Keywords
Angle’s malocclusions - disclusion time - T-Scan III
Introduction
The orthodontic therapeutic goal is an establishment of ideal or normal occlusion,[1] comprising static occlusion[2]
[3] and function aspects of the occlusion.[4] Dental occlusion is an important factor that determines the masticatory performance.
[1] Subjects with normal occlusion have a better masticatory performance than those
with malocclusions.[5] Class II and III relationships are reported to be predisposing factors to the contribution
of temporomandibular disorder (TMD).[6]
[7] Maxillary anterior teeth’s inclination affects condylar position and movement during
protrusion and their lingual inclinations in Class II Division 2 induce a posterior
position of the condyle.[8]
[9] However, an association between malocclusions and TMD has long been debated.[10]
[11]
In addition to the dental alignment, disclusion time (DT), the duration of time starting
after mandibular movement from maximum intercuspation to excursive movement until
all posterior teeth’s disclusion and anterior teeth’s contacts are observed,[12] is considered a TMD-related factor. Lack of anterior guidance in Class II and III
subjects with anterior crossbite causes a long contact among their posterior teeth
during protrusion,[9]
[13] and a prolonged duration of occlusal contact affects the amount of load distribution
on the masticatory system.[14] However, DT among malocclusion types has not been clearly reported.
A T-Scan I system has been firstly used to measure DT.[15] With an improved speed of recording and reporting the data in a 0.003 seconds time
increment, the T-Scan III system (Tekscan, Boston, Massachusetts, United States) is
the device’s newest version for computerized occlusion analyses. [16] The T-Scan III system has been widely used in dentistry, due to its advantages in
measuring the force-time sequences in mandibular static and dynamic positions.[17]
TMD is reported to relate to some occlusal features, that is, premature occlusal contacts,
Class II and III relationships, and over 5.0-mm overjet or overbite.[6]
[7]
[18] Among those with malocclusions, TMD subjects are reported to possess a longer DT
than non-TMD ones.[13] Despite the controversy of the possible effects of Angle’s Class II and III on TMD,
their DT is hypothesized to differ from that of Class I. Hence, the aim of this study
was to compare the DT during mandibular lateral excursions and protrusion among the
subjects with different Angle’s malocclusions by using a T-Scan III system.
Materials and Methods
This study was approved by Naresuan University Ethical Committee (IRB Number 0721/60).
Post-explanation of the procedures, written inform consents were obtained from all
subjects. An initial screening and a clinical examination were performed to exclude
the subjects with either of these criteria, that is, restoration(s) with dental implant
or fixed prosthesis, past or ongoing orthodontic treatment, molar relationship’s classification
on one side different from the other, TMD, and parafunctional habit. One hundred volunteers
were included and presented at least 28 permanent teeth with symmetrical dental arch
forms and equal number of teeth. Based on Angle’s classification of malocclusions,
they were divided into four groups (Class I, Class II division 1, Class II division
2, and Class III; n = 25 per group).
Each subject participated in two visits for the collections of data. The first one
included impression taking for diagnostic maxillary and mandibular models. Mesiodistal
width of the maxillary teeth was measured by a digital Vernier caliper (Mitutoyo,
Kanagawa, Japan) and recorded into a T-Scan III system, Version 9.1.9 (Tekscan) for
transferring individual dental arch dimension. The second one was 7 days later and
included the recording procedures of all participants’ DT by the same T-Scan III system.
With an upright sitting position on a dental chair and their Frankfort horizontal
plane parallel to the floor, the subjects were asked to perform mandibular protrusion
and movement to lateral sides. The correct motions were repeatedly conducted three
times. The T-Scan recording into the T-Scan sensor was done in each excursion, when
the subjects were asked to be in maximum intercuspation for 3 to 5 seconds before
excursion movement. By selecting left excursion, right excursion, or protrusion in
the program, the time difference between points C and D was recorded and designated
as DT shown in the graph of force versus time and timing table ([Fig. 1]). DT was measured three times in each excursion and their means and standard deviations
(SD) were calculated.
Fig. 1 Timing table with the recorded disclusion time (ΔC–D) shown in a T-Scan display screen
in two (-left) and three (-right) dimensional images by using a T-Scan III system.
The Universal Numbering System is shown on each maxillary tooth’s buccal side.
Statistical analyses of the obtained data were performed using PASW Statistics for
Windows, Version 17.0 (SPSS, Chicago, Illinois, United States). Standard descriptive
statistics were used for calculating means, SD, and range of DT in each excursion
(left excursion, right excursion, and protrusion). A one-way analysis of variance,
followed by a post hoc analysis by least significant difference (LSD) test, was used
for comparing mean DT in each excursion. The level of statistical significance was
the value of p < 0.05.
Results
Details of the subjects were shown in [Table 1]. There was no significant difference in the subjects’ mean age (p = 0.716).
Table 1
Sample characteristics in each malocclusion type
|
Malocclusion (n)
|
Gender
|
Age (y)
|
Overbite (mm)
|
Overjet (mm)
|
|
F (n)
|
M (n)
|
Mean ± SD
|
Range
|
Mean ± SD
|
Range
|
Mean ± SD
|
Range
|
|
Abbreviations: F, female; M, male; SD, standard deviation.
Note: Different lower case letters (a–d) indicate intracolumn statistically significant
differences (p < 0.05).
|
|
Class I (25)
|
14
|
11
|
21.5 ± 0.78a
|
18.0–25.0
|
2.42 ± 1.05a
|
0.50–5.00
|
2.52 ± 1.04a
|
0.50–4.00
|
|
Class II division 1 (25)
|
16
|
9
|
22.0 ± 0.55a
|
18.0–26.0
|
3.10 ± 1.11b
|
1.00–5.50
|
4.24 ± 1.50b
|
2.00–9.00
|
|
Class II division 2 (25)
|
13
|
12
|
23.5 ± 1.04a
|
18.0–31.0
|
4.70 ± 1.42c
|
2.50–8.00
|
1.62 ± 0.67c
|
0.50–3.50
|
|
Class III (25)
|
16
|
9
|
22.5 ± 0.44a
|
18.0–27.0
|
1.44 ± 0.95d
|
0.00–3.50
|
1.46 ± 1.11c
|
(–1.00)–4.00
|
|
p-Value
|
|
|
> 0.05
|
|
< 0.05
|
|
< 0.05
|
|
There were intergroup significant differences in the subjects’ mean overbite (p = 0.000). The significantly highest overbite value was seen in Class II division
2, while the significantly lowest one in Class III. There were intergroup significant
differences in the subjects’ mean overjet (p = 0.000), except for that between Class II division 2 and Class III (p = 0.614). The significantly highest overjet value was seen in Class II division 1,
while the significantly lowest one in Class III ([Table 1]).
Each excursion’s mean DT among the malocclusions was shown in [Table 2]. It was observed in all excursions that Class III possessed the significantly longest
mean DT, while Class I the shortest one. With respect to the excursions, there were
intergroup significant differences in the mean DT (left excursion, p = 0.000; right excursion, p = 0.003; and protrusion, p = 0.002). A post hoc LSD test revealed the significantly longest mean DT (p < 0.05) in Class III, when compared to those in other malocclusions ([Fig. 2]).
Table 2
Disclusion time(s) in each excursion among malocclusion types
|
Malocclusion (n)
|
Left excursion
|
Right excursion
|
Protrusion
|
|
Mean ± SD
|
Range
|
Mean ± SD
|
Range
|
Mean ± SD
|
Range
|
|
Abbreviation: SD, standard deviation.
|
|
Class I (25)
|
2.08 ± 0.65
|
0.50–3.21
|
2.15 ± 0.94
|
0.35–3.68
|
1.88 ±0.99
|
0.35–4.04
|
|
Class II division 1 (25)
|
2.13 ± 0.74
|
0.67–3.65
|
2.58 ± 1.16
|
0.90–5.71
|
2.08 ± 1.11
|
0.47–5.00
|
|
Class II division 2 (25)
|
2.12 ± 0.72
|
1.02–3.52
|
2.37 ± 1.07
|
0.56–4.11
|
2.07 ± 0.68
|
1.17–3.48
|
|
Class III (25)
|
3.19 ± 1.34
|
0.62–5.63
|
3.28 ± 1.25
|
0.67–5.25
|
3.01 ± 1.53
|
1.01–6.29
|
Fig. 2 Mean disclusion time of each excursion among malocclusion types. Asterisks (*) indicate
statistically significant differences (p < 0.05).
Discussion
DT is utilized for some investigations into the muscular activities and masticatory
performance, and the DT over 0.5 seconds can elevate the contraction levels of temporalis
and masseter muscles.[13]
[16] A prolonged DT leads to masticatory muscles’ higher electromyography activity and
abnormal stress distributions in the articular disc, thus facilitating TMD symptoms’
occurrences.[6]
[19] A successful method for reducing the symptoms is shortening the DT[13] and a reduction in DT to less than 0.5 seconds per excursion has been reported to
decrease muscular hyperactivity and their related symptoms. [19]
[20] In spite of the fact that the prolonged DT (over 0.5 s) of all excursions was detected
in our non- TMD subjects, it was explicable by a phenomenon of physiologic tolerance
that an individual might have an adaptive capability to malocclusion and functional
appearances.[21]
During excursions, there should be incisal contacts, which are determined by overbite
and overjet, to facilitate an achievement of the anterior guidance and to cause an
immediate disclusion of the posterior teeth.[22] Class II malocclusion was reported to have the longest DT, followed by Class III
and Class I malocclusions, respectively.[13] In this study, Class III malocclusion showed the significantly longest DT of all
excursions, followed by Class II division 1, Class II division 2, and Class I malocclusions,
respectively. Discrepancies between the above results might have contributed to our
larger sample size and usage of the T-Scan system’s newer version, apart from the
subjects’ racial group and age.
Some relationships between overjet and DT have been shown that subjects with an overjet
larger than 3 mm result in a prolonged DT.[23] However, no intergroup significant difference in the DT between Class I and Class
II malocclusions (except for those between Class III malocclusions) was detected in
our study, despite all intergroup significant differences in the overjet (except for
that between Class II division 2 and Class III malocclusions) and the overbite. Our
findings illustrated that neither overjet nor overbite affected the DT. Some further
investigation is needed to clarify such inconsistency.
Dental and skeletal characteristics are related to signs and symptoms of TMD. Not
only the dental factors but also the craniofacial skeletal structures are reported
to be among TMD contributing factors.[8]
[9]
[24] Associations between various morphological occlusions and TMD, including a prolonged
DT, have long been debated. Although some significant DT differences in Class III
malocclusion, lack of overjet- and overbite- DT relationships, and some possibilities
of detecting the prolonged DT in non-TMD subjects were shown in our study, it focused
solely on Angle’s classifications and was unable to relate such findings with the
skeletal structures. Consequently, it is necessary to prove a correlation, if any,
between DT and TMD subjects with various skeletal patterns.
Conclusion
When compared to Class I and II malocclusions, Class III possessed the significantly
highest mean DT of each excursion and the significantly longest DT in all excursions.
Different excursions caused no intragroup significant difference in their mean DT.
