Introduction
This study aimed to test whether control in maxillary and mandibular incisor position, during treatment with an acrylic splint Herbst appliance, could influence the mandibular response in growing patients with skeletal Class II malocclusion.
Methods
The lateral cephalograms of 61 patients (mean age, 12.3 years; standard deviation, 1.6) with skeletal and dental Class II malocclusion were retrospectively analyzed both at baseline and after Herbst appliance removal, using a modified Pancherz cephalomeric analysis. Forty-five patients had received miniscrew in the mandibular arch to control mandibular incisor anchorage. In 21 patients, the maxillary incisors had been proclinated before starting the treatment for deepbite and maxillary incisor lingual inclination. All the patients were categorized a posteriori into 2 homogeneous groups, according to dental overjet reduction: 30 patients with dental overjet reduction (DR) and 31 patients without dental overjet reduction (NDR).
Results
Both groups presented a significant skeletal correction. However, the change was significantly greater in the NDR group than in the DR group ( P <0.01). The mandibular bone base reached a median value of 4.0 mm (interquartile range, 2.5) in the NDR group vs 1.1 mm (interquartile range, 2.8) in the DR group ( P <0.001). The 2 groups were also significantly different in terms of the positional change of maxillary incisor, which was proclinated in group NDR and lingualized in group DR ( P <0.001).
Conclusions
The results showed that dental control of overjet was beneficial to improve the effectiveness of Herbst treatment in increasing mandibular length in growing patients with skeletal Class II malocclusion.
Highlights
- •
Herbst appliance treatment increased mandibular bone base length.
- •
Skeletal improvement was reduced as dental overjet correction increased.
- •
Anchorage control onto maxillary and mandibular incisors should be employed.
The majority of Class II malocclusions are due to mandibular deficiency, resulting in a retrusion of the chin. In 1970, Merrifield and Cross had already asserted that, concerning dental Class II malocclusion, it would be desirable that the lower part of the face comes forward to obtain a facial improvement. Therefore, in Tweed mechanics, the system of directional forces was designed to produce a counterclockwise-resultant vector onto the teeth, thus enhancing a favorable skeletal change and chin projection. The ideal correction of a Class II malocclusion should avoid point B dropping down and back to buccal procline mandibular incisors and lingual incline and extrude maxillary incisors.
In 1995, Gebeck and Merrifield, , analyzed a group of patients who were successfully treated with Tweed technique for Class II, Division 1 and Class I dentoalveolar protrusion malocclusions. They confirmed that the horizontal control (lingual up-righting) of the mandibular incisor was associated with an improvement in Frankfort mandibular incisor angle (FMIA), leading to the advancement of the mandible. In that group of patients, the cephalometric superimposition demonstrated that the maxillary incisors were retracted and intruded, although their inclination remained unchanged.
As such, the excess of overjet associated with the skeletal Class II malocclusion was not compensated by the dental displacement of the frontal teeth but corrected by mandibular advancement.
Therefore, over the years, when dealing with clinical practice for the treatment of Class II malocclusion, it has been accepted as a rule that incisor position control mechanics concerning maxillary and mandibular arches should be employed. The clinical rationale with this approach is the belief that dental displacement of the incisors in the overjet space generates an obstacle for the mandible to protrude.
For the same reason, in the example of maxillary retroinclined frontal teeth, some clinicians should be advised to correct the inclination while proclining the incisors—before starting treatment.
In contrast, the appliances employed for correction of Class II malocclusions, such as the Herbst appliance or multibracket appliance combined with Class II elastics, are anchored to the teeth. Although these types of devices protrude the mandible, the undesired side effects of proclination of mandibular incisor and retroinclination of maxillary incisor can occur because of a lack of complete anchorage control. The effect of Herbst appliance and other fixed functional appliances is an association of skeletal and dentoalveolar Class II correction. , , , In particular, it has been shown that the average advancement of the Pogonion after Herbst appliance was 1.45 mm with a 95% confidence interval from 0.43 to 2.47, a value lower than the total amount of overjet correction.
As a consequence of this finding, to hypothetically reduce the dental correction of skeletal Class II malocclusion, the original Herbst design was modified by adding cast splints , or recently skeletal anchorage. , However, none of them have been able to eliminate the proclination of the mandibular incisors completely.
However, it is noteworthy that, to our knowledge, not one paper published in literature has reported evidence that a lack of anchorage control onto mandibular incisors was directly correlated with a reduction of mandibular advancement in patients with a skeletal Class II malocclusion. Moreover, no data are available regarding the effect of lingual displacement of maxillary incisors on Herbst treatment effectiveness.
Despite this, after the clinical approach in Class II treatment, the focus is on the preservation of overjet space needed for the mandible to protrude. Moreover, this space results from the combination of position change of both maxillary and mandibular incisors produced during Herbst activation, not exclusively of mandibular incisors. Consequently, the question remains open regarding the influence of anchorage control of maxillary and mandibular incisors on Herbst treatment effectiveness.
Therefore, this study aimed to test whether the prevention of dental reduction of overjet during Herbst appliance treatment promotes an improvement in mandibular advancement in growing patients with a skeletal Class II malocclusion.
Material and methods
A group of growing patients in permanent or late mixed dentition were sequentially selected from the archive of an orthodontist (A.M), who performed the treatment. All patients had been treated with an acrylic splint Herbst appliance as the first step of full treatment for skeletal and dental Class II malocclusion. The sample also included patients with a deepbite treated with a fixed multibracket appliance to open the bite before starting Herbst treatment. The fixed appliance was bonded only to the maxillary arch, without involving the mandibular dental arch.
The employment of the Herbst appliance was chosen because the profile was characterized by a retrusive chin. Exclusion criteria were poor oral hygiene and motivation, tooth agenesis or premature loss of permanent teeth, transverse or vertical discrepancies, presence of a double bite at the end of Herbst treatment, and, finally, incomplete available records.
Cephalometric analysis was performed on lateral cephalograms undertaken at baseline (T1) and at the Herbst appliance removal (T2) (mean treatment time, 10.0 months; standard deviation [SD], 1.4) before starting the phase with a multibracket fixed appliance. All patients were tested for the presence of a dual bite before each radiographic examination. The operator (C.C), who performed the analysis, was blinded to the patients’ identification and the aim of the study.
To evaluate skeletal and dental changes, the sagittal occlusal analysis of Pancherz (analysis of changes in sagittal occlusion) was carried out for each cephalogram at T1 and T2, using the occlusal line and the occlusal line perpendicular (OLp) as reference for superimposition of the radiographs. Maxillary incisor inclination with respect to the sella-nasion plane (Is/SN), mandibular incisor proclination on the mandibular plane (ii/Go-Me), and skeletal divergence measured between the sella-nasion plane and mandibular plane (S-N/Go-Me) were also included in the analysis.
The cephalometric measurements recorded at T1 and T2 included the following:
- (1)
The vertical dimension (S-N/Go-Me): angle (in degrees) formed by the S-N and Go-Me lines.
- (2)
Skeletal Class relationship (A-N/N-Pg): angle (in degrees) formed by the A-N and N-Pg lines.
- (3)
Skeletal discrepancy: difference between distances (in mm) maxillary bone base (A/OLp) and mandibular bone base (Pg/OLp).
- (4)
Condylar position (Ar/OLp): distance (in mm) from point Ar to the OLp line.
- (5)
Maxillary bone base (A/OLp): distance (in mm) from point A to the OLp line.
- (6)
Mandibular bone base (Pg/OLp): distance (in mm) from point Pg to the OLp line.
- (7)
Maxillary incisor position (Is/Olp): distance (in mm) from point Is to the OLp line.
- (8)
Maxillary incisor proclination (Is/S-N): angle (in degrees) formed by the maxillary incisor axis and the S-N line.
- (9)
Mandibular incisor position (Ii/OLp): distance (in mm) from point Ii to the OLp line.
- (10)
Mandibular incisor proclination (Ii/Go-Me or IMPA), angle (in degrees) formed by the mandibular incisor axis and the mandibular plane (Go-Me).
- (11)
Interincisive angle: angle (in degrees) formed by the maxillary and mandibular incisor axes.
- (12)
Overjet: difference between distance (in mm) maxillary incisor position (Is/OLp) and mandibular incisor position (Ii/OLp).
- (13)
Maxillary molar position (Ms/OLp): distance (in mm) from point Ms to the OLp line.
- (14)
Mandibular molar position (Mi/OLp): distance (in mm) from point Mi to the OLp line.
- (15)
Molar relationship: difference between distance (in mm) maxillary molar position (Ms/OLp) and mandibular molar position (Mi/OLp).
On the basis of cephalometric dental changes in overjet, the patients were categorized into 2 groups: nondental reduction (NDR) of overjet and dental reduction of overjet (DR). More specifically, in the DR group, the incisors were dentally displaced within the space of the existing overjet. In contrast, in the NDR group, the total amount of overjet was preserved from teeth correction. Therefore, in this group of patients, the incisors were kept in their initial position and/or proclined in the maxillary arch and moved in the lingual direction in the mandible.
The assumptions given to classify the patients in group NDR were:
- (1)
The linear lingualization of the incisal edge of mandibular incisor and the linear buccal displacement of maxillary incisor edge: including the absence of displacement: Ii(d)-Pg(d) ≤ 0 and Is(d)-A(d) ≥ 0.
- (2)
The absolute value of linear lingualization of the mandibular incisal edge, higher than the absolute value of maxillary incisor edge buccal displacement: Ii(d)-Pg(d) < 0 and Is(d)-A(d) > 0 combined with |Ii(d)-Pg(d)| ≥ |Is(d)-A(d)|.
- (3)
The value of linear buccal displacement of the maxillary incisal edge, higher than the absolute value of mandibular incisor edge buccal displacement: Is(d)-A(d) > 0 and Ii(d)-Pg(d) > 0 and combined with Is(d)-A(d) ≥ Ii(d)-Pg(d).
The reliability of the measurements was tested using the intraclass correlation coefficient. The same operator repeated the measurement of Pg/OLp distance at T1 in the first 50 patients twice, with a 14-day interval between the 2 recordings. The coefficient reached 99%, representing a high agreement level between the repeated measurements.
Statistical analysis
All continuous cephalometric variables were tested for normality using the Shapiro-Wilk test. In the case of a significant result ( P <0.05), a nonparametric test was run. A comparison between 2 groups was performed with a 2-sample t test for independent data and Wilcoxon rank-sum test. The change in variables within each group was analyzed using a 1-sample t test for dependent data and Wilcoxon signed-rank test.
The difference in frequency between the 2 groups in sex distribution, in maxillary incisor proclination before Herbst treatment and in pretreatment procedures and/or auxiliary devices employed were tested with the Pearson chi-square and Fisher exact tests.
The correlation between dental correction of Class II malocclusion and Pogonion advancement in the full sample after treatment was measured by the Spearman correlation coefficient.
The level of significance was fixed at P = 0.05. All data were analyzed using STATA software (version 14.2; StataCorp, College Station, Tex).
The sample size was estimated a priori on the primary outcome mandibular bone base, Pg/OLp distance using the data published in the article by Manni et al regarding the 2 groups, standard Herbst and elastic chain, treated with the same design of Herbst appliance. Assuming the difference of 2.5 mm between the 2 groups and an SD of 3.1 mm, the sample size estimated for a 2 sample t test ( P <0.05; power of 0.8) was in a total of 50 patients (25 patients per group).
Results
The sample comprised 61 patients (mean age, 12.3 years; SD, 1.6), 39 boys (mean age, 12.0 years; SD, 1.5) and 22 girls (mean age, 12.8 years; SD, 1.5) treated with a Herbst appliance as an initial treatment.
The cephalometric analysis was performed on cephalograms at T1 and T2 (mean treatment time, 10.0 months; SD, 1.4) before starting the phase with a multibracket fixed appliance.
Twenty-one (34.4%) out of 61 patients presented a deepbite and maxillary incisor lingual inclination. The angle Is/S-N was 97.7° (SD, 9.4) in patients with a deepbite. In these patients, the maxillary incisors were proclinated before starting the Herbst treatment to create the overjet needed for Herbst appliance activation. More specifically, the treatment was performed exclusively in the maxillary arch, using brackets with a bidimensional prescription. The teeth were aligned with flat continuous archwires, in sequence 0.016 and 0.017 × 0.025 nickel-titanium and 0.018 × 0.022 stainless steel, without intrusion or extrusion mechanics. At T2, the clinician checked whether the canine and molar dental Class relationship were demonstrably the same as those at T1.
Patients without a deepbite showed a maxillary incisor inclination of 105.0° (SD, 7.6) on the S-N plane.
In 45 out of 61 patients, the anchorage in the mandibular arch was controlled with titanium miniscrews (Osstem Implant, Seoul, South Korea), 8 mm long, inserted between first and second premolars or second premolar and first molar. The screws were linked with a power chain to a button—bonded onto each canine crown. The lingual direction of force vector created the anchorage against the undesired side effect of incisor flaring caused by the Herbst appliance.
After having categorized the patients on the basis of dental overjet change ( Table I ), the NDR group comprised 30 patients, 22 males (73.2%) and 8 females (26.7%), and the DR group comprised 31 patients, 17 males (54.8%) and 14 females (45.2%). The sex distribution (Pearson chi-square test, P = 0.133) and mean age (2-sample t test; P = 0.6628) were not significantly different in the 2 groups.
| Baseline parameters | DR group | NDR group | Comparison, P value |
|---|---|---|---|
| Sample size, n | 31 | 30 | |
| Ratio boys to girls, n (%) | 17 (54.8) to 14 (45.2) | 22 (73.2) to 8 (26.7) | 0.133 ∗ |
| Age, mean (SD), y | 12.4 (1.8) | 12.2 (1.6) | 0.6628 † |
| Age of boys, mean (SD), y | 11.8 (1.7) | 12.2 (1.3) | 0.4362 † |
| Age of girls, mean (SD), y | 13.1 (1.6) | 12.2 (1.3) | 0.1818 † |
| Pretreatment procedures or auxiliary devices employed, n (%) | 0.006 ‡ | ||
| Pretreatment maxillary incisor proclination | 2 (6.4) | 3 (10.0) | |
| Skeletal anchorage in the mandibular arch | 15 (48.4) | 14 (46.7) | |
| Combined maxillary incisor proclination and skeletal anchorage | 4 (12.9) | 12 (40.0) | |
| No procedure or devices | 10 (32.3) | 1 (3.3) | |
| Treatment time with Herbst appliance, mean (SD), mo | 9.8 (1.5) | 10.2 (1.2) | 0.3169 † |
The distribution of patients treated with miniscrews for mandibular incisor anchorage control and/or maxillary incisor proclination before Herbst treatment was different between the 2 groups (Fisher exact test, P = 0.006; Table I ). Within the 2 groups, no procedure was performed on the incisors in 10 patients in the DR group and only 1 patient in the NDR group.
The treatment time was similar in the 2 groups: 9.8 months (SD, 1.5) in the DR group and 10.2 months (SD, 1.2) in the NDR group (2-sample t test; P = 0.3169).
In short, at baseline ( Table II ), the 2 groups were different with respect to divergence (2-sample t test; P = 0.0063) and pretreatment value of maxillary incisor inclination (2-sample t test; P = 0.0071). Similarly, the overjet reached different amounts in the NDR and DR groups (2-sample t test; P = 0.0006). However, in 21 patients the maxillary incisor inclination, and consequently the overjet, were modified before Herbst treatment for the presence of lingual inclination of the teeth and deepbite. The maxillary incisors were proclined in 6 patients (19.4%) of the DR group vs 15 patients (50.0%) in the NDR group (Pearson chi-square test, P = 0.012). In all of these patients, the cephalogram was not repeated at the beginning of the treatment with Herbst for ethical reasons. Therefore, the comparisons regarding overjet and maxillary incisor position at baseline between the 2 groups were not reliable.
| Cephalometric parameters | DR group | NDR group | Comparison, P value |
|---|---|---|---|
| Vertical dimension (S-N/Go-Me), mean (SD), ° | 35.1 (4.4) | 32.0 (4.3) | 0.0063 ∗ |
| Skeletal Class relationship (A-N/N-Pg), mean (SD), ° | 3.7 (2.0) | 3.3 (2.6) | 0.5286 ∗ |
| Skeletal discrepancy (A/OLp − Pg/OLp), median (IQR), mm | −0.6 (4.4) | 0.0 (4.1) | 0.6690 † |
| Condylar position (Ar/OLp), mean (SD), mm | 9.0 (2.4) | 9.1 (3.2) | 0.8554 ∗ |
| Maxillary bone base (A/OLp), mean (SD), mm | 67.1 (3.5) | 67.7 (3.9) | 0.5844 ∗ |
| Mandibular bone base (Pg/OLp), median (IQR), mm | 68.2 (7.7) | 68.2 (6.4) | 0.7449 † |
| Maxillary incisor inclination (Is/S-N), mean (SD), ° | 105.5 (6.9) | 99.4 (9.8) | 0.0071 ∗ |
| Maxillary incisor position (Is/OLp), mean (SD), mm | 74.9 (3.6) | 72.6 (4.9) | 0.0380 ∗ |
| Mandibular incisor inclination (IMPA), mean (SD), ° | 95.6 (7.7) | 97.9 (6.8) | 0.2128 ∗ |
| Mandibular incisor position (Ii/OLp), mean (SD), mm | 68.1 (4.2) | 67.5 (4.3) | 0.5727 ∗ |
| Interincisive angle, mean (SD), ° | 124.5 (10.4) | 131.0 (11.2) | 0.0229 ∗ |
| Overjet (Is/Olp − Ii/OLp), mean (SD), mm | 6.8 (1.9) | 5.1 (1.8) | 0.0006 ∗ |
| Maxillary molar position (Ms/OLp), mean (SD), mm | 46.7 (3.6) | 46.3 (4.0) | 0.6614 ∗ |
| Mandibular molar position (Mi/OLp), mean (SD), mm | 45.5 (3.7) | 44.6 (4.3) | 0.4039 ∗ |
| Molar relationship (Ms/OLp − Mi/OLp), mean (SD), mm | 1.2 (1.5) | 1.6 (1.5) | 0.2676 ∗ |
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