids, the net force vector shifts towards a more tangential direction relative to the joint surface, reducing joint compression and increasing translational loads on the humeral head.^8,14 To compensate for this effect, increased rotator cuff activity is required to maintain the humeral head centered.³³ In less superior inclination, in contrast, the compressive force component increases and with inferior inclination, the overall forces acting on the glenoid are reduced.^8,24 Therefore, the labral complex might contribute to optimizing load distribution within the joint by reducing glenoid inclination, potentially enhancing a stable joint configuration.

The transition between the LHBT and the superior labrum was defined as the superior border of the glenoid surface as both structures are deeply interconnected with fibers from the tendon merging with the labral complex.^4,19,42 According to Pagnani et al,^.38 this linkage could explain the findings of their cadaveric study, in which a simulated contraction of the LHBT resulted in decreased humeral head translations in both antero-posterior and supero-inferior directions. Due to the interconnecting fibers, tensile forces from the biceps tendon are transmitted to the labrum,²¹ which could constrain humeral head movement by tightening the labroligamentous complex.³⁸ Also, the stabilizing function of the superior labrum anterior to posterior complex relies on the integrity of the LHBT, emphasizing the necessity of considering these structures as a functional unit.³⁹ Although the role of the LHBT as an active humeral head depressor remains highly debated,^16,23,30,41 the tendon together with the labrum might contribute to passive stability against cranial humeral translations by prominently expanding the superior bony glenoid, thereby acting as a superior constraint.^15,26 This is important, as superior humerus translations increase the pressure load in the superior labrum anterior to posterior region, particularly at the biceps origin.²¹

The labral complex increased the concavity depth and reduced the radius of curvature of the glenoid fossa. Consequently, we can conclude an increase in overall glenoid concavity in the supero-inferior plane. The concavity is decisive for passive stability, as it influences the ability of the glenoid to stabilize decentralizing translational forces acting on the humeral head.^13,34 This concept is supported by previous studies demonstrating that an increased glenoid concavity depth by the labral complex contributes to joint stability as concavity-compression is improved and translational humeral head movements are limited through passive constraint.^17,18,28,31 In addition to increasing concavity, the labrum also contributed to greater joint congruency. The measured radii of the bony glenoid and the humerus are in agreement with findings of Zumstein et al.⁴⁵ In their study, it was demonstrated that the glenohumeral joint congruency is significantly enhanced by the presence of articular cartilage, which reduced the glenoid's radius and slightly increased the humeral head radius, leading to a highly congruent joint configuration in the supero-inferior plane.⁴⁵ Building upon these findings, we observed an even greater reduction in the glenoid's radius when additionally incorporating the labral complex into our measurements, resulting in more congruency between humeral and glenoid joint surfaces.

The observed influence of the labral complex on glenoid fossa morphology may also depend on the underlying bony morphology. We found a greater difference between bony glenoid and glenoid surface inclination, as well as between bony glenoid and glenoid surface radius in individuals with increased superior bony inclination and larger bony glenoid radii, respectively (Figs. 3 and 4). This suggests that in cases of higher superior glenoid inclination, which may negatively affect shoulder biomechanics, the labrum could function as a soft tissue compensator, optimizing the functional glenoid stability. This effect could be explained by the strong anatomical connection between the superior labrum and the LHBT. Increased tensioning of the LHBT has been shown to pull the labrum toward the humeral head,^2,21 potentially increasing its passive constraint function as a stabilizing mechanism. For instance, Kido et al observed increased LHBT activity in patients with rotator cuff tears and attributed a possible supplementary role to the tendon in recentralizing the humeral head to compensate for the deficient rotator cuff.²² Similarly, in individuals with larger radii of the bony glenoid, the labrum may improve joint congruency by conforming to the humeral head, thereby increasing articular contact area.

An even more extreme difference between bony and cartilaginous morphology can be seen when investigating early development of the glenoid. In infancy, the shape of the glenoid fossa is predominantly determined by cartilage.³⁶ While the underlying glenoid bone plate is convex, the adjacent cartilage forms a concave glenoid articular surface, conforming to the humeral head.^36,44 Interestingly, the cartilaginous contour of the early glenoid resembles the mature, fully developed bony glenoid.^25,36 This phenomenon can also be observed in later stages of childhood, where the bony glenoid is flat-shaped, but the cartilage provides concavity of the articular surface^25,36 (Fig. 5). Our findings show that, even after the bony glenoid is fully developed, the labrum and cartilage significantly shape the morphology of the glenoid fossa. This underscores the importance of considering both the bony anatomy and adjacent soft tissue structures when evaluating the true shape of the glenoid. Further studies are needed to explore the biomechanical implications of how soft tissue structures may compensate for variations in bony morphology of the shoulder joint.

Figure 5.

This image shows the X-ray and MRI scan of the shoulder of a 10-year-old individual. While the underlying bony glenoid is flat-shaped with pronounced superior inclination, the cartilage forms a concave articular surface of the glenoid fossa and neutralizes the inclination angle together with the superior labral complex. MRI, magnetic resonance imaging.

Our findings may also carry clinical relevance. Given the significant contribution of the superior labral complex to glenoid inclination, preserving this structure could be important not only in procedures such as anatomical arthroplasties but also in the context of rotator cuff repairs, where it may function as an upper stabilizer of the glenohumeral joint. Maintaining the integrity of the labro-bicipital complex may therefore optimize joint biomechanics and stability. Further studies are required to evaluate this hypothesis.

The study has several limitations. Variations in the multiplanar image reformatting process could affect the results by altering the standardized coronal image plane selected for the measurements. In addition, the transition between the superior labrum and the LHBT was more challenging to define compared to the distinct bony borders. This explains the lower reliability values for the glenoid surface inclination and surface depth measurements. However, all measurement parameters showed excellent ICC values according to the definition Cicchetti et al.⁹ The ICC is commonly reported in clinical studies to determine rater agreement, which is why we report it here. The 95% CI of the ICC should also be reported, which is not often the case in clinical studies; considering the lower 95% CI of the glenoid surface inclination results in only a “fair” inter-rater agreement. However, the ICC is sensitive to the range of true variation observed in the sample, since the ICC measures the between-rater variance compared to the total variance. In our study, the range of measured angles is low; therefore, this lowers the ICC compared to studies with a high range of angular measurements. A Bland-Altmann analysis was additionally performed to evaluate interrater agreement. Although the mean examiner difference was very low, we did observe large 95% limits of agreement relative to the effect size of the difference between the bony glenoid and the glenoid surface inclination. However, the mean of both raters still showed a significant effect of difference in inclination angles, suggesting that our results hold true regardless of raters. When analyzing the data of each rater separately, the effect of the labral complex on glenoid inclination was also very similar, ensuring that outliers in rater agreement did not affect the results of this study. The retrospective design is another limitation. We believe that the measurements obtained from MR arthrography scans did not differ from those of standard MRI scans. As only 4 cases were included in our sample, we do not expect this to have influenced the overall study results. Nevertheless, this aspect is acknowledged as a limitation. Finally, female patients are underrepresented in our study sample as all patients included were selected out of a patient registry for AC-joint dislocations, which predominantly affect males.³⁵

Conclusion

Incorporating the labral complex into inclination measurements decreases the inclination angle of the glenoid fossa when compared to bony glenoid inclination. The labral complex increases concavity depth and decreases the glenoid's radius of curvature, thereby enhancing glenoid concavity and joint congruency in the supero-inferior plane. Our findings suggest that the influence of the labral complex on the glenoid fossa may depend on the underlying bony glenoid morphology. We observed a greater difference between bony glenoid and glenoid surface inclination, as well as between bony glenoid and glenoid surface radius, in individuals with increased superior bony inclination and larger bony glenoid radii, respectively. This could indicate a compensatory role of the labral complex in supporting the underlying bony morphology.

Disclaimers:

Funding: No funding was disclosed by the authors.

Conflicts of interest: Markus Scheibel is a consultant for Stryker. Philipp Moroder serves as a consultant and receives royalties from Arthrex, Medacta, and Alyve Medical. Both the authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of this research. The other authors, their immediate families, and any research foundation with which they are affiliated have not received any financial payments or other benefits from any commercial entity related to the subject of this article.