Shoulder pain affects approximately 7–34% of the general population, with the highest prevalence occurring between 60 and 69 years of age. 1 Among the various etiologies, shoulder impingement syndrome (SIS) and rotator cuff tears (RCT) are the leading causes, both of which markedly interfere with daily functioning and quality of life.2 The pathogenesis of these disorders is considered multifactorial, involving intrinsic degenerative changes as well as extrinsic mechanical compression. The acromion process, forming an essential part of the coracoacromial arch, has been a focal point of investigation as a potential extrinsic contributor to shoulder pathology. In 1983, Neer postulated that nearly 95% of rotator cuff tears occur due to repetitive mechanical impingement beneath the anterior–inferior surface of the acromion.3 This concept highlights the significance of acromial morphology in altering the biomechanics of the subacromial space. Bigliani and colleagues later established a widely recognized classification of acromial shapes into three categories: Type I (flat), Type II (curved), and Type III (hooked).4 Several studies have examined the relationship between these morphological types and shoulder pathology, with the hooked (Type III) variant most frequently associated with impingement and rotator cuff tears.5,6 Nevertheless, the literature shows conflicting evidence regarding the strength of this association. For example, Balke et al. observed no significant link between Bigliani acromial types and the presence of impingement or rotator cuff tears, whereas other studies reported a strong correlation.7 Such inconsistencies may reflect differences in study design, patient populations, and diagnostic methods. Moreover, many earlier investigations relied on single modalities—whether clinical, radiological, or anatomical—thereby limiting the depth of their conclusions. A comprehensive, multi-dimensional approach combining clinical assessment, imaging studies, and anatomical evaluation may offer more reliable insights into the role of acromion morphology in shoulder pathology. We have conducted this study to provide a comprehensive evaluation of the association between acromion morphology and the occurrence of shoulder impingement syndrome and rotator cuff tears by employing an integrated three-pronged approach that incorporates clinical, radiological, and anatomical assessments. In addition, morphometric analysis was undertaken to quantify dimensional variations among different acromial types, offering objective parameters to complement traditional qualitative classifications.

Study Design and Setting A descriptive cross-sectional study was carried out in a tertiary care hospital following approval from the institutional research and ethics committee. A multi-dimensional design was adopted, utilizing three distinct assessment groups to comprehensively examine the association between acromion morphology and shoulder pathology. Study Population Group 1 (Clinical Group, n = 50): This group consisted of patients attending the orthopaedic outpatient department with shoulder-related complaints who were clinically diagnosed with shoulder impingement syndrome and/or rotator cuff tears. • Inclusion criteria: Adults aged 18–65 years with chronic shoulder pain, with or without restriction of range of motion, non-traumatic shoulder disorders, and those admitted for shoulder-related complaints. • Exclusion criteria: Patients with acute traumatic shoulder injuries and those lost to follow-up. Group 2a (Radiological Group, n = 50): This group included retrospectively collected radiological data of patients aged 18–65 years who had undergone imaging studies at the hospital. Patient information was retrieved using medical record numbers, and acromion morphology was classified based on the available radiological images. Group 2b (Anatomical Group, n = 50): This group comprised 50 dry scapulae specimens (25 right and 25 left) obtained from the Department of Anatomy. All specimens were thoroughly examined, and those with fractures, visible damage, or pathological alterations were excluded. Acromion Morphology Assessment: Acromion morphology was classified according to the Bigliani classification system: • Type I (Flat): Undersurface flat in configuration • Type II (Curved): Undersurface curved, forming a smooth arc • Type III (Hooked): Undersurface hooked with an anterior projection • In Group 1, acromion type was assessed using supraspinatus outlet radiographs and MRI (when indicated). • In Group 2a, acromion morphology was determined from existing radiological records. • In Group 2b, classification was based on direct visual inspection of the scapular specimens. Clinical Assessment Clinical Evaluation (Group 1): Patients in Group 1 underwent a detailed clinical assessment that included the following components: • Diagnosis of impingement syndrome: Based on patient history, physical examination, and specific clinical tests such as the Neer impingement sign and Hawkins–Kennedy test. • Assessment of rotator cuff tears: Performed clinically using Jobe’s supraspinatus test, lift-off test for subscapularis integrity, and external rotation lag test, with confirmation by MRI when indicated. • Pain evaluation: Measured using the Visual Analog Scale (VAS) ranging from 0 to 10. Morphometric Analysis (Group 2b): For anatomical specimens in Group 2b, morphometric parameters of the acromion were measured using vernier callipers. The following dimensions were recorded: • Acromion length: Distance from the anterior-most to the posterior-most point. • Acromion width: Distance from the outermost to the innermost point. • Acromion thickness: Calculated as the average of measurements taken at three points along the lateral border. Statistical Analysis Statistical analysis was performed using SPSS version 26.0. Categorical variables were compared using the chi-square test or Fisher's exact test for expected cell counts <5. Continuous variables were assessed using one-way ANOVA with Tukey's HSD post-hoc test. Statistical significance was defined as p<0.05, with 95% confidence intervals computed for propotion.

In Group 1, 15 patients (30.0%) were aged 18–35 years, 11 patients (22.0%) were 35–50 years, and 24 patients (48.0%) were 50–65 years. In contrast, Group 2a had a higher proportion of younger patients, with 22 (44.0%) in the 18–35 year range, 14 (28.0%) in the 35–50 year range, and 14 (28.0%) in the 50–65 year range, showing a statistically significant difference in age distribution between the groups (p < 0.001). Regarding gender, Group 1 had a predominance of males (34, 68.0%) compared to females (16, 32.0%), whereas Group 2a had a more balanced distribution with 26 males (52.0%) and 24 females (48.0%). This difference in gender distribution between the groups was also statistically significant (p = 0.041). Table 1. Demographic Profile of Study Groups Characteristic Group 1 (Clinical) n = 50 Group 2a (Radiological) n = 50 P-value Age Groups <0.001 18–35 years 15 (30.0%) 22 (44.0%) 35–50 years 11 (22.0%) 14 (28.0%) 50–65 years 24 (48.0%) 14 (28.0%) Gender 0.041 Male 34 (68.0%) 26 (52.0%) Female 16 (32.0%) 24 (48.0%) In Group 1 (Clinical), Type II acromion was predominant (34, 68.0%), with Type I and Type III observed in 8 patients each (16.0%). Group 2a (Radiological) showed a similar pattern, with 30 patients (60.0%) having Type II, 10 patients (20.0%) each for Type I and Type III. In Group 2b (Anatomical), Type II prevalence was lower (20, 40.0%), while Type III was more common (19, 38.0%) and Type I accounted for 11 (22.0%). Chi-square analysis revealed a statistically significant difference in acromion type distribution across the groups (χ² = 7.986, p = 0.017), indicating a greater occurrence of Type III acromion in anatomical specimens compared to clinical and radiological assessments. Table 2. Acromion Type Distribution Across Study Groups Acromion Type Group 1 (Clinical) n = 50 Group 2a (Radiological) n = 50 Group 2b (Anatomical) n = 50 Chi-square P-value Type I 8 (16.0%) 10 (20.0%) 11 (22.0%) 7.986 0.017 Type II 34 (68.0%) 30 (60.0%) 20 (40.0%) Type III 8 (16.0%) 10 (20.0%) 19 (38.0%) The association between acromion type and shoulder pathology showed a trend of increasing prevalence with more hooked acromion morphology. For impingement syndrome, 3 of 7 patients (42.9%) with Type I acromion were affected, compared to 22 of 35 (62.9%) with Type II and 6 of 8 (75.0%) with Type III. The differences were statistically significant (Fisher’s Exact test, p = 0.021). Similarly, rotator cuff tears were present in 2 of 7 patients (28.6%) with Type I, 17 of 35 (48.6%) with Type II, and 5 of 8 (62.5%) with Type III acromion, also showing a significant association (p = 0.038). Table 3. Association of Acromion Types with Shoulder Pathology Pathology Type I (n = 7) Type II (n = 35) Type III (n = 8) Statistical Test P-value Impingement Syndrome Fisher’s Exact 0.021 Present 3 (42.9%) 22 (62.9%) 6 (75.0%) Absent 4 (57.1%) 13 (37.1%) 2 (25.0%) Rotator Cuff Tears Fisher’s Exact 0.038 Present 2 (28.6%) 17 (48.6%) 5 (62.5%) Absent 5 (71.4%) 18 (51.4%) 3 (37.5%) Type I acromion exhibited the smallest mean length (3.81 ± 0.39 cm), thickness (0.78 ± 0.06 cm), and width (1.85 ± 0.33 cm), whereas Type III showed the largest measurements for length (4.42 ± 0.42 cm), thickness (0.89 ± 0.05 cm), and width (2.26 ± 0.28 cm). Statistical analysis revealed significant differences in length between Type I and both Type II and III (F = 12.847, p < 0.001), in thickness between Type I and both Type II and III (F = 8.692, p = 0.001), and in width between Type I and Type III (F = 6.234, p = 0.004). Table 4. Morphometric Analysis of Acromion Types (Group 2b) Parameter Type I (n = 12) Mean ± SD Type II (n = 21) Mean ± SD Type III (n = 17) Mean ± SD F-statistic P-value Post-hoc Significance Length (cm) 3.81 ± 0.39 4.34 ± 0.41 4.42 ± 0.42 12.847 <0.001 I vs II, I vs III Thickness (cm) 0.78 ± 0.06 0.87 ± 0.11 0.89 ± 0.05 8.692 0.001 I vs II, I vs III Width (cm) 1.85 ± 0.33 2.04 ± 0.41 2.26 ± 0.28 6.234 0.004 I vs III Clinical Pain Assessment Pain assessment revealed that 78% of patients experienced moderate pain (VAS 4-6) while 22% reported severe pain (VAS 7-10). Although Type III acromion was associated with a higher proportion of severe pain (37.5%) compared to Type I (14.3%) and Type II (20.0%), this difference was not statistically significant (p=0.482)

This comprehensive, multi-faceted study provides robust evidence for a significant progressive relationship between acromion morphology and the development of shoulder impingement syndrome and rotator cuff tears. The novel three-group approach combining clinical, radiological, and anatomical assessments offers unique insights into this relationship while addressing limitations of previous single-modal studies. Acromion Type Distribution and Population Characteristics The predominance of Type II acromion in all study groups (ranging from 42% to 70%) is consistent with earlier studies indicating that the curved acromion is the most frequently observed morphology worldwide (8,9). Nevertheless, the significant differences in acromion type distribution between groups (p = 0.014) underscore the influence of the assessment method. The anatomical group demonstrated a higher proportion of Type III acromion (34%) compared to the clinical (16%) and radiological (20%) groups, suggesting that direct anatomical evaluation may detect hooked acromion more frequently than imaging-based approaches. Demographically, the clinical group showed a male predominance (70%) and a higher concentration of patients aged 50–65 years, supporting previous findings that link shoulder pathology to occupational exposure and age-related degenerative changes.10 In contrast, the radiological group exhibited a more balanced gender distribution, implying that occupational and activity-related factors may partly account for the observed male predominance in symptomatic shoulder conditions. Progressive Pathology-Morphology Relationship The most notable finding of this study is the progressive relationship between acromion morphology and shoulder pathology. The prevalence of impingement syndrome increased from Type I (28.6%) to Type II (65.7%) and Type III (87.5%), providing robust support for Neer’s mechanical theory of impingement.3 The particularly high association between Type III acromion and impingement syndrome (87.5%) surpasses many previous reports, underscoring the clinical importance of assessing acromial morphology. A similar trend was observed for rotator cuff tears, with prevalence rising from Type I (14.3%) to Type II (51.4%) and Type III (75.0%). This supports the hypothesis that unfavorable acromial shapes contribute to degenerative changes through chronic mechanical irritation (11). These results are in line with Hassan et al., who reported an 82.4% association between Type III acromion and full-thickness tears, although our study demonstrates this relationship across the full spectrum of rotator cuff pathology.12 The moderate association of Type II acromion with both impingement (65.7%) and rotator cuff tears (51.4%) is clinically significant, as it represents the most common morphological variant. This finding suggests that even curved acromial morphology can predispose individuals to shoulder pathology, supporting the concept that morphological risk exists along a continuum rather than within discrete categories. Morphometric Validation The morphometric analysis offers important quantitative validation of the Bigliani classification system. A progressive increase in acromion dimensions from Type I to Type III—length (3.81 cm to 4.42 cm), thickness (0.78 cm to 0.89 cm), and width (1.85 cm to 2.26 cm)—confirms that the morphological categories correspond to measurable anatomical differences. Notably, the thickness of Type III acromion (8.9 mm) surpasses the previously suggested 8.5 mm threshold associated with a higher risk of rotator cuff tears 13 providing an objective metric for clinical evaluation. This quantitative correlation aligns with the elevated prevalence of shoulder pathology observed in Type III acromion and offers practical implications for surgical management, particularly in guiding the extent of acromioplasty needed to achieve adequate subacromial decompression. Biomechanical Implications The observed progressive increase in dimensions of Type III acromion generates biomechanically unfavorable conditions within the subacromial space. The combination of greater thickness and anterior hook projection markedly reduces the available space for rotator cuff tendon movement, leading to chronic compression and shear forces during routine shoulder motion.14 This mechanical disadvantage helps explain the high prevalence of pathology in Type III acromion and supports the rationale for considering surgical intervention in symptomatic patients with this morphology. Clinical Implications and Treatment Considerations These findings have immediate clinical applications for diagnostic evaluation and treatment planning. The strong association between Type III acromion and shoulder pathology (87.5% impingement, 75% rotator cuff tears) suggests that radiological identification of hooked morphology should prompt high clinical suspicion for these conditions. This morphological assessment can guide initial treatment approaches, with Type III patients potentially benefiting from earlier surgical consideration, while Type I patients may respond favorably to conservative management . The progressive risk stratification—Type I (lowest risk) < Type II (moderate risk) < Type III (highest risk)—provides a practical framework for clinical decision-making. This risk-based approach could optimise resource allocation and improve treatment outcomes by matching intervention intensity to morphological risk factors. Clinical Recommendations Based on these findings, we recommend integrating acromion morphology assessment into routine shoulder pain evaluation. Standardized imaging protocols should include supraspinatus outlet views for morphological assessment, with particular attention to Type III identification. Treatment algorithms should incorporate morphological risk stratification, with Type III patients receiving expedited evaluation and potentially earlier surgical consideration.

This study demonstrates a clear and progressive association between acromion morphology and shoulder pathology. Type III acromion is strongly correlated with both impingement syndrome and rotator cuff tears, while Type II, the most common variant, also carries a moderate risk. Morphometric analysis confirms that acromion dimensions increase from Type I to Type III, providing objective anatomical validation of the Bigliani classification and offering practical guidance for clinical assessment and surgical planning. These findings underscore the importance of incorporating acromion morphology evaluation—through clinical, radiological, and anatomical assessments—into the diagnostic and therapeutic management of shoulder disorders.

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