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Comparison of measurements made on digital
and plaster models
Margherita Santoro, DDS, MA,a Scott Galkin, DMD,b Monica Teredesai, DMD,c Olivier F. Nicolay, DDS, MS,d
and Thomas J. Cangialosi, DDSe
New York, NY
Measuring plaster models by hand is the traditional method of assessing malocclusion. Recent technologic
advances now allow the models to be digitized, measured with software tools, stored electronically, and
retrieved with a computer. OrthoCAD (Cadent, Fairview, NJ) performs this service. The purpose of this study
was to evaluate the reliability of the OrthoCAD system. Two independent examiners measured tooth size,
overbite, and overjet on both digital and plaster models. The results were compared, and interexaminer
reliability was assessed. The study sample consisted of 76 randomly selected pretreatment patients. The
results showed a statistically significant difference between the 2 groups for tooth size and overbite, with the
digital measurements smaller than the manual measurements. However, the magnitude of these differences
ranged from 0.16 mm to 0.49 mm and can be considered clinically not relevant. No difference was found
between the 2 groups in the measurement of overjet. Interexaminer reliability was consistent for both the
plaster and the digital models. (Am J Orthod Dentofacial Orthop 2003;124:101-5)
C
omputer technology is expanding to include
more areas in various scientific fields, and
orthodontics is no exception. Orthodontists use
computers for record keeping, practice management,
patient education, communication with colleagues, restorations fabrication, and many other tasks. Computers
have become a necessity rather than an option.
The introduction of digital models offers the orthodontist an alternative to the plaster study models
routinely used. Plaster study models are a standard
component of orthodontic records, and they are fundamental to diagnosis and treatment planning, case presentations, evaluation of treatment progress and results,
and record keeping.
Tooth size, crowding or spacing, overjet, overbite,
and Bolton analysis are typically measured by hand on
plaster models. Several other methods of measuring
have been proposed over the years.1-7 Schirmer and
Wiltshire1 and Champagne2 compared measurement
From the Division of Orthodontics, Columbia University, School of Dental and
Oral Surgery, New York, NY.
a
Assistant professor of clinical orthodontics.
b
Former postgraduate resident, now in private practice, NJ.
c
Former postgraduate resident, now in private practice, Conn.
d
Associate professor of clinical orthodontics.
e
Professor and chairman.
None of the authors has a financial interest in OrthoCAD or digital model
companies.
Reprint requests to: Margherita Santoro, DDS, MA, Division of Orthodontics,
Columbia University, School of Dental and Oral Surgery, 635W 168th Street,
P&S Box 20, New York, NY 10032; e-mail, [email protected].
Submitted, April 2002; revised and accepted, August 2002.
Copyright © 2003 by the American Association of Orthodontists.
0889-5406/2003/$30.00 ⫹ 0
doi:10.1016/S0889-5406(03)00152-5
made manually on casts with those made on digitized
casts obtained from a photocopier. They concluded
that, although photocopies are easy to handle, manually measuring teeth with a calibrated gauge produces the most “accurate, reliable, and reproducible”
measurements. The photocopier method, furthermore, still requires a traditional plaster model, and
only provides a 2-dimensional image of a 3-dimensional object. Bhatia and Harrison3 studied the performance of the “travelling microscope,” an apparatus modified to measure dental casts, and concluded
that the method was more precise than some alternatives. Martensson and Ryden4 investigated a holographic system for measuring dental casts. The
method was shown to be more precise than previous
methods, and the authors believed that it would also
save storage space. However, although microscope
and holographic systems had some advantages, they
did not prove to be practical in clinical practice, and
they never became popular.
OrthoCAD (Cadent, Fairview, NJ) is a patented
computer model system that creates digital images of
dental casts (Fig 1). To obtain the digital images, the
orthodontist sends alginate impressions and a wax bite
to the OrthoCAD laboratory. The impressions are
scanned, converted into digital images that are stored
on the company’s server, and made available for
downloading by the account holder. OrthoCAD provides software the orthodontist can use to make routine
measurements, such as tooth size, overjet, overbite, and
Bolton analysis, on the digital images.
101
102 Santoro et al
American Journal of Orthodontics and Dentofacial Orthopedics
July 2003
The study sample to compare plaster and digital
models was selected randomly from the patient records
at the Columbia University Orthodontic Clinic. Two
sets of alginate impressions were made, plaster models
were poured the same day, and 1 set was shipped
immediately to OrthoCAD via overnight courier.
The following selection criteria were used:
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●
●
●
Fig 1. Gallery 3-dimensional model images in OrthoCAD.
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●
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OrthoCAD offers many advantages, including elimination of model breakage and storage problems, instant retrieval of models, ease of communication with
patients and colleagues, and model access from many
locations. It enables the orthodontist to e-mail images if
desired and is a convenient presentation tool. Disadvantages include lack of tactile input for the orthodontist and time needed to learn how to use the system.
As with any new method, accuracy must be assessed by comparison with the existing gold standard,
in this case, measurements made manually on plaster
casts. The purpose of this study was to compare the
accuracy of measurements made by the OrthoCAD
system on digital models with measurements made by
hand on traditional plaster models. Because 2 separate
alginate impressions were needed, 1 for plaster models
and 1 for OrthoCAD, we also compared 2 consecutive
alginate impressions made during the patient’s first
visit, to determine whether any major bias could be
detected.
MATERIAL AND METHODS
The sample to compare plaster models from consecutive alginate impressions consisted of 20 randomly
selected subjects, each with all permanent teeth from
first molar to first molar erupted, no missing teeth from
first molar to first molar, and no existing orthodontic
appliances.
Two consecutive alginate impressions were taken
on each subject and poured immediately in plaster. The
bite was recorded using a wax wafer. A single examiner
() measured tooth width, overbite, and overjet on both
casts. The results were then statistically evaluated.
Plaster and digital models made from alginate impressions taken consecutively at the same visit
No appliances pretreatment
Permanent dentition erupted from first molar to first
molar
No missing teeth from first molar to first molar
Stable centric occlusion with at least 3 occlusal
contacts
No voids or blebs in the plaster or digital models
No fractures on the teeth on the plaster models
An initial sample of 100 patients was randomly
selected. After the above criteria were applied, the sample
size was reduced to 76 patients. Two examiners, working
independently, recorded tooth size, overbite, and overjet
on the plaster and digital models The sizes of the mandibular and maxillary teeth from first molar to first molar
were measured, and the maximum mesiodistal width was
recorded for each tooth. Overbite was measured as the
amount of vertical overlap of the mandibular incisor in
millimeters. This maximum overbite involving a maxillary central incisor was recorded. Overjet was measured in
millimeters from the labial surface of the mandibular
incisor to the labial surface of the maxillary incisor. In
case of different labial inclination of the maxillary incisors, the maximum overjet was recorded.
Tooth size was measured on the plaster model with
an orthodontic-style Boley gauge (Orthopli, Philadelphia, Pa), to the nearest 0.1 mm. Overjet and overbite
were measured with a graduated, calibrated periodontal
probe, to the nearest 0.5 mm.
Tooth size was measured on the digital models with
the analysis tools provided by OrthoCAD, to the
nearest 0.1 mm (Fig 2). The posterior teeth were
measured from the occlusal view and the anterior teeth
from the facial view. However, in case of rotated or
malpositioned anterior teeth, the images were rotated
on-screen, and the measurements were made from the
occlusal view to provide better visibility. For ease and
accuracy of measurements, the images were enlarged
on-screen 2 or 3 times using the built-in magnifying
tool. Overjet and overbite were also measured using the
analysis tools. Two windows appear during this process
(Fig 3). The right window has a cross-section plane
Santoro et al 103
American Journal of Orthodontics and Dentofacial Orthopedics
Volume 124, Number 1
Table. Repeated-measures analysis of variance
between plaster and digital models P values and
mean differences
Variable
Fig 2. Tooth size measurement tools (mesiodistal diameters) in OrthoCAD.
Mand R first molar
Mand R second premolar
Mand R first premolar
Mand R canine
Mand R lateral incisor
Mand R central incisor
Mand L central incisor
Mand L lateral incisor
Mand L canine
Mand L first premolar
Mand L second premolar
Mand L first molar
Max L first molar
Max L second premolar
Max L first premolar
Max L canine
Max L later incisor
Max L central incisor
Max R central incisor
Max R lateral incisor
Max R canine
Max R first premolar
Max R second premolar
Max R first molar
Overbite
Overjet
P
Mean difference
⬍.0001*
.0003*
⬍.0001*
⬍.0001*
⬍.0001*
⬍.0001*
⬍.0001*
⬍.0001*
⬍.0001*
⬍.0001*
⬍.0001*
⬍.0001*
.0012*
⬍.0001*
⬍.0001*
⬍.0001*
⬍.0001*
.0009*
.0002*
⬍.0001*
⬍.0001*
⬍.0001*
.0001*
.028*
.0124*
.9771
⫺0.3053
⫺0.2138
⫺0.3066
⫺0.2888
⫺0.3592
⫺0.2605
⫺0.2816
⫺0.3842
⫺0.2447
⫺0.3250
⫺0.2862
⫺0.3605
⫺0.1632
⫺0.2375
⫺0.2836
⫺0.2375
⫺0.2763
⫺0.2395
⫺0.2605
⫺0.3164
⫺0.2224
⫺0.2816
⫺0.2191
⫺0.2151
⫺0.4901
⫺0.00987
Mand, mandibular; Max, maxillary; R, right; L, left.
*Significant at P ⬍ .05.
RESULTS
Fig 3. Right, selection of section plane for overbite and
overjet measurements. Models can be rotated, which
facilitates cross-sectioning at point of maximum overjet.
Left, resulting cross-section. Overjet and overbite measurements.
tool, which can be dragged to the location at which the
overjet or overbite is to be measured. The maximum
overjet and overbite were determined by selecting the
most accurate cross-sectioning plane on the image in
the right window, while actual measurements were
made in the left window, where the resulting crosssection is shown. Once again, the images were enlarged
to facilitate ease and accuracy of measurement. OrthoCAD measures overjet and overbite to the nearest 0.1
mm, and these measurements were rounded to the
nearest 0.5 mm.
Student t tests based on equality of variances
revealed no significant difference between any of the
measurements made on the plaster models made from
consecutive alginate impressions (P values ranged between .83 and 1.00).
The 2 sets of measurements made by the 2 independent examiners on the plaster and digital models
were found to be significantly correlated, both for the
plaster and digital models, via Pearson correlation
coefficient (P ⬍ .0001), indicating good interexaminer
reliability for both methods. This finding allowed the
whole set of measurements to be treated as the product
of the work of a single examiner. A repeated-measures
analysis of variance (ANOVA) was then performed
(Table).
There was a statistically significant difference (significance set at P ⬍ .05) between tooth width measurements made by the 2 methods, with all the digital model
measurement smaller than the corresponding plaster
model measurements. The greatest mean difference
104 Santoro et al
was found for the mandibular left lateral incisor (0.38
mm).
ANOVA showed a statistically significant difference in overbite measurement between the plaster and
digital models (P ⫽ .0124). All digital measurements
were smaller than the manual measurements, with the
mean difference being 0.49 mm.
ANOVA showed no statistically significant difference between the overjet measurements by the 2 methods
(P ⫽ .98). The mean difference was 0.098 mm.
DISCUSSION
The plaster and digital model groups showed differences in measurements of tooth width for each of the
teeth measured. The mean differences were statistically
significant but fell within a small range (0.16-0.38
mm). The digital measurements were smaller than the
manual measurements. This finding cannot be attributed to differences between alginate impressions. The
comparison between the measurements made on plaster
casts from the 2 consecutive sets of alginate impressions showed no significant difference. Alginate shrinkage during transportation to OrthoCAD location and
different pouring times therefore remained the most
likely explanations for the differences.
Another possible cause of different tooth size measurement is the intrinsic difference between the 2
methods. OrthoCAD provides a 3-dimensional visual
pointing to interproximal contacts on an enlarged image
(Fig 2) and digital tools to measure diameters and
distances along selected planes. Depending on the
orthodontist’s training, abilities, and preferences, measuring on a computer screen can be more or less
accurate than the traditional gauge-on-cast method.
There was also a statistically significant difference
in overbite measurements between the 2 groups, with
the digital measurements smaller than the manual ones.
The difference could be attributed to the digital tooth
sizes being consistently smaller than the plaster measurements. A smaller overbite, in pure terms of millimeters, must therefore be expected if the teeth are
smaller. However, overbite expressed in terms of percentage will not be affected by measurements in millimeters. The magnitude of the difference (0.49 mm on
average), even if slightly larger than the difference
detected in tooth measurements, does not appear to be
clinically significant. In fact, earlier studies have shown
that the measurement error itself in the repeated single
operator clinical measurements of plaster casts averages 0.2 mm.8 Other factors could have introduced
some inconsistency in overbite measurements, eg, an
incorrect probe angulation during the traditional model
manipulation or rounding the digital measurements to
American Journal of Orthodontics and Dentofacial Orthopedics
July 2003
the nearest 0.5 mm. Even a different vertical plane used
to measure overbite in the 2 methods could have
contributed, because the plane is randomly selected in
the traditional manual measurements.
On the other hand, if the factors mentioned above
were actually responsible for measurement inconsistency, we should expect the same discrepancy to apply
to the evaluation of overjet. However, no significant
difference in overjet was found in this study between
the 2 groups. The finding suggests that the difference in
overbite measurement could be simply and safely
attributed to the smaller tooth sizes in the digital models.
As stressed before, as long as the smaller tooth size is
generalized and uniform, it is not a threat to the diagnostic
capability of the digital method, because it does not affect
proportional measurements (such as Bolton analysis and
overbite expressed as a percentage).
CONCLUSIONS
1. Tooth width and overbite measurements made on
plaster and digital models showed statistically significant differences; the magnitude of the differences
does not appear to be clinically relevant.
2. No significant difference was found in the measurement of overjet between the 2 samples.
3. Digital models seem to be a clinically acceptable
alternative to stone casts for the routine measurements used in orthodontic practice.
Further studies are needed to test the accuracy of
OrthoCAD in calculating Bolton ratio. The time needed
to measure digital models should also be evaluated and
compared with the time needed to measure plaster
models. There is a definite learning curve involved with
the use of OrthoCAD. Familiarity with the system, as
with any new method, can substantially improve measurement accuracy and reduce the time needed to
complete the measurements.
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