Analysis of osseous shoulder morphology and muscle volume in concentric and eccentric osteoarthritis of the shoulder

Abstract

Background

The etiology of primary eccentric osteoarthritis (OA) is multifactorial involving glenoid shape alterations, acromion abnormalities, and rotator cuff pathologies. However, none of the changes described for eccentric OA are either consistent or do satisfactorily explain the condition. Up to now, potential individual risk factors contributing to the development of concentric or eccentric OA have been studied mostly independently of each other. This study examined the differences of osseous shoulder morphology and muscle volume in concentric and eccentric OA of the shoulder as a potential risk factor for the development of posterior glenoid wear.

Methods

A retrospective, comparative study was conducted, analyzing computed tomography scans of 114 shoulders in 86 patients with primary OA at a single center between 2010 and 2023. These patients were divided into 2 groups—according to an underlying concentric or eccentric OA. As parameters, the osseous shoulder morphology (glenoid offset, glenoid version, posterior humeral head subluxation, anterior acromial coverage, posterior acromial coverage, posterior acromial tilt, posterior acromial height, and critical shoulder angle (CSA)) and muscle volume (subscapularis, infraspinatus/teres minor, supraspinatus), were measured and compared between the groups. Computed tomography images were classified according to the modified Walch classification.

Results

The mean age of the patients was 68.9 ± 9.9 years and 62.3% of the patients were female (54 of 86). A total of 25 shoulders were included in the concentric group and 89 shoulders in the eccentric group. Patients with eccentric OA had a significantly increased glenoid retroversion according to Friedmann (12.6° ± 8.2° vs. 4.3° ± 3.4°; P < .001) and relative to scapular blade axis (10.6° ± 7.6° vs. 3.1° ± 3.6°; P < .001), increased scapulohumeral subluxation index (0.67 ± 0.01 vs. 0.55 ± 0.05; P < 001), increased glenohumeral subluxation index (0.56 ± 0.06 vs. 0.52 ± 0.05; P = .004), and increased CSA (26.3° ± 5.0° vs. 23.1° ± 4.2°; P = .006) compared to patients with concentric OA. No significant differences in anterior glenoid offset and other parameters of acromial roof morphology were found between the 2 experimental groups. No significant differences in volumes of supraspinatus, subscapularis and infraspinatus/teres minor muscles could be detected between the 2 experimental groups.

Conclusion

Patients with primary eccentric OA show significant differences in glenoid retroversion, posterior scapulohumeral/glenohumeral subluxation, and CSA. However, there are no significant differences regarding the acromion roof morphology and rotator cuff volume compared to patients with concentric OA.

Among the many reasons for osteoarthritis (OA) development of the shoulder joint, mechanical factors such as individual variations of osseous shoulder morphology have been investigated lately; however, resulting in an unclear relation to the OA development.^7,40 In some of the common OA cases, the humeral head stays subluxated in relation to the glenoid articular surface, resulting in eccentric OA.^4,5,39 The underlying mechanisms of this static posterior humeral head subluxation and the etiology of consequent posterior glenoid wear in primary eccentric OA remain unknown.^7,14,42

A multifactorial etiology is suggested involving glenoid shape alterations, acromion abnormalities, and rotator cuff pathologies, but such changes are neither consistent, nor do they satisfactorily explain the condition.^6,7,14,17 Increased glenoid retroversion and increased anterior glenoid offset were identified as potential causes of static humeral subluxation.^{2,14,17,20,34} A flat acromial roof with less posterior bony coverage of the humeral head has been observed, which is strongly associated with posterior instability.^9,19,26 However, these glenoidal and acromial alterations associated with posterior instability have not been proven to be risk factors of long-term primary OA.^7,27 Interestingly, studies indicate that in patients with eccentric OA and an intact rotator cuff, the posterior part of the rotator cuff tends to have a larger area than the anterior part, suggesting possible structural adaptations in response to eccentric OA.^3,30 Additionally, other anatomical characteristics including anterior acromion morphology, lateral acromial angle, coracohumeral interval and glenoid inclination have been proposed as contributing factors in the development of rotator cuff tears.^{10,13,21,23,24,33,34} Considering the importance of eccentric OA, a comprehensive understanding of various factors contributing to posterior humeral head subluxation is crucial for achieving optimal outcomes in shoulder arthroplasty.¹⁵,¹⁷,⁴¹

To date, individual risk factors contributing to the development of concentric or eccentric OA have mostly been studied separately from each other. The aim of the present study was therefore to analyze the differences in bony shoulder morphology and muscle volume in concentric and eccentric OA. The hypothesis was that shoulders with eccentric OA would have flatter acromial orientation in the sagittal plane, less posterior coverage of the humeral head, and more posterior rotator cuff volume than the anterior rotator cuff, hypothetically causing decentralization of the humeral head.

Materials and Methods

Study design and cohort

In the present retrospective study, all available computed tomography (CT) scans of patients with primary shoulder OA who required anatomic or reverse total shoulder arthroplasty at our institution between 2010 and 2023 were included. Further inclusion criteria were supine position during CT imaging with arms at the side and elbows resting on the examination table with complete depiction of scapular blade in the axial images. Exclusion criteria included diagnosed concomitant rotator cuff tears and previous ipsilateral shoulder injuries. The study protocol was reviewed and approved by the institutional ethics committee before commencement (EA1/057/24). From 655 patients with primary shoulder OA, 452 patients were excluded because no CT images were available. Of the remaining 203 patients, 117 patients were excluded due to other previous ipsilateral shoulder injuries, no complete imaging of the scapula blade in the axial images or having Walch Type C/D glenoids. So, the study cohort consisted of 114 shoulders in 86 patients.

Walch classification

The image sets of all patients were classified according to the modified Walch classification. Patients with centric glenoid erosion (equal wear of the anterior and posterior glenoid rim) were classified as Walch A. Type A1 showed minor central wear or erosion, whereas Type A2 had severe or major central wear or erosion. The Type B glenoid is characterized by posterior humeral head subluxation with asymmetric wear. In contrast to the B1 subgroup in which there is posterior joint space narrowing, subchondral sclerosis and osteophytes without erosion, the B2 subgroup is found to have a posterior glenoid wear pattern, giving a characteristic biconcave glenoid appearance. Severe posterior glenoid erosion with or without anterior glenoid rim involvement and a pathologic glenoid retroversion of more than 15° was classified as B3. A glenoid exhibiting minor erosion, a clearly visible anterior and posterior rim, and excessive retroversion exceeding 25° was categorized as dysplastic (Walch C). In contrast, a glenoid with anterior humeral head subluxation of less than 45°, anteversion greater than 10°, or both was classified as Walch D.⁷

According to the modified Walch classification, the cohort is divided into 2 experimental groups, patients with concentric OA including all Walch A (A1 and A2) and patients with eccentric OA including all Walch B (B1-3) (Table I). As mentioned above, C and D type glenoids were excluded due to low number of type C glenoids and the focus of the present study on investigating concentric and eccentric OA.

Table I.

Demographic data with distribution according to modified Walch classification.

Variable	Concentric OA	Eccentric OA	P value
Shoulders	25	89	-
Age (yr, range)	70.5 (52-84)	68.9 (31-85)	.620
Height (cm, range)	164.1 (150-189)	168.9 (149-192)	.023
Weight (kg, range)	75.3 (55-117)	82.9 (52-150)	.085
BMI (kg/m2)	28.2 (19.7-49.3)	29.1 (18.9-49.3)	.796
Side			.122
Right	17	45	-
Left	8	44	-
Sex			.348
Women	18	55	-
Men	7	34	-
Modified Walch Classification			-
A1	15	-	-
A2	10	-	-
B1	-	40	-
B2	-	38	-
B3	-	11	-

BMI, body mass index; OA, osteoarthritis.

Data are presented as number or as mean (range).

Bold P values are statistically significant (P < .05).

Computed tomography measurements

All measurements were conducted with Visage radiological software (version 7.1; Visage Imaging GmbH, Berlin, Germany).

Osseous shoulder morphology

Glenoid offset according to the method demonstrated by Akgün et al,² glenoid version according to the method depicted by Friedman et al¹⁶ and relative to scapular blade axis as described by Hoenecke et al,²⁰ posterior humeral head subluxation using the scapulohumeral and glenohumeral subluxation indexes as described by Walch et al according to the glenoid plane at the level of the middle of the glenoid³⁹ were measured on a 2D standardized axial imaging plane using a multiplanar reconstruction as previously described¹ (Fig. 1).

Figure 1.

Measurement of the glenoid offset (a), glenoid version relative to the scapular blade axis (b); glenoid version relative to the Friedman-line (c); scapulohumeral subluxation index (d); glenohumeral subluxation index (e).

Measurements of acromial morphology included anterior acromial coverage, posterior acromial coverage, posterior acromial tilt, and posterior acromial height, according to Meyer et al,²³ and critical shoulder angle (CSA).²⁶ According to Akgün et al¹ the measurements on 3D models of the scapula from a true sagittal view were conducted, where the coracoid process and scapular spine appeared as symmetric upper limbs of a “Y,” in contrast to Meyer et al, who conducted the measurements on a true lateral radiograph (Fig. 2).

Figure 2.

Measurements of the acromial morphology on 3-dimensional computed tomography models arccoding to Akgün et al.¹ (a) The anterior acromial coverage is defined as the angle between a reference line connecting the inferior margin of the scapula with the center of the intersection of the “Y" arms and a line from this intersection to the most anterior point on the inferior aspect of the acromion (blue angle). (b) The posterior acromial coverage is the angle between the same reference line and a line from the intersection of the “Y" arms to the most posterior point on the inferior aspect of the acromion (red angle). (c) The posterior acromial tilt is measured as the angle between the reference line and a line connecting the most posterior to the most anterior point on the inferior aspect of the acromion (orange angle). (d) The posterior acromial height is the distance from the center of the “Y" intersection to a perpendicular line that connects the reference line with the most posterior point on the inferior aspect of the acromion. (e) The critical shoulder angle is the angle between a line connecting the inferior and superior borders of the glenoid fossa and a line connecting the inferior border of the glenoid fossa to the most inferolateral point of the acromion. AAC, anterior acromial coverage; PAC, posterior acromial coverage; PAT, posterior acromial tilt; PAH, posterior acromial height; CSA, critical shoulder angle.

Rotator cuff volume

Volumes of the following muscles were measured in the CT: subscapularis, infraspinatus/teres minor, and supraspinatus. As already depicted by Akgün et al¹ and Mitterer et al,²⁹ muscle contours were manually marked on every transverse slice for each measured muscle and the muscle volume was calculated automatically by the software (Fig. 3).

Figure 3.

Marking of rotor cuff muscles on transverse slices (a and b). M. subscapularis (red), M. infraspinatus/teres minor (purple), M. supraspinatus (orange).

Statistical analysis

Descriptive analysis of variables involved calculating mean, median, standard deviation, median, absolute and percentage frequency. The Kolmogorov–Smirnov test was used to analyze for normal distribution. The 2-sample independent t-test (for parametric distribution) or Mann–Whitney U test (for nonparametric distribution) was utilized to compare continuous variables between groups. IBM SPSS Statistics 29.0 software (IBM, Armonk, NY, USA) was employed for this analysis. A P value < .05 was considered significant.

Results

The mean age of the cohort was 68.9 ± 9.9 years and 62.3% (54 of 86) of the patients were female. A total of 25 shoulders were included in the concentric group and 89 shoulders in the eccentric group. The only significant difference between the groups was the height of the patients, with the eccentric group being significantly taller (168.9 cm vs. 164.1 cm; P = .023). All other data showed no significant difference between the 2 experimental groups of patients with centric and eccentric OA (Table I).

Osseous shoulder morphology

The results of the measurements and comparisons between groups are summarized in Table II. Patients with eccentric OA had a significantly increased glenoid retroversion according to Friedmann (12.6° ± 8.2° vs. 4.3° ± 3.4°; P < .001) and relative to scapular blade axis (10.6° ± 7.6° vs. 3.1° ± 3.6°; P < .001), increased scapulohumeral subluxation index (0.67 ± 0.01 vs. 0.55 ± 0.05; P < 001), increased glenohumeral subluxation index (0.56 ± 0.06 vs. 0.52 ± 0.05; P = .004), and increased CSA (26.3° ± 5.0° vs. 23.1° ± 4.2°; P = .006) compared to patients with concentric OA. No significant differences in anterior glenoid offset and other parameters of acromial roof morphology were found between the 2 experimental groups.

Table II.

Osseous shoulder morphology measurement comparison of the patients with concentric and eccentric osteoarthritis.

Variable	Concentric OA (n = 25)	Eccentric OA (n = 89)	P value
Glenoid retroversion according to Friedmann (°)	4.3 ± 3.4	12.6 ± 8.2	<.001
Glenoid retroversion blade axis (°)	3.1 ± 3.6	10.6 ± 7.6	<.001
Scapulohumeral subluxation index (points)	0.55 ± 0.05	0.67 ± 0.1	<.001
Glenohumeral subluxation index (points)	0.52 ± 0.05	0.56 ± 0.06	.004
Anterior glenoid offset (mm)	4.1 ± 2.4	4.7 ± 3.2	.362
AAC (°)	2.8 ± 7.5	3.5 ± 8.5	.800
PAC (°)	57.0 ± 8.1	56.4 ± 9.1	.940
PAT (°)	67.5 ± 7.2	67.1 ± 10.7	.795
PAH (mm)	23.5 ± 5.7	26.3 ± 14.1	.598
CSA (°)	23.1 ± 4.2	26.3 ± 5.0	.006

OA, osteoarthritis; AAC, anterior acromial coverage; PAC, posterior acromial coverage; PAT, posterior acromial tilt; PAH, posterior acromial height; CSA, critical shoulder angle.

Data are presented as number or as mean (range).

Bold P values are statistically significant (P < .05).

Rotator cuff volume

The results of the measurements and comparisons between groups are summarized in Table III. No significant differences in volumes of subscapularis, infraspinatus/teres minor, and supraspinatus muscles were found between the 2 experimental groups.

Table III.

Muscles of the shoulder girdle measurement comparison of the patients with concentric and eccentric osteoarthritis.

Variable	Concentric OA (n = 25)	Eccentric OA (n = 89)	P value
Subscapularis (cm3)	111.8 ± 28.2	115.6 ± 47.9	.619
Infraspinatus/teres minor (cm3)	115.5 ± 29.4	124.6 ± 51.8	.263
Supraspinatus (cm3)	38.1 ± 8.7	40.9 ± 16.7	.261

OA, osteoarthritis.

Data are presented as number or as mean (range).

Discussion

In addition to the current literature, this study showed some significant differences in osseous shoulder morphology, however there seems to be no differences in acromion roof morphology and rotator cuff volume in patients with eccentric OA compared to patients with concentric OA.

Since many years, glenoid version and CSA are debated as a potential risk factor for posterior humeral head subluxation.^{12,17,18,20,22,36,39} In the retrospective radiological study conducted by Beeler et al, it was shown that significant differences in glenoid retroversion (mean difference, 2.5; P < .01), posterior humeral head subluxation (mean difference, 0.08; P < .01), and CSA (mean difference, 3°; P < .01) could be seen in patients with eccentric OA compared to patients with concentric OA.⁷ Recently, Terrier et al were able to find a strong correlation between glenoid retroversion and scapulohumeral subluxation, showing association of each degree of glenoid retroversion with a percentage of subluxation in the same orientation.³⁸ In addition, multiple studies have already described the association of larger CSAs with increased severity of eccentric OA.^7,11,31 A larger CSA, the resulting force vector—primarily influenced by the ascending force component of the deltoid—acts directly upward against the rotator cuff, contributing to cuff degeneration, potential tears, and eventually eccentric arthritis.^30,31 Similar results were observed in the present study in terms of glenoid retroversion (mean difference, 10.4; P < .001), posterior glenohumeral/scapulohumeral subluxation (mean difference, 0.05 and 0.16; P < .001, respectively) and CSA (26.3° ± 5.0° vs. 23.1° ± 4.2°; P = .006).

Static posterior humeral head subluxation and posterior glenoid wear in primary OA remain with its unsolved pathogenesis as an unclear entity, even with our findings, and should be considered as multifactorial. Beeler et al and Meyer et al demonstrated in their retrospective radiological studies that eccentric OA is associated with flatter acromial orientation in the sagittal plane and less posterior coverage of the humeral head, hypothetically causing decentralization of the humeral head due to decreased posterior bony support and a less favorable force vector of the deltoid muscle.^7,26 However, in contrast to already described differences of the acromial roof morphology between patients with rotator cuff tears and OA,6, 7, 8^{,25,28,30,35,37} the present study demonstrated no significant differences in acromion morphology between patients with concentric and eccentric OA.

The present study showed no significant differences in rotator cuff volumes between patients with concentric and eccentric OA. In contrast to the results of Mitterer et al, who demonstrated a significantly higher muscle volume of the subscapularis muscle in patients with a static posterior humeral head, another study was unable to find any significant differences in the rotator cuff in patients with static posterior decentration of the humeral head.¹ In addition, the study by Werthel et al showed that patients with OA of the shoulder had lower overall muscle volumes compared to healthy control groups, but again no differences were found between the anterior and posterior muscle groups.⁴³ In contrast to the previously mentioned studies, there are studies concluding that the posterior part of the rotator cuff has a larger area than the anterior part. Moverman et al found that in patients with eccentric OA and an intact rotator cuff, hypertrophic posterior rotator cuff muscles and an increased rate of fatty infiltration in the infraspinatus muscle can be observed.³² In addition, Aleem et al found that in patients with eccentric OA, the posterior rotator cuff is larger in the transverse plane than the anterior rotator cuff.³ In comparison to our study, these 2 studies presented the rotator cuff only as an area, whereas in the present study the volume was determined. Further studies, including a larger cohort number and concentrating on dynamic components of this entity, are required to substantiate these findings and finding clinically relevant differences in its pathomechanism.

This study has several limitations which should be considered when interpreting the data. First, the measurements were performed by 1 rater independently in a standardized manner. However, Akgün et al had recently illustrated almost perfect inter-rater and intrarater reliability of all the measurements conducted in the present study.¹ It should be noted that in our study, CT data were utilized rather than magnetic resonance imaging data, which constitutes the primary difference between their methodology and the current one. Second, CT imaging allows for bony visualization and it is not the optimal method for precise measurement of the rotator cuff volumes. Third, the number of patients in the concentric OA group may have limited the ability to detect subtle anatomical differences. A larger cohort would improve the robustness and generalizability of our findings. In addition, many patients who did not meet the inclusion criteria of this study were excluded, which could lead to a potential bias in the data. Fourth, the 2 experimental groups could not be matched according to the age, sex, weight, height and affected side due to the relatively small sample size of both cohorts. This may underpower the results of the present study, hindering to find out substantial results with statistical significance. Fifth, including a healthy control group with premorbid anatomical data would have been beneficial to demonstrate “normal” anatomy or a “low risk shoulder”. Sixth, due to its retrospective nature, no clinical outcomes of the included subjects in terms of analyzing dynamic components contributing to this entity could have been evaluated, potentially limiting the interpretation of the radiological results in the present study and their clinical impact.

Conclusion

Patients with primary eccentric OA show significant differences in glenoid retroversion, posterior scapulohumeral/glenohumeral subluxation, and CSA; however, no differences in other acromion roof morphology parameters and rotator cuff volume compared to patients with concentric OA. These findings may play a crucial role in understanding the pathomechanism of primary eccentric OA and identifying intrinsic risk factors for its development.

Disclaimers

Funding: No funding was disclosed by the authors.

Conflicts of interest: The authors, their immediate families, and any research foundations 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.