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.
