1 Introduction

Fragility fractures of the sacrum have emerged as a major challenge in orthopaedic trauma care, driven by demographic aging and the increasing prevalence of osteoporosis. These injuries typically occur after low-energy trauma or even spontaneously and are frequently associated with prolonged pain, impaired mobilization, and loss of functional independence in older adults ^1–4. Owing to the limited sensitivity of conventional radiographs for posterior pelvic ring injuries, diagnostic delay and underestimation of fracture extent remain common, often resulting in prolonged symptoms and delayed initiation of appropriate management.^1,2

To characterize fracture morphology and mechanical stability in this population, several classification systems have been established. The Fragility Fracture of the Pelvis (FFP) classification and the OF-Pelvis system are widely used and emphasize posterior pelvic involvement as the principal determinant of instability rather than trauma mechanism or fracture displacement alone.^3,4 Within these frameworks, fracture patterns range from unilateral, nondisplaced posterior lesions to complex bilateral posterior disruptions. Notably, such variability is observed even among patients with similar injury mechanisms and comparable osteoporotic status, suggesting that factors beyond bone quality may influence fracture morphology and instability.

Bone mineral density (BMD) and CT-derived bone density are well-established contributors to fracture risk and severity in osteoporotic patients. Reduced bone quality is a prerequisite for the development of sacral fragility fractures and has been associated with more complex fracture patterns. However, bone density alone does not fully explain why some patients develop bilateral posterior instability while others sustain limited unilateral fractures under similar conditions.⁵ This discrepancy indicates that additional intrinsic factors may modulate the biomechanical environment of the posterior pelvic ring.

Sagittal pelvic morphology represents one such intrinsic factor. Pelvic incidence (PI) is a fixed anatomical parameter describing the spatial relationship between the sacrum and the pelvis and constitutes the morphological cornerstone of spinopelvic alignment.⁶ A higher PI is associated with increased sacral slope and altered load transmission across the lumbosacral junction, resulting in greater horizontal shear forces acting on the sacrum.^7,8 In osteoporotic bone, these biomechanical conditions may predispose to more extensive fracture propagation across the sacral ala and posterior pelvic ring. While PI has been extensively investigated in the context of degenerative spinal disorders, sagittal imbalance, and adjacent segment pathology, its role in sacral fragility fractures has not been comprehensively evaluated.

Previous studies addressing spinopelvic parameters in fragility fractures of the pelvis have primarily focused on posture-dependent parameters such as pelvic tilt and compensatory mechanisms related to pain and sagittal balance. However, these parameters may reflect adaptive responses rather than intrinsic morphology and are influenced by patient positioning, pain, and mobility limitations. In contrast, PI is a posture-independent anatomical constant that can be reliably measured on supine computed tomography (CT), making it particularly suitable for assessment in elderly patients who are often unable to tolerate standing radiographs.⁹

The present study was therefore designed to investigate whether sagittal pelvic morphology—specifically pelvic incidence—is associated with posterior instability patterns in sacral fragility fractures. Using standardized CT-based morphometric analysis, we examined the relationship between spinopelvic parameters and fracture severity according to the FFP classification, as well as unilateral versus bilateral posterior involvement according to the OF-Pelvis system. To account for bone quality as a potential confounder, CT-derived bone density was incorporated into multivariable analyses. We hypothesized that higher pelvic incidence would be associated with more advanced fracture patterns and bilateral posterior instability, independent of bone density and demographic factors.

2 Materials and methods

2.4

2.4 Measurement of spinopelvic parameters

Spinopelvic parameters were measured on midsagittal CT reconstructions using a standardized digital workstation (Fig. 1). The following parameters were evaluated:•Pelvic Incidence (PI): measured as the angle between a line perpendicular to the midpoint of the superior endplate of S1 and a line connecting this point with the bicoxofemoral axis.•Pelvic Tilt (PT): the angle between the vertical and the line joining the bicoxofemoral axis to the midpoint of the sacral endplate.•Sacral Slope (SS): the angle between the sacral endplate and the horizontal.•Sacral Kyphosis: the angle between the superior endplate of S1 and the anterior sacral cortex at the level of S3–S4.

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Fig. 1 Measurement of pelvic incidence (PI) on a midsagittal computed tomography reconstruction in the supine position. The bicoxofemoral axis is defined by the midpoint between the centers of both femoral heads. Pelvic incidence is measured as the angle between the line connecting the midpoint of the S1 superior endplate to the bicoxofemoral axis and a line perpendicular to the S1 endplate.

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The bicoxofemoral axis was determined by identifying the centers of both femoral heads on coronal CT images and projecting their midpoint onto the midsagittal plane. Measurements were performed independently by two fellowship-trained spine surgeons blinded to fracture classification. Pelvic incidence, as a posture-independent anatomical constant, was considered the primary morphometric parameter of interest.

3 Results

3.1

3.1 Study population and fracture characteristics

We identified 164 eligible patients. Morphometric measurements could not be obtained in 10 cases due to incomplete visualization/unsuitable midsagittal reconstruction. A total of 154 patients met the inclusion criteria. The mean age was 80.4 ± 8.0 years (range 61–95 years), and the cohort was predominantly female (130 women, 84.4%). Injury mechanisms consisted of low-energy falls from standing height in 118 patients (76.8%) and atraumatic or spontaneous fractures in 36 patients (23.2%). No patient sustained a high-energy injury. All patients underwent CT and MRI within one week of presentation, confirming acute fracture morphology and minimizing diagnostic delay.

3.1.1

3.1.1 Fracture distribution

According to the Fragility Fracture of the Pelvis (FFP) classification, fractures were distributed as follows: FFP II in 63 patients (40.9%), FFP III in 17 patients (11.0%), and FFP IV in 74 patients (48.1%). No FFP I fractures were included in this sacral fracture cohort. Based on the OF-Pelvis classification, 47 patients (30.5%) presented with unilateral posterior involvement (Type 3), whereas 107 patients (69.5%) exhibited bilateral posterior involvement (Type 4). Patient characteristics and fracture distribution are summarized in Table 1.

Table 1 Patient characteristics and fracture distribution.

Variable	Value	Percentage
Patients (n)	154	100%
Age (years)	80.4 ± 8.0	—
Female sex	130	84.4%
FFP II	63	40.9%
FFP III	17	11.0%
FFP IV	74	48.1%
OF Type 3	47	30.5%
OF Type 4	107	69.5%

3.1.2

3.1.2 Spinopelvic parameters and reliability

Mean spinopelvic parameters for the entire cohort were: pelvic incidence (PI) 62.4° ± 10.0°, pelvic tilt (PT) 20.2° ± 6.9°, sacral slope (SS) 42.2° ± 9.0°, and sacral kyphosis 45.3° ± 14.6°. Interobserver reliability was excellent for all measured parameters, with intraclass correlation coefficients of 0.94 for PI, 0.88 for PT, 0.92 for SS, and 0.93 for sacral kyphosis.

3.1.3

3.1.3 Association with fracture instability

Spearman rank correlation analysis demonstrated a significant positive association between pelvic incidence and fracture severity according to the FFP classification (ρ = 0.236, p = 0.003). Sacral slope (ρ = 0.221, p = 0.006) and sacral kyphosis (ρ = 0.229, p = 0.004) were also positively correlated with increasing FFP stage. No significant correlation was observed between pelvic tilt and FFP classification (p = 0.43). See Fig. 2A and Table 2.

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Fig. 2 Fig. 2A Distribution of pelvic incidence across fracture severity according to the Fragility Fracture of the Pelvis (FFP) classification in the final analyzed cohort (n = 154). Higher pelvic incidence values were associated with increasing fracture severity (Spearman ρ = 0.236, p = 0.003). Fig. 2B Comparison of pelvic incidence between unilateral posterior fractures (OF-Pelvis Type 3) and bilateral posterior fractures (OF-Pelvis Type 4) in the final analyzed cohort (n = 154). Patients with bilateral posterior involvement exhibited significantly higher pelvic incidence values.

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Table 2 Correlation of sagittal spinopelvic parameters with FFP classification.

Parameter	Spearman ρ	p-value
Pelvic incidence	0.236	0.003
Pelvic tilt	0.065	0.43
Sacral slope	0.221	0.006
Sacral kyphosis	0.229	0.004

3.1.4

3.1.4 Comparison between unilateral and bilateral posterior fractures

Patients with bilateral posterior pelvic involvement (OF-Pelvis Type 4) exhibited significantly higher pelvic incidence values than those with unilateral posterior fractures (Type 3) (63.7° ± 10.2° vs. 59.5° ± 9.0°, p = 0.012). Pelvic tilt was numerically higher in patients with bilateral involvement (20.9° ± 7.1° vs. 18.4° ± 6.1°), but this difference did not remain statistically significant in adjusted analyses. Differences in sacral slope and sacral kyphosis between the two groups did not reach statistical significance. See Fig. 2B.

3.1.5

3.1.5 Multivariable analysis of predictors for bilateral posterior instability

Multivariable logistic regression analysis was performed to identify independent predictors of bilateral posterior pelvic involvement (OF-Pelvis Type 4). After adjustment for age, sex, pelvic tilt, and CT-derived bone density (HU), pelvic incidence remained independently associated with bilateral posterior instability (odds ratio 1.048 per degree increase; 95% confidence interval 1.003–1.096; p = 0.035). This corresponds to an approximate odds ratio of 1.6 per 10° increase in pelvic incidence. CT-derived HU was not independently associated with posterior instability in the adjusted model. Increasing age was independently associated with a lower likelihood of bilateral posterior involvement, whereas sex and pelvic tilt were not significant predictors (Table 3).

Table 3 Multivariable logistic regression for bilateral posterior instability (OF-Pelvis Type 4).

Variable	Odds ratio	95% CI	p-value
Pelvic incidence (per degree)	1.048	1.003–1.096	0.035
Age (per year)	0.942	0.896–0.990	0.018
Pelvic tilt	1.025	0.964–1.090	0.422
Sex (female)	0.418	0.128–1.363	0.148
CT-derived HU	1.000	0.990–1.011	0.927

The magnitude and direction of the association between pelvic incidence and bilateral posterior involvement remained consistent across adjusted models, supporting the robustness of the observed relationship.

4 Discussion

The present study demonstrates a significant association between pelvic incidence (PI) and posterior instability patterns in sacral fragility fractures. Patients with bilateral posterior involvement according to the OF-Pelvis classification exhibited higher PI values than those with unilateral fractures, and PI remained independently associated with bilateral posterior instability after adjustment for demographic factors, pelvic tilt, and CT-derived bone density. These findings suggest that sagittal pelvic morphology may contribute to posterior pelvic instability patterns in sacral fragility fractures, alongside established factors such as bone quality and trauma mechanism.

Pelvic incidence represents the fundamental morphologic determinant of spinopelvic alignment and governs the spatial orientation of the sacrum relative to the pelvis.⁶ A higher PI is associated with increased sacral slope and a greater horizontal shear component across the lumbosacral junction, resulting in altered load transmission through the sacral ala and posterior pelvic ring.^7,8 In the setting of osteoporotic bone, these biomechanical conditions may facilitate bilateral fracture propagation or progressive instability following minor trauma or spontaneous fracture onset. The observed association between PI and posterior instability in the present cohort extends established biomechanical concepts from degenerative spinal pathology to sacral fragility fractures.^6,11,12

Previous investigations into fragility fractures of the pelvis have primarily emphasized bone quality, trauma mechanism, and posture-dependent parameters such as pelvic tilt and compensatory mechanisms related to sagittal balance.^10,13 While these factors are undoubtedly relevant, they do not fully account for the marked variability in fracture patterns observed among patients with similar osteoporotic status and comparable injury mechanisms.^3,4 In the present study, pelvic tilt showed no independent association with posterior fracture extent in multivariable modeling, suggesting that static pelvic morphology rather than positional compensation is more closely associated with posterior fracture complexity.

The inclusion of CT-derived bone density as a covariate represents a key methodological strength of this study. Reduced bone quality is a prerequisite for sacral fragility fractures and has been associated with increased fracture complexity in prior work.⁵ However, the persistence of the association between PI and bilateral posterior instability after adjustment for CT-derived HU suggests that pelvic incidence does not merely act as a surrogate marker for osteoporosis severity. Instead, PI appears to represent an independent morphologic factor that modifies the biomechanical environment of the posterior pelvic ring.

Assessment of spinopelvic parameters is traditionally performed using standing radiographs.¹⁴ In elderly patients presenting with acute pelvic pain, however, upright imaging is often impractical or unsafe. Computed tomography provides accurate and reproducible morphometric data in the supine position, and pelvic incidence, as a posture-independent anatomical constant, can be reliably measured on CT reconstructions.^6,14 In contrast, posture-dependent parameters such as pelvic tilt and sacral slope are influenced by pain, positioning, and compensatory mechanisms and should therefore be interpreted with caution when derived from supine imaging.

Diagnostic delay and fracture progression represent important considerations in studies of pelvic fragility fractures. Progressive instability from unilateral to bilateral posterior involvement has been described, particularly when posterior pelvic injuries are initially underestimated or missed.¹⁴ In the present cohort, all patients underwent both MRI and CT within one week of presentation, minimizing the likelihood that fracture classification reflected delayed progression rather than the initial extent of posterior instability.^3,4,14

Interestingly, increasing age was independently associated with a lower likelihood of bilateral posterior instability. This finding may reflect survivor bias, differences in activity patterns, or unmeasured clinical confounders and should be interpreted with caution. Nevertheless, it highlights the complex interplay between morphology, biology, and patient-related factors in sacral fragility fractures and warrants further investigation.

From a clinical perspective, pelvic incidence is a fixed anatomical parameter that cannot be modified therapeutically. Nevertheless, recognition of a morphology-related predisposition may assist in identifying patients at increased risk for bilateral posterior involvement. In individuals presenting with sacral pain after low-energy trauma, a higher PI may justify a lower threshold for comprehensive cross-sectional imaging or closer surveillance to detect occult or progressive posterior instability. Importantly, the present findings do not support the use of PI as a standalone decision-making tool but rather as a complementary factor within a broader risk assessment framework incorporating bone quality, frailty, and clinical presentation.

Several limitations must be acknowledged. The retrospective single-center design introduces the potential for selection and referral bias. Functional outcomes and longitudinal progression were not assessed. Bone mineral density data were incomplete, necessitating reliance on CT-derived HU as a surrogate for bone quality.² Although HU measurements were obtained using a standardized and previously validated protocol, they cannot fully substitute for dual-energy X-ray absorptiometry. Furthermore, posture-dependent spinopelvic parameters were assessed in the supine position, limiting conclusions regarding dynamic sagittal balance and compensatory mechanisms.¹³ Finally, the observational nature of the study precludes causal inference.

Despite these limitations, the present study has several strengths, including a relatively large cohort, standardized CT-based morphometric assessment, excellent interobserver reliability, early cross-sectional imaging with MRI and CT, and the use of two complementary fracture classification systems. These features support the robustness of the observed association between pelvic incidence and posterior instability patterns.

In summary, the findings of this study suggest that sagittal pelvic morphology, as reflected by pelvic incidence, is associated with posterior instability patterns in sacral fragility fractures independent of bone quality. These results support the concept of a morphology-related risk phenotype and provide a rationale for future prospective studies integrating pelvic morphology, bone density, frailty, and clinical outcomes to improve risk stratification in elderly patients with pelvic fragility fractures.

5 Conclusion

Pelvic incidence is associated with posterior instability patterns in sacral fragility fractures and remains independently related to bilateral posterior involvement after adjustment for bone quality and demographic factors. As a fixed anatomical parameter reliably measurable on routine CT, pelvic incidence reflects an intrinsic morphologic characteristic that may contribute to the biomechanical environment predisposing to complex posterior fracture patterns in osteoporotic bone.

Although pelvic incidence cannot be modified therapeutically, awareness of a morphology-related susceptibility may support earlier recognition of patients at increased risk for bilateral posterior instability and justify closer surveillance or a lower threshold for comprehensive cross-sectional imaging in elderly individuals presenting with sacral pain after low-energy trauma. Integration of pelvic morphology with bone quality and clinical factors may enhance future risk stratification strategies in the management of sacral fragility fractures.

Guardian/patient's consent

Due to the retrospective nature of the study and the use of anonymized data, the requirement for informed consent from patients or their legal guardians was waived by the ethics committee.

Ethical statement

Ethical approval was obtained from the ethical committee of the Medical Association of Hessen, under research no. 2024 - 3959-evBO. The study was performed in accordance with the Declaration of Helsinki.

Credit author statement

Martin Naisan: Conceptualization, Methodology, Formal analysis, Writing – original draft.

Simon Harsch: Data curation, Investigation, Visualization, Writing – review & editing.

Marco Brenneis: Investigation, Data curation, Writing – review & editing.

Yazan Noufal: Investigation, Data curation, Writing – review & editing.

Felix Schmitz: Investigation, Resources, Writing – review & editing.

Yama Afghanyar: Formal analysis, Validation, Writing – review & editing.

Marcus Richter: Supervision, Methodology, Writing – review & editing.

Philipp Drees: Supervision, Resources, Writing – review & editing.

Philipp Hartung: Conceptualization, Methodology, Supervision, Writing – review & editing.

Andreas Kramer: Supervision, Project administration, Writing – review & editing.

Declaration of generative AI and AI-assisted technologies in the manuscript preparation process

During the preparation of this work the author(s) used ChatGPT (OpenAI) to improve language and readability and to assist with manuscript formatting. After using this tool, the author(s) reviewed and edited the content as needed and take full responsibility for the content of the manuscript.

Funding statement

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.