Introduction:
Sutural bones (SBs) or wormian bones (WBs) are a rare skeletal anomaly, which either forms as accessory bone(s) located inside the sutures of the cranium or in the rarest scenario, it could be even observed within the fontanelles. They contain independent ossification centers and usually penetrate to both outer and inner tables of bones forming cranial vault [1-3]. In normal cases when sutural bones exist, the number is limited to two or three. Whereas in skulls of hydrocephalus [4] it’s usually found in greater numbers. There is not much information available on what leads to the formation of SBs. According to a few authors, external influences play an important role in the formation of SBs [5] while others claim that they are derived from normal development process and are genetically influenced [6, 7].
Amongst the occurrences of SBs, they are most commonly present in the lambdoid suture [8] and least at bregma. They are very rarely present in other sutures like the coronal, sagittal, and squamosal sutures [9]. As they vary in number and shape from person to person these bones usually remain unnamed [10]. Variations or alterations in the normal development of the flat bones of the skull or abnormal suture closure results in the formation of WBs. These are regarded as “epigenetic” and “hypostotic” traits [11].
The SBs or WBs act as a marker for various diseases and hence are important in diagnosing various problems like brittle bone disease and osteogenesis imperfecta [12]. SBs can be seen through microcomputed tomography [13]. Congenital pathological conditions might lead to multiple sutural bones [14]. WBs can also be found at the cranial vault, obliterating the anterior fontanelle. One such case with an epidermoid cyst projecting intracranially between the cerebral hemispheres has been reported [15]. Jeanty et al. [16] reported four fetuses with WBs but none of the associated anomalies. WBs occur in deformed and undeformed skulls with no significant differences [17]. Interparietal bones were found in 1.315 % cases in the skulls of central India. Interparietal bones are also called Inca bones or Gothe’s ossicles [18]. The exact mechanisms behind the formation of WBs are unknown, though some of the studies have indicated that their presence may be associated with some anomalies of the central nervous system. They also occur in large numbers associated with skeletal dysplasia’s [19].
In the literature, very few studies on WBs have been reported from southern part of India. Main objective of our study is to document the percentage occurrence of WBs in the south Indian skulls and also to know their exact numbers and location in the sutures and other landmarks of the skull. Results of the current study could be of importance to anthropologists, radiologists, orthopedic surgeons, and neurosurgeons.
Methodology
A total of 27 dry, adult human skulls of either sex were observed for the presence of SBs. The damaged skulls were excluded from the study. All the sutures of the skull were observed for the existence of any sutural bones. Total number of SBs in each suture were noted down. The skulls with SBs were photographed. Later, the percentage occurrence of the SBs at coronal suture, sagittal suture, lambdoid suture, bregma, pterion, lambda and asterion were calculated.
Results
Among the 27 skulls studied, 24 (88.8%) skulls had SBs and 3 skulls were without any SBs. Asterion was the commonest region of the skull to have SBs. 18 (66.6%) skulls had at least one sutural bone at the asterion. Among these 18 skulls, 3 had more than one SBs. Maximum number of SBs found at asterion was 4 and was found in 1 (3.7%) skull. SBs in lambdoid suture were found in 6 (22.22%) skulls. 2 (7.4%) skulls had 2 SBs each, 1 (3.7%) skull had 8 and 1 (3.7%) skull had 20 SBs at the lambdoid suture. 4 (14.8) skulls had SBs at lambda. Among them, one skull had 4 SBs at lambda 1 (3.7%). 2 (7.4%) skulls had one each SBs at the coronal suture.
Occurrence of Sutural bones in the present study
Total number of skulls: 27.
| Table 1: Showing the percentage occurrence of sutural bones |
Sutural bones |
Number of skulls |
Percentage |
Present |
24 |
88.8% |
Absent |
3 |
11.11% |
| Table 2: Topographical distribution and frequency of the sutural bones |
Site of sutural bone |
Total number of skulls |
Percentage (%) |
Pterion |
2 |
7.4 |
Coronal suture |
2 |
7.4 |
Lambda |
4 |
14.8 |
Lambdoid suture |
6 |
22.22 |
Asterion |
18 |
66.6 |
 |
 |
Figure 1. Posterolateral view of the skull showing a single sutural bone at asterion. (SB – sutural bone; PB – parietal bone; OB – occipital bone; TB – temporal bone; MP – mastoid process) |
Figure 2. Posterolateral view of the skull showing two sutural bone at asterion. (SB – sutural bone; PB – parietal bone; OB – occipital bone; TB – temporal bone) |
 |
 |
Figure 3. Lateral view of the skull showing one sutural bone at pterion. (SB – sutural bone; PB – parietal bone; FB – frontal bone; GSB – greater wing of sphenoid; TB – temporal bone; ZB – zygomatic bone) |
Figure 4. Posterolateral view of the skull showing a large interparietal bone at the lambda. (SB – sutural bone (interparietal bone); PB – parietal bone; OB – occipital bone; TB – temporal bone) |
 |
 |
Figure 5. Posterosuperior view of the skull showing four interparietal sutural bones at the lambda. (SB – sutural bone (interparietal bone); PB – parietal bone; OB – occipital bone) |
Figure 6. Superior view of the skull showing a single sutural bone at the coronal suture. (SB – sutural bone; PB – parietal bone; FB – frontal bone) |
 |
 |
Figure 7. Posterosuperior view of the skull showing a single sutural bone at the lambdoid suture. (SB – sutural bone; PB – parietal bone; OB – occipital bone) |
Figure 8. Posterolateral view of the skull showing multiple sutural bones at the labdoid suture. (* – sutural bone; PB – parietal bone; OB – occipital bone; TB – temporal bone) |
 |
Figure 9. Posterior view of the skull showing multiple sutural bones at the lambdoid suture. (* – sutural bone; PB – parietal bone; OB – occipital bone) |
Discussion
The WBs or SBs can occur in both normal and pathological skulls. As the reason behind their formation is still not clearly understood, it is believed to be related to the genetics. Also in terms of population groups, external forces such as stress conditions [20] also influence the formation of the sutural bones. During the process of ontogenesis, the imbalance between the cranial changes in size and shape can also result in the formation of SBs [21]. The number of WBs does not depend on the cause of enlargement, instead it increases with the capacity of the skull [16]. Healthy individuals have also been diagnosed with WBs as a normal variant. [22]. The major difference between their presence as normal variant and that of skeletal dysplasias is their size and the number [19]. But many authors are of the opinion that WBs are not always pathognomonic, as they can occur in some normal individuals too. But the occurrence of more than ten WBs is considered unusual.
The dentations are more complex on the exterior of the skull, than on the internal side as the sutural (wormian) articulate with the surrounding bones by sutures. They act as supplementary points of ossification and do not have any vital role in the mechanism of supporting the cranium [23]. According to Brothwell’s [24] study, percentage frequency of WBs in various population groups ranged between 55.56 and 80.32. The incidence rate of WBs in the present study is significantly higher. WBs are more frequently observed in Chinese population due to their traditional supine infant sleep position causing brachycephalic deformations in their crania [25].
| Table 3: showing the incidence of wormian bones in different populations according Brothwell (1963) |
Population |
Incidence (%) |
Anglo-Saxon |
55.56 |
Lachish |
63.41 |
Melanesian |
64.41 |
Romano-British |
71.03 |
Australian |
72.58 |
German |
75 |
Chinese |
80.32 |
Indian (Present study) |
88.8 |
In the present study, sutural bones were found in 88.8% of south Indian skulls. This number is higher than all the previous studies. Bergman et al. [21] have reported that nearly 40% of skulls have sutural bone in the lambdoid suture. The location of wormian bones at different location has different importance. They occur most frequently in the lambdoid suture. Normally the number of wormian bones does not exceed two or three in a normal skull but they present in large number in hydrocephalus [26].
| Table 4: Shows the different percentages of incidence of wormian bones in various studies. |
| Incidence Of Sutural Bones In Various Studies |
Veeresh kumar et al. [27] |
32% |
Walukar et al. [28] |
34.22% |
Radha et al. [29] |
34.84% |
Satpathi et al. [30] |
43.30% |
Shivleela et al. [31] |
43.52% |
Patel et al. [32] |
44.40% |
Showri & Suma [33]. |
52.4% |
Vathsala et al. [34] |
52.77% |
Sreekanth et al. [35] |
53.15% |
Uday Kumar et al. [36] |
56.5% |
Lekshmy vijay et al. [37] |
61.5% |
Murlimanju et al. [23] |
73.1% |
Shantharam et al. [38] |
81.89% |
Present study |
88.8% |
In the present study, the most common site of occurrence of WBs was asterion (66.6%). Second most common site was lambdoid suture (22.22%) and the least common site for WBs was pterion (7.4%) and Coronal suture (7.4%) WBs were also found in lambda (14.8%). None were found in bregma and sagittal suture.
Asterion – In infants, it is called as a posterolateral fontanelle. It is closed at 12 months after birth. In adults, asterion is the junction of the parietal, temporal and occipital bones. It is the surgical landmark to the transverse sinus location, which is of great significance in the surgical approaches to the posterior cranial fossa [39, 40]. Presence of sutural bones at asterion may complicate the surgical orientation [41, 42]. Lekshmy vijay et al. [37] have found highest occurrence (79.67%) and Satpathi et al. [30] lowest occurrence (1.33%) of WBs in the asterion. In the present study, we found more skulls and higher number of WBs at this region.
Pterion – In infants, it is called as anterolateral fontanelle. It closes at 2-3 months after birth. In adults, pterion is the junction of the parietal, frontal, greater wing of the sphenoid, and squamous portion of the temporal bones. Wormian bone at pterion is called “pterion ossicle” or “epipteric bone” or Flower’s bone [43-45]. Incidence of such bones is high in Indians (11.79%) shown in studies done by Nayak B et al. [10]. Shantharam et al. [38] have found highest occurrence (20.91%) and Patel et al. [32] lowest occurrence (0.03%) of WBs in this region. Walukar et al. [28] have not found any WBs in the pterion in their study. In the present study the occurrence of Wormian bone at this point is 7.4 %. It was reported that the presence of WBs at the pterion may lead to complications in making burr holes at the pterion [39].
Lambda – At birth it is called as posterior fontanelle and it is closed at 2-3 months after birth. In adults it forms meeting point of sagittal and lambdoid sutures. Veeresh Kumar et al. [27] observed 6 % large sized WBs at the lambda, called “Inca bone”. Uday Kumar et al. [36] found highest occurrence (46.0%) and Patel et al. [32] lowest occurrence (0.07%) of WBs in the lambda. Brasilli et al. [46] reported incidence of WBs at lambda to be more in male skulls, but studies done by Manjula and Santosh [47] showed incidence more in female skulls. The site of least occurrence of WBs in the present study is in lambda (7.4%).
Bregma – Bregma is the meeting point of the sagittal and coronal sutures. It is represented by the anterior median fontanelle in the fetal life. The fontanelle obliterates usually by 18th postnatal month [48]. The SBs are common in the skull but presence of a sutural bone at bregma is certainly a rare occurrence. In the present study, WBs in this region was not found among 27 skulls investigated. Satpathi et al. [30] and Lekshmy vijay et al. [37] reported that incidence of WBs in bregma in 1.33% and 2.43% respectively. Patil and Sheelavant [44] and Brasili et al. [46] stated that WBs at bregma is more frequent in male skulls. The presence of the sutural bone at the bregma may be because of appearance of an abnormal ossification center in the fibrous membrane at the anterior median fontanelle of foetal life. Johal et al. [49] describes a rare incidence of WBs within the anterior fontanelle of an infant who had concurrent craniosynostosis. Hussain Saheb et al. [50] found a case of an unusual sutural bone, in the region of the bregmatic frontanelle.
Coronal suture – Coronal suture is a rare site to have any WBs. The studies by Radha et al. [29], Vathsala et al. [34], Veeresh kumar et al. [27], Sreekanth et al. [35] and Uday Kumar et al. [36] have not documented presence of any WBs in the location of coronal suture. When compared to these studies, our study has documented coronal WBs in 2 (7.4%) skulls. Some of the earlier studies have found WBs on coronal suture in very few skulls. Showri & Suma [33] found highest occurrence (6%) and Patel et al. [32] lowest occurrence (0.03%) of WBs in the coronal suture. Brasilli et al. [46] reported incidence of 3% with more frequency among male skulls. Vasi [51] reveals that the WBs are unilateral in occurrence on the left side of the coronal suture which is a rare condition. When compared to all the above studies, south Indian skulls have a higher prevalence of WBs as documented by the current study.
Lambdoid suture – Jones [52] has stated that lambdoid suture being always the most complicated suture of all the cranial sutures is by far the commonest site for the development of WBs. Lekshmy vijay et al. [37] found the highest (91.05%) and Radha et al. [29] least (18.1%) incidence of WBs in lambdoid suture. WBs are most common in lambdoid suture because of their rugged nature compared to all the other sutures [53]. From the review of literature, it is evident that lambdoid suture is the commonest site of skull to have WBs (Table 5). But, in a study by Vathsala et al. [34], which comprised of 180 skulls, none of the skulls had any WBs at the lambdoid suture. In the present study, 22.22% of skulls had WBs at lambdoid suture, which is the second most common site to have WBs.
The sutural bone at lambdoid suture is also termed as pre-intraparietal bone or Inca bone. If there are more than two or three Inca bones, it may lead to complications while performing craniotomy surgeries when done by posterior approach. Such bones may also create confusion among radiologist to misdiagnose it as skull fractures during the severe head injuries.
Sagittal suture - In the present study, WBs were not found in this region. Patel et al. [32], Veeresh kumar et al. [27] and Uday Kumar et al. [36] also did not observe any WBs in the sagittal suture. Satpathi et al. [30] found maximum incidence (5.33%) and Sreekanth et al. [35] minimum incidence (0.90%) of WBs in the sagittal suture.
| Table 5: Comparison of distribution of wormian bones at various locations of the skull as observed in the present study and previous ones. |
| Population |
Sample size |
Asterion |
Pterion |
Lambda |
Bregma |
Coronal suture |
Lambdoid suture |
Sagittal suture |
Costal south India
Murlimanju et al. [23] |
78 |
17.9 |
11.5 |
0.0 |
0.0 |
1.3 |
56.4 |
1.3 |
Vidarbha (India)
Walukar et al. [28] |
225 |
9.09 |
0.0 |
10.38 |
0.0 |
1.29 |
74.2 |
5.19 |
Gulbarga (India)
Shivleela et al. [31] |
180 |
11.11 |
1.85 |
8.33 |
0.0 |
2.78 |
33.33 |
0.92 |
South indian
Radha et al. [29] |
66 |
9.09 |
3.03 |
12.12 |
0.0 |
0.0 |
18.1 |
1.51 |
Central India
Satpathi et al. [30] |
150 |
1.33 |
1.33 |
8.0 |
1.33 |
1.33 |
24.66 |
5.33 |
Tamilnadu (India)
Vathsala et al. [34] |
180 |
8.3 |
12.0 |
28.3 |
0.0 |
0.0 |
0.0 |
3.8 |
Surath (India)
Patel et al. [32] |
27 |
18.50 |
0.03 |
0.07 |
0.0 |
0.03 |
48.14 |
0.0 |
Bangalore (India)
Showri & Suma [33] |
132 |
5.30 |
1.5 |
13.63 |
0.0 |
6.0 |
45.45 |
4.54 |
(Veeresh kumar et al. [27] |
50 |
6.0 |
2.0 |
6.0 |
0.0 |
0.0 |
44.0 |
0.0 |
Telangana (India)
Sreekanth et al. [35] |
111 |
2.7 |
1.80 |
8.10 |
0.9 |
0.0 |
53.15 |
0.90 |
North Karnataka
Uday Kumar et al. [36] |
200 |
40.70 |
5.30 |
46.0 |
0.0 |
0.0 |
56.63 |
0.00 |
Mangalore (India)
Lekshmy vijay et al. [37] |
200 |
79.67 |
16.26 |
23.57 |
2.43 |
53.65 |
91.05 |
27.64 |
South Indian
Shantharam et al. [38] |
110 |
38.18 |
20.91 |
22.73 |
0.0 |
10.0 |
48.18 |
3.64 |
Present study |
27 |
66.6 |
7.4 |
7.4 |
0.0 |
7.7 |
22.22 |
0.0 |
The WBs may infrequently cause confusion during the radiological examination of the skull [54]. Sometimes, the WBs look like fractures and may confuse the radiologist or surgeon [10]. It is more challenging if the fracture of skull is misinterpreted as a wormian bone and the patient may miss the suitable treatment at a right time. Hence the basic awareness about these SBs are important for the doctors in day to day clinical practice.
In conclusion, the current study reports, 88.8% as the incidence rate of WBs in South Indian skulls. This is slightly higher compared to other reports indicating a possibility of racial variations in the existence and distribution of them. The WBs were more frequent at the asterion in the current study, second highest in lambdoid suture and less common at the pterion and coronal sutures. We hope that the present study has provided extra information on the morphology and topography of the WBs. The clinical significance of these variant bones were emphasized with relevant review of literature. Knowledge of WBs is of importance to the neuroanatomists, neurosurgeons, radiologists, anthropologists and morphologists.
Conflict of Interest: None
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