Dental Asia November/December 2023

CLINICAL FEATURE
Digital palate analysis to verify the
mirror twin phenomenon
By Dr Botond Simon DMD, Dr George Freedman BSc, DDS, DiplABAD, and János Vág, DMD, PhD, Med Habil
INTRODUCTION
Mirror-image twin is a unique phenomenon
where the features on one identical, or
monozygotic, twin’s left side are mirrored
on the other twin’s right side. For example,
one twin may have a birthmark on their left
cheek, while the other twin has a birthmark
in the exact location but on the right cheek.
The prevalence of mirror twins can only
be estimated from indirect evidence. 1,2
Approximately 25% of monozygotic twins
(n=75) in the US, 3 and 23% of monozygotic
twins (n=375) in Belgium, 4 had discordant
handedness, where one sibling is righthanded
and the other is left-handed.
However, handedness is determined mainly
by the non-shared environment with fewer
genetic influences.
While mirror twins share identical DNA, they
may have distinct personalities, interests
and abilities, and they may also experience
different health issues throughout their lives.
Numerous studies have reported mirrorimage
dental anomalies in twins: aplasia, 5
supernumerary teeth, 6 or dental fusion. 7
Contralateral craniofacial lesions such as
cleft lip and palate, 8 hemifacial microsomia, 8
and unilateral condylar hyperplasia have
been reported. 9 The pathological phenomena
suggest the importance of mirror-imaging. 2
Recently, it has been demonstrated that
palatal geometry measured on the intraoral
scan can distinguish strangers and even
identical twins with moderate certainty,
facilitating human and victim identification. 10
After selecting possible matches, an
additional superimposition of the palatal
scan can confirm the identity. 11,12 However,
recognising a twin relationship might be
concealed by the opposite laterality of
siblings. Therefore, this study’s primary aim
was to assess the effect of sagittal mirroring All three scans of the 174 subjects, 522
on the fit between twin siblings’ scans.
total scans, were cropped, and teeth were
removed in the GOM Inspect Suite software.
Compared to DNA analysis, the strength The cropped palatal model was duplicated,
of the intraoral ante-mortem scan-based and one was mirrored in the y-axis, or the
identification method is growing due to sagittal plane (Figs. 1A and B). The palatal
the rapid spread of intraoral scanning.
scans were aligned by the best-fit function
However, the limitation is that dentists may using the iterative closest point algorithm. 17
focus on quadrant rehabilitation, limiting The mean absolute distance between aligned
the scan to only one palate side. Similarly, a scans’ surfaces was calculated by dividing the
severe accident may damage the half palate. integrated absolute distance by the area of
Although the palatal side-to-side asymmetry the valid distance (Fig. 1C).
is relatively small, 13 the total geometry and
surface morphology may differ between sides. Assessment of twin mirroring: Betweensiblings
comparison
Therefore, the study’s secondary aim was to
assess the fit between the mirrored scan and The three scans of one sibling were
the original scan of the same individuals. superimposed with three scans of the
other sibling in the original group, for nine
MATERIAL AND METHODS
superimpositions per subject. Additionally,
Sample collection and preparation
the three original scans of one sibling were
A total of 174 participants were recruited superimposed with three mirrored scans of
through the National Twin Registry: 61
the other sibling in the mirror group, for nine
monozygotic twin pairs and 26 same-sex superimpositions per twin pair. A total of
dizygotic twin pairs. 14,15 All participants agreed 2,610 measurements were made.
to study participation by written informed
consent and ethical approval 36699-2/2018/ Assessment of asymmetry: Between-replicates
EKU was obtained. Zygosity was determined comparison
using questionnaires. 16 Evidence of previous The original three scans within a subject were
orthodontic treatment was also recorded. superimposed on each other in the original
Each participant’s palate was scanned three group, for three superimpositions. The three
times using a Planmeca Emerald intraoral mirrored scans were superimposed with a
scanner, version number Romexis 5.2.1, as non-mirrored replicate in the mirror group,
previously described. 11 for three superimpositions.
Fig. 1A-C: The original scan (A) was mirrored on the y-axis (B). The complete surface comparison is shown
after the best-fit alignment of the two scans (original vs mirrored) (C). The table indicates the integrated
absolute distance (977.08mm 3 ) and the area of the valid distance (1,622.60mm 2 ). Their ratio makes up the
mean absolute distance (0.602mm)
36 DENTAL ASIA NOVEMBER / DECEMBER 2023