Fusion patterns of major calvarial sutures on volume-rendered CT reconstructions (2024)

Keywords: craniosynostosis; metopic suture; coronal suture; sagittal suture; lambdoid suture; craniofacial; congenital

Craniosynostosis is estimated to occur in about 1 per 2100–2325 live births,^3,9 with sagittal synostosis being the most common type at 1 in 5250–6667 live births.^3,10 Recently, we investigated the fusion of minor lateral calvarial sutures (sphenoparietal, squamosal, parietomastoid) on the volume-rendered head CT reconstructions in 337 pediatric trauma patients. While reviewing these reconstructions, we discovered many more fused sagittal sutures than the expected 1 in 5000–6000 subjects. In the present study, using the same head CT reconstructions, we investigated abnormal fusion of the sagittal as well as the lambdoid, coronal, and metopic sutures.

Several publications have reviewed normal fusion of the metopic suture on CT.^{1,12,15,22,23} However, there is less information on normal fusion of other major sutures. Most existing information derives from studies of adult cadaveric skulls,^{2,5,14,16–21} although there is at least one recent CT study in the forensic literature.⁶ With the present publication, we hope to increase the information available about the normal fusion of major calvarial sutures.

Methods

In 2010, Children’s Hospital Colorado began performing most head CTs as spiral scans, allowing high-resolution, “3D” volume-rendered reconstructions to be made of the skull. That same year, Children’s Colorado began performing the initial head CT for all trauma patients using a new protocol, “CT brain without contrast for trauma,” which includes volume-rendered reconstructions. All scans in this study were obtained using this protocol on a Siemens SOMATOM Definition Flash CT scanner. Scans were acquired helically with a slice thickness of 0.5 mm. Volume-rendered reconstructions of the skull were created by the CT technologist for each case. Under a Colorado Multiple Institutional Review Board protocol, we reviewed all initial volume-rendered head CT reconstructions performed at Children’s Colorado in the period from January 2010 through mid-2012 for trauma patients aged 0–21 years.

Each sagittal, lambdoid, coronal, and metopic suture in every reconstruction was evaluated independently by a board-certified pediatric neuroradiologist (N.V.S.) and pediatric neurosurgeon (C.C.W.). Each suture was graded as open, partially fused, or fused. A length of suture was considered fused when no discernible suture was seen over that length on a volume-rendered reconstruction of the outer skull surface. A suture length was considered open when it did contain clearly visible suture. A suture was considered partially fused when any length except its entire length was fused. When no length of the suture was fused, the suture was called “open”; when the entire length was fused, the suture was called “fused.” When there was a discrepancy between the grades assigned to a particular suture by the two reviewers, the reviewers reevaluated the suture jointly and assigned a consensus grade. When it was difficult to determine the extent of fusion of certain sutures and when it was suspected that sutures were fused early and/or atypically, we also evaluated the source images. Although we consider each normal cranium to have two coronal and two lambdoid sutures, left and right for each, for this study, we considered each cranium to have a single bilateral coronal and a single bilateral lambdoid suture.

Individual sutures were excluded if they were obscured by artifact, incompletely imaged, imaged with insufficient quality to evaluate their extent of fusion, or directly involved by fracture. If there was a fracture elsewhere in the calvaria that did not directly involve a particular suture, that suture was still included. All sutures were excluded when no volume-rendered reconstructions were available or when the available reconstructions were of insufficient quality, when the subject had a cerebrospinal fluid shunt, or when the subject had radiological evidence of previous cranial or intracranial surgery or significant abnormalities of the brain.

The only records reviewed for most subjects were their head CT images. However, for subjects with fused or partially fused sagittal sutures, we also searched lists of existing diagnoses for craniosynostosis. Only one CT scan was reviewed per subject.

For subjects with fused or partially fused sagittal sutures, the cephalic index (CI) was calculated from their CT scans. To calculate the CI, the distance between the left and right euryon was divided by the distance between the glabella and opisthocranion and multiplied by 100. The percentages of sutures that were open, partially fused, or fused were then tabulated and graphed by month and year of age. The degree of suture fusion was tabulated by month from 1 month through 36 months of age, rounding up or down to the nearest whole month. After the age of 36 months, fusion was tabulated only by year. For ages 1 and 2 years, we evaluated the scans of all subjects aged 9 through 15 months and 21 through 27 months, respectively. For the age of 3 years, we evaluated the scans of all subjects aged 33 through 42 months. For the age of 4 years and older, all subjects were included and their ages were rounded up or down to the nearest whole year.

For statistical analysis, the data were organized and analyzed using JMP (JMP Pro, version 12, SAS Institute Inc.). Comparisons were made using Wilcoxon’s rank-sum test (continuous variables) and Fisher’s exact test (categorical variables). Significance was set at p < 0.05.

Results

We initially reviewed 337 volume-rendered head CT reconstructions. Two subjects were excluded because of preexisting ventriculoperitoneal shunts. Two were excluded because of an age > 21 years (30 and 31 years). As there were only two 19-year-olds (and no 20- or 21-year-olds), we excluded them too. One coronal suture was excluded because of involvement by fracture. Thus, 331 reconstructions and 1323 sutures were ultimately included (Supplemental Table 1). The subject ages ranged from 11 days to 18 years. There were 215 males (65%) and 116 females (35%).

The sagittal suture was fused or partially fused in 21 subjects (6%; Table 1). In 4 subjects, the medial sections of adjacent sutures were also fused (Fig. 1D–G): the lambdoid in 2, the coronal in 1, and the lambdoid and coronal in 1. None of these 21 subjects were found to have preexisting diagnoses of craniosynostosis.

Other than in the aforementioned 21 subjects, no sagittal or lambdoid sutures were even minimally fused and no coronal sutures were fused medially. Medial lambdoid fusion occurred in 0.9% of subjects and medial coronal fusion in 0.6% (Table 1). Several older subjects had narrow sutures with substantial ridging that were still open throughout their lengths (Fig. 2A). This pattern was called “lapsed suture union” by Todd and Lyon.²⁰

Of all subjects with sagittal suture fusion, whether complete or partial, 17 (81%) were male and 4 (19%) were female (Table 2). After adjustment for the proportion of males (65%) versus females (35%) in the entire series, there were 70% males and 30% females. Of the subjects with isolated sagittal suture fusion (without medial fusion of adjacent sutures), 14 (82%) were male and 3 (18%) were female. After adjustment for the proportion of males versus females in the entire series, there were 72% males and 28% females. Of the subjects with medial fusion of coronal and/or lambdoid sutures, 3 (75%) were male and 1 (25%) was female (Table 3).

Of the subjects with isolated sagittal suture fusion, 10 (59%) had complete fusion, whereas 7 (41%) had partial (but nearly complete) fusion (Table 4). When fusion was partial, the anterior end was minimally open in all 7 subjects; in 4 subjects, a short section of the posterior end was also open (Table 1). Of the subjects with complete fusion, 8 (80%) were male and 2 (20%) were female. Of the subjects with only partial fusion, 6 (86%) were male and 1 (14%) was female. There was no significant difference in the proportions of males and females with complete versus partial fusion (p = 0.9999; Table 4). The mean age of subjects with complete fusion was 11.3 years, whereas that of the subjects with partial fusion was 5.6 years; the difference between the two was statistically significant (p = 0.0444).

The mean age of subjects with isolated sagittal suture fusion was 8.9 years, whereas the mean age of subjects with medial fusion of adjacent sutures was 9.3 years; the difference between the two was not statistically significant (p = 0.9641; Table 3).

All 4 subjects with medial fusion of coronal and/or lambdoid sutures had complete fusion of the sagittal suture (Table 1). In each of these subjects, fusion of the coronal and/or lambdoid sutures was partial and medial only.

Of all subjects with any sagittal suture fusion, 14 (67%) had complete fusion and 7 (33%) had partial fusion (Table 4). Of the subjects with complete sagittal suture fusion, 11 (79%) were male and 3 (21%) were female. In the entire group of 21 subjects, there was no significant difference in the proportion of males and females with complete versus partial sagittal suture fusion (p = 0.9999). The mean age of all subjects with complete sagittal suture fusion was 10.7 years, whereas that of the subjects with partial fusion was 5.6 years, a difference that was statistically significant (p = 0.0393).

The mean age of all male subjects with any sagittal suture fusion was 8.4 years, whereas the mean age of females was 11.8 years; the difference between the two was not statistically significant (p = 0.2078; Table 2). The mean age of males with isolated sagittal suture fusion and no medial fusion of adjacent sutures was 8.1 years, whereas the average age of females was 12.7 years; these ages were not significantly different (p = 0.1454).

In all subjects with any sagittal suture fusion, the average CI was 78.0 (range 62–96). In subjects with isolated sagittal suture fusion, the average CI was 77.6 (range 62–88); in subjects with medial fusion of adjacent sutures, the average CI was 81.3 (range 74–96; Table 3). The difference between the two was not significant (p = 0.6526). In subjects with isolated sagittal suture fusion, subjects with complete fusion had an average CI of 75.8, whereas those with partial fusion had an average CI of 80.3; the difference between the two was not significant (p = 0.2018; Table 4).

Coronal sutures were often bilaterally partially fused just above the pterion early in the 2nd decade of life (Figs. 2B–C and 3 and Supplemental Table 2), and sporadic sutures were fused bilaterally in the same spots during the 1st decade. As the teenage years went on, fusion progressed symmetrically superiorly and medially and was present in progressively more subjects, with over 90% of coronal sutures partially fused by 18 years of age. However, none were ever completely fused even at 18 years. Of 60 partially fused coronal sutures, 57 had bilateral inferior fusion, 1 was fused inferiorly only on the right, and 2 (3%) were fused medially. The latter 2 sutures were in subjects with sagittal suture fusion.

The metopic suture remained completely open in all subjects through 2 months of age (Fig. 4 and Supplemental Table 3). By 3 months, 29% were partially fused. Fusion was first seen just above the glabella and progressed inferiorly and superiorly as subjects aged (Fig. 2D). By 1 year of age and thereafter, nearly all metopic sutures were at least partially fused. However, completely open metopic sutures were sporadically found in subjects as old as 14 and 15 years (Figs. 2E–F and 4C). By 18 months, most metopic sutures were completely fused, although it was not unusual to find partially fused sutures even in the 3rd year of life. At 4 years and thereafter, 193 of 198 metopic sutures were completely fused (Fig. 4C); the exceptions were completely open. The last sections of the suture to fuse were just above the nasion and just in front of the anterior fontanel/bregma. The metopic suture was completely fused in all subjects with sagittal suture fusion. No metopic sutures were considered to be prematurely fused (i.e., no reconstruction was believed to demonstrate trigonocephaly).

Discussion

Conclusions

The sagittal and lambdoid sutures do not normally begin to fuse before 18 years of age. However, the sagittal suture fuses in children and adolescents more often than generally thought. The significance of this finding is unknown. Children and adolescents with sagittal suture fusion and normal skull growth do not need cranial surgery.

The coronal suture often begins to fuse inferiorly early during the 2nd decade of life but does not usually complete fusion before 18 years of age. Children and adolescents with bilateral inferior coronal suture fusion do not need cranial surgery.

The metopic suture often starts to fuse by 3 months of age. Normal metopic sutures may not completely fuse until several years of age, if at all.

Disclosures

The authors report no conflict of interest concerning the materials or methods used in this study or the findings specified in this paper.

Author Contributions

Conception and design: Wilkinson, Stence, French. Acquisition of data: Wilkinson, Stence, Serrano, Graber, Batista-Silverman, Schmidt-Beuchat. Analysis and interpretation of data: Wilkinson, Stence, Graber, Schmidt-Beuchat, French. Drafting the article: Wilkinson. Critically revising the article: all authors. Reviewed submitted version of manuscript: all authors. Approved the final version of the manuscript on behalf of all authors: Wilkinson. Statistical analysis: Graber. Administrative/technical/material support: Graber, Batista-Silverman. Study supervision: Wilkinson.

Supplemental Information