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Pterion its location and clinical implications- a study compared.

INTRODUCTION: Pterion is a significant region which is marked by the junction of frontal bone, parietal bone, squama temporalis and the greater wing of sphenoid bone and forms the floor of temporal fossa. This cranio-metric point on the lateral side of skull is used by neurosurgeons and maxillo-facial surgeons due to its structural and anatomical importance. It is an area of bone junction in the anterior part of the temporal fossa. It is usually indicated by an H-shaped formation of sutures that unite the frontal, parietal, sphenoid (greater wing), and temporal bones. Less commonly, the frontal and temporal bone articulate, sometimes all bones meet at a point (1).

The pterion corresponds to the site of anterolateral fontanelle of neonatal skull which closes in the third month after birth (2). The joints of the cranial vault are sutural joints which ossify in membranes. As the bones are growing, the unossified sutural membranes connect the periosteum covering the outer and inner surfaces of the bone, which helps in growth as well as binding the bones together to their apposed margins (3). A sutural bone is sometimes present at the pterion (4). This bone is called pterion ossicle or Epipteric bone or flower's bone.

It is the region mostly used as a guiding point where the position of deeper structures and their relations to the surface of the head are explained. This point is an important clinical landmark because the calvarium is thin and gets fractured easily. It overlies anterior branch of the middle meningeal artery which is the most common artery to be damaged producing extradural haematoma, requiring burr hole surgery to evacuate haematoma (5).

The pattern of bone articulation at pterion however can be varied and small epipteric bones may be present. Its center is approximately 4.0 cm superior to the zygomatic arch and 3.0-3.5 cm posterior to the frontozygomatic suture. (6)

The Pterion was first classified into three types (sphenoparietal, frontotemporal and stellate) by Broca in 1875. Four types of Pterion (sphenoparietal, frontotemporal, stellate, and epipteric) were defined by Murphy in 1956. Lastly Wang et al. proposed six types of pterion (sphenoparietal, frontotemporal, stellate, epipteric zygomatico-parietal and zygomatico-temporal). Pterion lies two fingers superior to the zygomatic arch and a thumb's breadth posterior to the frontal process of the zygomatic bone (1). It is the point where the greater wing of the sphenoid meets the anterio-inferior angle of the parietal bone and is not marked by an eminence or a depression (7). In neurosurgery, it is important to have the most suitable bony aperture in order to be minimally invasive (8).

In present study 40 dry skulls were examined for morphology & variation of the pterion with relation to the 2 bony landmarks. The distance between the pterion with respect to landmarks is measured by the stainless steel sliding caliper and the data obtained was analyzed statistically. This knowledge is mandatory for the surgeons in the pterional approach used for various microsurgeries and surgeries. Due to scarcity of the data on the morphology & location of the Pterion in the Uttarakhand dry skulls this study was undertaken.

MATERIAL AND METHODS: Present study is based on observation of 40 skulls of unknown sex. The study is conducted in the department of Anatomy, Shri Guru Ram Rai Institute of Medical & Health Sciences (SGRRIM & MS), Patel nagar, Dehradun. The dry skull belongs to the department of Anatomy, SGRRIM & MS. Morphology of the pterion on both the sides of each skull and the sutural pattern of the pterion was determined.

The type of Pterion was identified in all 40 skulls on both the sides. The center of pterion was determined by drawing a circle of smallest radius connecting all the four bones taking part in formation of pterion, a center of which was taken as a center of pterion. This method is also used in the previous studies done on Pterion.

Stainless steel sliding caliper with an accuracy of 0.1 cm was used to measure linear distances between the pterion and specific identifiable bony landmarks. All the Measurements were taken twice and then averaged so as to minimize bias errors. Parameters used in the present study are as follows:

PF: distance from the center of the pterion to the anterior aspect of the frontozygomatic suture.

PT: distance from the center of the pterion to the superior aspect of the temporozygomatic suture.

The linear measurement were taken for the PF with the help sliding caliper, one pointed jaw was kept at anterior aspect of the frontozygomatic suture and other at the center of Pterion. Similarly all the skulls were measured for the particular variable bilaterally. Data collected is summarized in table I.

PT variable was also measured by keeping the pointed jaws of sliding caliper at the center of the pterion and at the superior aspect of the temporozygomatic suture. All skulls were measured for this particular variable bilaterally and data collected is tabulated in table II.

Fig 3 shows that the sphenoparietal type is a sutural pattern in which the sphenoid and parietal bones are in direct contact, preventing the frontal and temporal bones making contact with one another.

Fig 4 shows that frontotemporal type is a sutural pattern in which the frontal and temporal bones are in direct contact, preventing the sphenoid and parietal bones making contact with one another.

Fig 5 shows that stellate type is characterized by articulation of four bones (frontal, parietal, temporal and sphenoid) at a point.

RESULT: Sphenoparietal type of pterion accounted 86.25% (90% on the right side and 82.5% on the left side), frontotemporal 11.25% (12.5% on the right side and 10% on the left side) and stellate 2.5% (5% on the right side) in all 40 skulls.

The Student's t test was employed in the assessment of each variable on either side of the skull A p value [less than or equal to] 0.05 was considered significant.

The pterion is located 3.25 [+ or -] 1.05 cm behind the anterior aspect of the fronto-zygomatic suture on right side and 3.25 [+ or -] 1.05 cm on left side of the skulls. This is comparatively same on either side. The p value for this variable is 0.43824 which is higher than the p = 0.05, this is statistically insignificant.

The Pterion is located 3.825 [+ or -] .625 cm above the superior aspect of the temporo-zygomatic suture on the right side and 3.7 [+ or -] .7 cm on the left side of skulls. Pterion on the left side is slightly higher than the right side. The p value for this is 0.27527 which is statistically insignificant.

DISCUSSION: In primate evolution, the anterosuperior segment of the squamous part of the temporal bone of lower primates became detached from its parent and incorporated into the posterosuperior angle of the greater wing of the sphenoid bone of humans, thereby changing the pterion pattern from the frontotemporal type of nonhuman primates to the sphenoparietal type of humans (9).

Sphenoparietal type of pterion is most common in all the regions. It's occurrence is higher in Indians (95.1%)(10), Northern Indians (87.72%)(11), Southern Indians (93.55%)(12), western Indian (91.7%)(9) and Nigerians (87.79% and 82.1%)(10,13), while it was significantly lower in Korean (76.5%)(14) and Kenyan (66%)(15) populations as compared to this study.

The occurrence of frontotemporal type of Pterion also shows dissimilarity among different groups, it accounts 10%-23.6% in Nigerians (10,13) and 15% of Kenyans (15), which are significantly higher than present study in which the pterion is 11.25% being closest to that reported in other populations of India.

Stellate variety was least present in the skulls of Uttarakhand region, study revealed it's occurrence nearly 2.5% (both sides). In the study done in Gujrat it's extremely as low as 0.02% but in the other study done in the region of Awadh area around Lucknow it accounts 5.17% which is higher than the present study (11). It can be concluded that the occurrence of stellate variety increases from east to west in India.

Epipteric type of pterion was not encountered in the present study, this is significant as in Nigerians it accounts 23.6% (13), Australian Aborigines 18.5% (16), and in other Indian studies it ranges from 6.74% to 11.79% which was mostly associated with sphenoparietal type (10,17).

The pterion lie 4.0cm above the arcus zygomaticus and 3.5 cm behind the sutura Frontozygomatica, this is in agreement with the present study where the distance from the arcus zygomaticus ranges from 3.0 cm to 4.45 cm and the distance from the sutura frontozygomatica ranges from 2.2 cm to 4.3 cm.

In the 1970s Yasargil laid to the foundation of the pterional approach. There are many variants for the pterional approach. Mainly it is a trepanation which permits access to the frontal and to the temporal lobe as well as the Sylvian fissure (18, 19). Pterion is a keyhole approach to such kind of intracranial surgeries (20,21).The combination of both a vital artery in this area and the relatively thin bone structure has lent itself to the name "God's little joke" by some physicians and clinicians (22). The study done on Korean population also adds up that the bone thickness at pterion of the left side was significantly thicker than the right side of Korean adult skulls (23).

The variation in type and location of the pterion from different bony landmarks have been studied and explained in different populations, the findings of this study might be useful in providing information and data to the anthropologists, forensic pathologist neurosurgeons and maxilla-facial surgeons.

ACKNOWLEDGEMENT: First of all, I am grateful to the almighty god for establishing me to complete this article. I take this opportunity to record my sincere thanks to all the faculty members of the department of anatomy for their help, guidance and encouragement in completing this article. I am extremely grateful and indebted to them for their expert, sincere and valuable guidance extended to me.

I would like to thank statistician (Raman Nautiyal) for helping me in statistical analysis. I also place on record, my sincere gratitude to one and all who, directly or indirectly, have lent their helping hand in this article. These acknowledgments would not be complete without thanking my family especially my wife (Gunjan) and my daughter (Sugun) for their constant support and care.

REFERENCES:

(1.) Moore KL, Dalley AF. Clinically oriented anatomy, 4th edn. Lippincott Williams & Wilkins, Baltimore, 1999;836-840

(2.) Williams PL, Bannister LH, Berry MM, Collins P, Dyson M, Dussek JE, et al. The skull. In. Gray's Anatomy, 38th ed London: Churchill Livingstone; 1995. p. 583-606.

(3.) Natekar PE, DeSouza FM, Natekar SP. Pterion: An anatomical variation and surgical landmark. Indian J Otol 2011; 17:83-5.

(4.) Hussain SS, Haseena S, Prasanna LC. Unusual Wormian bones at Pterion--Three case reports: JBiomed Sci and Res. 2010; 2 (2):116-18

(5.) Snell Richard S. Clinical Anatomy by Regions, 8th edi. Lippincott. Williams and Wilkins, 2008; p.673, 686

(6.) Apinhasmit W, Chompoopong S, Chaisuksunt V, Thiraphatthanavong P, Phasukdee N. Anatomical consideration of pterion and its related references in Thai dry skulls for pterional surgical approach. Journal Of The Medical Association Of Thailand Chotmaihet Thangphaet. 2011; 94(2): 205-214.

(7.) Richard S. Snell. Clinical Anatomy. 7th ed. Lippincott Williams & Wilkins, Baltimore, 2004;896

(8.) Ersoy M, Evliyaoglu C, Bozkurt M, Konuskan B, Tekdemir I. Epipteric bone in the pterion may be a surgical pitfall. Minim. Invas. Neurosurg. 2003; 46:363-5.

(9.) Zalawadia DA, Vadgama DJ, Ruparelia DS, Patel DS, Patel DSV. Morphometric Study Of Pterion In Dry Skull Of Gujarat Region. NJIRM. 2010; 1(4): 25-29.

(10.) Saxena SK, Jain SP, Chowdhary DS. A comparative study of pterion formation and its variations in the skulls of Nigerians and Indians. Anthropol Anz 1988;46:75-82

(11.) Saxena R, Bilodi A, Mane S, Kumar A. Study of pterion in skulls of Awadh area-in and around Lucknow. Kathmandu University Medical Journal (KUMJ). 2003; 1(1): 32-33.

(12.) Manjunath KY, Thomas IM. Pterion variants and epipteric ossicles in South Indian skulls. J Anat Soc India. 1993; 42:85-94.

(13.) Asala SA, Mbajiorgu FE. Epigenetic variation in the Nigerian skull: sutural pattern at the pterion. East Afr Med J 1996;73:484-6

(14.) Lee UY, Park DK, Kwon SO, Paik DJ, Han SH. Morphological analysis of the pterion in Korean. Korean J Phys AnthropoL 2001; 14:281-9.

(15.) Mwachaka PM, Hassanali J, Odula P. Sutural morphology of the pterion and asterion among adult Kenyans. Braz J Morphol Sci. 2009; 26:4-7

(16.) Murphy, T. The pterion in the Australian aborigine. Am. J. Phys. Anthropol, 14:225-44, 1956

(17.) Gopinathan K., Dhall U., Chhabra S. Sutural bones in the North Indian population, J. Anat. Soc. India. 1998; 47(2):91-96.

(18.) Scholz M, Parvin R, Thissen J, Lohnert C, Harders A, Blaeser K. Skull base approaches in neurosurgery. Head & Neck Oncology. 2010; 216.

(19.) Chao S, Shen C, Cheng W. Microsurgical removal of sylvian fissure lipoma with pterion keyhole approach-case report and review of the literature. Surgical Neurology. 2008; 70 Suppl 1S1:85-90.

(20.) Cheng W, Lee H, Sun M, Shen C. A pterion keyhole approach for the treatment of anterior circulation aneurysms. Minimally Invasive Neurosurgery: MIN. 2006; 49(5): 257-262.

(21.) Samson D, Hodosh R, Clark W. Microsurgical evaluation of the pterional approach to aneurysms of the distal basilar circulation. Neurosurgery. 1978; 3(2): 135-141.

(22.) Praba AMA, Venkatramaniah C. Morphometric Study of different types of Pterion and it's relation with middle meningeal artery in dry skulls of Tamil Nadu. JPBMS. 2012; 21 (04):1-4.

(23.) Hwang K, Kim JH, Baik SH. The thickness of the skull in Korean adults. J Craniofac Surg. 1999; 10: 395-9.

Suchit Kumar [1], Anurag [2], Shashi Munjal [3], Puja Chauhan [4], Alok Chaudhary [5], Sanjeev Kumar Jain [6].

[1.] Post Graduate, Department of Anatomy, Shri Guru Ram Rai Institute of Medical and Health Sciences, Dehradun.

[2.] Professor, Department of Anatomy, Shri Guru Ram Rai Institute of Medical and Health Sciences, Dehradun.

[3.] Associate Professor, Department of Anatomy, Shri Guru Ram Rai Institute of Medical and Health Sciences, Dehradun.

[4.] Associate Professor, Department of Anatomy, Shri Guru Ram Rai Institute of Medical and Health Sciences, Dehradun

[5.] Lecturer, Department of Anatomy, Shri Guru Ram Rai Institute of Medical and Health Sciences, Dehradun

[6.] Professor & HOD, Department of Anatomy, Shri Guru Ram Rai Institute of Medical and Health Sciences, Dehradun

CORRESPONDING AUTHOR:

Dr. Suchit Kumar, Shri Guru Ram Rai Institute of Medical and Health Sciences. Patel nagar, Dehradun Uttarakhand.

E-mail: skumar1422@yahoo.com

Table I

Parameter   No: of skulls    Mean      Median    Minimum   Maximum

PF (r)      40               3.44875   3.47500   2.20000   4.30000
PF (l)      40               3.46250   3.50000   2.20000   4.30000

Table II

Parameter   No: of skulls    Mean      Median    Minimum   Maximum

PT (r)      40               3.75375   3.70000   3.20000   4.45000
PT (l)      40               3.69563   3.65000   3.00000   4.40000

TABLE III: MEASUREMENT OF PTERION FROM BONY LANDMARKS

Sl.No OF   PF (r)   PF(l)   PT (r)   PT(l)
SKULL

1          3.2      3.55    4.05     4.05
2          3.5      3.35    3.4      3.4
3          3.8      3.05    4.2      3.5
4          3.25     3       3.8      3.5
5          3.65     3.65    4.3      4.25
6          3.75     3.85    3.85     3.6
7          3.9      4.25    3.6      4
8          3.45     3.7     3.9      3.55
9          4.05     4.1     4.25     4.05
10         3.65     3.65    4.45     4.2
11         3        3.3     3.5      3.7
12         3.85     3.85    3.9      3.9
13         3.6      3.4     3.65     3.6
14         4.15     3.75    4.45     3.95
15         4.3      3.9     3.65     3.5
16         3.55     3.95    3.8      4.05
17         2.95     2.7     3.2      3.7
18         4.3      3.6     4.4      4
19         3.4      2.25    3.75     3
20         2.4      3.3     3.3      3.3
21         3.5      3.7     3.7      3.7
22         3.2      3.1     3.4      3.3
23         3.2      3.5     3.7      3.6
24         3.3      3.3     3.4      3.5
25         3.8      3.7     3.6      3.5
26         3.3      3.4     3.9      3.4
27         2.2      2.3     3.4      3.6
28         3.5      3.4     3.7      3.9
29         3.25     3.3     3.8      3.9
30         3.65     3.8     3.7      3.8
31         3.3      3.2     3.3      3.5
32         3.5      3.4     3.8      3.7
33         3.6      3.9     3.9      3.7
34         3.4      3.5     3.5      3.5
35         3.3      3.2     3.3      3.4
36         4.1      3.9     3.9      3.8
37         3.6      3.6     4.3      4.25
38         2.9      2.6     3.2      3.7
39         4.2      3.6     4.4      4.1
40.        3.5      2.2     3.7      3.1

Table IV: Percentage of pterion observed on right and left
side of skulls

Type of Pterion    Right side   Left side   Both sides

                   n = 40       n = 40      n = 80

Sphenoparietal     90%          82.50%      86.25%
Frontotemporal     12.5%        10%         11.25%
Stellate           5%           0%          2.5%

Keys: n-number of skulls

TABLE: V Analysis of linear distances of Pterion from bony
landmarks.

Variables   Descriptive Statistics (STAmaster.sta)

            Valid N   Mean      Median    Minimum

PF (r)      40        3.5       3.47500   2.20000
PF (l)      40        3.41      3.50000   2.20000
PT (r)      40        3.7775    3.70000   3.20000
PT (l)      40        3.69375   3.65000   3.00000

Variables   Descriptive Statistics (STAmaster.sta)

            Maximum   Variance  S.D.                S.E

PF (r)      4.30000   0.164176  [+ or -] 0.44922    0.0710
PF (l)      4.30000   0.186551  [+ or -] 0.48287    0.07635
PT (r)      4.45000   0.116505  [+ or -] 0.35752    0.05653
PT (l)      4.40000   0.088430  [+ or -] 0.30153    0.04768

Keys:

S.D-standard deviation
S.E-standard error
(r)-right side
(l)-left side
N-number of skulls.

Table VI: Types of Pterion in different populations (in percentage)

Study, year          Population, n
                     (skulls), sex

Saxena et al,        Nigerian, n = 40,
  1988                 unknown sex
Saxena et al,        Indian, n = 72, unknown
  1988                 sex
Manjunath et al,     Southern Indian, n = 172,
  1993                 unknown sex
Asala et al, 1996    Nigerian, n = 212,
                       unknown sex
Lee et al, 2001      Korean, n = 149,
                       unknown sex
Saxena et al,        Northern Indian, n = 203,
  2003                 both sex
Mwachaka et al,      Kenyan, n = 50, both sex
Prabha et al,        Southern Indian, n = 50,
  2012                 both sex
Present              Uttarakhand, n = 40,
  study, 2013        unknown sex

Study, year          Type of pterion

                     Sphenoparietal    Frontotemporal

                     In %              In %

Saxena et al,        87.79             10.11
  1988
Saxena et al,        95.3              3.46
  1988
Manjunath et al,     93.55             3.52
  1993
Asala et al, 1996    82.1              23.6

Lee et al, 2001      76.5              --

Saxena et al,        87.72             10.01
  2003
Mwachaka et al,      66                15
Prabha et al,        74                3
  2012
Present              86.25             11.25
  study, 2013

Study, year          Type of pterion

                     Stellate   Epipteric

                     In %       In %

Saxena et al,        5.06       3.79
  1988
Saxena et al,        1.38       11.79
  1988
Manjunath et al,     2.93       17.3
  1993
Asala et al, 1996    --         5.7

Lee et al, 2001      --         40.3

Saxena et al,        5.17       0
  2003
Mwachaka et al,      7          12
Prabha et al,        9          14
  2012
Present              2.5        0
  study, 2013

Keys: n-number of skull
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Title Annotation:ORIGINAL ARTICLE
Author:Kumar, Suchit; Anurag; Munjal, Shashi; Chauhan, Puja; Chaudhary, Alok; Jain, Sanjeev Kumar
Publication:Journal of Evolution of Medical and Dental Sciences
Article Type:Report
Date:Jun 24, 2013
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