The sacroiliac joint (SIJ) is a complex synovial and fibrocartilaginous articulation that plays a vital role in transmitting axial loads between the vertebral column and the lower extremities while permitting limited but essential motion [1-3]. Because of its unique anatomical configuration and functional importance, the SIJ has been the focus of increasing clinical and radiological interest. Variations in its morphology—including accessory joints, bipartite iliac bones, crescentic shapes, and degenerative alterations—have been identified more frequently with the widespread use of computed tomography (CT) and magnetic resonance imaging (MRI) [4-7]. Clinically, the SIJ is of great significance as it contributes to 15–30% of cases of chronic low back pain [6–8]. Degenerative changes such as joint space narrowing, subchondral sclerosis, and cyst formation are commonly observed in older populations, and the prevalence of these alterations has been shown to differ between males and females [9,10]. Female pelvic morphology, hormonal influences, and obstetric history are additional factors that may predispose women to specific SIJ variants [11-13]. Age-related changes in SIJ morphology are well established. Several studies have demonstrated increasing prevalence of accessory SI joints, bipartite iliac configurations, and degenerative variants with advancing age [13–15]. Gender differences are equally noteworthy: males are more likely to exhibit degenerative sclerosis, while females more often present with bipartite or crescentic iliac variants, likely reflecting adaptive responses of the female pelvis to reproductive functions [16–18]. Despite these observations, reported prevalence rates across studies remain inconsistent, which may be due to heterogeneity in study design, imaging techniques, and population characteristics. Importantly, there is a paucity of systematic studies that comprehensively evaluate SIJ variations across both age and gender in South Asian populations, where differences in lifestyle, body habitus, and occupational or reproductive factors may significantly influence SIJ morphology [19–21]. Such region-specific data are essential to improve anatomical understanding, guide radiological interpretation, and optimize clinical diagnosis of SIJ-related disorders. The present study was undertaken to analyze age- and gender-related differences in sacroiliac joint variants and degenerative changes using CT imaging. By statistically comparing these morphological features across demographic groups, this study aims to provide robust evidence that can strengthen anatomical and clinical perspectives on SIJ variations.

Study Design and Setting This was a retrospective, cross-sectional observational study conducted in the Department of Anatomy and Department of Radiodiagnosis, IGMC, Shimla based on computed tomography (CT) imaging of the sacroiliac joints. The study was approved by the Institutional Ethics Committee, and all procedures conformed to the ethical standards outlined in the Declaration of Helsinki. Study Population A total of 108 adult patients (58 males and 50 females) who underwent pelvic CT scans were included in the study. The age of participants ranged from 18 years to over 60 years, and they were stratified into three groups for analysis: • ≤40 years • 41–60 years • 60 years Patients with known pelvic fractures, sacroiliitis due to infection or inflammatory disease, or previous pelvic surgery were excluded to avoid confounding pathological changes. Imaging Protocol CT examinations were performed using a multi-detector CT scanner with thin-section axial images obtained through the pelvis. High-resolution images were reconstructed in coronal and sagittal planes using bone algorithms, which provided detailed visualization of the sacroiliac joints. Evaluation Parameters Each scan was independently reviewed by two experienced radiologists, and discrepancies were resolved by consensus. The following anatomical variants were assessed: • Accessory sacroiliac joint (ASI) • Bipartite iliac bone • Crescentic-shaped iliac bone • Semi-circular defects • Iliosacral complex • Intra-articular new bone formation (IANBF) • Other morphological variations The following degenerative changes were also documented: • Subchondral sclerosis • Subchondral cysts • Joint space narrowing • Ankylosis In addition, joint space width was measured bilaterally in millimeters at the narrowest point of the articular cavity. Data Categorization The study specifically focused on: 1. Age-related differences in the prevalence of anatomical variants and degenerative changes. 2. Gender-related differences in the prevalence of the above variations and in mean joint space measurements. Statistical Analysis All data were tabulated and analyzed using the Epi Info software version 7. Categorical variables (presence or absence of variants) were expressed as frequencies and percentages, while continuous variables (age, height, weight, BMI, and joint space measurements) were expressed as mean ± standard deviation (SD). Comparisons between categorical variables were performed using the chi-square test or Fisher’s exact test where appropriate. For continuous variables, the independent samples t-test (for gender) and one-way analysis of variance (ANOVA) (for age groups) were applied. A p-value of <0.05 was considered statistically significant.

Table 1 presents the age- and gender-wise demographic distribution of the study cohort. Out of 108 participants, 33 (30.6%) were ≤40 years, 48 (44.4%) were between 41–60 years, and 27 (25.0%) were >60 years. Among females, half (50.0%) belonged to the 41–60-year group, whereas among males, the largest proportion (37.9%) was ≤40 years. Although females were slightly older on average than males, no statistically significant difference in age distribution between genders was observed (p = 0.201). Table 1. Demographic Distribution by Age and Gender Age group (years) Female n (%) Male n (%) Total n (%) p-value ≤40 11 (22.0) 22 (37.9) 33 (30.6) 0.201 41–60 25 (50.0) 23 (39.7) 48 (44.4) >60 14 (28.0) 13 (22.4) 27 (25.0) Total 50 (100) 58 (100) 108 (100) Table 2 summarizes the prevalence of anatomical variants of the sacroiliac joint according to gender. Accessory sacroiliac joints (ASI) were more common in females than males on both sides, although not statistically significant. In contrast, bipartite iliac bones were markedly more frequent among females on both the left (28.0% vs. 3.4%) and right (30.0% vs. 3.4%) sides, with highly significant differences (p < 0.001). Crescentic iliac bones also showed a significant female predominance (left: 30.0% vs. 10.3%, p = 0.010; right: 18.0% vs. 5.2%, p = 0.034). Semi-circular defects were more common in females (40.0% vs. 20.7% on the left, p = 0.028), whereas other variants, including the iliosacral complex, intra-articular new bone formation (IANBF), and miscellaneous forms, did not differ significantly by gender. Table 2. Anatomical Variants by Gender Variant Female Left n (%) Male Left n (%) p-value Female Right n (%) Male Right n (%) p-value Accessory SI Joint (ASI) 8 (16.0) 7 (12.1) 0.556 14 (28.0) 8 (13.8) 0.068 Bipartite Iliac Bone 14 (28.0) 2 (3.4) <0.001 15 (30.0) 2 (3.4) <0.001 Crescentic Iliac 15 (30.0) 6 (10.3) 0.010 9 (18.0) 3 (5.2) 0.034 Semi-Circular Defect 20 (40.0) 12 (20.7) 0.028 19 (38.0) 13 (22.4) 0.077 Iliosacral Complex 7 (14.0) 5 (8.6) 0.375 6 (12.0) 5 (8.6) 0.563 Intra-articular New Bone Formation (IANBF) 7 (14.0) 7 (12.1) 0.766 9 (18.0) 9 (15.5) 0.730 Other Variations 5 (10.0) 7 (12.1) 0.733 5 (10.0) 6 (10.3) 0.953 Table 3 highlights the gender-based distribution of degenerative sacroiliac changes. Overall degenerative changes were slightly higher among females on the left side (24.0% vs. 15.5%), but without statistical significance. Subchondral cysts demonstrated a notable gender difference on the right side, being observed in 18.0% of females but absent in males (p = 0.001). Other degenerative changes, including ankylosis, joint space narrowing, and subchondral sclerosis, were found more frequently in males, though these differences did not reach statistical significance. Table 3. Degenerative Variants by Gender Variant Female Left n (%) Male Left n (%) p-value Female Right n (%) Male Right n (%) p-value Degenerative Changes 12 (24.0) 9 (15.5) 0.267 11 (22.0) 12 (20.7) 0.868 Subchondral Cysts 4 (8.0) 3 (5.2) 0.552 9 (18.0) 0 (0.0) 0.001 Ankylosis 1 (2.0) 3 (5.2) 0.384 0 (0.0) 3 (5.2) 0.103 Joint Space Narrowing 3 (6.0) 6 (10.3) 0.415 5 (10.0) 6 (10.3) 0.953 Subchondral Sclerosis 4 (8.0) 8 (13.8) 0.339 4 (8.0) 9 (15.5) 0.231 Table 4 compares the presence of symmetry and asymmetry in the sacroiliac joints across genders. Symmetry was more frequently observed in males (75.9%) compared to females (66.0%), while asymmetry was higher in females (34.0% vs. 24.1%). However, these differences were not statistically significant (p = 0.259). Table 4. Symmetry vs Asymmetry by Gender Parameter Female n (%) Male n (%) Total n (%) p-value Symmetry 33 (66.0) 44 (75.9) 77 (71.3) 0.259 Asymmetry 17 (34.0) 14 (24.1) 31 (28.7) 0.259 Table 5 explores the distribution of anatomical variants according to age groups. Accessory sacroiliac joints demonstrated a strong age-related trend, being absent in participants ≤40 years, but rising to 18.8% in the 41–60-year group and 22.2–40.7% in those >60 years (p = 0.020 left, p = 0.004 right). Semi-circular defects also showed a sharp age-related increase, from 9.1–12.1% in those ≤40 years to 35.4–44.4% in older groups (p = 0.006 left, p = 0.027 right). Iliosacral complexes on the left side (p = 0.045) and IANBF on the right (p = 0.041) were significantly associated with age. Other variants, such as bipartite and crescentic iliac bones, showed increasing trends with age but did not reach statistical significance. Table 5. Anatomical Variants by Age Variant ≤40 yrs n (%) 41–60 yrs n (%) >60 yrs n (%) p-value ASI (Left) 0 (0.0) 9 (18.8) 6 (22.2) 0.020 ASI (Right) 2 (6.1) 9 (18.8) 11 (40.7) 0.004 Bipartite Iliac (Left) 2 (6.1) 8 (16.7) 6 (22.2) 0.191 Bipartite Iliac (Right) 3 (9.1) 9 (18.8) 5 (18.5) 0.453 Crescentic Iliac (Left) 5 (15.2) 11 (22.9) 5 (18.5) 0.680 Crescentic Iliac (Right) 2 (6.1) 8 (16.7) 2 (7.4) 0.256 Semi-Circular Defect (Left) 3 (9.1) 17 (35.4) 12 (44.4) 0.006 Semi-Circular Defect (Right) 4 (12.1) 17 (35.4) 11 (40.7) 0.027 Iliosacral Complex (Left) 0 (0.0) 7 (14.6) 5 (18.5) 0.045 Iliosacral Complex (Right) 1 (3.0) 5 (10.4) 5 (18.5) 0.142 IANBF (Left) 2 (6.1) 6 (12.5) 6 (22.2) 0.178 IANBF (Right) 1 (3.0) 11 (22.9) 6 (22.2) 0.041 Other Variations (Left) 1 (3.0) 7 (14.6) 4 (14.8) 0.208 Other Variations (Right) 1 (3.0) 7 (14.6) 3 (11.1) 0.236 Table 6 details age-related distribution of degenerative changes and symmetry. Degenerative changes increased significantly with age, affecting 37.0% of individuals >60 years (p = 0.027 left, p = 0.045 right). Subchondral cysts on the right side were also significantly associated with older age (p = 0.010). Subchondral sclerosis on the right increased notably in the >60 years group (25.9%, p = 0.023). Asymmetry rose sharply with advancing age, from 15.2% in ≤40 years to 48.1% in >60 years (p = 0.018), confirming that asymmetry is significantly age-related. Table 6. Degenerative Variants and Symmetry by Age Variant ≤40 yrs n (%) 41–60 yrs n (%) >60 yrs n (%) p-value Degenerative Changes (Left) 4 (12.1) 7 (14.6) 10 (37.0) 0.027 Degenerative Changes (Right) 7 (21.2) 6 (12.5) 10 (37.0) 0.045 Subchondral Cyst (Left) 2 (6.1) 3 (6.3) 2 (7.4) 0.974 Subchondral Cyst (Right) 1 (3.0) 2 (4.2) 6 (22.2) 0.010 Ankylosis (Left) 0 (0.0) 3 (6.3) 1 (3.7) 0.343 Ankylosis (Right) 1 (3.0) 1 (2.1) 1 (3.7) 0.914 Joint Space Narrowing (Left) 2 (6.1) 4 (8.3) 3 (11.1) 0.780 Joint Space Narrowing (Right) 2 (6.1) 3 (6.3) 6 (22.2) 0.058 Subchondral Sclerosis (Left) 2 (6.1) 6 (12.5) 4 (14.8) 0.517 Subchondral Sclerosis (Right) 1 (3.0) 5 (10.4) 7 (25.9) 0.023 Asymmetry 5 (15.2) 13 (27.1) 13 (48.1) 0.018 Symmetry 28 (84.8) 35 (72.9) 14 (51.9) 0.018 Table 7 provides a comparison of demographic and joint space parameters by gender. No significant differences were observed between males and females for mean age, height, weight, or BMI. The mean left joint space was similar in both sexes (2.84 mm). However, the right joint space was significantly narrower in males (2.62 ± 0.45 mm) compared to females (2.93 ± 0.75 mm), with statistical significance (p = 0.011). Table 7. Gender-Wise Comparative Statistics of Demographic and Joint Space Parameters Parameter Male (Mean ± SD) Female (Mean ± SD) p-value Age (years) 47.02 ± 16.87 51.26 ± 16.53 0.191 Height (cm) 159.72 ± 4.87 158.08 ± 4.30 0.068 Weight (kg) 71.22 ± 9.42 80.72 ± 83.30 0.390 BMI (kg/m²) 28.09 ± 2.74 27.86 ± 2.28 0.638 Joint Space Left (mm) 2.84 ± 0.75 2.84 ± 0.97 0.984 Joint Space Right (mm) 2.93 ± 0.75 2.62 ± 0.45 0.011 Table 8 presents age group–wise comparative statistics for demographic and joint space parameters. No significant age-related differences were found for height, weight, or joint space measurements. BMI showed a trend of gradual increase with age (27.22 in ≤40 years to 28.73 in >60 years), but this did not reach statistical significance (p = 0.066). Similarly, mean joint space values on both sides remained comparable across the three age groups (p > 0.05). Table 8. Age Group-Wise Comparative Statistics of Demographic and Joint Space Parameters Parameter 18–40 yrs (Mean ± SD) 41–60 yrs (Mean ± SD) >60 yrs (Mean ± SD) p-value Height (cm) 158.64 ± 4.26 159.33 ± 4.28 158.70 ± 5.80 0.764 Weight (kg) 67.64 ± 6.59 83.02 ± 84.85 72.22 ± 10.02 0.464 BMI (kg/m²) 27.22 ± 2.31 28.09 ± 2.44 28.73 ± 2.77 0.066 Joint Space Left (mm) 2.79 ± 0.61 2.83 ± 0.58 2.92 ± 1.39 0.834 Joint Space Right (mm) 2.76 ± 0.61 2.77 ± 0.60 2.83 ± 0.78 0.893

The present study provides detailed insight into the prevalence of sacroiliac joint (SIJ) variants and degenerative changes in relation to both age and gender, based on high-resolution CT imaging. The findings highlight significant demographic influences on SIJ morphology, offering a valuable addition to the literature, particularly in the context of South Asian populations where data remain limited [19–21]. One of the most striking observations of this study was the higher frequency of bipartite and crescentic iliac bones in females. On both the left and right sides, bipartite iliac variants were present in nearly one-third of women, compared to less than 5% of men, a highly significant difference (p < 0.001). Similarly, crescentic variants were also more common in women (p = 0.010 on the left; p = 0.034 on the right). These findings are consistent with earlier reports, which attribute such female predominance to pelvic adaptations for childbirth [11,12,16–18]. During pregnancy, hormonal factors—particularly relaxin and estrogen—lead to increased ligamentous laxity and altered SIJ biomechanics. Repeated exposure to these changes across multiple pregnancies may induce remodeling, resulting in higher prevalence of such morphological variants in women [13–15]. Furthermore, the broader female pelvic morphology, designed to accommodate parturition, inherently modifies load transmission across the SIJ, predisposing to unique variants such as crescentic and bipartite configurations. In contrast, degenerative variants were more commonly seen in males. Although not all differences reached statistical significance, subchondral cysts demonstrated a distinct gender disparity, being present in 18% of females on the right side but absent in males (p = 0.001). Degenerative sclerosis and ankylosis were more frequent in men, reflecting the higher axial loading and occupational stress typical of male-dominated physical labor in many populations [9,10]. Previous studies have suggested that males are prone to degenerative SIJ changes due to greater body mass, mechanical strain, and reduced pelvic flexibility [17,20,21]. These findings are further supported by biomechanical analyses demonstrating that the male pelvis, being narrower and less compliant than the female pelvis, transmits higher focal loads to the sacroiliac articulation [16,19]. The age-wise analysis revealed additional important associations. Accessory SI joints, absent in participants under 40 years, were increasingly frequent in the 41–60 and >60 year groups (p = 0.020 on the left; p = 0.004 on the right). This finding parallels earlier work demonstrating that accessory articulations represent adaptive or degenerative phenomena that emerge with age as the joint remodels in response to chronic stress [13–15]. Similarly, semi-circular defects showed a steep age-related rise, affecting over 40% of older participants compared to under 12% in the youngest group (p = 0.006, 0.027). These changes suggest progressive osseous remodeling of the iliac surface with advancing age, an interpretation that has also been supported by histological studies documenting increased bony irregularities and fibrocartilaginous degeneration in older SIJs [17,19]. Degenerative changes followed an expected trajectory with age. Individuals over 60 years had significantly higher prevalence of sclerosis, cysts, and asymmetry, highlighting the impact of skeletal aging. Subchondral sclerosis on the right was particularly associated with advanced age (p = 0.023), and asymmetry increased from 15.2% in ≤40 years to nearly half of participants above 60 years (p = 0.018). These results corroborate reports by previous authors that degenerative SIJ asymmetry is a hallmark of aging and may confound radiological evaluation for pathological sacroiliitis [14,17]. The rise of subchondral cysts with age is consistent with microtrabecular stress remodeling described in earlier CT-based studies [9,10,]. Analysis of joint space width provided further insights. While mean left joint space was nearly identical between sexes, males demonstrated a significantly narrower right joint space (2.62 ± 0.45 mm) compared to females (2.93 ± 0.75 mm, p = 0.011). This observation has not been widely reported in prior literature, making it a noteworthy finding of this study. Possible explanations include gender differences in pelvic tilt, sacral orientation, and occupational biomechanics. Male-dominated occupations such as carpentry, farming, and driving, which involve asymmetrical loading and repetitive strain, may predispose to right-sided narrowing [19–21]. It is also possible that inherent anatomical differences in the male sacrum and iliac surface contribute to this asymmetry, although further biomechanical studies would be required to confirm this hypothesis. BMI, height, and weight did not show significant differences across age or gender groups in this cohort. However, BMI trended upward with age, from 27.2 in younger participants to 28.7 in those >60 years, echoing global observations that increased adiposity contributes to SIJ stress and degenerative changes [22-25]. Despite this, BMI did not emerge as a significant predictor of specific variants in the present analysis, suggesting that age and gender exert stronger influences than body habitus on SIJ morphology in this population. Clinical Implications The findings of this study have direct and important clinical implications for radiologists and musculoskeletal physicians. Anatomical variants such as bipartite and crescentic iliac bones, more commonly seen in females, may mimic pathological lesions on CT or MRI and could be misinterpreted as erosions or inflammatory changes. Awareness of these normal variants is crucial to avoid unnecessary further investigations or inappropriate therapy. Similarly, age-related increases in accessory SI joints, semi-circular defects, and degenerative changes such as sclerosis and cysts underscore the importance of interpreting imaging findings within the demographic context of the patient. Gender-specific differences in joint space narrowing, particularly the significant right-sided narrowing in males observed in this study, further highlight the need for cautious interpretation, as such changes may represent physiological remodeling rather than true pathology. These insights are clinically valuable when evaluating patients with chronic low back pain, where the sacroiliac joint accounts for up to one-third of cases. Correct recognition of normal variation versus disease can reduce misdiagnosis of sacroiliitis, ankylosing spondylitis, and degenerative SIJ disorders, leading to more accurate patient management and treatment planning. Strengths and Limitations The present study has several notable strengths. It is one of the few CT-based evaluations of sacroiliac joint variants in a South Asian population, providing region-specific data where literature remains sparse. The use of high-resolution imaging allowed detailed visualization of both anatomical and degenerative variants, while systematic age- and gender-wise analysis offered comprehensive demographic insights. Furthermore, independent evaluation by two radiologists minimized observer bias. However, certain limitations must be acknowledged. The retrospective design may introduce inherent selection bias, and the study was conducted at a single center, which may limit generalizability. Occupational and lifestyle factors, which are known to influence SIJ morphology, were not analyzed in detail. Hormonal influences and parity history were not incorporated into the age- and gender-specific analysis, though these are likely important contributors to SIJ variation in females. Finally, while the sample size was adequate to detect major differences, it may not have been sufficient to identify more subtle associations.

This CT-based study demonstrates that sacroiliac joint morphology is significantly influenced by both age and gender. Anatomical variants such as bipartite and crescentic iliac bones were markedly more frequent in females, whereas degenerative changes were more common in males. With advancing age, the prevalence of accessory joints, semi-circular defects, sclerosis, and asymmetry increased significantly, underscoring the role of aging in SIJ remodeling. Importantly, right-sided joint space narrowing was significantly greater in males, a novel observation in this population. These findings emphasize the necessity of considering demographic context when interpreting sacroiliac imaging. Recognizing normal anatomical variants and age-related changes is crucial to distinguish them from pathological processes, thereby enhancing diagnostic accuracy and optimizing clinical management of patients presenting with SIJ-related disorders.

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