DOI: 10.18410/jebmh/2015/644
ORIGINAL ARTICLE
INCIDENCE OF INTERPARIETAL (INCA) BONES IN THE POPULATION OF
ASSAM
Rubi Saikia1, Bijoy Kumar Borah2, Jyotirmayee Lahon3, Natasha Gohain4
HOW TO CITE THIS ARTICLE:
Rubi Saikia, Bijoy Kumar Borah, Jyotirmayee Lahon, Natasha Gohain. ”Incidence of Interparietal (INCA)
Bones in the Population of Assam”. Journal of Evidence based Medicine and Healthcare; Volume 2,
Issue 31, August 03, 2015; Page: 4574-4578, DOI: 10.18410/jebmh/2015/644
ABSTRACT: The occipital bone forms the region of the back of the head. It consists of three
parts- the squamous part, the basilar part and the two condylar parts on either side of the
foramen magnum. The squamous part of the occipital bone consists of two parts – the upper
interparietal and the lower supraoccipital part. Usually the interparietal part fuses with the
supraoccipital part but sometimes it may remain separated by a suture. Then it is called
interparietal bone or Inca bone. AIM & OBJECTIVES: The aim of the present study is to find
out the incidence of interparietal or Inca bones in this part of north east India as data regarding
its incidence rate and number of fragments of Inca bones from Assam is still underreported.
MATERIALS & METHOD: In the present study, a total of 126 dry adult skulls were examined to
know the incidence of Inca bones. All the specimens were procured from various departments in
Assam Medical College, Dibrugarh, Assam. They were first examined for presence or absence of
Inca bones and then for number of fragments. RESULTS & OBSERVATIONS: The incidence of
Inca bone was found to be 1.58%. One skull presented with a single triangular Inca bone while
another skull presented with two fragments of Inca bones. CONCLUSIONS: The knowledge
regarding interparietal bones and that it may present with many fragments is certainly very useful
for neurosurgeons, radiologists and anthropologists.
KEYWORDS: Interparietal bones, Inca bones, Squamous, Suture, Occipital bone, Supraoccipital,
cartilaginous.
INTRODUCTION: The squamous part of the occipital bone consists of two parts- the upper
interparietal part & the lower supraoccipital part. Usually these two parts fuse but sometimes the
interparietal part may remain separated from the supraoccipital part by a well-defined suture. It
is then called the interparietal bone or Inca bone.(1) The interparietal part develops in membrane
whereas the supraoccipital part develops both in cartilage and membrane. The segment of
supraoccipital bone between the highest and superior nuchal line forms the intermediate
segment. It ossifies in membrane and probably never separates from the cartilaginous
supraoccipital part.(2) The interparietal bones or Inca bones are bounded superolaterally on either
side of the median plane by the lambdoid suture and inferiorly by sutura mendosa (transverse
occipital suture).(3) Inca bones in humans were first found in the skulls of contemporary
indigeneous people of southern Andes as well as in those of mummies of the Inca civilization.
Although Inca bone was originally encountered as a variation in South American and Latin
American cranial remains, this variation does occur in people from all geographic regions of the
world. Inca bones were previously known as os-Incae, os-interparietale, Gothe’s ossicle.(4) Inca
bones resembles a triangular architectural monument design of the Inca tribe.(5) The members of
J of Evidence Based Med & Hlthcare, pISSN- 2349-2562, eISSN- 2349-2570/ Vol. 2/Issue 31/Aug. 03, 2015 Page 4574
DOI: 10.18410/jebmh/2015/644
ORIGINAL ARTICLE
Royal family of Inca tribe had a crown like configuration on their head. Hence thereafter these
ossicles have been known as Inca.(6) The occurrence of Inca bone is rare as compared to sutural
or wormian bones. Data on the incidence and number of fragments of Inca bones in Assamese
population is under-recorded. Keeping this in mind, the present study was undertaken and the
data collected was compared with other relevant literature.
MATERIALS & METHODS: The study was carried out in 126 dry human adult skulls of unknown
sex, procured from the department of Anatomy, department of Forensic science & the Institute of
Pharmacy at Assam Medical College, Dibrugarh, Assam. The selected skulls were without any
antemortem or postmortem injuries. The skulls were examined macroscopically to determine the
presence or absence of interparietal or Inca bones. Also the number of fragments of Inca bone
was noted. Next, the length and width of the Inca bones were measured. The data collected was
tabulated, analyzed and compared with others.
Exclusion & Inclusion Criteria: Adult skulls without any antemortem or postmortem injuries
were only considered fit for the present study. Skulls with fractures were discarded.
RESULTS AND DISCUSSION: Among the 126 skulls, only 2(1.58%) skulls showed the
presence of interparietal or Inca bones. Of these, one skull had a single large triangular
interparietal bone (photograph A) while the other skull had two fragments, right & left, of
interparietal bones (photograph B).
No. of fragments found
1
2
No. of skulls
1
1
Type
Median
Bipartite
Table 1: Showing the number of fragments of interparietal or Inca bones
J of Evidence Based Med & Hlthcare, pISSN- 2349-2562, eISSN- 2349-2570/ Vol. 2/Issue 31/Aug. 03, 2015 Page 4575
DOI: 10.18410/jebmh/2015/644
ORIGINAL ARTICLE
Type
Median
Bipartite
Maximum length (cm)
6.4
4.1
3.3
Maximum width (cm)
12.3
3.5
2.2
Table 2: Showing the measurements of interparietal or Inca bones
Available literature suggests that the occurrence of interparietal or Inca bones is rare.(3)
Studies also show that its incidence rate varies among different population around the world. It is
found to be 1.2% in Europeans, 0.8% in Australians, 2.4% in Indians, 15% in Nigerians & 2.8%
in Turkish population.(1,7) The Inca bone may exist as a single bone, two symmetrical halves, in
three pieces or even four in which case the upper two constitutes the preinterparietal bones.(8)
According to Srivastava HC,(2) the membranous portion of squamous part of occipital bone
develops from three pairs of Centre’s. The first pair of Centre forms the intermediate segment,
which form the portion of the bone between the superior and highest nuchal lines and always
fuses with the cartilaginous supraoccipital part. The second and third pairs of centers form the
interparietal or Inca bone which lies above the highest nuchal line. The second pair consists of
medial and lateral nuclei and it forms the lateral plate. The third pair consists of upper and lower
nuclei which forms the medial plate. (Fig. 1)
Figure 1: Diagrammatic representation of ossification centres and their nuclei in the
membranous part of the occipital bone above the supraoccipital bone (SO) - paired centres of the
intermediate segment (IS), medial and lateral nuclei of second pair of centres (II) which will form
the lateral plate and upper and lower nuclei of third pair of centres (III) which will form the
medial plate.(2)
J of Evidence Based Med & Hlthcare, pISSN- 2349-2562, eISSN- 2349-2570/ Vol. 2/Issue 31/Aug. 03, 2015 Page 4576
DOI: 10.18410/jebmh/2015/644
ORIGINAL ARTICLE
Table 3: The incidence of interparietal or Inca bones as reported by various workers are as
follows;
Worker
Year
Srivastava HC(9)
Singh et al(7)
Pal et al(10)
Saxena et al(1)
Gopinathan(11)
Katkiei et al(12)
Yucel et al(13)
Zambare BR(14)
Marathe et al(3)
Nirmale et al(15)
Walulkar et al(16)
Present study
1977
1979
1984
1988
1992
1995
1998
2001
2010
2012
2013
2015
No. of skulls
studied
620
500
348
40
125
302
540
310
380
148
175
126
Table 3
Incidence of Inca
bone found
0.8%
1.6%
2.6%
2.5%
0,8%
0.99%
2.8%
0.99%
1.31%
4.054%
2.86%
1.58%
In the present study carried out on 126 skulls, interpareital or Inca bone was found in 2
(1.58%) skulls only. One of these skulls presented with a single, triangular interparietal bone and
the other presented with two asymmetrical fragments of interparietal bones. Keith(17) stated that
a separate single interparietal bone in man is an extremely rare anomaly. Our findings is similar
to the findings of Singh et al and Marathe et al.(7,3)
CONCLUSION: It can be concluded that Inca bones may exist as one, two or more fragments
and they may be of various sizes. It can also be concluded that the incidence of Inca bones is
variable in different population. Inca bone may give rise to false appearance of skull fracture in xrays. Its mere presence may lead to complications during burr-hole surgery. Meticulous
knowledge about their incidence rate in different population and that they might exist in more
than one fragment would be useful to the radiologists, clinicians and neurosurgeons. This
information could also be useful in anthropological and anatomical studies.
REFERENCES:
1. Saxena SK et al. Interparietal bones in Nigerian skulls. Journal Anat. 1988; 144: 235-237.
2. Srivastava HC. Ossification of the membranous portion of the squamous part of the occipital
bone in man. Journal Anat. 1992; 180: 219-224.
3. Marathe RR et al. Inca-interparietal bones in neurocranium of human skulls in central India.
J Neurosci Rural Pract. 2010; 1: 14-18
4. Williams PL et al. Grays Anatomy, 38th ed, London: Churchill Livingstone; 1995 p. 583-606
5. Gordon S. The book of knowledge. 6th ed. London: Waveday Book Co. Ltd; 1963 p.161-6
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DOI: 10.18410/jebmh/2015/644
ORIGINAL ARTICLE
6. Moseley M. The Incas and Their Ancestors. In. The Archaeology of Peru, London: Thames &
Hudson; 1992 p.8
7. Singh et al (1979): Incidence of interparietal bones in adult skulls of Agra region.
Anatomical Anzeles 145: 528-531.
8. Brash JC. Cunningham’s text book of Anatomy 9th ed. London: oxford university press,
1951, p.220.
9. Srivastava HC (1977): Development of ossification centres in the squamous portion of the
occipital bone in man. Journal Anat, 124: 634-649.
10. Pal et al (1984): The ossification of membranous part of the squamous occipital bone in
man. Journal Anat 138: 259-266.
11. Gopinathan (1992): A rare anomaly of 5 ossicles in the pre-interparietal part of the
squamous occipital bone in North Indian. Journal Anat 180: 201-202.
12. Katkiei et al (1995): A study of incidence of the interparietal bone. Journal Anat Soc.
India,44 (1): 1-17.
13. Yucel et al: A study of interparietal bone in man. Turk J Med Sci. 1998; 28: 505-509.
14. Zambare BR(2001) : Incidence of interparietal bone in Adult skulls. Journal Anat. Soc. India
50(1) 11-12.
15. Nirmale VP: Variation of frequency of Os Incae in human skull. Indian Journal of Biomedical
Science 2012; 28:05-09
16. Walulkar et al: Study of Inca bone in Vidarbha region of Maharashtra. PJMS- Volume 3 no.
1:January –June, 2013
17. Keith A. Human embryology & morphology. 6th ed. London, Edward Arnold & Co. 1943; 223224.
AUTHORS:
1. Rubi Saikia
2. Bijoy Kumar Borah
3. Jyotirmayee Lahon
4. Natasha Gohain
PARTICULARS OF CONTRIBUTORS:
1. Associate Professor, Department of
Anatomy, Assam Medical College,
Dibrugarh.
2. Assistant Professor, Department of
Anatomy, Assam Medical College,
Dibrugarh.
3. Professor & HOD, Department of
Anatomy, Assam Medical College,
Dibrugarh.
4. Demonstrator, Department of Anatomy,
Assam Medical College, Dibrugarh.
NAME ADDRESS EMAIL ID OF THE
CORRESPONDING AUTHOR:
Dr. Rubi Saikia,
Associate Professor,
Department of Anatomy,
AMCH, Dibrugarh.
E-mail: drrubisaikia@gmail.com
Date
Date
Date
Date
of
of
of
of
Submission: 21/07/2015.
Peer Review: 22/07/2015.
Acceptance: 28/07/2015.
Publishing: 31/07/2015.
J of Evidence Based Med & Hlthcare, pISSN- 2349-2562, eISSN- 2349-2570/ Vol. 2/Issue 31/Aug. 03, 2015 Page 4578