Os parietale divisum - Department of Anatomy & Neurobiology

CASE REPORT
Os Parietale Divisum
DEVRA B. BECKER, 1 JAMES M. CHEVERUD, 2 DANIEL P. GOVIER, 1 AND ALEX A. KANE 1 *
1 Cleft Palate and Craniofacial Deformities Institute, St. Louis Children’s Hospital, Washington University Medical Center,
St. Louis, Missouri
2 Department of Anatomy and Neurobiology, Washington University School of Medicine, St. Louis, Missouri
INTRODUCTION
Divisions of the parietal bone are extraordinarily rare, and have been described previously
only in skull specimens in the anatomical literature. We report a case of os parietale divisum
(divided parietal bone) in an infant with plagiocephaly. A 36-weeks gestation male
with multiple congenital anomalies presented to the deformational plagiocephaly clinic at
11 weeks of age. CT imaging demonstrated a suture dividing the left parietal bone into
two segments. He had a plagiocephalic appearance, with the greater occipital and parietal
protuberances ipsilateral to the abnormal left parietal suture, and a correspondingly flatter
right occiput. Head circumference was in the normal range, and the anterior fontanelle was
patent and of normal size. He had mild right-sided right frontal and malar protuberance.
Mandibular asymmetry was noted with a chin-point to the right of midline. He had a leftsided
head tilt. He was treated with cranial molding helmet therapy for his head shape and
physical therapy for his head tilt. After 16 months of helmet therapy, he had little
improvement of his head shape. This case is an in vivo presentation of a rare variant, os
parietale divisum. Clin. Anat. 18:452–456, 2005. VC 2005 Wiley-Liss, Inc.
Key words: os parietale divisum; parietal bone; plagiocephaly
Divisions of the parietal bone are extraordinarily
rare and have been described previously in skull
specimens in anatomical literature. We report a case
of a divided parietal bone presenting as deformational
plagiocephaly with associated muscular torticollis.
The patient was treated with passive molding
helmet therapy for his head shape, and physical
therapy for his muscular torticollis.
Names for parietal bone divisions have included
divided parietal bone, os parietale partitum, double
parietal bone, sutura parietalis transversa, sutura parietalis,
anostosis, and os parietale divisum (Hauser
and De Stefano, 1989). The lack of consistent
nomenclature is likely related to the rarity of the
condition. Although the condition is considered to
have ‘‘no functional consequences’’ (Hauser and De
Stefano, 1989) the case we present did have morphological
consequences that required treatment. The
morphological consequences paralleled deformational
plagiocephaly.
VC 2005 Wiley-Liss, Inc.
CASE REPORT
Clinical Anatomy 18:452–456 (2005)
Consent has been obtained from the patient’s
parents for the publication of this case report. The
patient was born at 36-weeks gestation to a 34-yearold
G3P2 female by cesarian section for placenta
previa. At birth he was noted to have multiple congenital
anomalies including a sacral dimple, spinal
cleft, accessory ribs, a renal anomaly, hypospadias,
right thumb duplication, and head asymmetry. Computed
tomography (CT) scan of the head showed an
os parietale divisum (Fig. 1). He was seen in plastic
surgery clinic at 11 weeks of age, when the parents
reported that he was positioned on his back during
*Correspondence to: Alex A. Kane, M.D. Pediatric Plastic Surgery,
Ste 11W7, St. Louis Children’s Hospital, One Children’s
Place, St. Louis, MO 63110. E-mail: kanea@msnotes.wustl.edu
Received 13 September 2004; Revised 3 January 2005; Accepted
17 January 2005
Published online 13 July 2005 in Wiley InterScience (www.
interscience.wiley.com). DOI 10.1002/ca.20139

Fig. 1. Three-dimensional reconstructions of head CT carried out shortly after birth. The accessory
parietal suture is marked by the black arrow.
sleep, and that they noted that his head shape was
worse than at birth. On exam, he was noted to have
a head circumference of 39.5 cm (22nd percentile
corrected). He had right-sided occipital flattening
and a right frontal and malar protuberance (Fig. 2).
Mandibular asymmetry was noted with a chin-point
to the right of midline. Anterior fontanelle was patent
and non-bulging. Neck range of motion was full.
CT imaging of the head demonstrated a suture completely
dividing the left parietal bone into two segments
in the antero-posterior dimension, and no
other abnormalities. Parietal formina were not noted
to be enlarged (Fig. 3).
He was seen again in follow-up at 5 months of age, at
which time his head circumference was noted to be 41.5
cm (12th percentile) and he was noted to have continued
right occipital flattening with right malar protuberance.
He was begun on a course of molding-helmet
therapy per the deformational plagiocephaly protocol.
After 1 month of helmet therapy, the patient had
some improvement of his right occipital flattening
on physical exam. He was, however, noted to have
developed left-sided head-tilt with constraint noted
in his left sternocleidomastoid muscle. He was
started on physical therapy.
CT imaging of the head was repeated at 10
months of age, and demonstrated persistence of the
accessory parietal suture (Fig. 4). At 1 year of age,
the patient continued helmet therapy at approximately
23 hrs/day. His occipital contour was noted
to be improved from baseline, although his frontal
contour was unchanged.
Fig. 2. Appearance of the patient in frontal (left) and birds eye
(right) views.
Os Parietale Divisum
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454 Becker et al.
Fig. 3. CT scan 3D reconstructions of the subject at 3.5 months of age. The accessory parietal suture is marked by the black arrow.
At 16–17 months of age, he continued to wear
the helmet for 15 hrs/day, and his family reported
positive changes in the shape of his head. Despite
this report, on clinical exam his head circumfer-
ence was 45.5 cm (4th percentile), and his
head shape was noted to have a persistent mild
to moderately severe right occipital flattening
(Fig. 5).
Fig. 4. CT scan 3D reconstructions at 10 months of age. There has been some interval closure of the accessory parietal suture.

DISCUSSION
Hrdlic˘ka’s encyclopedic 155-page manuscript on
divisions of the parietal bone, published in 1903,
belies the extraordinary rarity of the variant. Though
he reported to cite every case in the literature, the
number of complete antero-posterior divisions of the
parietal bone in the very young totaled three. It is
not surprising, then, that 7 years later Berry noted,
in his description of an Australian aboriginal skull,
that ‘‘...[os parietale partitum] is a condition of sufficient
rarity to justify the description of any specimen
in which it occurs.’’ Berry examined 3,400 human
crania and found only two with os parietale divisum
(Berry, 1910). Berry’s exhortation did not portend
further descriptions; for years the literature remained
largely silent on this condition. There has been,
however, some relatively recent work. Shapiro (1972)
reviews the literature and reports his own findings.
He confirms the condition is rare, and notes that in
his review of 25,000 radiographs he found only three
cases of complete parietal divisions. In addition,
Fenton et al. (2000) report two cases of divided parietal
bones found at autopsy. In these cases, radiographs
of two infants were obtained in the context
of workup for bleeding disorders, and the divided
parietal bones were initially interpreted as linear
parietal fractures.
Nomenclature associated with the condition has
not been uniform. There are no fewer than six terms
for the condition: os parietale divisum, os parietale,
partitum, double parietal bone, sutura parietalis
transversa, and sutura parietalis transversa (Hauser
and De Stefano, 1989). The lack of consistency in
Os Parietale Divisum
Fig. 5. Appearance of the patient at 16 months in frontal (left) and birds eye (right) views. Note his torticollis.
455
nomenclature is likely related to the rarity of the
condition.
Furthermore, anatomic variation exists. The divisions
can be either complete or incomplete, and
can further be antero-posterior (parallel with the sagittal
suture), supero-inferior, or oblique. All of these
subtypes were described by Hrdlic˘ka in his article
(1903). Hauser and De Stefano (1989) also include
the number of subdivided parts in their classification.
Some authors have reported a preference for the left
side. The condition is known to occur in other mammals,
specifically apes and monkeys, as well as
humans.
There has been debate over the presumed
embryologic processes that cause the condition, and
the most popular is the two-center theory of ossification,
which holds that there are two centers of ossification.
The two-center theory of ossification is attributed
to Toldt in 1883 by Shapiro (1972). During
development, intramembranous bone forms in mesenchymal
tissue. First, mesenchymal cells differentiate
into osteoblasts. Those osteoblasts form ossification
centers, which generate radial ossification to the
bone edges. Failure of the ossification centers to
meet may lead to an accessory suture. The two-center
theory of ossification was used as the basis for an
explanation of os parietale divisum in an Australian
skull by Turner in 1890 (Turner, 1890).
The phenotype of our case is consistent with
other reported cases of os parietale partitum (Bessell-
Browne and Thonell, 2004), and with what is
expected based on our knowledge of craniosynostosis
and deformational plagiocephaly. ‘‘Virchow’s Law’’
predicts that growth will occur parallel to a synostosed

456 Becker et al.
suture and will be inhibited perpendicular to it. This
explains the asymmetries associated with unicoronal
synostosis, as well as some of the key differences
between synostotic plagiocephaly (of the unicoronal
and unilambdoid types) and non-synostotic deformational
plagiocephaly. Chin deviation will be contralateral
in synostosis and ipsilateral in deformational
plagiocephaly, and the ipsilateral cheek is forward in
synostosis and backward in deformational plagiocephaly
(resulting in a relative prominence of the
contralateral cheek). In this case of os parietale divisum,
the patient had ipsilateral chin deviation, and contralateral
cheek prominence, consistent with deformational
plagiocephaly. It is interesting to note, and
not simple to explain why, the patient’s occipital
flattening was contralateral to the divided parietal
bone.
Although we noted an association of os parietale
divisum with deformational plagiocephaly the exact
causal relationship between the two is not known.
Plagiocephaly has not been reported in skull specimens
with os parietale divisum. It is possible that os
parietale divisum predisposes a supinely positioned
child to deformational plagiocephaly. An accessory
parietal suture certainly provides an outlet for abnormal
force vectors, and the ultimate phenotype is predictably
consistent with deformational plagiocephaly.
Given that our patient was born with multiple congenital
anomalies, he might have been predisposed
to the development of deformational plagiocephaly,
and therefore the presence of an accessory parietal
suture was not causally related.
CONCLUSION
Os parietale divisum is a rare variant in which the
parietal bone is divided. Our patient presented with
deformational plagiocephaly.
REFERENCES
Berry RJA. 1910. A case of os parietale bipartitum in an Australian
aboriginal skull. J Anat Physiol 44:73–82.
Bessell-Browne RJ, Thonell S. 2004. Bipartite parietal bone:
a rare cause of plagiocephaly. Australas Radiol 48:248–250.
Fenton LZ, Sirotnak AP, Handler MH. 2000. Parietal pseudofracture
and spontaneous intracranial hemorrhage suggesting
nonaccidental trauma: report of 2 cases. Pediatr
Neurosurg 33:318–322.
Hauser G, De Stefano GF. 1989. Epigenetic variants of the
human skull. Stuttgart: Schweizerbart.
Hrdlic˘ka A. 1903. Divisions of the parietal bone in man and
other mammals. Bull Am Mus Nat Hist 29:231–386.
Shapiro R. 1972. Anomalous parietal sutures and the bipartite
parietal bone. Am J Roentgenol Radium Ther Nucl Med
115:569–577.
Turner W. 1890. Double right parietal bone in an Australian
skull. J Anat Physiol 25:473–474.