Age estimation by spheno-occipital synchondrosis fusion in South Indian population using computed tomography

Abstract

Background

Age estimation has paramount importance for the forensic practitioner. Depending on the timing of fusion, the spheno-occipital synchondrosis will be open during childhood and then later fuse during adulthood. Since there have been a lot of inconsistencies in terms of fusion degree, that can be attributed to variability in population or methodology. The aim of the present study was to determine the age of beginning of the spheno-occipital synchondrosis in the South Indian population using computed tomography.

Results

The study reported a mean age of 8.38 ± 1.21 for males and 8.2 ± 1.16 for females in the stage 0 (completely open) category. Spheno-occipital synchondrosis began to fuse (stage 1) at the mean age of 11.64 ± 1.19 for males and 10.21 ± 1.19 for females.

Conclusions

Spheno-occipital synchondrosis begins to close at the age of less than 15 years in males. Similarly, for females, the spheno-occipital synchondrosis begins to close at the age of less than 14 years. Also, a strong sexual dimorphism is seen as each stage of synchondrosis and starts earlier in females compared to males. In addition, the study also showed the degree of fusion increasing with age. This estimation will not only help in forensic and anthropological studies but also in assessing craniofacial growth in orthodontics.

Background

Age estimation plays a crucial role in forensic and anthropological contexts. They help in estimating the age at death of bodies deteriorated, damaged or fragmentary (Ubelaker and Khosrowshahi 2019; Alkass et al. 2010). In living individuals, age estimation is required for logical affirmation of the judicial authority and certain administrative departments (Pate et al. 2018). Estimation of age is more accurate for the immature skeletal system (Cardoso et al. 2014). Various skeletal markers, such as the root of the third molar tooth, the diaphysis-epiphysis fusion of the clavicle, tibia, calcaneus, occipital condyles, the fusion of skull sutures, cervical vertebrae and ossification of wrist joint have been studied for the age estimation (Cardoso et al. 2014; Hisham et al. 2018). Among these, the examination of skull sutures and ossification of the wrist joint may not always provide accurate results to estimate the age at the time of death (Can et al. 2014a; Akhlaghi et al. 2010). However, the degree of fusion of spheno-occipital synchondrosis (SOS) is considered to be a good indicator of age (Pate et al. 2018; Can et al. 2014a; Akhlaghi et al. 2010). The SOS is considered an active growth site that lengthens the skull base during the postnatal period (Yang et al. 2019; Ford 1958). The fusion of the sphenoid and occipital bones proceeds from endocranial towards ectocranial, and the gap between the bones is completely closed in adulthood (Demirturk Kocasarac et al. 2016). Therefore, analysing the degree of fusion of the spheno-occipital synchondrosis will be useful for forensic age assessment due to its relatively late ossification (Sinanoglu et al. 2016; Krishan and Kanchan 2013). However, the literature reports a great amount of inconsistency in terms of fusion degree that can be ascribed to the variability in populations and various methodologies employed for examination (Sinanoglu et al. 2016; Shirley and Jantz 2011; Can et al. 2014b). The various methods include direct examination of skeletal material, dry skull, postmortem, bone histology, and imaging modalities. Although direct examination techniques are cost-effective, they are not always accurate. A synchondrosis may look completely closed in dry skull,however, it may not be so in the CT images of the skull (Bassed et al. 2010). Similarly, conventional radiography is also inexpensive and accessible, but there is a problem of superimposition and a lack of resolution that can interfere with the interpretation of the image. Therefore, imaging modalities like computed tomography that provide three-dimensional information have better resolution for visualising anatomical structures, thereby having a greater accuracy in analysing the various degrees of fusion of spheno-occipital synchondrosis (Sahni et al. 1998; Okamoto et al. 1996; Alhazmi et al. 2017). Therefore, this study aimed to assess the time of fusion of spheno-occipital synchondrosis for age estimations in the South Indian population using computed tomography.

Methods

The study was conducted retrospectively in a tertiary care hospital in South India after approval from the Institutional Ethical Committee [IEC 547]. A total of 123 male and 123 female participants, aged from 7 to 15 years, and having previously undergone a CT scan of the brain, PNS, temporal bone, cervical spine, orbits and facial bones were included in the study. The CT images that showed artefacts at the base of the skull were excluded from the study.

All the CT examinations were performed on a 128-slice Philips Incisive CT and a 16-slice Big Bore CT scanner. Scans were performed using 120 kVp with automatic tube current modulation. The axial images of the CT scan were acquired in 1 mm, and all the images were reconstructed into coronal and sagittal sections.

The images collected were retrieved from the PACS server, Mediff PACS (InstaRISPACS version 4.1.0). The midsagittal section was considered the choice for viewing. The window level and window width of the obtained images were changed to 800 and 1600, respectively.

The fusion of SOS was assessed using a modified 3-stage system referred from Bassed et al. (Bassed et al. 2010). The stages are as follows, as shown in Fig. 1. Stage 0 indicates a completely open SOS. In stage 1, the SOS is beginning to close, starting with the fusion of the superior border. In stage 2, the synchondrosis is fused for more than half the length.

Fig. 1

Stages of SOS fusion stages in the mid-sagittal plane

The images were assessed by two subject experts. In cases of disagreement between the observers, a third observer was asked to rescore the image.

Statistical analysis

Descriptive statistics were used to find out the mean age and standard deviation for each stage. The homogeneity of variances was checked using the Levene test. As the age (in years) was positively skewed, a log transformation was used to be able to fit a two-way ANOVA model for analysis. Therefore, to evaluate the significant difference between the groups and biological sex among each group, a two-way ANOVA test was performed. For inter-observer agreement, Cohen’s Kappa test was performed. A Kappa value less than 0.20 indicated the poorest agreement. Value between 0.21 and 0.40 indicated slight agreement. Value between 0.41 and 0.60 and 0.61 and 0.80 indicated moderate and good agreement, respectively, and value above 0.81 indicated almost perfect agreement (Rosner 2011).

Results

The study analysed the degree of SOS for 246 subjects ranging from 7 to 15 years. The distribution of the age sample is shown in Table 1.

Table 1 Age distribution of the sample population

Inter-observer agreement

The Kappa values showed almost perfect agreement (k = 0.854) between observer 1 and observer 2.

The study reported a mean age of 8.38 ± 1.21 for males and 8.2 ± 1.16 for females in the stage 0 (completely open) category. The spheno-occipital synchondrosis began to fuse (stage 1) at the mean ages of 11.64 ± 1.19 for males and 10.21 ± 1.19 for females. With regards to semi-closed (stage 2), the mean age was found to be 13.07 ± 1.17 and 11.8 ± 1.14 for males and females, respectively (Fig. 2). The descriptive properties of various stages of both sexes are shown in Table 2.

Fig. 2

Mean degree of fusion for each age group for males and females. Standard deviation is represented by error bars

Table 2 Descriptive statistics for SOS in the study population

According to the two-way ANOVA test, the study result showed a significant difference among various stages of SOS fusion with age (p < 0.001). Similarly, the results also showed a significant difference among sex in all 3 categories (p < 0.001). Therefore, the study depicted that SOS closure occurs earlier in females (10.07) compared to males (11.03).

Discussion

Age estimation has paramount importance for the forensic practitioner. Identifying the biological age of the individual depends on the skeletal indicator. The spheno-occipital synchondrosis is one such indicator that reflects the biological age. Information on the timing of spheno-occipital synchondrosis reported in literature shows that synchondrosis will be open during childhood and then later merge during adulthood (Ford 1958). A study conducted by Hariprasad et al. reported that there was no significant progression in SOS fusion after an age of 15 years in females and 16 years in males with a mean age of 20.3 ± 3 and 20 ± 3.1 in males and females, respectively among the South Indian population. However, this study only investigated the timing of complete closure of spheno-occipital synchondrosis (Nambiar et al. 2021). Hence, this study was conducted to assess the timing of beginning of the fusion of SOS. Based on the mean age of the study sample, we found that fusion starts at an age of 11.64 ± 1.19 years for males and 10.21 ± 1.19 years for females. These results were found to be similar to a study conducted by Salina Hisham et al., where the mean age at which SOS begins to fuse is 10.82 in females (Hisham et al. 2018). However, for males, the SOS begins to fuse at a mean age of 13.74 years. Even though SOS status was assessed using CT, the study was conducted in a Malaysian population. Several studies report morphometric parameter differences among inter- and intra-populations (Pate et al. 2018; Bassed et al. 2010; Sharma et al. 2020). The common factors affecting inter- and intra-populations are geographical, biological, age-related, biological sex-related, ecological, and environmental aspects such as diet, climate and weather (Durtschi et al. 2009; Buretic-Tomljanovic et al. 2007).

The current study also shows a sexual dimorphism during the beginning of fusion, which is in accordance with various studies reported in literature (Pate et al. 2018; Hisham et al. 2018; Sinanoglu et al. 2016; Shirley and Jantz 2011; Can et al. 2014b; Okamoto et al. 1996). In the present study, the oldest male with an open SOS was 14 years old, whereas the oldest female with an open SOS was 12 years old. Table 3 demonstrates a summary of various studies reported in literature indicating the beginning of fusion for males and females in different populations.

Table 3 SOS fusion timing among genders found in literature in contrast with the current study showing the mode of assessment, sample size and place

As shown in the table, there is a variation in the time of fusion. These differences could be attributed to the differences in population. This highlights the requirement for a population specific data. The literature also reports methodological differences that cause variation in the time of fusion. The most procedural variation depends on whether the research is conducted on live individuals or post-mortem. Several studies report a variation in results due to differences in assessment methods (Pate et al. 2018; Demirturk Kocasarac et al. 2016; Shirley and Jantz 2011; Okamoto et al. 1996). To analyse age, macroscopic and histological methods are used for post-mortem, whereas conventional radiography, CT, and MRI methods have been used for live individuals (Pate et al. 2018; Demirturk Kocasarac et al. 2016; Shirley and Jantz 2011; Okamoto et al. 1996; Ekizoglu et al. 2015). It is shown that a macroscopic assessment of SOS generally overestimates the age. On the other hand, various studies have reported the advantages of CT scans over conventional radiographs and macroscopic techniques. Due to its superior image quality in terms of resolution, obtaining three-dimensional images, and enabling earlier detection and accurate interpretation, CT can be used effectively to demonstrate various stages of spheno-occipital synchondrosis (Shirley and Jantz 2011; Bassed et al. 2010; Okamoto et al. 1996; Sharma et al. 2020). However, cost, radiation exposure, and accessibility are a few limitations of CT scans (Demirturk Kocasarac et al. 2016). The variability in SOS among different populations and studies can also be attributed to maturational events. A study conducted by Alhazmi A et al. reported a significant relation between the onset of puberty and SOS closure. The systemic and hormonal changes in a growing adolescent can be the reason affecting the closure (Alhazmi et al. 2017). However, the present study did not take into account the onset of puberty and its relationship to SOS closure. Also, as the current study collected the data retrospectively, it did not take into account the socio-economic status or ethnicity background,therefore, further studies can be done to understand the relationship of SOS closure with respect to these factors as well. Another limitation was that the study had a limited sample size. Therefore, studies involving a larger sample can be done to represent a diverse population. Also, there is a high chance that the fusion of SOS can vary among the South and North Indian populations due to morphometric differences caused by ecological and environmental factors such as climate and diet. Therefore, studies can be conducted in this region to provide a unique perception of the variability in population throughout the country.

Conclusions

The study concluded that in the South Indian population, when the SOS begins to close, the age is less than 15 years in males. Similarly, for females, if SOS begins to close, the age is less than 14 years. Also, a strong sexual dimorphism is seen as each stage of synchondrosis starts earlier in females compared to males. This estimation will not only help in forensic and anthropological studies but also in assessing craniofacial growth in orthodontics.