PTERION TYPES AND MORPHOMETRY IN MIDDLE AND SOUTH ANATOLIAN ADULT SKULLS

Pterion is an irregular H letter shaped sutural confluence in the temporal fossa formed by frontal, parietal bones, great wing of sphenoid bone and temporal squama. Pterion is classified in 4 types as follows: sphenoparietal, frontotemporal, epipteric and stellate. The pterion represents: anterior branch of the middle meningeal artery, middle cerebral artery, Broca’s motor speech area, insula and stem of the lateral cerebral sulcus. This pterion junction has been used as a common extra-cranial landmark for surgeons in microsurgical and surgical approaches pertaining to important pathologies of this region. In the present study, our aim was to determine pterion types, to estimate distances between pterion and some special landmarks by which means to contribute to the related literature by comparing the data with other studies focusing on various populations. Pterion types identified by observation and measurements were taken by steel Vernier caliper. This study was conducted with 75 adult skulls (both sides 150 pterion). Skulls were classified with regard to gender as: 47 male and 28 female. Pterion types observed in both genders were classified as: sphenoparietal type 82% (84.04% in male, 78.57% in female), frontotemporal type 4.66% (5.31% in male, 3.57% in female), epipteric type 10.66% (8.51% in male, 14.28% in female) and stellate type 2.66% (2.12% in male, 3.57% in female). These findings will be usefull for clinicians, anthropologists and forensics.


INTRODUCTION
Pterion is usually characterized by an H letter shape sutural structure formed by frontal, parietal, temporal and greater wing of sphenoid bones on temporal fossa. Four pterion types were classified as: sphenoparietal, frontotemporal, epipteric and stellate by Murphy (1956). The center of the pterion is located approximately 3.5 -4 cm above arcus zygomaticus and 3 -3.5 cm behind the suture frontozygomaticus. In practice, the pterion is located two finger widths above the arcus zygomaticus and a thumb-width behind the processus frontalis of os zygomaticus (Moore, 1992). Pterion has a different morphology because of meeting point of skull base, calvaria and facial skeleton (Urzi et al., 2003). Pterion also forms the base of temporal fossa. Pterion is known as the Sylvian point. This is the point where the sulcus lateralis is divided into ramus anterior, ascendens and posterior branches. Pterion corresponds to the fontanella anterolateralis (sphenoidalis) in the newborn cranium that disappeares 3 months after birth (Moore, 1992). Pterion is crucial intraoperative field in surgical approaches. In neurosurgery, pterional approaches are used in such various interventions and treatments as removal of complicated tumors, medial cerebral artery, internal carotid artery and traumatic optic neuropathy (Mori et al., 2007). This pterion junction has been used as a common extracranial landmark for surgeons in microsurgical and surgical approaches pertaining to important pathologies of this region (Urzi et al., 2003). In the present study, our aim was to determine pterion types, to estimate distances between pterion and some special landmarks by which means to contribute to the related literature by comparing the data with other studies focusing on various populations.

MATERIAL AND METHOD
In the present study, 75 (both side 150 pterions) adult skulls, which have shown non-trauma, were examined. Examined skulls were obtained from the laboratories: Anthropology Department of the Faculty of Letter and Anatomy Department of the Faculty of Medicine of Cumhuriyet University and Anatomy Department of the Faculty of Medicine, Erciyes University. Morphometrical mesurements were taken by steel Vernier caliper and outer diameter compass. This study was done by determining the sutural patterns of the pterion on both sides of each skull based on the description of Murphy (Fig. 1a,1b,1c,1d). In the sphenoparietal type: sphenoid and parietal bones are directly in contact (Fig. 1a). In the frontotemporal type: frontal and temporal bones are directly in contact (Fig. 1b). In the epipteric type, the presence of sutural bones is located between sphenoid and parietal bones (Fig. 1c). In the stellate type: frontal, temporal, sphenoid and parietal bones meet at same junction point (Fig. 1d). Distance parameters were taken as: between pterion center (P) and posterior aspect of sutura frontozygomaticus (SFZ), midpoint of the arcus zygomaticus (AZ), posterior edge of the orbita margo lateralis (OML), inferior pole of the processus mastoideus (PM), upper edge of the meatus acusticus externus (MAE), glabella (G), inion (I), asterion (A). Also distance between glabella (G) and inion (I) was measured (Fig. 2). www.anatclinar.com.ar The data obtained from our study were loaded on the SPSS (Ver: 22.0) program, and the t-test was used in independent subjects to compare the parameters in the evaluation of the data. ANOVA was used in the comparison of the pterion types and the error level was taken as 0.05.

P-SFZ P-AZ P-OML P-PM P-MAE P-G P-I P-A G-I
Mean distance between pterion and glabella was measured as 78.2±4.6 mm, 74.0±4.6 mm in male and female skulls (Table 2) respectively and totally 76.6±5.0 mm (Table 3). www.anatclinar.com.ar Mean distance between pterion and inion was measured as 135.5±6.2 mm, 126.2±6.3 mm in male and female skulls (Table 2) respectively and totally 132.0±7.7 mm (Table 3). Mean distance between pterion and asterion was measured as 89.3±5.4 mm, 83.5±3.9 mm in male and female skulls (Table 2), respectively and totally 87.1±5.6 mm (Table 3). Mean distance between glabella and inion was measured as 169.0±9.6 mm. In the determination whether the skeletal length correlates with the pterion type, we concluded that there is no link between the two parameters. When the male and female values of the mean distance between the pterion and orbita margo lateralis were examined, it was seen that there is no significant difference between the right and left sides but when the average values were taken into consideration, it is concluded that the difference between male and female is significant (p=0.048) ( Table 2). When the mean distance between the pterion and the processus mastoideus was examined, it was concluded that there was a significant difference between the male and female (p=0.000) in terms of the mean values of both sides (Tablo 2). When the mean distance between the pterion and meatus acusticus externus was examined, it was found that between male and female; there was no significant difference in the right-sided values, but a significant difference was found between the left-sided and total mean values between the two gender types (p = 0.001) ( Table  2). When the mean distance between pterion and glabella was examined, it was concluded that there was a significant difference between the both-sided values and the mean values between male and female (p = 0.000) ( Table 2). When the mean distance between the pterion and the inion was examined, it was concluded that there was a significant difference between the two-sided values and the mean values between male and female (p = 0.000) ( Table 2). When the mean distance between the pterion and the asterion was examined, it was concluded that there was a significant difference between the two-sided values and the mean values between male and female (p = 0.000) ( Table 2). When the mean distance between glabella and inion was examined, it was concluded that there was a significant difference between male and female (p = 0.000) ( Table 2). In our symmetry study; in skulls used in this study, pterion types were classified as 78.72% in males and 78.57% in females bilaterally. Sphenoparietal type was found to be the most symmetrical type with 74.46% in males and 67.85% in females. The stellate type pterion was not seen symmetrically.

DISCUSSION
Pterion is an important landmark for optic nerve patologies, orbit, ridge of the sphenoid lesser wing (Lang, 1984) and, it also represents anterior ramus of the middle meningeal artery, middle cerebral artery, Broca's motor speech area on left side, anterior pole of the insula and stem of the sulcus lateralis (Urzi et al., 2003). A blow on the side of the head can cause the breakdown of the thin bones forming the pterion and the tearing of the ramus anterior to the medial meningeal artery that crosses the pterion. This may cause hematoma that presses the underlying cerebral cortex. Medial meningeal artery hemorrhage may cause death if not stopped within a few minutes (Moore, 1992). Pterion keyhole approach may achieve the best operative effect for the treatment of intracranial anterior circulation aneurysms in a selective group of patients with several advantages over traditional craniotomy including minor tissue damage, less brain retraction, a superior cosmetic result and a shorter duration of surgery (Cheng et al., 2006). The pterional approach used in brain surgery is defined as the most popular intervention (Yasargil et al., 1987). Nowadays, the technique, which is characterized by opening a hole that is less than 1.5 cm at the pterion, provides minimal bleeding in patients with coronal craniosynostosis, less operation time, the minimization of the bony defects and protection of the dural structures (Choi et al., 2009). Pterion is an important guide for age and sex determination as well as archaeological and forensic estimation (Lovejoy et al., 1985). According to the common result of research on the incidence of pterional types, it is seen that the one with the highest incidence among the four types is the sphenoparietal type. The incidence of frontotemporal type in our study is compatible with the studies performed in other populations and it has the third frequency among the four pterion types. When we compare our study with studies in other populations, the incidence of epipteric type is compatible with the studies in other populations. The incidence of stellate type in our study is the lowest among the pterion types. It is seen that the incidence of stellate type is low in our study and, our results are compatible with the studies in other populations (Table 4). When the samples used in the study are compared between male and female; the sphenoparietal type appears to be the most dominant type in both sexes. However, the www.anatclinar.com.ar incidence of sphenoparietal type was found to be higher in males in comparison to females. The incidence of frontotemporal type does not show a significant difference between male and female.  The epipteric type rate is the second most common type of pterion observed in both genders. The incidence in females is higher than males. Stellate type is the least common pterion type in our study and there is no significant difference between male and female incidence. It has been reported in literature that the pterion is located 3.0-3.5 cm behind the sutura frontozygomaticus (Moore, 1992). When we compare the results with the studies conducted in other populations, our data show compatibility with others (Table 5). It has been reported in literature that the pterion is 3.5-4 cm above the arcus zygomaticus (Moore, 1992). When we compared our data with the studies performed in other populations, the data show compatibility with each other (Table 5). Ersoy et al. (2003) measured the mean distance between pterion and orbita margo lateralis; the values gathered were 27.9 ± 5.6 mm for epipteric propria, 24.3 ± 1.7 mm for epipteric anterius, 33.2 ± 5.0 mm for epipteric posterius and, 27.4 ± 4.8 mm for ossa epipterica. Aksu et al. (2014) measured the mean distance between the pterion and the orbita margo lateralis only in the skulls with epipteric type pterion and reported 31.0 ± 5.8 mm on the right and, 32.3 ± 5.8 mm on the left. When we compare our data with other studies, our results are higher than the data of Ersoy et al. (2003) and, Aksu et al. (2014). A high incidence of epipteric type pterion may cause differences in mean values. The distance between these two parameters and the possibility of the presence of sutural ossicles in the pterional region are important for surgical intervention.
Because of the presence of the sutural bones, the pterion center may be miscalculated and lead to various intraoperative problems. These sutural www.anatclinar.com.ar bones, which can be found in the pterional region during pterional intervention used in accessing the orbit, can cause various complications. For this reason, the possibility of epipteric type pterion should be considered for surgical interventions.   Ari et al. (2009) measured mean distance between pterion and processus mastoideus is by manual measurements in Byzantine samples as 83.0 ± 3.4 mm on the right and, 85.0 ± 2.6 mm on the left, in the contemporary samples as 82.0 ± 5.0 mm on the right and 81.0 ± 7.7 mm on the left. Aksu et al. (2014) measured pterion with proc. mastoideus were 82.5 ± 5.5 mm on the right and, 81.8 ± 5.5 mm on the left. Our results are consistent with the results of these two studies. Aksu et al. (2014) measured mean distance between pterion and meatus acusticus externus is 53.3 ± 4.6 mm on the right and, 56.2 ± 4.6 mm on the left. Comparisons with our study indicate consistencies. Eboh and Obaroefe (2014) measured mean distance between pterion and glabella 78.3 ± 3.8 mm on the right, 76.7 ± 4.2 mm on the left. When the studies are compared, the results show compatibility with our findings. Ari et al. (2009)  Mean distance between glabella and inion was measured as 169.0±9.6 mm. In the determination whether the skeletal length correlates with the pterion type, we concluded that there is no link between the two parameters. Mwachaka et al. (2008) measured the mean distance between pterion and suture frontosygomaticus in Kenyan men as 30.7 ± 3.7 mm. Ma et al. (2012) measured the mean distance between pterion and sutura frontozygomaticus in New Zealand men 26 ± 4 mm. Sunday et al. (2013) measured the mean www.anatclinar.com.ar distance between the pterion and the sutura frontozygomaticus in Nigerian men as 31.9 ± 0.6 mm. Oğuz et al. (2004) measured mean distance between pterion and sutura frontozygomaticus in the study performed in Turkish men 33.0 ± 4.0 mm on the right and, 34.4 ± 3.9 mm on the left. Ari et al. (2009) measured the mean distance between the pterion and the suture frontozygomaticus in male samples of the Byzantine period by manual measurements 39.0 ± 4.0 mm on the right and, 39.0 ± 4.2 mm on the left. According to the results of our study, male pterion and sutura frontozygomaticus values are higher than the New Zealanders, lower than the Byzantine group, while they are compatible with the Kenyan and Nigerian populations. Mwachaka et al. (2008) measured the mean distance between pterion and arcus zygomaticus in Kenyan men as 39.3 ± 3.3 mm. Ma et al. (2012) measured the mean distance between pterion and arcus zygomaticus in New Zealand men as 34 ± 4 mm. Sunday et al. (2013) measured the mean distance between pterion and arcus zygomaticus in Nigerian male as 39.7 ± 0.5 mm. Oğuz et al. (2004) measured mean distance between pterion and arcus zygomaticus in the study carried out in the samples of Turkish men, 40.5 ± 0.4 mm on the right and, 38.5 ± 0.3 mm on the left. Ari et al. (2009) measured the mean distance between pterion and arcus zygomaticus in male samples of the Byzantine period by manual measurements 37.0 ± 2.0 mm on the right and, 39.0 ± 4.6 mm on the left. According to the results of our study, pterion and arcus zygomaticus values in males, in compareison to different populations, are higher than New Zealanders, which is significantly lower than the values obtained in both of the studies in Turkey. Ari et al. (2009) measured the mean distance between the pterion and the processus mastoideus in the male samples of the Byzantine period by manual measurements 83.0 ± 3.4 mm on the right and, 85.0 ± 2.6 mm on the left. These two studies show comparability of data when compared. Ari et al. (2009) determined the mean distance between pterion and asterion in male samples of Byzantine period by manual measurements 89.0 ± 4.0 mm on the right and, 93.0 ± 6.5 mm on the left. When the studies are compared, it is observed that while the right side shows compatibility, the left side shows that our values are lower. Ari et al. (2009) measured the distance between pterion and inion in male samples of Byzantine period 138.0 ± 5.0 mm on the right and, 137.0 ± 4.0 mm on the left. When studies compared results show compatibility. Mwachaka et al. (2008) measured the mean distance between pterion and sutura frontozygomaticus in Kenyan women as 29.7 ± 3.4 mm. Ma et al. (2012) measured the mean distance between pterion and sutura frontozygomaticus in New Zealand women as 25 ± 4 mm. Sunday et al. (2013) measured the mean distance between pterion and sutura frontozygomaticus in Nigerian women as 30.34 ± 0.8 mm. When the studies were compared, it was observed that our results were higher than the other three populations. Mwachaka et al. (2008) measured the mean distance between pterion and arcus zygomaticus in Kenyan women as 37.4 ± 3.0 mm. Ma et al. (2012) measured the mean distance between pterion and arcus zygomaticus in New Zealand women as 34 ± 4 mm. Sunday et al. (2013) measured the mean distance between pterion and arcus zygomaticus in Nigerian women as 38.0 ± 0.7 mm. When we compare the results, it is observed that our results are higher than the New Zealand women. In conclusion, identification and morphometrical measurements of pterion types are important studies frequently performed in various populations. It is clinically important to know the attachment types of the bones that shape the pterion to the main axis, and the distances of the pterion to various special landmarks. Different measurement results in different populations it may be the indicate that the geographic, genetic, evolutionary differences. We believe that these findings will be usefull for clinicians, anthropogists and forensic specialized.

Conflict of Interest
Authors state that there is no conflict of interest.

Fundings
There were no fundings to conduct this study.

Ethical Approval
This study approved by Cumhuriyet University Ethical Committee for Non-Interventional Clinical Trials. Decision number 2016-02/15.

Informed Consent
It does not correspond.

Contributions
The authors in this article have made the following contributions: with collaboratively MC, IO, KC: study design and manuscript drafting; KC: data acquisition, literature research, manuscript editing; IO: statistical analysis; MC: figure drawings.