Introduction

With the development of modern times, sports have become an indispensable part of people’s lives, and the incidence of shoulder joint pain has increased accordingly. Among the primary causes of shoulder joint pain is rotator cuff injury1. Neer2 suggested that most rotator cuff tears are caused by mechanical impingement between the rotator cuff and the acromion, making the shape of the acromion particularly important. Bigliani et al.3 classified the acromion into three types: flat, curved, and hooked, with the hooked type being the most prone to causing rotator cuff tears. However, in practice, some patients with hooked acromions do not develop rotator cuff tears, while some patients with curved or flat acromions do experience rotator cuff tears (Fig. 1). Therefore, we propose a new acromial angle in this study. By observing and measuring the new acromial angle in both normal individuals and patients with rotator cuff tears, we aim to investigate the potential relationship between them.

Fig.1
figure 1

X-ray of acromion. (a) hooked acromion without rotator cuff tears; (b) curved acromion with rotator cuff tears.

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Materials and methods

Clinical data

This was a retrospective cohort study. We retrospectively selected complete imaging data from 148 individuals who underwent scapular outlet view X-rays and shoulder MRIs in our hospital between January 2023 and September 2024. The inclusion criteria for the subjects were as follows: (1) no infectious diseases in the shoulder joint; (2) no neoplastic diseases in the shoulder joint; (3) no traumatic diseases in the shoulder joint; (4) no congenital deformities of the shoulder joint; (5) no history of shoulder joint surgery. The study included 148 cases, with the normal group comprising 38 males and 32 females, with a mean age of 58.97 ± 16.04 years, and the injury group comprising 41 males and 37 females, with a mean age of 58.06 ± 12.06 years.

Measurement method of the new acromial angle

The method for measuring the new acromial angle (Fig. 2): On the scapular outlet view X-ray, select the anterior and posterior edges of the base of the acromion as points A and B, respectively. Then, mark point C as the point located at the anterior third of line AB. From point C, draw a perpendicular line to AB, intersecting the base of the acromion at point D. Connect points A and D, as well as points B and D. The angle ADB is defined as the new acromial angle. All measurement data were blindly assessed by two radiologists who were unaware of the research purpose. Both radiologists have been practicing in the field for over 10 years. The measurement results they obtained differed by no more than 5°, and there was no significant difference between their data (P > 0.05) (Fig. 3 and Table 1). The average of the measurements from the two observers was calculated to minimize inter-observer variability.

Fig. 2
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Measurement of the new acromial angle. A: Anterior edge of the base of the acromion; B: Posterior edge of the base of the acromion; C: One-third point along the AB line from the front; D: A point at the base of the acromion, connected to C and perpendicular to AB.

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Fig. 3
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Comparison of new acromial angle measurements by different observers.

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Table 1 Comparison of new acromial angle measurements by different observers.
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Statement

All methods were performed in accordance with the relevant guidelines and regulations. The study protocol was approved and a waiver to obtain informed consent from study participants was obtained by the Ethics Committee of Yueqing Hospital Affiliated to Wenzhou Medical University.

Statistical methods

Data were statistically analyzed using SPSS 25.0 software. Measurement data that followed a normal distribution were expressed as mean ± standard deviation (x̅ ± s). A two-sample independent t-test was used for group comparisons, with a significance level set at two-sided α = 0.05.

Results

In this study, the basic information of subjects in the normal group and the injury group is shown in (Table 2). The comparisons were made and the differences were not statistically significant (P > 0.05). There were 70 subjects in the normal group and 78 subjects in the injury group. The mean new acromial angle in the normal group was 149.1 ± 5.96°, while it was 142.3 ± 6.56° in the injury group. The difference between the two groups was statistically significant (P < 0.001) (Table 3 and Fig. 4). Since both groups’ data followed a normal distribution, we chose the mean value of the new acromial angle in the injury group as a standard for comparison. For those with a new acromial angle below this mean (Fig. 5), there were 9 cases in the normal group and 34 cases in the injury group, making a total of 43 cases, with the injury cases accounting for 79.07%. When the angle of the acromion is lower, hooked and curved acromions are more prevalent. For those with a new acromial angle above this mean (Fig. 6), there were 61 cases in the normal group and 44 cases in the injury group, making a total of 105 cases, with the injury cases accounting for 41.90% (Table 4). The angle of the acromion is higher, and its shape tends to be flatter. In the injury group, there were 71 cases with supraspinatus muscle injury, 1 case with infraspinatus muscle injury, 8 cases with subscapularis muscle injury, 20 cases with biceps tendon injury, and 26 cases with biceps tendon effusion. Among them, 16 cases had both supraspinatus muscle injury and biceps tendon injury, and 26 cases had both supraspinatus muscle injury and biceps tendon effusion. Based on these findings, we aimed to further explore the significance of the new acromial angle size in predicting rotator cuff tears within the population. We selected the average value of the new acromial angle among all study subjects as a standard to investigate the relationship between the size of the acromial angle and rotator cuff tears (Fig. 7). Among patients with partial-thickness supraspinatus tendon tears, 17 had a new acromial angle smaller than the population average, and 13 had an angle larger than the average. In patients with full-thickness supraspinatus tendon tears, 28 had a new acromial angle smaller than the population average, and 13 had an angle larger than the average. Among patients with supraspinatus muscle injury combined with biceps tendon injury, 11 had a new acromial angle smaller than the population average, and 5 had an angle larger than the average.

Table 2 Comparison of basic information.
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Table 3 Comparison of new acromial angles.
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Fig. 4
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Difference in novel acromial angle between the normal group and the rotator cuff tears group.

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Fig. 5
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New acromial angle below the average. A: Anterior edge of the base of the acromion; B: Posterior edge of the base of the acromion; D: A point at the base of the acromion.

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Fig. 6
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New acromial angle below the average. A: Anterior edge of the base of the acromion; B: Posterior edge of the base of the acromion; D: A point at the base of the acromion.

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Table 4 Distribution of values above and below the average of the new acromial angle.
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Fig. 7
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Distribution in new acromial angles between the two groups. Partial: partial injury of the supraspinatus muscle; Total: total injury of the supraspinatus muscle; Combine: Supraspinatus muscle injury combined with injury to other muscles.

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Discussion

Rotator cuff tears

The rotator cuff is a complex of tendons that originate from the scapula and insert at the upper end of the humerus. It includes the supraspinatus, infraspinatus, teres minor, and subscapularis muscles. These four muscles stabilize the humeral head within the glenoid cavity, playing an essential role in maintaining shoulder joint stability. Rotator cuff tears refer to the damage to any of these four muscles. The incidence of rotator cuff tears has been steadily increasing. Clinically, rotator cuff tears can be classified into the following three types: Type 1: Partial rotator cuff tears. Type 2: Small to medium full-thickness tears, with tear diameters (either anteroposterior or mediolateral) ranging from 1 to 3 cm. Type 3: Large and massive tears, where large tears have diameters of 3 to 5 cm, and massive tears exceed 5 cm4.With advances in healthcare and food availability, the average human lifespan is increasing. Maman et al.5 conducted a retrospective study on 54 patients with rotator cuff tears and found that age over 60 is a risk factor for the worsening of rotator cuff tears. This finding aligns with the research by Lu Yi et al.6, who discovered that the dominant shoulder exhibits a higher incidence across all types of rotator cuff tears, with rates of 64.7, 63.0, and 70.3%, respectively. This may be due to the fact that the dominant upper limb is not only used in daily activities but also engaged more intensively during sports.

However, some researchers believe that most rotator cuff tears are caused by mechanical impingement between the rotator cuff and the acromion2, making the shape of the acromion particularly important. Bigliani et al.3 classified acromion shapes into three types: flat, curved, and hooked, with the hooked type being the most prone to causing rotator cuff tears. However, some studies have shown that there is no correlation between acromion morphology and rotator cuff injury7,8,9,10. Because Bigliani et al. provided only a simple description of acromion morphology without objective standards, which led to different interpretations of acromion types by different researchers. Moreover, some scholars believe that an excessively hooked acromion with a slope greater than 43° is associated with rotator cuff injury11. This further confirms the relationship between rotator cuff injury and acromion types. Therefore, this study aimed to further objectify acromion morphology.

Advantages of the new acromial angle

The new acromial angle proposed in this article is determined by the acromial position corresponding to the first third of the line connecting the anterior and posterior edges of the acromial base. This position is chosen because Snyder’s research12,13 suggests that the acromial morphology should be classified based on the thickness at the junction of the anterior and middle thirds of the acromion. This method can help guide acromioplasty, as the main area targeted in acromioplasty is the anterior third of the acromion. There are already some traditional classification methods for the acromion. In 1986, Bigliani et al.14 proposed three types of acromial classification: type I (flat), type II (curved), and type III (hooked). However, this classification is based solely on the description of the acromial shape without using objective data standards. As a result, studies have found varying proportions for these three types of acromions15,16. Clinically, patients with a hooked acromion are not uncommon, but some may have a larger subacromial space without causing rotator cuff injury, and the overall curvature may be relatively flat, yet they are classified as having a hooked acromion due to its shape alone.

In this study, we propose a new acromial angle that not only provides a quantitative standard but also has higher sensitivity than the traditional classifications mentioned above. Park et al.17 proposed another method to differentiate between type II and type III acromions, which has higher repeatability and reliability compared to that of Bigliani et al. However, some type II acromions can still lead to acromial impingement syndrome and rotator cuff tears. This standard does not clearly specify which type II and type III acromions can result in acromial impingement syndrome and rotator cuff tears. The Park criteria take into account the position of the humeral head, suggesting that the upward migration of the humeral head can cause changes in the acromion morphology, making impingement syndrome and rotator cuff tears more likely. However, in the same patient, if the humeral head dislocates forward, a type II acromion according to the Park criteria might be classified as a type III acromion. The newly proposed acromial angle in this study can be more objective and quantifiable. Aragão et al.15 conducted a study on 90 scapular specimens and found a correlation between acromial curvature, subacromial space, and acromial types, but the method they used was more complex, and clinical imaging may not always provide the necessary reference points. In contrast, the new acromial angle proposed in this study is simpler, easier to operate, and easier to locate. Through imaging studies, Balke et al.11 found that a slope angle of the acromion greater than 43° is associated with an increased risk of rotator cuff injury. Caffard et al.18 conducted a retrospective study on 92 patients with supraspinatus injuries and found that a high preoperative acromial slope angle was associated with postoperative re-tears of the supraspinatus tendon. These findings support the significance of the acromial slope angle. However, the acromial slope angle is measured by connecting the anterior and posterior edges of the acromial base to the center point, which to some extent ignores the acromial morphology, and the area with the most significant acromial shape variation is the anterior third. The new acromial angle proposed in this study focuses on the anterior third of the acromion.

Significance of the new acromial angle

In this study, the average value of the new acromial angle in the normal group was (149.1 ± 5.96)°, while in the injury group it was (142.3 ± 6.56)°. The acromial angle in the injury group was generally smaller than that in the normal group, and the difference was statistically significant (P < 0.001). Among the study subjects with an acromial angle smaller than 142.3°, 9 were in the normal group and 34 in the injury group, totaling 43 subjects, with the injury group accounting for 79.07% of the cases. Additionally, the curvature of the entire acromion is relatively large, with more hooked and curved acromions. In comparison, Bigliani et al.14 found that among 140 anatomical specimens, 73% of those with rotator cuff tears had a type III acromion. The injury ratio in this study was higher, indicating that the new acromial angle could serve as an indicator for predicting rotator cuff tears. Some subjects had an acromial angle smaller than the average but did not have rotator cuff tears, possibly due to their younger age. Fallahpour et al.19 believe that there is a correlation between age and rotator cuff tears. This may be because aging leads to degenerative changes in the rotator cuff, making tendons more prone to tearing. Additionally, with age, the occurrence of acromial bone spurs also increases the incidence of rotator cuff tears. In this study, some of the subjects with smaller new acromion angles exhibited rotator cuff tears. They may develop rotator cuff tears in the future, requiring long-term follow-up studies. Moor et al.20 believe that a critical shoulder angle (CSA) greater than 35° is likely to result in rotator cuff tears. This may be because as the CSA angle increases, the upward force vector of the deltoid muscle on the humeral head increases during shoulder abduction. To maintain the stability of the shoulder joint, the supraspinatus muscle needs to exert more pressure. If the supraspinatus muscle is consistently under excessive load, it becomes weaker and more prone to rotator cuff tears. A tear in the supraspinatus muscle causes the humeral head to move upwards, increasing the likelihood of impingement with the acromion, thereby altering the acromion’s shape. There has been no definitive conclusion on whether changes in acromion shape or rotator cuff tears occur first, but they inherently influence each other. The study in this paper on the influence of acromion shape on rotator cuff tears may have limitations, yet the impact of acromion shape on such injuries cannot be denied.

In this study, there were 71 cases of supraspinatus injuries, of which 16 were accompanied by long head biceps tendon injuries. This could be because the function and stability of the long head biceps tendon depend on the pulley structure within the rotator interval. This structure is formed by the interweaving muscle fibers of the lower margin of the supraspinatus and the upper margin of the subscapularis, along with the coracohumeral ligament and the superior glenohumeral ligament, which together stabilize the long head biceps tendon21. During activity, the stability of the shoulder joint largely relies on surrounding soft tissue structures, which is why rotator cuff tears often lead to long head biceps tendon damage. Reducing the occurrence of rotator cuff tears can help protect the long head biceps tendon to some extent.

Additionally, we found that the new acromial angle may be related to the severity of rotator cuff tears. In patients with full-thickness supraspinatus tears or those with supraspinatus tears combined with long head biceps injuries, the proportion of those with a new acromial angle smaller than the average was significantly higher than those with an angle larger than the average. This suggests that a smaller new acromial angle is more likely to lead to more severe rotator cuff tears.

Conclusion and limitations

In this study, the new acromial angle in the injury group was generally smaller than that in the normal group, with a probability of 79.07% for rotator cuff tears in those with a new acromial angle smaller than the average. Moreover, smaller acromial angles were associated with more severe rotator cuff tears. Therefore, by measuring the new acromial angle, it is possible to easily and objectively assess the likelihood of rotator cuff tears. Early intervention for individuals with a high probability could help reduce the occurrence of such injuries. However, there are some limitations to this study. First, the sample size is insufficient, and more measurement data are needed to further analyze the effectiveness of the new acromial angle. Additionally, despite the involvement of two observers during the measurement process, measurement errors could not be completely avoided. Additionally, some subjects with smaller acromial angles did not develop rotator cuff tears, but they may do so over time with continued shoulder use and wear. Finally, rotator cuff tears may also affect acromion morphology, and the study subjects could not distinguish whether a smaller new acromion angle or rotator cuff injury occurred first. Follow-up is needed for patients with a smaller new acromion angle.

In summary, this study indicates a correlation between the new acromial angle and the occurrence of rotator cuff tears.