It is a type of fibrous joint that occurs only in the bones of the head.
Cranial sutures are the intimate union that exists between bone and bone, with age the sutures are joined (ossifying) until the bone growth of the newborn is completed.
The main sutures are:
- Coronal or frontal metopic suture: it is established between the frontal and both parietal bones.
- Sagittal suture: the one between the two parietal bones.
- Posterior or lambdoid suture: the one between the occipital bone and both parietal bones.
The suture between the frontal bone is known as a metopic suture, it is a type of serrated suture. Fusion of the suture during ossification proceeds from the bregma to the nasion.
Each half of the frontal bone ossifies from a primary center in the membrane in the frontal tubercle by the eighth week of intrauterine life.
The ossification extends from this center in different superior, posterior, and inferior directions.
The term methopic is a Greek word that means “in the middle of the face.”
It is located anterior to the coronal suture. Remnants of this suture may persist in the skull.
Sutures play an important role in the growth of the brain and also for the normal growth of the skull. The persistence of the metopic suture may not be pathological, but its anatomy and incidence are clinically important.
Incidence of the ethnic group in the closure of the metopic suture
At birth, both halves of the frontal bone are joined by the metopic suture. This middle suture between the two halves of the frontal bone usually closes in the first postnatal year, but may persist as the metopic suture in some individuals and in various ethnic groups.
The incidence of metopic suture varied in different races in Europeans by 7 to 10%, from 4 to 5% in the yellow races, while 1% in the African population.
Classification of the metopic suture
The metopic suture is a dense, serrated fibrous joint that extends from the nasion craniometric point (the meeting point of the nasofrontal and internal sutures) to the bregma (the intersection point of the coronal and sagittal suture).
The classification according to its extension is as follows:
- Complete metopic suture : A metopic suture that extends from the bregma to the nasion.
- Incomplete metopic suture: extends a short distance, either from the nasion or from the bregma.
The additional incomplete metopic suture is subclassified, depending on the site from which the metopic suture arises, into either the Nasion incomplete metopic suture type and the Bregma incomplete metopic suture type.
Based on the shape, Nasion’s incomplete metopic suture type is described as linear type, V-shaped, and U-shaped.
The frontal bone is a flat, pneumatic bone of the calvaria. It has two parts, a squamous part involved in the formation of the forehead and an orbital part that forms the roof of the orbit.
The metopic suture runs midline through the frontal bone from the nasion to the bregma, although it can often be incomplete.
Normally, the closure of this suture takes place between 1 and 8 years of age. Failure of this closure beyond 8 years leads to a persistent metopic suture.
The metopic suture can simulate a skull fracture and can confuse an inexperienced forensic expert.
The neurosurgeon must also be aware of this anatomical variation when performing a frontal craniotomy, as the persistent metopic suture can simulate a vertical skull fracture.
A premature fusion of the suture is called a metopic synostosis, which is a type of craniosynostosis and can lead to trigonocephaly.
Premature closure of the metopic suture results in a growth restriction of the frontal bones, leading to a malformation of the skull known as trigonocephaly.
Over the past decades, its incidence has been increasing, making it the second most common type of craniosynostosis today.
Metopic synostosis is associated with an increased level of neurodevelopmental delays.
Theories about the etiology of these delays range from reduced volume of the anterior cranial fossa to intrinsic malformations of the brain.
The term trigonocephaly is derived from the Greek words “trigonon”, which means triangle, and “kephale”, which means head.
The term trigonocephaly was first proposed by Welcker in 1862, who used it to describe a child who had a wedge-shaped skull combined with a cleft lip.
Characteristics of metopic synostosis
This type of craniosynostosis is characterized by a wedge-shaped or triangular forehead, which results from premature fusion and subsequent ossification of the metopic suture.
A premature fusion results not only in an obvious midline ridge of the forehead due to ossification of the suture, but also in lateral growth restriction of the frontal bones.
This wedge shape is further enhanced by the increased compensatory growth of the remaining skull sutures, while the skull continues to expand.
The final product is a skull with a triangular forehead, a bony midline ridge, and a shortening of the anterior cranial fossa. There is often a certain degree of excess soft tissue along the same lines.
In 55% of cases, the anterior fontanelle closes prematurely. Lateral deficient orbital margins add to supraorbital retrusion and bitemporal clefts.
In severe cases, the lateral canthal angles are high. At the level of the medial orbital walls, there is hypotelorism combined with ethmoidal hypoplasia.
Epicantal folds are often present. The orbits are teardrop-shaped and angled toward the midline of the forehead.
Vertical growth restriction expressed in reduced height of the auricular head is one of the most significant components of midline growth abnormalities.
The cephalic index (maximum skull width / maximum skull length) remains within normal limits, although there is bitemporal shortening and biparietal widening.
Treatment of metopic synostosis
Since growth restriction results in reduced intracranial volume.
Treatment consists of a cranioplasty, which is usually performed before the age of 1 year.
This surgery is indicated to restore the volume of the skull and its appearance.