The cranial sutures and normal variants:
Large sutures:
- Coronal suture
- Sagittal suture
- Lambdoid suture
- Squamosal sutures (temporosquamosal and sphenosquamosal sutures)
- Parietomastoid suture
Developmental sutures/syncondroses:
- Metopic
- Sphenofrontal
- Occipitomastoid
- Innominate
- Intra-occipital syncondroses
- Spheno-occipital syncondroses
Normal variants:
- Accessory occipital suture (Mendosal)
- Accessory parietal suture
- Persistent occipital suture
- Wormian bones
Fig. 1: 3D volume rendered CT images and plain radiograph demonstrating the various cranial sutures.
Fig. 2: Axial CT images of the calvarium and base of skull, demonstrating the cranial sutures.
Fig. 3: 3D volume rendered CT and Axial images demonstrating the skull of a 6 month old infant (Images a & b) and a 3 year old child (Images c & d)
Note the differences in sutural widths of a 6 month old infant and a 3 year old child.
Closure timeline:
Suture/syncondroses
|
Average Time of closure
|
Metopic
|
3 – 9 months**
|
Anterior fontanelle
|
18 - 24 months
|
Innominate
|
4 years
|
Mendosal
|
6 years
|
Sphenosquamosal
|
6 – 10 years
|
Intra-occipital syncondroses
|
12 years
|
Sphenofrontal
|
15 years
|
Occipitomastoid
|
16 years
|
Spheno-occipital syncondroses
|
18 years
|
Sagittal
|
22 years
|
Coronal
|
24 years
|
Lambdoid
|
26 years
|
Squamosal
|
60 years
|
** Note: Can persist until adulthood in up to 10% [4]: Sutura Frontalis Persistens (Figure 4). Can be mistaken for a frontal bone fracture.
Fig. 4: 3D volume rendered CT image of a persisting metopic suture (Sutura frontalis persistens). It is usually obliterated by 7 years of age but can persist to adulthood in 10%.
References: S. Idriz, J.H Patel, SA. Renani, R. Allan, I. Vlahos. CT of Normal Developmental and Variant Anatomy of the Paediatric Skull: Distinguishing Trauma from Normality. RadioGraphics RSNA. July 2015; 35(5). DOI : https://doi.org/10.1148/rg.2015140177
Accessory Sutures:
Most commonly seen in the parietal and occipital bones due to incomplete union of the ossification centers. Parietal bone - 2 occification centers; Occipital bone - 6 occification centers.
- Accessory parietal suture (Figure 5) - Usually extends anteriorly from (or lateral to) the lambdoid. Usualy bilateral but can be unilateral and assymmetric (Figure 6), which can cause diagnostic dilemma.
Fig. 5: 3D volume rendered CT image demonstrating bilateral accessory parietal sutures.These may also be unilateral and can appear longer.
References: S. Idriz, J.H Patel, SA. Renani, R. Allan, I. Vlahos. CT of Normal Developmental and Variant Anatomy of the Paediatric Skull: Distinguishing Trauma from Normality. RadioGraphics RSNA. July 2015; 35(5). DOI : https://doi.org/10.1148/rg.2015140177
Fig. 6: 3D volume rendered CT image demonstrating a unilateral accessory parietal suture extending lateral to the lambdoid suture. Note the appearance of this is much longer than the previous image. Also, can be seen here is an accessory occipital suture medial to the lambdoid suture.
References: T. Sanchez, D. Stewart, M. Walvick, L. Swischuk. Skull fracture vs accessory sutures: how can we tell the difference? Emergency Radiology. May 2010; 17(5): 413-418. DOI: 10.1007/s10140-010-0877-8
- Accessory occipital suture (Mendosal) (Figure 7) - usually lies medial to the lambdoid suture and superior to the occipito-mastoid suture. Usually bilateral but can be unilateral as well and it usually closes by 6 years of age.
Fig. 7: 3D volume rendered CT image demonstrating bilateral accessory occipital (mendosal) sutures.
- Persistent midline occipital suture (Figure 8) - Extends cranially from the dorsal aspect of the foramen magnum. Usually closed by 4 years of age. Appearances can mimic a fracture - appearing wide and sharp. However, the suture should not extend more than 2cm or persist later than 4 years of age [5]. If not seen following these 2 criteria, a fracture should be considered.
Fig. 8: 3D volume rendered CT image demonstrating a remnant of the midline occipital suture. Again, bilateral mendosal sutures can be seen here.
References: T. Sanchez, D. Stewart, M. Walvick, L. Swischuk. Skull fracture vs accessory sutures: how can we tell the difference? Emergency Radiology. May 2010; 17(5): 413-418. DOI: 10.1007/s10140-010-0877-8
Other variant to note: Wormian bones (aka intrasutural bones) (Figure 9)
- Additional small bones found between cranial sutures - most commonly in the lambdoid suture.
- Normal variant but some considered abnormal if greater than 10 in number.
- Can be associated with various pathologies and syndromes (eg, hypothyroidism, rickets, osteogenesis imperfecta, Down syndrome, etc).
Fig. 9: 3D volume rendered and axial CT images demonstrating wormian bones. On the axial image, additional suture lines can be seen, which may sometimes result in confusion. 3D reconstruction is helpful here in picturing the whole skull.
Fracture vs Suture: How do we differentiate them
Table 1: Summary table showing characteristic appearances and differences between cranial fractures and sutures.
Fig. 10: Axial and 3D volume rendered CT images of a 1.5 years old child who sustained a major trauma, resulting in multiple skull fractures bilaterally.
Image a: Note the difference between the sharp lucency in keeping with a left parietal bone fracture (white arrow) vs the sclerotic zig-zag pattern of a normal lambdoid suture (Purple arrow head). Note also the soft tissue haematoma (red arrow head) adjacent to the fracture.
Image b: Axial section more superiorly demonstrates widening of the fracture (white arrow) as it approaches the left coronal suture (blue arrow head). (see video below - Figure 11)
Image c: More fractures (white arrow heads) are seen on the right frontal bone as we approach the vertex. They appear comminuted with depressed fragments.
Image d: Further assessment with 3D volume rendering (VR) demonstrates the fractures on the right and can be clearly seen as a fracture crossing the right coronal suture (black arrow heads) and involves both the right frontal and parietal bones. The known left parietal fracture (white arrow) can also be easily visualised here on the 3D VR.
Fig. 11: Red arrow head demonstrates widening of the left parietal fracture as it approaches the left coronal suture. Note also additional fractures on the right side.
Fig. 12: Axial CT image of the same child, demonstrating a left frontal extradural haematoma (white arrow) and also a shallow left subdural haematoma (white arrow head). Extracranially, there is a large left subgaleal haematoma (red arrow head).
Tips:
- Understand the normal closure patterns and evolution of the developmental sutures.
- Although not to be used in isolation, 3D VR is helpful in problem solving and visualising complex fractures. It is good at providing an overview, course, linearity, symmetry and separation, which are helpful features in distinguishing fractures from sutures.
- Fractures normally come with other ancillary signs, such as extracranial haematoma, cortical incongruity, intracranial haemorrhage, contusions, or contra-coup injuries. Although one does not necessarily need to have a fracture to have intracranial injuries (especially in the paediatric population).
- Always ensure proper alignment and reconstruction before assessment (which will aid greatly in assessing for things like symmetry).
****Fracture or suture ? Have a look at the images below and decide what you would call them.****
Image I
Fig. 13: Axial CT image of a 2 month old infant.
I: FRACTURE = Traumatic diastasis and displacement of the right coronal and lambdoid sutures with associated soft tissue haematoma.
Note the zig-zag pattern and well defined borders (especially the more anterior lucency) suggesting that these are sutures. The incongruity and assymmetry compared to the left side suggests an element of diastasis.
Axial slices more superiorly and 3D VR demonstrated a large right parietal fracture extending into the sutures. (Figure 14).
Fig. 14: Axial CT image more superiorly demonstrating diastasis of the coronal suture and a large fracture in the right parietal bone. 3D volume rendered image demonstrates the large parietal bone fracture, with diastasis of the left lambdoid suture.
Image II
Fig. 15: Axial CT image of a 4 month old infant.
II: SUTURE = Asymmetry of the calvarium in keeping with right coronal synostosis (a unilateral coronal suture can be seen on the left). The red arrowhead points to a normal metopic suture, where there is no associated soft tissue swelling or intra-cranial findings.
Fig. 16: 3D VR demonstrating coronal synostosis in a 4 month old infant and a 3 year old child. Note the orbital distortion and superolateral elevation (white arrows), which can usually also be seen on plain radiographs (aka the Harlequin eye deformity). There is associated asymmetrical growth of the calvarium (white stars).
Image III
Fig. 17: Axial CT images of a preterm 33/40 weeks neonate.
III: SUTURE = Molding in a pre-term neonate; a physiological mechanism to reduce the circumference of the head during vaginal delivery.
Usually there is posterior and superior displacement of the parietal bones and elevation of the occipital bone as shown in the images above. These tend to resolve by day 7 post-delivery [4] and it is important not to mistake these for depressed skull fractures.
However, as you can see, they can be associated with intracerebral injuries (Figure 18). In this particularly case, the patient had an assisted forceps delivery, which resulted in injury.
Fig. 18: Axial CT images of the same child demonstrating concurrent subdural haematoma (red arrow head) and subgaleal haematomas (white arrow heads), associated with a forceps-assisted delivery.
Image IV
Fig. 19: Axial CT image of a 2 month old infant.
IV: SUTURE = Left posterior interdigitating line joining with the left Lambdoid/mastoid fontanelle in keeping with a mendosal suture. Note that there is no adjacent soft tissue haematoma next to this.
Image V
Fig. 20: Axial and 3D volume rendered CT images of a 2 month old infant
V: ARTEFACT = Although there is cortical incongruity on the left, there is also a sharp step in the soft tissue; an artefactual appearance (Figure 21). This should not be mistaken for an open fracture.
Note also how artefact on the 3D VR is reconstructed; mimicing a large right parietal fracture.
Fig. 21: Further axial and 3D volume rendered CT images demonstrate symmetrical step artefacts resulting in bony and soft tissue incongruity.