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 Table of Contents  
ORIGINAL ARTICLE
Year : 2022  |  Volume : 11  |  Issue : 2  |  Page : 84-89

Morphometric and morphological variants of pterion with its clinical importance in South Indian population


1 Assistant professor, Department of Anatomy, Srinivasa medical college and hospital, Samayapuram, Tiruchirappalli, Tamilnadu, India
2 Associate Professor, Department of Anatomy, All India Institute of Medical Sciences, Bhubaneswar, Odisha, India

Date of Submission28-Nov-2021
Date of Decision16-Mar-2022
Date of Acceptance23-Mar-2022
Date of Web Publication26-May-2022

Correspondence Address:
Sipra Rout
Associate Professor, Department of Anatomy, AIIMS Bhubaneswar, Odisha
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/NJCA.NJCA_149_21

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  Abstract 


Background: Pterion is a critical neuroanatomical landmark in various neurosurgical approaches, including for the placement of burr holes. In addition, it also holds significance from radiological, anthropological, and forensic point of view. Thus, we undertook the current study to investigate the prevalence of different morphological types of pterion along with their morphometric parameters from adjacent bony landmarks in the south Indian population. Methodology: A total of 64 adult dry human skulls of unknown gender and age were included for the study. All skulls were grossly normal. The types of pterions were classified on both sides based on Murphy's classification. The vertical linear distance from the center of the pterion to the zygomatic arch (PZ) and to the anterior border of the frontozygomatic suture (PF) was measured with a digital vernier caliper. Data were statistically analyzed. Results: All four varieties of pterion were noticed. The sphenoparietal variety was found as most predominant bilaterally (81%), followed by epipteric (12.5%) stellate (3.9%) and frontotemporal (2.39%). One of the epipteric varieties consists of an unusually large ossicle. Pterion was typically located 3.59 ± 0.43cm behind the fronto zygomatic suture and 3.71 ± 0.34 cm above the zygomatic arch. Conclusion: The sphenoparietal variant of pterion was predominant on both sides, followed by epipteric variety.

Keywords: Frontozygomatic suture, keyhole neurosurgery, pterion, zygomatic arch


How to cite this article:
Prasad M, Rout S. Morphometric and morphological variants of pterion with its clinical importance in South Indian population. Natl J Clin Anat 2022;11:84-9

How to cite this URL:
Prasad M, Rout S. Morphometric and morphological variants of pterion with its clinical importance in South Indian population. Natl J Clin Anat [serial online] 2022 [cited 2022 Jul 4];11:84-9. Available from: http://www.njca.info/text.asp?2022/11/2/84/346072




  Introduction Top


Craniofacial sutures serve as an important articulation of bones helping in the typical growth of the skull.[1] The skull shows four distinct bony elements, namely, the greater wing of the sphenoid bone, the frontal bone, the parietal bone, the squamous part of the temporal bone coming together to form a bony landmark called the pterion. It serves as a crucial craniometric point and constructs a reliable reference point on the skull for neurosurgical approaches for temporal lobe, lateral Sylvian fissure, optic nerve pathologies, and anterior cerebral circulation aneurysms or locating anterior branch of middle meningeal vessels.[2],[3] Thus surgical approach to these structures would be possible by identifying this bony landmark on the skull followed by creating a secure window to enter into the cranial cavity. Pterional keyhole approaches have gained acceptance yielding improved results in terms of efficacy in approach, better cosmetic outcome, lesser tissue injury, and reduction in duration of surgery. Undoubtedly with minimal surgical site exposure and with minimal brain retraction possibility, a pterional keyhole surgery is a method of preference as compared to traditional craniotomy approaches in certain surgical procedures like treatment of anterior circulation aneurysm.[4] The successful approach to keyhole craniotomy depends on creating a well-defined surgical window over this area to avoid inadvertent injury to adjacent structures. However, the position of the pterion cannot be observed readily undercover of scalp tissue. Hence, determining the pterion location in respect to palpable bony landmarks can be supportive and obligatory. Although it is formed by the fusion of four distinct skull bones, the sutural variability in its formation is often encountered. At times, a small but discrete Wormian bone may participate in articulation and brings about varying types of patterns. This has been used as a criterion for the classification of morphological types of pterion.[5] In addition, using the articulation closure pattern of sutures at the pterion, age and sex determination from osteological remains can be predicted, which is of great archeological and forensic significance.[6]

The sutural pattern forming pterion remains consistent to a species, presuming a genetic basis to it.[7] Nevertheless, ethnic variations have been described by various authors across the globe in different population due to a variable degree of union between these four bones.[8] A meticulous knowledge of variations of morphology would help in the selection of the appropriate site for placing craniotomy and multiple burr holes during keyhole surgeries. This too would substantiate the data to validate anthropological and forensic remains. The presence of more than 2 epipteric bones in the pterion region might be mistaken by radiologists as a fracture line with inadequate data. Hence, we undertook this study to investigate the prevalence of different morphological types of pterions along with their morphometric parameters from nearest bony landmarks in the south Indian population.


  Materials and Methods Top


The study was taken up as a cross-sectional observational study on 64 dry skulls from the Department of Anatomy, Christian medical college, Vellore to determine the morphological variants of pterion. The study was approved by the Institutional Review Board (IRB/11870). The prevalence of the most common type of pterion is sphenoparietal is 80% as per one previous study done on the south Indian population.[9] Thus to detect at least 80% of prevalence with 10% precision and 95% confidence interval using the standard formula, it was calculated that a number of 64 number of skulls are needed for this study. Thus, 64 dry adult human skulls with intact calvaria of unknown age and gender, were selected for the current study. These bones were procured from human cadavers who have been donated for the purpose of teaching and research to the department of anatomy. Only such skulls that showed the articulation of the bones at the pterion clearly on both sides were included in the study. The skulls that showed pathological changes and ill-defined sutures were excluded from the study. Then, the skulls were observed for the morphological pattern of pterion based on Murphy's classification.[10] For the morphometric observations, the center of the pterion was marked drawing a small circle including all the participating bones with the help of a divider. The vertical distance from the center of the circle to the posterolateral aspect of the frontozygomatic suture and to the zygomatic arch was measured [Figure 1]. All these measurements were taken with the help of a digital Vernier caliper with an accuracy of 0.1 mm. Data were entered in Microsoft Excel (version 2010, Microsoft Inc., United States) and statistically analyzed. The incidence of the most common morphological type of pterion on both sides was identified. The variants were expressed in frequency and percentage, while the morphometric parameters were expressed in mean with standard deviation and range. The comparison between the sides was made using unpaired t-test.
Figure 1: Linear distances measured from pterion is represented by a circle (black). Yellow arrow marks the center of the pterion. P-FZA: Linear distance (orange line) from the center of pterion to frontozygomatic suture. P-ZA: Linear distance (purple line) from the center of pterion to the zygomatic arch

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  Results Top


Morphological types

All the four types of morphological variants namely, sphenoparietal [Figure 2]a, epipteric [Figure 2]b, stellate [Figure 2]c, frontotemporal [Figure 2]d were observed in the current study. The most common type of pterion observed was the sphenoparietal type (61%) followed by epipteric (9.5%). The least common was the frontotemporal type (1.78%) [Table 1]. The epipteric variety has been seen constituting single small ossicles but rarely it consists of two small ossicles. One case of epipteric variety showed large ossicles in between the skull bones, as shown in [Figure 2]e. In the current study, sphenoparietal and frontotemporal types were more commonly found on the right side while epipteric and stellate types were more frequent on the left side. Bilaterally symmetrical types of pterions were observed in 78% of dry skulls, among which sphenoparietal bilaterally was more common (71.8%) [Table 2]. The sphenoparietal in combination with epipteric was the most common asymmetrical representation (57%) [Table 2]. Among the asymmetrical presentation sphenoparietal with epipteric variety of pterion was seen in 8 skulls, followed by SP-ST combination (4 out of 14).
Figure 2: Photographs showing different morphological variants of pterion. (a) Sphenoparietal, S: Greater wing of the sphenoid bone, T: Temporal bone (squamous part), P: Parietal bone, F: Frontal bone. (b) Epipteric (EP), S: Greater wing of the sphenoid bone, T: Temporal bone (squamous part), P: Parietal bone, F: Frontal bone. (c) Stellate type (black arrow head), S: Greater wing of the sphenoid bone, T: Temporal bone (squamous part), P: Parietal bone, F: Frontal bone. (d) Frontotemporal, S: Greater wing of the sphenoid bone, T: Temporal bone (squamous part), P: Parietal bone, F: Frontal bone. (e) An unusually large wormion/sutural bone forming the epipteric variety (black arrow head), S: Greater wing of the sphenoid bone, T: Temporal bone (squamous part), P: Parietal bone, F: Frontal bone

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Table 1: Frequency of different morphological types of pterions on both sides

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Table 2: Incidence of symmetrical and asymmetrical distribution among morphological variants

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Morphometric parameters of pterion

The linear distances from the center of the pterion to the frontozygomatic suture and zygomatic arch were taken, as shown in [Figure 1]. The mean linear distance of pterion from the frontozygomatic suture was 3.65 ± 0.43 cm on the right side and 3.55 ± 0.43 cm on the left side. There was no statistically significant (P = 0.212) difference found between the mean perpendicular distance between the pterion to the zygomatic arch is 3.73 ± 0.35 cm on the right side and 3.69 ± 0.34 cm on the left side. There was also no significant difference (P = 0.490) observed between the sides [Table 3].
Table 3: Location of pterion on both the sides from nearest bony points (distances in cm)

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  Discussion Top


Broca[5] was the first person to classify the pterion based on its morphology into three main types which was further revised by Murphy[10] into four types [Figure 3]. He observed four types of morphological variants, i.e., sphenoparietal, frontotemporal, stellate, and epipteric variety. In the present study, all four types of pterions according to Murphy's classification were observed. This is in accordance with studies on the same population[11],[12] yet, studies on the North Indian population, Turkish population, and Nigerian population observed only three varieties of pterion.[13],[14],[15],[16]
Figure 3: Murphy's morphological classification of Pterion based on the skull bones taking part in formation of Pterion (Schematic representation). (a) Sphenoparietal type: Articulation between greater wing of sphenoid and parietal bone in a classical “H” pattern. (b) Frontotemporal type: Articulation between the frontal bone and temporal bone (squamous part). (c) Stellate: Articulation between all four bones i.e., frontal bone, parietal bone, greater wing of the sphenoid and temporal bone. (d) Epipteric: Presence of small sutural bones in the horizontal limb of the H-shaped pterion

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The sphenoparietal (SP) type is the predominant type in the present study similar to previous studies by various authors [Table 4]. The prevalence of SP type varies between 66% and 96% in the previous studies.[10],[11],[15],[17],[18],[19],[20],[21],[29],[30] In the present study, the occurrence of sphenoparietal type was found to be 61% which is lower in respect to previous studies on current population[10],[19],[31] [Table 4]. The high prevalence of SP type in humans has been explained based on the progress of human evolution. The SP type of pterion was found to be the dominant form in human beings, whereas the frontotemporal type is the major type in chimpanzees and monkeys. As the evolution continues the anterosuperior segment of the squamous part of the temporal bone which is present in lower primates is separated and incorporated into the greater wing of the sphenoid in higher primates.[31] This initiates an adaptation of frontotemporal morphology to sphenoparietal type in higher primates like humans.[31] This is intended to create space that will accommodate the increasing brain size in higher primates like human beings.[31]
Table 4: Comparison of morphological variants (incidence in percentage) of pterion in different studies

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The second-most common variety is the epipteric type of pterion (9.5%) in this population. This is in accordance with earlier studies on the south Indian population.[11],[19],[22] The frequency in this population varied from 3.7% to 17.3%. This variety was also observed to be the second-most common type in the Western Indian population.[17] On the contrary, studies on the North Indian population did not show the epipteric variety of pterion[13],[14] [Table 4]. The epipteric type was demonstrated a highest of 40.3% prevalence in skulls (out of 148) of Korean origin.[21]

The epipteric variety involves variable number and sizes of sutural bones based on which pterion is further classified Ersoy et al.[23] The sutural configuration of epipteric variety is crucial because it can confuse the surgeons regarding the precise area for craniotomy after raising the skin flap. The formation of epipteric bones can be explained with various hypotheses. These patterns develop with the development of an independent center of ossification along the posterior superior margin of the greater wing of the sphenoid, resulting in formation of an additional separate bone trapped in between the suture.[32] Thus, the number, size, and shape of sutural bones contributing to epipteric variety do not fall into any permanent pattern. One such sutural epipteric variety in the current study was observed which was larger and extended all along the inferior border of the parietal bone, which might give a misleading impression of a fracture line [Figure 2]e. Furthermore, these sutural bones have been reported to occur in infants affected by hydrocephaly, rickets, Down's syndrome, cleidocranial dysostosis.[33] It is also proposed that the rapid expansion of the skull may lead to the separation of tips of ossifying fibers which ossifies later separately with independent ossification centers.[34] Although the sutural bones are universally associated and reported around the lambdoid suture, the sutural bones contributing various pattern of epipteric variety needs to be investigated still further.[35] No two epipteric variety appears identical, thus reporting various possible types will improve the understanding.

The third-most common type observed in the study is the stellate type of pterion (2.97%) and the observation is in accordance with the previous reports from the Indian population and this subpopulation and falls within the observed prevalence of 2%–9.7%.[11],[12],[24],[25],[26],[29] However, studies on the Turkish and Nigerian population did not yield any stellate variety in the samples studied[15],[16] [Table 4].

The frontotemporal type is usually the dominant variety in nonhuman primates.[13],[15],[16] As long as the evolution progresses with the increased size of the brain in the anteroposterior plane, the frontotemporal articulation and thus the frequency of FT variety steadily declines. This condition was observed at the least, i.e., 3 out of 128 sides in the present study. Only one skull under the current study had frontotemporal pterion with bilateral representation. However, Hussain Saheb et al.[12] have reported FT variety as high as 17.35% in this population along with few other reports across the Indian population[11],[13],[14],[22],[24] where the prevalence of FT type was much higher as compared to the current study finding. A study done on the east Indian, Ethiopian, and Korean population did not report any FT variety.[21],[27],[28] Although, from an evolutionary perspective, the FT variety should be the least prevalent type in humans, still, it has not shown any consistent result among different population to confirm this. Thus, tracking its prevalence in future studies can provide any conclusive data on it corroborating its predominance in the nonhuman primate. The predominant bilateral symmetrical representation found in the current study was for the sphenoparietal type (SP-SP) (78%) [Table 2] same as most previous findings on the Indian population,[11],[14],[17],[18],[19],[20],[21],[29] followed by epipteric variety (EP-EP).

The diverse presentation of the pterion changes its possible relation to neighboring bony landmarks, which have been described in different populations. This knowledge would be valuable during the planning of various neurosurgical procedures, specifically with the pterional approach which is now a widespread approach to enter the cranial cavity.[36],[37]

Conventionally, the pterion was reported to be 4 cm above the zygomatic arch and 3.5 cm rear to the frontozygomatic suture. The results of the current study did not differ significantly from the overall findings reported in previous studies [Table 5].
Table 5: Comparison morphometric parameters of pterion from bony landmarks

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With a precise surgical corridor made at pterion, the structures of the anterior and parts of the middle and posterior cranial fossa can be easily assessed.[38] The occurrence of more than two epipteric bones of varying shape in this region of the pterion is not an uncommon finding. In those conditions, this landmark pterion could be misinterpreted to be at the anterior-most junctions of bones while placing a keyhole which may cause inadvertent penetration into the orbit.[23] According to Murphy, the sutural fusion pattern at pterion is regulated by genetic and environmental effects which influence each other.[10] Thus, population-based differences have been reported by various authors even within the same ethnic groups. This confirms various genetic plays in humans which could regulate the sutural configurational patterns of the pterion[7] Asala and Mbajiorgu[16] have concluded that these variations could be attributed as ”epigenetic factors.” Examinations on the sutural patterns at pterion in rhesus monkey pedigrees have shown familial aggregation, supporting the genetic theory. One of the genes from the homeobox gene family, i.e., the MSX 2 gene which resides on 5q35.2 has been presumed to be regulating the fusion of cranial bones.[7] This has been further confirmed by studies on mice models where it has shown that MSX 2 gene. Various mice models have proved that MSX 2 gene regulates the pterion type and asymmetry and any mutations of the MSX 2 gene have been linked to clinical conditions like craniosynostosis type 2 and persistent parietal foramen Type-1.[39] If cranial sutures fuse early, it tends to give rise to condition like craniosynostosis. However, the definitive influences of genetic and epigenetic factors regulating sutural morphogenesis at pterion in various populations and its evolutionary impact on the growth of brain size need further research to understand this crucial joint.

Limitations

In the present study, we have not classified different types of epipteric variety because of fewer samples size, which is of more clinical significance as it could significantly affect the morphology, and morphometric of a classically described “H” shape of pterion. Knowledge of different subtypes of pterion would help in placing multiple burr holes, which is required during keyhole surgeries. In addition, the current study could not comment on the gender difference.


  Conclusions Top


It was noticed that SP variety of pterion formed the most predominant variety followed by epipteric variety. Regarding the patterns of epipteric variety, though single small ossicles are common findings still large sutural bone can be usual, mimicking well-defined fracture lines.

The authors sincerely thank those who donated their bodies to science so that anatomical research could be performed. Results from such research can potentially increase humankind's overall knowledge that can then improve patient care. Therefore, these donors and their families deserve our highest gratitude.[40]

Financial support and sponsorship

The study was funded by Fluid Research Grant, Christian Medical College, Vellore.

Conflicts of interest

There are no conflicts of interest.



 
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    Figures

  [Figure 1], [Figure 2], [Figure 3]
 
 
    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5]



 

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