Mastoid process – A tool for sex determination, an anatomical study in South Indian skulls

Studies on human skeletal remains for sex determination have been a topic of interest among researchers. Osteometric studies using individual bones exhibiting sexual dimorphism has been reported among different populations.1 The pelvis and skull bones are widely used reporting almost 100% accuracy 2-3 followed by other bones such as femur4, humerus5 etc. Dimorphism in skulls is based on its size and robustness. The male crania have well developed supraorbital ridges, broader palates, thicker zygomatic and larger mastoid process than those of females.6-7 Skull bones such as mastoid, hard palate, craniofacial parts 8 have been analyzed for sex determination. A study by Demoulin9 reported dimorphic traits in mastoid process. It is a bone less prone to damage due to its safe anatomical position. 9 Mastoid region is one of the slowest and late growing regions of cranium 10 and such regions show higher degree of sexual dimorphism in adulthood.11 The differences in size of the mastoid process between sexes could be attributed to the variable growth duration in males IJBR (2013) 04 (02) www.ssjournals.com Abstract Objective: The aim of this study is to evaluate the use of mastoid process as a tool for sex determination in unidentified skeleton.

Studies on human skeletal remains for sex determination have been a topic of interest among researchers. Osteometric studies using individual bones exhibiting sexual dimorphism has been reported among different populations. 1 The pelvis and skull bones are widely used reporting almost 100% accuracy 2-3 followed by other bones such as femur 4 , humerus 5 etc. Dimorphism in skulls is based on its size and robustness. The male crania have well developed supraorbital ridges, broader palates, thicker zygomatic and larger mastoid process than those of females. [6][7] Skull bones such as mastoid, hard palate, craniofacial parts 8 have been analyzed for sex determination. A study by Demoulin 9 reported dimorphic traits in mastoid process. It is a bone less prone to damage due to its safe anatomical position. 9 Mastoid region is one of the slowest and late growing regions of cranium 10 and such regions show higher degree of sexual dimorphism in adulthood. 11 The differences in size of the mastoid process between sexes could be attributed to the variable growth duration in males and females, along with relatively greater development of the mastoid process in males in response to stronger muscle actions of the sternocleidomastoid, splenius capitis and longissimus capitis. 11 So this region can be considered vital for diagnosis of sex from visual assessment as well as osteometric basis. The objective of this study is to validate the use of mastoid process for assessing the sex of the human skull and signify the use of statistical approach in estimation of sex from fragmentary crania.

Materials and Methods
The study sample includes 80 adult skulls, 40 males and 40 females aged between 35-60yrs at time of death from the Department of Anatomy, Yenepoya Medical College, Yenepoya University, South India. Skulls with good conditions with gender identified in book record were included and those which were damaged, incomplete or without identification were excluded from the study. In each cranium, from the right mastoid the following measurements were recorded using vernier calipers (Forbes, 0.01mm precision) (Fig 1). The straight distance from the tip of the mastoid process to the upper rim of the zygomatic arch was measured for the mastoid length (ML). For Mastoid breadth (MB), distance from the posterior end of the incisura mastoidea (PEIM) to the nearest point on the posterior border of the external auditory meatus was measured. Asterion-porion (AST-PO) is the length of the mastoid process measured from the asterion to the porion . In Posterior end of incisura mastoidea-depression of suprameatal triangle (PEIM-DSMT), the distance from the posterior end of incisura mastoidea to the depression in the supra meatal triangle was measured .The length of mastoid process measured from the porion to the posterior end of the incisura mastoidea is taken for PEIM-PO. In Asterion-depression of suprameatal triangle (AST-DSMT), the distance from asterion to the depression of the suprameatal triangle was measured. The distance from asterion to the mastoidale was taken for Asterion-mastoidale (AST-MS) and the height of the mastoid process from Mastoidale to porion was measured for Mastoidale-porion (MS-PO). IJBR (2013) 04 (02) www.ssjournals.com

Statistical analysis
The data were analyzed using statistical software package SPSS 13.0 program (SPSS Inc., Chicago, IL). 12 Descriptive statistics, including mean and standard deviations were obtained for each of the measurements. Stepwise, univariate and multivariate direct discriminant function analysis were performed to calculate specific discriminant function equation for all parameters.

Results
The descriptive statistics for all the measurements with t-values and p-values are presented in Table 1. The t-value indicates that the five measurements except AST-PO, PEIM -DSMT and AST -DSMT shows highly significant differences between male and female (p<0.000).   Table 2 shows the results of multivariate analysis using all variables. Direct method indicates that overall accuracy of sex determination by using all parameters is 82.5%. For males the average accuracy is 85.0% and for females it is 80.0%.  Table 2) with Wilk's lambda 0.642. Using stepwise discriminant score, an average accuracy of 65.0% was obtained ( Table  2). The formula derived is ---y = (0.281 MS-PO) + (-7.550). For males average accuracy is 80.0% and for females it is ˣ 100% ( Table 2).The discriminant function equation obtained in this study is unique to skulls of South Indian population.

Discussion
The results of this study shows that there is a sexual dimorphism in the dimensions of the mastoid region and establish its value as a sex indicator in South Indian population. Discriminant function analysis is exclusively objective and is used extensively for sex determination. 13 As a non-metric approach is entirely subjective and accurate in hands of experienced and trained people, few researchers studied the sexual dimorphism in mastoid process by metric approach. 13 The average accuracy by direct method in our study was 85% in males and 80% in females. Compared to literature data (male% vs female%) 61% vs 52%, 14 60% vs 72%, 14 60% vs 60% 15-16 our values are higher except in a study in North Indian population (males 92.3%, females 70.2%). 17 The overall accuracy is higher in direct method (82.5% vs 76.7%) and lower in stepwise analysis(65% vs 66.7% ) in our study compared to North Indian population. 18 In the present study stepwise analysis selects MS-PO as the best discriminant which is in agreement with a study among Japanese population with an accuracy of 69.6%. 19 In a North Indian study 18 ML (mastoid length) was best determinant which is similar to Gupta's finding. 20 In another study among same population yield AST-MS and MB as best discriminants. 17 The variations in the result in different population may be due to the inconsistency in the position of landmarks of the skull in different populations. 21 Craniofacial growth like mastoid region, zygomatic process and the ridges of occipital bone are influenced by nutrition, environment and genetic factors. 22

Conclusion
This is an attempt to validate the osteological criterion for sex determination based on the mastoid process. The proper identification of landmarks, careful measurements and strict statistical methods will yield reliable results.