|Year : 2021 | Volume
| Issue : 3 | Page : 340-347
Effects of fixed twin-block and forsus fatigue resistant device on mandibular third molar angulation – A comparative study
Pooja Milind Tendulkar, Tejashri Pradhan
Department of Orthodontics and Dentofacial Orthopaedics, KLE Vishwanath Katti Institute of Dental Sciences, KLE Academy of Higher Education and Research, Belagavi, Karnataka, India
|Date of Submission||27-Sep-2020|
|Date of Acceptance||20-Apr-2021|
|Date of Web Publication||30-Sep-2021|
Dr. Pooja Milind Tendulkar
Department of Orthodontics and Dentofacial Orthopaedics, KLE Vishwanath Katti Institute of Dental Sciences, KLE Academy of Higher Education and Research, Belagavi - 590 010, Karnataka
Source of Support: None, Conflict of Interest: None
INTRODUCTION: Fixed functional appliances such as fixed twin-block (FTB) and forsus fatigue resistant device (FRD) improve Class II skeletal relationships by increasing the length of the mandible, which subsequently leads to an increase in the retromolar space. This would bring about a positional and angular change in the mandibular third molar. These appliances also produce forward movement of the mandibular buccal segments, which has also been cited as an important factor providing space and therefore improving the angulation and eruption potential of the third molars.
OBJECTIVE: To evaluate and compare the effects of FTB and forsus FRD treatment on mandibular third molar angulation.
MATERIALS AND METHODS: Pretreatment and posttreatment lateral cephalograms and orthopantomograms of 25 patients with Class II division 1 malocclusion treated with FTB were compared with those of 25 patients treated with forsus FRD. Sagittal and vertical skeletal relationships and mandibular length were assessed on lateral cephalogram, whereas linear parameters such as retromolar space; space width ratio (SWR); distance of Xi point from the distal surface of the lower second molar; and angular parameters such as α, β, and ϒ; and gonial angles were measured using orthopantomograms. Intragroup comparison was done using paired t-test, while unpaired t-test was done for intergroup comparison of the observed changes.
RESULTS: Mandibular length increased significantly in the FTB group, whereas mesialization of the mandibular dentition was significantly high in the FRD group. Both groups showed a significant increase in retromolar space, SWR, Xi point to 7 distance, α angle, and a decrease in β angle. These changes were significantly more pronounced in the FTB group.
CONCLUSION: There was a significant improvement in the third molar angulation following treatment with fixed functional appliances, which was comparatively more in the FTB group than that in the FRD group, owing to an increase in mandibular length.
Keywords: Fixed functional appliances. fixed twin-block, forsus fatigue resistant device, third molar angulation
|How to cite this article:|
Tendulkar PM, Pradhan T. Effects of fixed twin-block and forsus fatigue resistant device on mandibular third molar angulation – A comparative study. Indian J Health Sci Biomed Res 2021;14:340-7
|How to cite this URL:|
Tendulkar PM, Pradhan T. Effects of fixed twin-block and forsus fatigue resistant device on mandibular third molar angulation – A comparative study. Indian J Health Sci Biomed Res [serial online] 2021 [cited 2022 Aug 14];14:340-7. Available from: https://www.ijournalhs.org/text.asp?2021/14/3/340/327259
| Introduction|| |
One of the most unpredictable events in the course of development of the human dentition is the eruption of the mandibular third molar and its subsequent occlusal location in the dental arch. As mandibular third molars are the last teeth to develop, they are particularly prone to impaction. Hard-tissue formation of these teeth begins between the ages of 8 and 10, with enamel formation completed by the ages of 12–16 years. They erupt at 17–21 years, whereas root completion occurs at roughly 18–25 years of age.
Successful eruption of the mandibular third molar occurs by means of the tooth continuing to upright with respect to the mandibular plane and moving occlusally into sufficient space. Authors like Richardson, have reported that uprighting occurs by differential mesial root growth.
For a long time, insufficient development of retromolar space has been considered the most important factor contributing to the high impaction rate of lower third molars. Chances of eruption increase with an increase in available retromolar space. Björk et al. have suggested that sufficient space is produced by mandibular growth and a forward movement of the dentition as it erupts. Considerable jaw growth is required to allow sufficient room for these teeth. This growth occurs by bone resorption of the anterior-facing surface of the ramus and deposition on the posterior-facing surface, resulting in the lengthening of the mandibular corpus. A lack of eruptive space in the mandibular corpus will produce errant eruptive paths and consequent third molar impaction.
Many studies found greater chances of lower third molar impaction in subjects with decreased mandibular length, as is seen in Class II skeletal malocclusions with shorter, narrower mandibles.
Janson et al. showed that the available retromolar space could differ between Class II and Class I skeletal malocclusions, indicating that sagittal skeletal relationships might also have an impact on the fate of these teeth. Furthermore, they also reported less space for mandibular third molars on the sides of skeletal Class II malocclusion compared with the sides of skeletal Class I malocclusion, which was supported by authors like Jakovljevic et al.
With these linear and angular indicators aside, patient age is another factor that cannot be overlooked. Authors like Kruger have noted changes in the position of third molar teeth after the age of 18, which have promoted their better eruption. These findings may be attributed to further skeletal growth occurring after the onset of adulthood, which might be a contributor to the increase in retromolar space.
Class II functional appliances bring about the correction of mandibular deficiencies by holding the mandible forward and/or downward, thus allowing mandibular postural changes., These appliances cause the muscles and soft tissues to stretch, and the pressure generated is transmitted to the skeletal and dental structures, which causes skeletal growth modification and tooth movement.,
Both fixed and removable Class II functional appliances are used to improve skeletal Class II malocclusions. Because the success with removable appliances largely depends on patient's compliance, using a fixed appliance can increase the chances of a favorable outcome as it negates the compliance of the patient from the treatment. These appliances can also be used concurrently with fixed orthodontic therapy, as they can be cemented or attached directly to the main archwire, thus saving the time of secondary postfunctional mechanotherapy.
The fixed twin-block (FTB) and forsus FRD are two commonly used fixed functional appliances employed in the correction of skeletal Class II malocclusions.
The twin-block appliance, developed by Clark, consists of maxillary and mandibular acrylic plates with inclined planes at an angle of 70°, which encourages the forward and downward displacements of the mandible. A FTB appliance that can be fixed with dental cement in the patient's mouth, would produce more favorable treatment outcomes, because reliance on compliance in these patients is eliminated completely.
The forsus FRD is a fixed Class II appliance developed by Vogt, which essentially is “a three-piece, hybrid telescoping system incorporating a superelastic nickel-titanium coil spring.” The apparatus consists of a telescoping cylinder into which the distal end of the push rod is inserted and a hook on the mesial end which is crimped directly onto the archwire near the canine or premolar bracket.
While the FTB is used during the growth phase and has been known to bring about definite skeletal changes rather than dental, the forsus FRD which is used toward the end of the growth phase has shown to bring about a greater degree of dental changes than skeletal.,
These appliances improve Class II skeletal relationships by increasing the length of the mandible. Hence, as the length of the mandible increases, subsequently the retromolar space also increases, which would bring about a positional and angular change in the mandibular third molar. They also produce forward movement of the mandibular buccal segments, which has also been cited as an important factor providing space and therefore improving the eruption potential of the third molars.
Therefore, the purpose of this study was to evaluate and compare the changes in the retromolar space and mandibular third molar angulation, in skeletal Class II division 1 patients treated with FTB versus forsus FRD.
| Materials and Methods|| |
Patients were divided into two equal groups – Group A: 25 patients treated with FTB appliance and Group B: 25 patients treated with forsus FRD appliance Forsus™ Fatigue Resistant Device (FRD; 3M Unitek, Monrovia, CA, USA).
Pretreatment lateral cephalograms and orthopantomograms of the patients were collected. The patients were recalled for the purpose of obtaining postretention lateral cephalograms and orthopantomograms 3–4 years after completion of their treatment, that is, in the age group of 16–25 years, to evaluate the retromolar space and late mandibular growth. For the purpose of evaluating the skeletal parameters, acetate matte sheets were placed over the lateral cephalograms and the cephalometric landmarks were traced. Similarly, for the purpose of evaluating the retromolar space and the mandibular third molar angulation, the orthopantomograms were traced, and the outlines of the mandibular body and ramus, mandibular second and third molar teeth, and their long axes were drawn on the tracing sheet. As there was a significant time gap between some pretreatment and postretention records, some radiographs had to be digitally standardized. Standardization was done by scanning the radiographs and uploading them on Adobe Photoshop CS5, on which they were digitized, and a standard scale was set.
Linear and angular parameters evaluated on lateral cephalograms and orthopantomograms are shown in [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5].
|Figure 1: Diagrammatic representation of cephalometric skeletal linear measurement|
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|Figure 2: Diagrammatic representation of cephalometric dental linear measurements Pancherz analysis|
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|Figure 3: Diagrammatic representation of cephalometric skeletal angular measurements|
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|Figure 4: Diagrammatic representation of linear measurements done on panoramic radiograph|
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|Figure 5: Diagrammatic representation of angular measurements done on panoramic radiograph|
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Ethical Clearance obtained from Research and Ethics Committee, KLE VK Institute of Dental Sciences, KLE University, Ref. no. 1232 dated 24 June 2019.
| Results|| |
Intragroup comparison of pre- and postintervention values using paired t-test showed statistically significant differences between most of the linear and angular parameters in the FTB group, except β angle on the right side [Table 1] and [Table 2].
|Table 1: Intragroup comparison of different linear parameters pre- and postintervention in fixed twin-block group using paired t-test|
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|Table 2: Intragroup comparison of different angular parameters pre- and postintervention in fixed twin-block group using paired t-test|
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Intragroup comparison in the FRD group showed statistically significant changes for all linear parameters except the mesiodistal widths (MDWs) of the lower third molar on both sides, and the space width ratio (SWR) and distance between Xi point to the distal surface of the lower second molar on the right side [Table 3]. All angular parameters in this group except the α and β angles on both sides showed statistically significant changes [Table 4].
|Table 3: Intragroup comparison of different linear parameters pre- and postintervention in forsus fatigue resistant device group using paired t-test|
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|Table 4: Intragroup comparison of different angular parameters pre- and postintervention in forsus fatigue resistant device group using paired t-test|
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Intergroup comparison between the two groups was done using unpaired t-test. Statistically significant changes were observed with all linear parameters except MDW of the lower third molar and the distance between Xi point to the distal surface of the lower second molar on both sides [Table 5]. α and β angles also showed statistically significant differences on both sides between the two groups [Table 6].
|Table 5: Intergroup comparison of linear parameters between fixed twin-block group and forsus fatigue resistant device group using unpaired t-test|
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|Table 6: Intergroup comparison of angular parameters between fixed twin-block group and forsus fatigue resistant device group using unpaired t-test|
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| Discussion|| |
The mandibular third molar is by far the most frequently impacted tooth, followed by the maxillary third molar, with a prevalence rate ranging from 9.5% to 39%. The great amount of variability in size, shape, position, root formation, time of calcification, and path of eruption of these teeth makes their emergence one of the most unpredictable events in the course of development of the human dentition.
The mandibular third molar continually changes its angular position in relation to the mandibular plane and the adjacent teeth and undergoes pre-eruptive rotational changes while uprighting itself into the dental arch, which occur as soon as the developing bud comes in contact with the distal aspect of the second molar. These pre-eruptive changes are of utmost importance to the emergence of these teeth in the dental arch, the absence of which increases the chances for impaction, which occurs when the space available is deficient.
Lack of space posterior to the mandibular second molars has been cited as one of the most common reasons of mandibular third molar impaction. Over the years, several authors have suggested various factors that might be responsible for the contribution of developmental space for the mandibular third molar. Brash suggested that resorption of the anterior border of the ramus of the mandible and the backward slope of the anterior border of the ramus in relation to the alveolar bone might play an important role in the creation of space posterior to the mandibular second molars, while in 1953, Brash and Scott suggested that forward movement of the dentition might be an important contributor. According to Björk et al., growth in the length of the mandible, sagittal direction of mandibular growth, and sagittal direction of eruption of the dentition were also important factors leading to this increase in the developmental space.
The present study noted a statistically significantly overall greater (P < 0.005) increase in the retromolar space of 2.95 mm on the right side and 1.99 mm on the left side in the FTB group as compared to the FRD group, which increased by 0.86 mm on the right side and by 2.36 mm on the left side [Table 5].
Besides retromolar space, several studies have also researched the correlation between the growth in the length of the mandible as suggested by Björk et al., with the risk of impaction. Richardson found that the proportion of impacted mandibular third molars was more in Class II skeletal cases with shorter, narrower, more acute-angled mandibles. The author also noted that impacted third molar cases showed reduced amount of mandibular growth. Capelli also found that chances of third molar impaction are greatly increased in cases where the length of the mandible is deficient. Sagittal skeletal relationships might also have a role in the eruption of these teeth, as suggested by Janson et al. They reported a difference in available retromolar space in skeletal Class II and Class I malocclusions. These findings were also corroborated by Jakovljevic et al., who found the greatest available space in Class III subjects and the least amount of retromolar space in Class II subjects.
Treatment effects of fixed functional appliances are a combination of skeletal and dental modifications. Comparative studies between these two appliances, have shown that while both bring about Class II correction by inducing mandibular growth, the twin-block appliance produced a larger effect on the growth and position of the mandible than did the FRD or what occurred in controls. These studies noted greater increase in mandibular length in patients treated with twin-block as compared to FRD, indicating that the former produced greater skeletal effects in terms of mandibular advancement and growth stimulation than the latter. Similar result was found in the present study, with statistically significant improvements in the Go-Gn, CoGn, and Co-Go parameters, all of which were comparatively more in the FTB group than that in the FRD group [Table 5].
Forsus FRD has been shown to produce comparatively more dentoalveolar effects, a combination of mesialization of the lower molars along with distalization of the upper molars, contributing significantly to Class II molar correction. Overjet correction was also brought about majorly by lower incisor proclination and upper incisor retroclination. These findings were also reflected in the current study [Table 6].
A potential problem of previous studies that have evaluated third molar impactions is the age of the subjects included in the study. Third molars erupt between 17 and 21 years of age, but root formation is not completed till the age of 18–25 years. Case studies have suggested that third molars that show apparent signs of development of impaction at an earlier stage might even have the potential for eruption during the final stages of root formation, indicating the risk of overdiagnosis of impaction if the age factor is not considered.
Moreover, although mandibular growth is completed at 18 years of age, there might be some amount of late mandibular growth that occurs during early adulthood, which may have a favorable effect on retromolar space, mandibular third molar position, and angulation. Chen et al. reported moderate increase in retromolar space through the ages of 16–20 years, while Kruger et al. found positional changes of the third molars after the age of 18 years, leading to their eruption. Taking into consideration all these factors, patients who completed their treatment before reaching adulthood were recalled 3–4 years after completion of active treatment, that is, in the age group of 16–25 years of age, for postretention records, which were then evaluated and compared with their pretreatment records.
ϒ angle and gonial angle did not show statistically significant differences between the FTB and FRD groups. Haavikko et al. suggested that “the favorable erupting path of lower third molars cannot be predicted from the size of the gonial angle or the second molar angle (ϒ angle) alone and the most valuable variable was the initial angulation of the third molar (α angle).”
Some authors have suggested that there is a stronger correlation between eruption and α and β angles than that with retromolar space, and that even in the presence of enough retromolar space, the third molars may remain impacted if the α and β angles are inconvenient. The concept of resorption at the anterior border of the ramus contributing to the development of retromolar space has been discussed. Bony trabeculae adapt to the stresses and strains caused by external forces, as a part of the bone-remodeling process. Türköz suggested that “third molar teeth with appropriate α and β angles may maintain the necessary external force to remodel the retromolar region by expanding the bone in all the three dimensions and form resorption of the ramal region, thus increasing the retromolar space.” This concept was backed by Jakovljevic et al. The current study found a statistically significant increase in α angle (P < 0.05) [Table 6] on both sides and a decrease in β angle on both sides, indicating uprighting of the third molar in relation to the mandibular plane and to the long axis of the lower second molar.
Furthermore, Niedzielska et al. reported that “the retromolar SWR/crown width ratio and third molar angulations in relation to second molar inclination and to the lower border of the mandible are determinants of the ultimate third molar position in the dental arch,” all of which were changed significantly in the FTB as compared to the FRD group in the present study [Table 5].
Regression models in the study of Jakovljevic et al. showed that along with β angle, length of the mandibular corpus (Go-Gn) had a significant influence on the level of mandibular third molar eruption, which would explain the greater degree of favorable changes in the FTB group.
Measurement of third molar angulation on lateral cephalograms, as seen in previous studies, might be subject to errors because of the superimposition of contralateral images of third molars. This problem might be eliminated by using panoramic radiographs of the same magnification. Tronje et al. suggested that rotational panoramic radiography causes inbuilt distortion effect; however, they also stated that panoramic radiographic images can be reliable for geometric measurements in clinical practice, as long as they are recorded on the same machine at different times. Stramotas et al. noted that “linear vertical measurements, ratio calculations, and angular measurements can be accurately made on panoramic radiographs.” Larheim et al. and Olive et al. also vouched for the reliability of panoramic radiographs in assessing the third molar position. Therefore, in this study, while the skeletal measurements were done on lateral cephalograms, important linear and angular measurements evaluating third molar position and angulation were done on panoramic radiographs.
This study recognizes the skeletal and dental modifications brought about by fixed functional appliances such as FTB and FRD in the correction of Class II skeletal malocclusions, and the effect these changes have on the position and angulation of mandibular third molar teeth. Mandibular growth and forward movement of the mandibular dentition were found to have profound effect on the fate of these teeth. A comparison between these two appliances showed that the third molars show more favorable positional and angulation changes in patients treated with FTB, over those treated with FRD. These findings indicate that true skeletal changes, as seen more after treatment with FTB than FRD, play a greater role than dentaolaveolar changes, in the creation of an environment conducive to third molar eruption.
| Conclusion|| |
- Both FTB and FRD are effective in the treatment of Class II malocclusion, and they act through a combination of skeletal and dentoalveolar changes
- FTB produced a greater effect on the growth and position of the mandible, by inducing lengthening of the mandibular corpus, whereas Class II correction by FRD was mainly a result of mesialization of the mandibular molars and proclination of the mandibular incisors
- While both FTB and FRD groups showed an increase in retromolar space, SWR, and the distance of Xi point from the distal surface of the mandibular second molar, these changes were more pronounced in the FTB group than that in the FRD group
- Both groups showed improvement in mandibular third molar angulation in terms of an increase in α angle and a decrease in β angle, which was significantly more in the FTB group than that in the FRD group.
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Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]