Determination of Sex from the Sternum and Fourth Rib Measurements (A Cross-Sectional Study Using Thoracic Computed Tomography (CT)

The accurate determination of sex from human skeletal remains is one of the most fundamental tasks in forensic anthropology and medicolegal investigations. Establishing sex not only narrows down the pool of possible identities in cases of unidentified bodies but also forms the foundation for constructing the biological profile of an individual, which subsequently includes age, ancestry, and stature estimations. In archaeological contexts, reliable sex estimation contributes to understanding demographic composition, migration, social structure, and cultural practices of past population(1)s. Thus, sex determination is rightly regarded as the keystone of forensic and anthropological analysis, with direct implications for legal, humanitarian, and scientific purposes. Traditionally, the pelvis and skull have been considered the most sexually dimorphic bones in the human skeleton. The pelvis, adapted for childbirth, exhibits wide and consistent differences between males and females, while cranial traits such as brow ridges, mastoid processes, and mandibular robustness provide additional discriminatory value. However, reliance on these bones can become problematic in situations where skeletal remains are incomplete, fragmented, or taphonomically altered. In mass disasters, advanced decomposition, or violent trauma, it is not uncommon for pelvic and cranial elements to be absent or too damaged for reliable analysis. These limitations underscore the necessity of identifying alternative skeletal structures that are resilient, consistently preserved, and capable of reflecting sexual dimorphism. The sternum and ribs, located in the protective thoracic cavity, meet many of these requirements. The sternum is a flat bone comprised of the manubrium, mesosternum (body), and xiphoid process, articulating with the clavicles and upper seven ribs. It not only plays a protective role for vital organs such as the heart and lungs but also demonstrates measurable differences between sexes. Male sterna are generally longer, broader, and more robust due to greater skeletal mass and hormonal influences during growth and maturation. Ratios such as the manubrio-body index and variations in total sternal length have been shown to be useful for sex determination across populations. Similarly, the ribs—particularly the fourth rib are valuable due to their mid-thoracic position, articulation with the sternum, and relatively high preservation in forensic contexts. Male ribs tend to be longer, thicker, and more curved, reflecting differences in thoracic architecture and respiratory mechanics(2). The increasing availability of multidetector computed tomography (MDCT) has revolutionized the analysis of skeletal dimorphism. MDCT enables high-resolution, three-dimensional reconstructions of bones, allowing for precise morphometric measurements without the need for invasive or destructive procedures. Unlike traditional osteometric methods, CT imaging minimizes inter-observer variability and provides reproducible digital records that can be re-evaluated. These advantages are particularly critical in forensic settings where chain of custody, evidence preservation, and objectivity are paramount. Moreover, CT imaging allows for the collection of data from living populations undergoing routine medical scans, thereby overcoming some of the limitations of skeletal collections that may be outdated or demographically unrepresentative(3). Several studies across diverse populations have validated the utility of thoracic bones in sex estimation. Torimitsu and colleagues demonstrated over 85% accuracy in sex determination using sternal measurements in Japanese populations. Significant sexual dimorphism in sternal dimensions in French groups, while Bacci and co-workers highlighted rib curvature and thickness as reliable indicators in South American populations. Importantly, these studies also emphasize that patterns of sexual dimorphism are population-specific, shaped by genetic, nutritional, and environmental factors(4). Equations derived in one population may not be directly applicable to another, reinforcing the need for region-specific standards. In the Indian context, published data on CT-based thoracic measurements for sex estimation remain limited, creating a clear gap in forensic reference databases. Another notable challenge in previous research has been the reliance on dry bone collections and cadaveric samples. Although invaluable, such material is subject to postmortem changes, decomposition, and taphonomic processes that may distort measurements and reduce applicability to living populations. In contrast, CT imaging of living individuals provides a contemporary and more accurate representation of human variability. It also ensures larger, more diverse datasets that can account for the heterogeneity of modern populations. The present study was therefore designed to explore the discriminative potential of sternal and fourth rib morphometric parameters obtained from thoracic CT scans in an Indian population. By employing a cross-sectional design with balanced male and female representation, the study aimed to quantify key measurements, assess their sexual dimorphism, and derive discriminant function equations for sex classification. Advanced statistical tools, including independent t-tests, discriminant function analysis, and receiver operating characteristic curve analysis, were used to evaluate the predictive value of each parameter. By providing population-specific standards and highlighting the utility of imaging-based morphometrics, this research contributes to bridging the gap between classical forensic osteology and modern radiological techniques. Its findings hold particular relevance for forensic casework where traditional skeletal markers are absent, as well as for medico-legal investigations in hospital settings where CT imaging is routinely performed. Ultimately, this study emphasizes the growing role of radiological sciences in forensic anthropology and advocates for the integration of thoracic morphometrics into the standard toolkit for sex estimation.

MATERIALS AND METHODS

This investigation was designed as a cross-sectional observational study using retrospective thoracic computed tomography (CT) scans. The methodological approach was quantitative, allowing for precise numerical analysis of morphometric parameters of the sternum and fourth rib to determine sexual dimorphism. A quantitative design ensured objectivity, reproducibility, and statistical rigor, making it suitable for the development of discriminant function equations applicable in forensic practice.

Study Setting

The study was conducted in the Department of Radiodiagnosis of a tertiary-care teaching hospital. The institution is equipped with a 64-slice multidetector CT (MDCT) scanner, providing high-resolution imaging of thoracic anatomy, which is essential for morphometric evaluation.

Study Population

The target population consisted of adult male and female patients aged 20–70 years who underwent thoracic CT scans for non-traumatic and non-pathological reasons such as routine pulmonary or cardiovascular evaluation. Scans were selected retrospectively from the hospital’s Picture Archiving and Communication System (PACS) database.

Inclusion Criteria

• Adult patients aged 20–70 years.
• High-quality thoracic CT scans with complete visualization of the sternum and fourth rib.
• Scans performed using standard diagnostic protocols.
• Absence of congenital, traumatic, or pathological anomalies involving the bony thorax.

Exclusion Criteria

• Patients with a history of thoracic trauma, fractures, or surgical procedures (e.g., sternotomy, rib resection).
• Cases with congenital or acquired deformities such as scoliosis or pectus excavatum.
• CT scans showing bony pathologies such as osteomyelitis, metastasis, or gross degenerative changes.
• Poor-quality scans with artifacts, incomplete coverage, or motion blur obscuring bony landmarks.

Sample Size and Sampling Method

A total of 200 thoracic CT scans were included in the study, comprising 100 males and 100 females. The sample size was determined using power analysis, with a 95% confidence level and 80% power to detect significant sex-based differences in morphometric parameters. A purposive sampling technique was employed to ensure balanced representation of both sexes and adherence to inclusion criteria. Scans were retrospectively collected from a 12-month period, ensuring adequate sample diversity and relevance.

Imaging Modality and Protocol

Thoracic scans were acquired using 64-slice MDCT scanners (Siemens Somatom/GE Healthcare) following departmental protocols. Slice thickness: ≤1 mm for high-resolution reconstruction. Scan planes: Axial images reconstructed into sagittal and coronal multiplanar views. Window settings: Bone window (WW ≈ 2000 HU, WL ≈ 500 HU) to maximize cortical and trabecular bone visibility. Reconstruction: Multiplanar reconstructions (MPRs) and 3D volume rendering were employed where necessary.

Measurement Tools and Parameters

Quantitative data were obtained using the digital caliper tool of the PACS workstation. Measurements were rounded to the nearest 0.1 mm for precision. All measurements were performed in millimeters (mm) except angles (degrees). Sternal Parameters: Manubrium Length (ML) distance from jugular notch to manubriosternal joint. Mesosternum (Body) Length (MSL), Distance from manubriosternal joint to xiphisternal junction. Sternal Body Width (SBW), Maximum transverse width of the mesosternum at mid-body. Manubriosternal Angle (MSA), Angle between the manubrium and body of sternum (sagittal plane). Xiphoid Process Length (XPL), Distance from xiphisternal junction to tip of xiphoid process.

Fourth Rib Parameters

1. Anteroposterior (AP) Breadth: Maximum AP dimension at the sternal end of the rib.
2. Superoinferior (SI) Height: Maximum vertical dimension at the sternal end of the rib.

Observer Reliability

• Intra-observer reliability: One observer repeated measurements on 30 randomly selected scans after 2 weeks.
• Inter-observer reliability: A second observer independently measured the same 30 scans.
• Statistical measure: Intraclass correlation coefficient (ICC), with values >0.85 considered good agreement.

Data Collection Procedure: Retrieval of eligible thoracic CT scans from PACS. Screening for inclusion/exclusion criteria. Measurement of parameters using digital calipers in axial, sagittal, and coronal planes. Recording of results in a pre-designed Microsoft Excel data sheet, coded to remove identifiers. Quality control random re-checking of 10% of cases.

RESULTS

Study Sample

A total of 200 thoracic CT scans were included in the study, consisting of 100 males (50%) and 100 females (50%), aged between 20 and 70 years. All cases fulfilled inclusion criteria, and no pathological or traumatic alterations of the thoracic skeleton were observed. One case initially categorized incorrectly was excluded from final analysis after quality control.

Descriptive Statistics

The morphometric data of the sternum and fourth rib revealed distinct patterns between the sexes.

Table 1 Sex-based differences in thoracic morphometric parameters

These descriptive findings highlight a clear trend of larger thoracic dimensions in males, except for the MSA, which is more obtuse in females.