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Table of Contents
Year : 2022  |  Volume : 71  |  Issue : 2  |  Page : 140-145

Sub-Acromion Impingement Syndrome: Scapular Morphometric Analysis: A Study On Dry Bones among Eastern Indian Population


Date of Submission01-Aug-2020
Date of Decision19-Oct-2021
Date of Acceptance03-Apr-2022
Date of Web Publication30-Jun-2022

Correspondence Address:
Ratnadeep Poddar

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Source of Support: None, Conflict of Interest: None

DOI: 10.4103/JASI.JASI_143_20

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Introduction: Among various factors responsible for the development of chronic shoulder pain worldwide, the role of scapula, as a bony factor, is very important. This study focuses on evaluating the scapular shape and contour as a determinant of sub-acromion impingement syndrome. This was a cross-sectional observational study conducted on dry bones. Material and Methods: Dry scapulae (42 right sided and 38 left sided) were studied by taking digital photographs in different views and analyzing various parameters (critical shoulder angle (CSA), glenoid inclination, shape of acromion process, etc.) using ImageJ analyzer. Results were analyzed using measures of central tendency, and statistical significance was analyzed by measuring P values with the help of SPSS software (v25). Results: There were 40% Type I, 38.75% Type II, and 21.25% Type III scapulae, respectively. The Type I and III scapulae showed significant variations on the basis of various acromion overhangs (anterior overhang was 9.03 mm and 11.08 mm in Types I and III, respectively, while for the lateral overhang, the values were 9.73 mm and 6.25 mm in Types I and III, respectively) and angles (lateral acromion angle was 79.5° and 71.9° for Types I and III, respectively, whereas the coraco-acromion angle was 37° and 30.8° in Types I and III, respectively). The glenoid inclination and CSA were also significantly variable between all three types of scapulae. Discussion and Conclusion: The scapular morphology plays a pivotal role which can be extrapolated on a radiological basis in pertinent patients to determine the chances of developing pathological shoulders in future.

Keywords: Acromion overhangs, critical shoulder angle, glenoid inclination, rotator cuff, scapula, sub-acromion impingement syndrome

How to cite this article:
Dutta M, Poddar R. Sub-Acromion Impingement Syndrome: Scapular Morphometric Analysis: A Study On Dry Bones among Eastern Indian Population. J Anat Soc India 2022;71:140-5

How to cite this URL:
Dutta M, Poddar R. Sub-Acromion Impingement Syndrome: Scapular Morphometric Analysis: A Study On Dry Bones among Eastern Indian Population. J Anat Soc India [serial online] 2022 [cited 2022 Aug 17];71:140-5. Available from: https://www.jasi.org.in/text.asp?2022/71/2/140/349530

  Introduction Top

Rotator cuff (RC) injury is a predominant disease affecting multiple persons globally. The prevalence of the disease among US population was 33.8% unilaterally and 30.1% bilaterally.[1] In Japanese population, the prevalence was 20.7%.[2] Subacromial impingement syndrome (SIS) includes a spectrum of pathologies involving the subacromial space.[3],[4],[5] Neer describes the subacromial space, bounded inferiorly by the humeral head and superiorly by the coraco-acromial arch.[6] Both intrinsic (intra-tendinous) and extrinsic (extra-tendinous) factors play an important role in SIS.[7],[8],[9],[10],[11] Most of the studies relating the scapular angles (critical shoulder angle [CSA] and glenoid inclination) were based on radiological imaging previously. These two parameters were mostly related to pathologies around the shoulder including osteoarthritis, SIS, and RC tears of various degrees.[12],[13],[14] The focus of this study is on the assessment of bony factors of scapula, which can contribute significantly in development of SIS.

  Material and Methods Top

Eighty dry scapulae (42 right sided and 38 left sided) were collected from the museum of Kolkata-based medical college for purpose of the morphometric study. Bones with any morphological deformity or breakage were excluded from the study. The scapulae were divided into three groups following Bigliani et al.'s classification: Type I – flat acromion, Type II – curved acromion, and Type III – hooked acromion.[15] For each scapula, four views were obtained with digital camera: lateral, anterior, posterior, and superior (with each scapula being held in anatomical position by a tripod). With the help of image analyzer (ImageJ), the following parameters were studied and analyzed.

Glenoid inclination

On posterior view, four points were marked. The angle formed at superolateral quadrant by intersection of two lines was considered glenoid inclination. This is shown in [Figure 1].
Figure 1: Glenoid inclination angle was calculated as the angle formed between line 1 and line 2. Line 1 was defined as connecting (A) the intersection of the scapular spine with the scapula's medial border and (B) the middle of the spinoglenoid notch. Line 2 was defined as connecting (C) the superior-most point on the glenoid rim and (D) the inferior-most point on the glenoid rim

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Critical shoulder angle

This defines the position of glenoid fossa relative to the lateral-most point of acromion process. This angle is very much important from radiological point of view, as it determines the translation of humeral head against the scapula. This parameter was studied radiologically on multiple occasions previously.[12],[13] [Figure 2] depicts the CSA.
Figure 2: S – Supraglenoid tubercle; I – Infraglenoid tubercle; A – Lateral-most point on Acromion process

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Acromion overhangs

The extensions of acromion process over the sub-acromion space were represented by the anterior, posterior, and lateral acromion overhangs. They were determined in the superior view in relation to the posterior margin of the glenoid fossa, as shown in [Figure 3]. The posterior acromial overhang was the distance between the posterior edge of the glenoid and the anterior edge of the acromion on a line passing through the anterior and the posterior edges of the glenoid. The lateral acromial overhang was the distance between the posterior edge of the glenoid and the medial edge of the acromion on a line perpendicular to the glenoid surface. The anterior acromial overhang was the distance between the posterior edge of the glenoid and the intersection of lines passing through the anterior and posterior edges of the glenoid and a perpendicular line passing through the anterior end of the acromion.
Figure 3: The posterior (A), lateral (B), and anterior (C) acromial overhangs were all measured on the superior view. Acromial overhangs, (D) reference line perpendicular to glenoid fossa for measuring Anterior Acromion Overhang, passing through anterior end of acromion process

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Lateral acromion angle

This is the angle formed at the intersection of two lines – one drawn along the undersurface of acromion process and the other along the surface of glenoid fossa. This is shown in [Figure 4].
Figure 4: (A and P) Anterior-most point and posterior-most point of tip of acromion process, respectively; S and I: Supraglenoid and infraglenoid tubercles, respectively. Line 1 joins A and P along undersurface of acromion, while line 2 joins S andf I along surface of glenoid fossa. Lateral acromial angle

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Coraco-acromion angle

The coraco-acromial angle was the angle formed between a line passing through the axis of the coracoid process and a line perpendicular to the glenoid surface, joining the anterior end of the acromion on a superior view. This angle also indicates indirectly the shape and volume of sub-acromion space. It is calculated in the superior view, as shown in [Figure 5].
Figure 5: Coraco-acromion angle (shown in red) on superior view

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Lateral coracoid angle

The lateral coracoid angle was measured on the lateral view. It was the angle between a line passing through the axis of the base of the coracoid process and a line passing through the vertical axis of the glenoid. This is again an indirect measure of the sub-acromion space, as evident from [Figure 6].
Figure 6: Lateral coracoid angle (shown in white line) on lateral view

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  Results Top

The scapulae were primarily divided into three categories, on the basis of shapes of their acromion processes: Type I (40%), Type II (38.75%), and Type III (21.25%), respectively. When laterality was taken into consideration, around 50% of the scapulae of either side fell into each subtype, as shown in [Table 1].
Table 1: Prevalence of different types of scapulae (based on their shapes) in either side

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The mean overhang readings, based on the type of acromion process, noted in our study are depicted in [Table 2]. The measurements were also categorized according to laterality, as shown in [Table 3].
Table 2: Comparison of the acromion overhangs of various types of scapulae among themselves

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Table 3: Variations in measurements of acromion overhangs of either side, without considering the individual types of scapula

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As evident from the [Table 2], lateral overhang measurements varied significantly between the acromion types, the maximum being in Type II (10.2 ± 3.5 mm) and minimum being in Type III (6.25 ± 2.4 mm). Similarly, the anterior overhang measurements varied significantly between Type I and III acromion processes. There was no significant variation of the overhang measurements on scapulae of either side.

The different angles in relation to the acromion process were compared between different types of scapulae, as shown in [Table 4]. The mean lateral coracoid angle measurements were 53.5°, 50.7°, and 55.1° in Types I, II, and III, whereas the mean lateral acromion angle measurements were 79.5°, 78.4°, and 71.9°, respectively.
Table 4: Comparative data regarding variations in angles with respect to the acromion process in scapulae of different types

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Type III scapulae varied from both Types I and II with respect to the lateral acromion angle while the lateral coracoid angle did not vary significantly between the three types of scapulae. On the other hand, the type I scapulae was varying significantly from both the other types in respect to the coraco-acromion angle. Between the right and left scapulae, only the coraco-acromion angle varied significantly while others had no significant variation. This is evident from [Table 5].
Table 5: Variations in angles around acromion process among scapulae of either side

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Apart from the acromion process, another two important angles were measured in relation to the glenoid cavity. The mean glenoid inclination was measured to be 92.48° (±5.2°), 88.9° (±3.9°), and 96° (±3.3°), respectively, in Type I, II, and III scapulae, whereas the mean CSA was noted to be 31.5° (±3.8°), 34.3° (±3.8°), and 37.7° (±7.1°), respectively. These are evident from [Table 6]. Laterality-wise variation in these angles was also analyzed on the basis of measurements obtained, and they are reflected in [Table 7].
Table 6: Differences in angles around glenoid fossa among various types of scapulae

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Table 7: Comparison between scapulae of either side, with respect to the angles around glenoid fossa

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Both the glenoid inclination and CSA showed significant variations between different types of scapulae, as shown in the above [Table 6]. However, the angles in relation to glenoid fossa did not vary significantly between the right and left sides.

  Discussion Top

Bigliani et al. in their study argued that due to the shape and resultant damage, the Type II and Type III acromion had a greater predisposition to a RC tear and hence SIS.[15] Multiple anatomical factors including trauma or overuse of Rotator Cuff and allied structures can lead to Sub-acromion Impingement Syndrome.[16] Anatomical factors include acromial morphology variations such as shape of acromion process, lateral extension of acromion and coracoid processes, and degenerative changes at the inferior surface of the acromion, acromio-clavicular joint (ACJ), or coraco-acromial ligament (CAL). However, due to poor inter-observer reliability, this theory has been widely criticized.[17]

In our study, the incidence of Type I acromion was more on the right side which coincides with Penni et al. (1996) and Schippinger et al. (1997).[18],[19] On the left side, the incidence of Type II acromion was more than the other two types, which is similar to the findings of Bigliani et al., Paraskevas et al., and many more.[20] [Table 8] shows the incidence of acromial types in previous studies.[15],[18],[19],[20],[21],[22],[23],[24],[25],[26],[27]
Table 8: Comparison between previous studies showing different types of scapulae

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Analyzing the relationship between the acromial shapes and acromial overhang, significantly greater anterior acromial overhang was found in Type III than Type I and Type II. On the other hand, lateral acromial overhang was significantly less in Type III acromion than Type I and Type II. No significant relationship was found between posterior acromial overhang and types of acromion process. This study result was similar to that of Le Reun et al.[28]

Glenoid inclination angle varies significantly between the three types of acromion processes in our study. Glenoid inclination angles were 92.48°, 88.9°, and 96°, respectively, in Types I, II, and III in this study. Hughes et al. in their study found that glenoid inclination was greater in cadaver shoulders having full-thickness RC tears (98.6°) than in shoulders without tears (91.0°).[29]

The presence of degenerative shoulder pathologies may be indicated in a new radiographic measurement which is CSA. This parameter is not only associated with degenerative RC tears but also with the presence of glenohumeral osteoarthritis. Degenerative RC tear is associated with CSA greater than 35° while angle below 30° is common in osteoarthritis.[30],[31] In this study, CSA of Type I scapula varies significantly from Types II and III, the maximum being in Type III (37.7°) and minimum being in Type I (31.5°).

  Conclusion Top

The CSA and glenoid inclinations, in addition to the shapes of scapular acromion processes and overhangs, can be early predictors for the SIS and other shoulder pathologies. This anatomical study on dry bones can be used for extending the results upon patients based on their clinical presentations, which might be a cornerstone for orthopedicians and radiologists while dealing with shoulder pathologies, particularly the SIS.


The study has been possible due to the immense co-operation of Prof. (Dr.) Asis Kumar Ghosal, Head of the Department (Anatomy), IPGME and R, Kolkata, as well as the support and guidance of Prof. (Dr.) Avijit Hazra, Professor, Department of Pharmacology, IPGME and R, Kolkata. Furthermore, the Department of Anatomy of Murshidabad Government College and Hospital has played an important role for the support.

Financial support and sponsorship

This study was financially supported by the Department of Anatomy of Murshidabad Government College and Hospital as well as Rampurhat Government Medical College and Hospital.

Conflicts of interest

There are no conflicts of interest.

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  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6]

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7], [Table 8]


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