Age estimation from the sternal end of left fourth rib in the Nepalese population

Abstract

Background

Forensic anthropological examinations help in identification of unidentified human remains. This study aims to establish population-specific standards for age estimation from the sternal ends of left fourth ribs in the Nepalese population. A quantitative, observational study was conducted on 387 Nepalese deceased (106 females, 281 males) between November 2021 and March 2023. Left fourth ribs were collected, macerated and examined for the study. The variables examined included pit depth, pit shape, rim and wall configurations and rib phase.

Results

Pit depth significantly predicted age, β = 0.642, t(385) = 16.42, p < 0.001. Pit depth also explained a significant proportion of variance in age, R² = 0.41, F(1, 385) = 269.54, p < 0.001. The age estimates from pit depth stage were 7–36 (stage 0), 7–49 (stage 1), 16–51 (stage 2), 10–58 (stage 3), 22–70 (stage 4) and 32–81 (stage 5). Age estimates for pit shape were 17–20 (stage 0), 13–48 (stage 2), 15–55 (stage 3), 24–68 (stage 4) and 39–82 (stage 5). Age estimates for rim and wall configurations were 17–20 (stage 0), 13–47 (stage 2), 17–53 (stage 3), 24–68 (stage 4) and 40–82 (stage 5). Similarly, age estimates for rib phase were 17–20 (phase 0), 18–20 (phase 1), 13–46 (phase 2), 18–50 (phase 3), 23–60 (phase 4), 32–71 (phase 5), 50–76 (phase 6), 44–81 (phase 7) and 62–85 (phase 8). The study also examined the use of transitional analysis to develop posterior probability distributions for estimation of age using rib phases.

Conclusion

This study found significant differences in the age estimates from previous studies and shows the importance of developing population-specific models for use in forensic anthropology.

Background

Forensic anthropology helps in establishing the identity of unidentified human remains to assist in civil, legal and statistical purposes. Identification is necessary in a variety of situations, including mass disasters, migration or conflicts (Acharya et al. 2017).

Developments in forensic sciences have led to the validation of four primary methods for establishing identification—DNA profiling, dactylography, forensic odontological examination and comparison of medical and surgical records (INTERPOL 2018). However, these primary identifiers have a massive caveat, in that these are comparative techniques and require antemortem information regarding the missing persons for the establishment of identity. Additionally, these primary methods also require significant infrastructure and expertise, which is often lacking in developing countries (Acharya et al. 2017).

Forensic anthropological examination on the other hand develops a biological profile of the missing person through examination of the human remains to estimate ancestry, sex, age and stature. This biological profile is used to generate a hypothesis of identity (Spradley 2016). The estimation of sex, age and stature can be performed through a variety of osteological, radiological and somatological examination of the body and body parts. However, these anthropological methods require the establishment of prior probabilities through research, which have been found to be population specific, owing to the genetic and environmental differences between populations (İşcan and Steyn 2013).

Age is an important aspect of an individual’s biological profile. Age may need to be estimated in the living, including for sporting events, child labour, criminal prosecution and asylum (Shedge et al. 2021). This is especially true in developing countries with numerous instances of undocumented or unregistered births. Additionally, estimation of age at death is essential to a criminal investigation. The age estimation methods used also follow one of two methods, morphological or morphometric. Morphological examinations include gross examination as well as modern techniques, including radiography. Gross anthropological examinations examine the sequence of development and fusion of bones. The traditional methods used in adults for estimation of age from skeletal elements include grading of closure of cranial sutures (Meindl and Lovejoy 1985), developmental changes in the pubic symphysis (Brooks and Suchey 1990) and auricular surface (Lovejoy et al. 1985). Similarly, ribs have also been examined for estimation of age at death (İşcan et al. 1984a; 1984b; 1985). Newer methods include the use of periodontosis and transparency of teeth (Lamendin et al. 1992), bone loss in the femur and second metatarsal (Curate et al. 2022) and radiography of clavicle (Benito et al. 2014). Additionally, microscopic examination of osteons has also been used to estimate age (Stout and Paine 1992). Age estimation in the Nepalese population has been studied for the pisiform (Pangeni et al. 2020) and wrist (Parajuli et al. 2021).

The left fourth ribs are frequently analysed in forensic anthropological examination. Therefore, the left fourth ribs were collected for this study to standardise the analysis and facilitate comparison with previous studies. Therefore, the main objective of the present study is to develop standards for conducting age estimation from the sternal ends of left fourth rib in a contemporary Nepalese population. These parameters are population specific, owing to the genetic and environmental variations between populations (Krishan and Kanchan 2013). Additionally, intra-population nonconformities have also been reported due to temporal variations between populations (Saini et al. 2014).

Methods

The reported study is a quantitative, non-probability (convenience) sampling, observation study. The samples were collected at the Department of Forensic Medicine, Maharajgunj Medical Campus, Institute of Medicine, Kathmandu, Nepal, from November 2021 till March 2023. All deceased brought for autopsy were assessed for compliance with inclusion criteria and non-conformity with exclusion criteria. The family members of all those that met the criteria were counselled about the study, its implications, importance and their option to forego inclusion in the study. The family members were specifically informed that they could withdraw from the study at any time.

The reported study is part of the research conducted for the doctoral dissertation in Ph.D. in Anthropology titled “Forensic anthropological examination of the sternal ends of ribs in a Nepalese population.” To this end, the ethical clearance was obtained from the Institutional Ethics Committee, Panjab University, Chandigarh, India (vide EC-D-2103–44 dated 06 April 2021), and Institutional Review Committee (IRC), Institute of Medicine, Kathmandu, Nepal (vide 114 (6–11) E2 – 078/079 dated 10 September 2021) as well as from the National Health Research Council, Nepal (vide 1080 dated 17 November 2021) for conducting this study.

Sample size calculation

The sample size for the study was calculated to be 384 following Cochran’s method (Cochran 1977). While trauma or deformity of the sternal ends of ribs excluded the case from specific analysis, the details were still be recorded for overall documentation. The maceration process led to the destruction of some ribs making them unsuitable for analysis. As a result, additional samples were collected to ensure that adequate samples were available. A total of 387 samples were analysed. The samples comprised of 106 females (27.4%) and 281 males (72.6%).

Data collection

The dependent variables collected for this study includes age of the individuals. The thoracic cavity was dissected as per regular autopsy protocol. A midline incision was made from the supra-sternal notch to the symphysis pubis, sparing the umbilicus. The skin, subcutaneous tissue and muscles overlying the ribs were reflected and dissected. The ribs were examined for any deformity or trauma. The intercostal muscles between the ribs were dissected to remove as much of the soft tissue as possible by gross dissection. The costal cartilages were cut 2 cm medial to the costochondral junction using a scalpel, while the ribs were cut 5 cm lateral to the costochondral junction using rib shears. Where the costal cartilages had calcified, the cartilages were cut using rib shears.

For the present study, left fourth rib of each body was collected. Once collected, the ribs were macerated in water. The ribs were stored in water filled glass jars and examined every week till the soft tissue and cartilage loosened and were separated easily. Once the soft tissue and cartilage had separated, the ribs were boiled in water for 15–30 min to further clear any adherent debris. Finally, the ribs were dried (away from sunlight) for 24 h and assessed for suitability for inclusion in the study. If the articular surface of the sternal end of rib was free from damage, the rib was analysed. Where the articular surface of the sternal end of rib was damaged, the sample was excluded from the analysis.

Parameters of left fourth rib taken for the study

The independent morphometric variable used in the study included pit depth. The independent morphological variables include pit shape, rim and wall configurations and rib phase.

Pit depth of the sternal ends

The maximum pit depth of the sternal ends of left fourth rib was measured “where the distance between the base of the pit and the adjacent anterior or posterior wall is the greatest” (İşcan et al. 1984b). The measurement was recorded using a depth micrometre (to the nearest 0.02 mm), by holding it perpendicular to the base of the pit. The depth was not measured at the superior and inferior ends of the ribs, where bony projections may have been present (Fig. 1).

Fig. 1

Measurement of pit depth of sternal end of left fourth rib

Pit depth stage of the sternal ends

Pit depth stage was divided into six stages as described by İşcan et al. (1984a): stage 0—“flat to slightly billowy extremity with no indentation (pit) greater than 1.1 mm”; stage 1—“definite pit formation with a depth ranging from 1.1 to 2.5 mm”; stage 2—“pit depth ranging from 2.6 to 4.5 mm”; stage 3—“pit depth ranging from 4.6 to 7.0 mm”; stage 4—“pit depth ranging from 7.1 mm to 10.0 mm”; stage 5—“pit depth of 10.1 mm or more”.

Pit shape of the sternal ends

The pit shape of the sternal end of left fourth ribs was divided into six stages, based on the progression of the indentation and the thinning of the wall, as described by İşcan et al. (1984a): stage 0—describes sub-adult specimens with no pit; stage 1—describes a “shallow, amorphous pit”; stage 2—pit is “V-shaped with thick walls”; stage 3—pit is “U-shaped with fairly thick walls”; stage 4—pit is “wide U-shaped with thin walls”; stage 5—pit is “wide U-shaped” (with brittle, disintegrated bone) (Fig. 2).

Fig. 2

Morphological features of six stages of pit shape of sternal end of left fourth rib: stage 0—describes sub-adult specimens with no pit; stage 1—describes a “shallow, amorphous pit”; stage 2—pit is “V-shaped with thick walls”; stage 3—pit is “U-shaped with fairly thick walls”; stage 4—pit is “wide U-shaped with thin walls”; stage 5—pit is “wide U-shaped” with brittle, disintegrated bone

Rim and wall configurations of the sternal ends

The rim and wall configurations of the sternal ends of left fourth ribs are divided into six stages, based on the morphological changes as described by İşcan et al. (1984a): stage 0—rib ends have “smooth regular rim, with no wall”; stage 1—ribs ends with “thick smooth regular rim” showing beginning of wall; stage 2—wall is “thick and smooth, with scalloped or slightly wavy rim”; stage 3—thin but sturdy walls with disappearing scalloped edges “without significant deterioration in the texture of the bone”; stage 4—“sharper and increasingly irregular” rim “with more frequent bony projections often most pronounced at the cranial and caudal margins of the rib”, wall shows “further thinning and are less sturdy with noticeable deterioration in texture”; stage 5—extremely friable and porous bone, with very sharp, brittle and highly irregular rim having long bony projections, windows may be formed (Fig. 3).

Fig. 3

Morphological features of six stages of rim and wall configurations of sternal end of left fourth rib: stage 0—rib ends have “smooth regular rim, with no wall”; stage 1—ribs ends with “thick smooth regular rim” showing beginning of wall; stage 2—wall is “thick and smooth, with scalloped or slightly wavy rim”; stage 3—thin but sturdy walls with disappearing scalloped edges “without significant deterioration in the texture of the bone”; stage 4—“sharper and increasingly irregular” rim “with more frequent bony projections often most pronounced at the cranial and caudal margins of the rib”, wall show “further thinning and are less sturdy with noticeable deterioration in texture”; stage 5—extremely friable and porous bone, with very sharp, brittle and highly irregular rim having long bony projections and windows may be formed

Rib phase of the sternal ends

Rib phase of the sternal ends of left fourth ribs is divided into nine stages, based on the morphological changes as described by İşcan et al. (1984b; 1985): phase 0—“flat or billowy surface,” with “regular rim and rounded edges”, bone is firm, smooth and very solid; phase 1—amorphous pit, with rounded rim, bone is still firm, smooth and solid”; phase 2—“deeper V-shaped pit, with thick walls, having scalloped, rounded edges”, bone is still firm and solid; phase 3—‘deep pit is U-shaped, with thick walls having rounded edges’, rim is more irregular, with scalloping and bone is still firm and solid; phase 4—“pit depth increases further, with the sternal end being wider U-shaped, having thin walls with rounded edges”, the rim is more irregular and the quality of bone is still good, but with decreased weight and firmness; phase 5—“rib end is wide U-shaped, with further thinning of walls and sharp edges”, the rim become more irregular, with irregular bony projections and the bone is fairly good, with increased porosity and decreased density; phase 6—“deep, wide U-shaped pit with thin wall”, irregular rim having sharp edges and bony projections; phase 7—deep, wide to very wide U-shaped pit with thin and fragile walls having sharp irregular edges and bony projections, bone is light, thin and porous; phase 8—very deep and very wide U-shaped pit, with thin, fragile and brittle rim, bone is very light, thin, friable, brittle and porous (Fig. 4).

Fig. 4

Morphological features of nine phases of rib phase of sternal end of left fourth rib—phase 0—“flat or billowy surface,” with “regular rim and rounded edges”; phase 1—amorphous pit, with rounded rim; phase 2—deeper V-shaped pit, with thick walls, having scalloped, rounded edges; phase 3—deep pit is U-shaped, with thick walls having rounded edges, rim is more irregular, with scalloping; phase 4—pit depth increases further, with the sternal end being wider U-shaped, having thin walls with rounded edges, the rim is more irregular; phase 5—rib end is wide U-shaped, with further thinning of walls and sharp edges, the rim become more irregular, with irregular bony projections; phase 6—deep, wide U-shaped pit with thin wall, irregular rim having sharp edges and bony projections; phase 7—deep, wide to very wide U-shaped pit with thin and fragile walls having sharp irregular edges and bony projections; phase 8—very deep and very wide U-shaped pit, with thin, fragile and brittle rim

Manual, computer and statistical analyses

The data collected were analysed using Statistical Package for Social Sciences (SPSS, Version 21, Armonk, NY: IBM Corp.) The variables were tested for normality of distribution using visual evaluation, kurtosis and skewness as well as Kolmogorov–Smirnov test and Shapiro–Wilk test for females and males as well as combined population. The data was examined using non-parametric tests, based on the results of the normality test.

The data was explored to develop age estimates from the pit depth, pit shape, rim and wall configurations as well as rib phase for females and males separately as well as for the combined population. The independent variables were examined for correlation with age using Spearman’s rho test. Pit depth was used to develop linear regression models for estimating age. Following this, statistical analysis was performed using pit shape, rim and wall configurations, rib phase and composite score to develop 95% confidence intervals for age, by estimating the age range within two standard deviations of the mean. These variables were examined separately for females and males as well as the combined population. To finish the analysis, transitional analysis was used to develop probability distribution function models for estimation of age from rib phase, analysed separately for males and females. The statistical significance of these models was tested using independent-samples Kruskal–Wallis test.

Correctness, bias and inaccuracy

The frequency of correct assessment, bias and inaccuracy were examined for the age estimates developed. The age estimates were considered correct if the observed age was found to be within the age estimates developed. Bias was calculated as the mean difference between the observed age and estimated age. Inaccuracy was calculated as the average difference (absolute value) between the observed age and estimated age.

For example, a 60-year-old female with stage 1 of pit depth stage was found to have false correctness, with bias of 10 and an inaccuracy of 10, considering that the age estimates for stage 1 of pit depth stage developed from this study was 8–50 years. Similarly, A 30-year-old female with stage 4 of pit depth stage was found to have false correctness, with bias of − 2 and inaccuracy of 2, considering that the age estimates for stage 4 of pit depth stage developed from this study was 32–66 years. Finally, the bias of pit depth stage 1 was calculated as the mean of the bias of all individuals who were observed to have pit depth stage 1. Similarly, the inaccuracy was calculated as the mean of the inaccuracy of all individuals who were observed to have pit depth stage 1.

Results

Descriptive statistics

Left fourth ribs of 387 individuals were analysed for the present study. The samples comprised of 106 females (27.4%) and 281 males (72.6%). The age of the sample population was found to range between 18 and 84 years, with a mean of 38.52 years and a standard deviation of 14.08 years. Similarly, the pit depth was found to range between 0.02 and 14.98 mm, with a mean of 6.13 mm and standard deviation of 3.15 mm.

The categorical variables—pit depth stage and pit shape as well as rim and wall configuration—were analysed for frequency. Similarly, composite scores were computed by adding the scores of pit depth stage and pit shape as well as rim and wall configurations. Additionally, rib phases were also analysed for frequency (Table 1).

Table 1 Frequency distribution of categorical variables—pit depth stage, pit shape, rim and wall configurations, composite score and rib phase of sternal ends of left fourth ribs

The distribution of the continuous variables, namely age, maximum superior-inferior height, maximum anterior–posterior breadth and pit depth, were found to show non-normal distribution (Table 2).

Table 2 Test for normality for continuous variables (n = 387)

Development of models for estimation of age from morphometric and morphological variables of the sternal end of left fourth ribs

Age estimation can be performed through multiple methods. We examined four methods—linear regression using pit depth, component analysis using pit depth stage, pit shape, rim and wall configurations as well as composite score, phase analysis using rib phases and finally transitional analysis method using posterior probability distribution of rib phases.

Linear regression model (LRM)

Pit depth was found to have statistically significant correlation with age for females, males and combined, using Spearman’s correlation test. Pit depth was found to be statistically significant for predicting age in females, β = 0.702, t(104) = 10.07, p < 0.001. The variance in age was found to be explained by R² = 0.49, F(1, 104) = 101.30, p < 0.001. Pit depth was also found to be statistically significant for predicting age in males, β = 0.622, t(104) = 13.27, p < 0.001. The variance in age was found to be explained by R² = 0.39, F(1, 279) = 176.07, p < 0.001. Similarly, pit depth was found to be statically significant for estimating age in the combined sample population, β = 0.642, t(385) = 16.42, p < 0.001. Pit depth also explained a significant proportion of variance in age in the combine sample population, R² = 0.41, F(1, 385) = 269.54, p < 0.001. The linear regression models for estimating age and statistical tests are provided in Table 3.

Table 3 Linear regression models for estimating age using pit depth of sternal ends of left fourth ribs for males, females and both sexes

Component analysis

The Kruskal–Wallis tests for inter-group differences between the stages showed statistically significant differences between the age of the deceased between the pit-depth stage (p < 0.001), pit shape (p < 0.001) and rim and wall configurations (p < 0.001) as well as composite score (p < 0.001). The differences were found to be statistically significant amongst females and males as well as combined sample population. The results are provided in Fig. 5 for females, Fig. 6 for males and Fig. 7 for both sexes combined.

Fig. 5

Independent-samples Kruskal–Wallis test for difference in age distribution, in females, between the stages for categorical variables (pit depth stage, pit shape, rim and wall configurations and composite score) of left fourth ribs. The age distribution between the groups was found to be statistically significant for all four variables (p < 0.001)

Fig. 6

Independent-samples Kruskal–Wallis test for difference in age distribution, in males, between the stages for categorical variables (pit depth stage, pit shape, rim and wall configurations and composite score) of left fourth ribs. The differences in the age distribution between the groups was found to be statistically significant for all four variables (p < 0.001)

Fig. 7

Independent-samples Kruskal–Wallis test for difference in age distribution, in the entire sample population, between the stages for categorical variables (pit depth stage, pit shape, rim and wall configurations and composite score) of left fourth ribs. The differences in the age distribution between the groups was found to be statistically significant for all four variables (p < 0.001)

Following the confirmation of inter-group differences in the distribution of age, 95% confidence intervals were developed for estimating age using the pit shape, rim and wall configuration, rib phases and composite score.

The age estimates for pit depth stage in females were found to be 8–50 (stage 1), 12–50 (stage 2), 8–61 (stage 3), 32–66 (stage 4) and 66–72 (stage 5). Similarly, the age estimates from pit depth stage in males were found to be 5–41 (stage 0), 5–49 (stage 1), 19–51 (stage 2), 11–58 (stage 3), 18–71(stage 4) and 31–80 (stage 5). The age estimates for pit depth stage in the combined population were found to be 7–36 (stage 0), 7–49 (stage 1), 16–51 (stage 2), 10–58 (stage 3), 22–70 (stage 4) and 32–81 (stage 5). The details of the results are provided in Table 4. The age estimates derived for pit depth stage in females (stage 5), in males (stage 0) and the combined population (stage 0) should be evaluated with caution.

Table 4 Descriptive statistics and 95% confidence interval of age, using pit depth stage of sternal ends of left fourth ribs for males, females, and combined (both males and females)

The age estimates for pit shape in females were found to be 14–42 (stage 2), 14–56 (stage 3), 29–70 (stage 4) and 42–82 (stage 5). Similarly, the age estimates from pit shape in males were found to be 13–51 (stage 2), 15–54 (stage 3), 23–67 (stage 4) and 38–82 (stage 5). The age estimates for pit shape in the combined population were found to be 17–20 (stage 0), 13–48 (stage 2), 15–55 (stage 3), 24–68 (stage 4) and 39–82 (stage 5). The results are summarised in Table 5. The age estimates derived for pit shape in females (stage 5) and the combined population (stage 0) should be evaluated with caution.

Table 5 Descriptive statistics and 95% confidence interval of age, using pit shape of sternal ends of left fourth ribs for males, females, and combined (both males and females)

The age estimates for rim and wall configurations in females were found to be 12–44 (stage 2), 18–54 (stage 3), 28–71 (stage 4) and 42–81 (stage 5). Similarly, the age estimates from rim and wall configurations in males were found to be 13–49 (stage 2), 17–53 (stage 3), 23–66 (stage 4) and 39–82 (stage 5). The age estimates for rim and wall configurations in the combined population were found to be 17–20 (stage 0), 13–47 (stage 2), 17–53 (stage 3), 24–68 (stage 4) and 40–82 (stage 5). The detailed results are provided in Table 6. The age estimates derived for rim and wall configurations in females (stage 5) and the combined population (stage 0) should be evaluated with caution.

Table 6 Descriptive statistics and 95% confidence interval of age, using rim and wall configurations of sternal ends of left fourth ribs for males, females, and combined (both males and females)

The age estimates for composite score in females were found to be 15–29 (score 5), 13–48 (score 6), 17–42 (score 7), 16–40 (score 8), 11–57 (score 9), 20–71 (score 10), 35–72 (score 11), 33–62 (score 12), 37–78 (score 14) and 66–72 (score 15). Similarly, the age estimates for composite score in males were found to be 5–44 (score 4), 12–40 (score 5), 17–52 (score 6), 12–51 (score 7), 17–41 (score 8), 15–56 (score 9), 22–53 (score 10), 16–71 (score 11), 22–69 (score 12), 33–66 (score 13), 40–75 (score 14) and 38–85 (score 15). Lastly, the age estimates for composite score in the entire sample population were found to be 5–44 (score 4), 13–35 (score 5), 15–51 (score 6), 13–49 (score 7), 17–41 (score 8), 15–56 (score 9), 20–58 (score 10), 20–73 (score 11), 25–67 (score 12), 33–66 (score 13), 39–76 (score 14) and 39–85 (score 15) (Table 7). The age estimates derived for composite score in females (score 5, score 8, score 9, score 10, score 11, score 12, score 14 and score 15), in males (score 4, score 5 and score 14) and the combined population (score 4, score 5 and score 14) should be evaluated with caution.

Table 7 Descriptive statistics and 95% confidence interval of age, using composite score of sternal ends of left fourth ribs for males, females, and combined (both males and females)

Phase analysis of the sternal end of the left fourth rib

Similar to the component analysis, age estimation was attempted from the phase analysis of sternal end of the ribs. The Kruskal–Wallis tests for inter-group differences between the stages showed statistically significant differences between the age of the deceased between the rib phases (p < 0.001). The rib phase was analysed separately for females, males and combined (Fig. 8).

Fig. 8

Independent-samples Kruskal–Wallis test for difference in age distribution, in males, females and the entire sample population, between the rib phases of left fourth ribs. The differences in the age distribution between the groups was found to be statistically significant (p < 0.001)

The age estimates for rib phase in females were found to be 11–40 (phase 2), 19–43 (phase 3), 18–59 (phase 4), 31–71 (phase 5), 49–75 (phase 6), 49–67 (phase 7) and 66–72 (phase 8). Similarly, the age estimates from rib phase in males were found to be 18–20 (phase 1), 14–47 (phase 2), 18–52 (phase 3), 26–60 (phase 4), 32–71 (phase 5), 50–77 (phase 6), 43–82 (phase 7) and 68–87 (phase 8). The age estimates for rib phase in the combine population were found to be 17–20 (phase 0), 18–20 (phase 1), 13–46 (phase 2), 18–50 (phase 3), 23–60 (phase 4), 32–71 (phase 5), 50–76 (phase 6), 44–81 (phase 7) and 62–85 (phase 8) (Table 8). The age estimates derived for rib phase in females (phase 5, phase 6, phase 7 and phase 8), in males (phase 1, phase 6 and phase 8) and the combined population (phase 0, phase 1, phase 6 and phase 8) should be evaluated with caution, considering the number of participants present in these groups.

Table 8 Descriptive statistics and 95% confidence interval of age, using rib phase of sternal ends of left fourth ribs

Transition analysis of the sternal end of the left fourth rib

Continuing with age estimation, the rib phase was examined using transitional analysis. Posterior probability distribution functions for age were derived from rib phase, separately for females, males and combined (males and females). The graphical representations are presented in Fig. 9A for females, Fig. 9B for males and Fig. 9C for combined.

Fig. 9

A Probability distribution curves for age estimation using rib phases of left fourth ribs in females. B Probability distribution curves for age estimation using rib phases of left fourth ribs in males. C Probability distribution curves for age estimation using rib phases of left fourth ribs in the entire sample population

Correctness, bias and inaccuracy

Pit depth was found to correctly estimate the age in 72.6% of females, 70.5% of males and 80.6% of the combined population. Meanwhile, pit depth stage was found to estimate the correct age in 94.3% of females, 94.7% of males and 93.8% of the combined population. Pit shape was found to correctly estimate the age in 95.2% of females, 92.9% of males and 94.6% of the combined population. Rim and wall configurations were found to correctly estimate the age in 95.2% of females, 93.2% of males and 94.8% of the combined population. Composite score was found to estimate the correct age in 93.4% of females, 95.0% of males and 95.3% of the combined population. Lastly, rib phase was found to correctly estimate the age in 95.3% of females, 95.08% of males and 96.4% of the combine population. The details of the bias and inaccuracy are provided in Table 9.

Table 9 Correctness, bias, and inaccuracy of age estimates of sternal ends of left fourth ribs

The statistical analysis of the variables demonstrate that ribs can be used to develop statistically significant models for estimation of age.

Discussion

Forensic anthropological examination for estimation of age focuses primarily on dentition, long bones of the limbs and pubic symphysis, amongst others. All the above examination, while highly reliable, require the examination of significantly large skeletal elements or parts thereof, for estimation of age. It is not uncommon for these skeletal elements to be significantly damaged due to injury, erosions or taphonomy. As a result, this study aims to develop standards for estimation of age from the sternal end of left fourth rib.

The use of component method for estimation of age as prescribed by İşcan et al. (1984b, 1985) has been used to examine the reliability of the method for estimation of age in India (Meena and Rani 2014; Doshi and Doshi 2016), Mexico (Cerezo-Román and Espinoza 2014) and Thailand (Siriphimolwat et al. 2021). Studies have also examined using regression models (Verzeletti et al. 2010; Macaluso Jr and Lucena 2012; Verzeletti et al. 2013) dividing the age into groups (Gupta et al. 2007) as well as also using imaging techniques for geometric examinations (Fanton et al. 2012).

All these studies found that the rib end is a good indicator for age estimation in Asian populations. The studies from India (Meena and Rani 2014; Doshi and Doshi 2016) and Thailand (Siriphimolwat et al. 2021) found similarities in the age estimates developed by İşcan et al. (1984b; 1985). In contrast, this study found significant differences in the estimates for Nepalese population as compared to those established priors.

As with the component system, age estimation using the phase analysis have been studied in a multitude of populations, including in Turkey (Yavuz et al. 1998), India (Padmakumar et al. 2011; Meena et al. 2012, 2013), Hungary (Wolff et al. 2012) and Tunisia (Salem et al. 2014). Meena et al. (2012; 2013) show similarities in the age estimates as those recommended by İşcan et al. (1984b; 1985). However, Padmakumar et al. (2011) found significant differences between the age estimates in Indian females, when compared to those for American females. This study also found significant differences between the age estimates for Nepalese population as compared to the American estimates. In contrast, the estimates of this study were similar to those established for Indian population by Padmakumar et al. (2011) The results of this study, using transitional analysis and using probability distribution function, present similarities with the study by Muñoz et al. (2018).

All studies found that the age estimates became broader with increasing age. Similarly, the accuracy also decreases towards the extremes of age. This can be attributed to the relatively less samples in these age groups. Thus, it can be concluded that it is essential to conduct population specific studies or validation studies before using the results of studies on other populations.

While İşcan et al. (1984b, 1985) and other subsequent studies have used the age range of the samples to prescribe a suggested age range, this study uses the 95% confidence interval of the mean to prescribe the suggested age. As such, the age estimates developed using the present study was found to be accurate in more than 70% of cases using linear regression from the pit depth and in more than 95% of the cases using the component and phase analysis methods from pit depth stage, pit shape, rim and wall configurations, composite score and rib phase. When examined along with the probability distribution developed using the transitional analysis of rib phase, this provides extremely valuable resources to the forensic practitioner in providing testimony. However, age estimates in some of the stages, scores and phases were found to be unreliable, owing to the scarcity of data and therefore, further research should ensure adequate representation of the different stages of the component analysis or phase of the phase analysis. This can be done by undertaking research on specific stages of the component analysis or phase of the phase analysis.

It is pertinent to state that the use of statistical methods using sternal ends of the left fourth ribs for estimation of age should consider the possibility of individual variations. For example, in this study, while the age range of samples for stage 1 in pit depth of females was observed between 19 and 60 years, the suggested age range is 8–50 years. It is important to remember that the age estimates were developed by using 95% confidence intervals (within two standard deviations) of the mean of observed ages. As a result, while there were no samples below 18 years, the suggested age range goes below 18 years. Similarly, while the maximum age of the suggested range is 50 years, this study included a sample of 60 years of age. As a result, while this method has been tested and found to be statistically reliable, as with any statistical procedure, there is always a possibility that the examined individual does not confer with the suggested age estimates.

Conclusions

The sternal end of the left fourth rib is a valuable tool for forensic anthropological investigations to establish identity. They can be used to develop reliable estimates of age. The sternal ends of left fourth ribs were found to be reliable for estimation of age of individuals. However, the ages were found to be different from those found by western authors.

This study demonstrates the importance of developing population-specific models to accurately predict the primary indicators of biological profile. The results of this study can thus be of great assistance in the management of the dead following disasters and other events of mass fatality.