Metopism and Its Clinical Relevance: A Persistent Suture

Abstract

Background:

Metopic suture is a dense fibrous joint extending from nasion to bregma. Normally, closure of this suture takes place between 1-8 years of age. Failure of this closure beyond 8 years leads to persistent metopic suture. Persistent metopic suture may mimic skull fracture.

Aim and Objective:

To differentiate metopism from frontal bone fracture.

Material and Method:

54 dry skull of adult humans in were studied. These skulls were observed for the presence (complete or incomplete suture) or absence of metopic suture.

Result:

Metopic suture was found in 3.7% skulls. Both complete and incomplete metopic suture were found in 1.85% of the dry skulls.

Conclusion:

Neurosurgeons, radiologists, otorhinolaryngologists, physicians, anatomists, forensic specialists, anthropologists and evolutionary biologists should have knowledge of this anatomical variation. This case series focusses on the importance of metopism. Metopism is a topic, which has not been much studied.

INTRODUCTION

Cranial sutures lead to the development of entire skull.[1] Variations of cranial sutures have their significance in medicine and surgery. Metopic suture (MS) is a dense connective tissue extending from nasion to bregma. MS has other names such as sutura frontalis persistens, caput cruciatum, sutura interfrontalis, sutura frontalis, and sutura mediofrontalis.[2] Fusion of MS starts at anterior fontanelle and terminates at nasion.[3] This fusion generally occurs between the 1^st and 3^rd year of life.[4] However, various studies have shown that the age of fusion varies from 1 to 8 years.[5] If there is no fusion till 8 years of age, then, it is considered persistent MS.[6] Suture may be incomplete or complete. This condition of persistent MS is known as metopism.[7]

Frontal bone occupies the most anterior part of the skull. It is a pneumatic bone, presents singly. This bone contributes in the union of neurocranium with viscerocranium.[8] Embryologically, there are two frontal bones. These two frontal bones join along the median plane. This suture is known as MS. Metopion is a Greek word which means space between the eyebrows.[9] Sometimes, wormian bones may be present in this median frontal suture (present between two superciliary arches).[10]

Metopism can occur singly or can be part of malformative syndromes such as craniofacial Tessier or cleidocranial dysostosis.[11] This persistent suture can be confused for frontal bone fracture or sagittal suture.[12] An important feature to differentiate metopism from frontal bone fracture is its sclerotic borders seen in the X-ray of anteroposterior view of the skull.[13] Knowledge of metopism is very crucial for medical professionals and paleoanthropologists. This study on dry crania of adult humans is noteworthy for the understanding of persistent MS. Metopism is a topic, which has not been much studied.

METHODOLOGY

Fifty-four dry skulls of adult humans in the department of anatomy were studied. These skulls were of Western Indian origin. These skulls were observed for the presence (complete or incomplete suture) or absence of MS. Suture running continuously from nasion to bregma has been defined as complete suture, whereas suture being present but not over this entire length has been defined as incomplete suture.

RESULTS

Out of the 54 dry skulls examined, MS was found in 3.7% (2 out of 54) skulls. Complete MS was found in 1.85% (1 out of 54) of the dry skulls. Moreover, incomplete suture was also found in 1.85% (1 out of 54) of the dry skulls [Figure 1].

Figure 1.

Figure showing Metopism. (a) Complete metopic suture, (b) Incomplete metopic suture, (c) Metopic suture as visualized from above, (d) Metopic suture seen on inner table of cranial vault) (*indicates metopic suture)

DISCUSSION

According to the present study, metopism was found in 3.7% of the Western Indian population. Among the studies done on the Indian population, results of the present study were similar to the findings of Das et al.[14] Whereas results of Chandrasekaran’s study (5% metopism) and Agarwal et al.’s study (2.66% metopism) refute the present study’s observations.[15,16]

Studies done on dry skull bone of different countries have different opinions. In Agarwal et al.’s study on the European and Mongolian population, metopism was found to be 8.7% and 5.1%, respectively.[16] Romanes observed 8% prevalence of metopism in the European population.[17] Whereas Ajmani et al. found 3%–4% of the Nigerian population with persistent MS.[12] While studies of da Silva Ido et al. and Castilho et al. on the Brazilian population observed metopism to be 4.48%, 2.75%, and 7.04%, respectively.[18,19,20]

There are various theories for the persistent MS. Cause of metopism has been discussed to be regulating factors and/or active expression of cytokines during cranial fusion and/or active resorption of chondriodal tissue and/or genetic influence.[19,21] Equilibrium among cell proliferation, differentiation, and apoptosis is needed for the formation and persistence of suture. Several factors such as fibroblast growth factors (FGF), their FGF receptors, transforming growth factors-beta, bone morphogenetic proteins (BMP), Noggin an antagonist of BMP, homeobox MSX2, a basic helix-loop-helix transcription factor TWIST1, and runt-related transcription factor RUNX2 are responsible for osteogenesis, suture patency and closure.[22]

There are few studies which have correlated metopism with cephalic index. Some studies said; incidence of metopism to be more in dolichocephalic type, whereas according to others, incidence is more in brachycephalic type.[18,20] According to the latest study by Bryce, incidence of persistent MS is identical for both dolichocephalic and brachycephalic cranial types.[23]

MS is a small structure in lower mammals (smaller than the prominent internasal suture). In humans, MS is larger because of hypertrophy of frontal area of the brain.[24] In great apes, MS fuses almost at birth, whereas prevalence of metopism is lower in apes than in humans. Metopism is related to huge postnatal growth of the brain in humans. Studies on paleoanthropology have observed that Australopithecus africanus, Homo habilis, and Homo ergaster showed the presence of incomplete closure of MS, whereas metopism was a usual feature in Neanderthals.[25]

CONCLUSION

Metopism is clinically relevant anatomical variation. There are chances of it getting muddled up with skull fractures. It can also help in the identification of humans. Neurosurgeons should be aware of the presence of metopism before carrying out craniotomy. Forensic experts should have the understanding of differentiating midline skull fracture from persistent MS. Inaccuracies in its diagnosis can lead to fallacious treatment. Hence, knowledge of metopism is crucial for anatomists, forensic experts, physicians, neurosurgeons, radiologists, otorhinolaryngologists, anthropologists, and evolutionary biologists.

Conflicts of interest

There are no conflicts of interest.

Funding Statement

Nil.