Long-term effects of Class II Herbst treatment on the pharyngeal airway width

Long-term effects of Class II Herbst treatment on the pharyngeal airway width Summary Objective The aim was to assess the long-term effects of Class II malocclusion treatment with the Herbst appliance on the pharyngeal airway (PA) width in comparison to untreated individuals with Classes I and II malocclusion. Methods Lateral cephalometric radiographs of 13 male Class II patients from before (T1) and after (T2) treatment with the Herbst appliance as well as after the end of growth (T3) were retrospectively analyzed and compared to two untreated age- and gender-matched samples with Class I (n = 13) or Class II (n = 13) malocclusion. The PA dimensions were measured using the parameters p (narrowest distance between the soft palate and the posterior pharyngeal wall) and t (narrowest distance between the base of the tongue and the posterior pharyngeal wall). In addition, standard cephalometric measurements were performed. Results Relevant changes in PA dimensions were only seen for the post-treatment period, during which the distances p and t showed a significant increase in the Herbst group only (∆p: 2.3 mm, ∆t: 3.3 mm) while remaining similar in both untreated groups (∆p: 0.5 mm, ∆t: 0.5 mm, respectively, ∆p: 0.7 mm, ∆t: 1.6 mm). During the same period, posterior face height showed a significantly larger increase in the Herbst group than in both control groups (8.2 versus 5.8 mm, respectively, 5.4 mm), whereas anterior face height (NL-Me) showed a similar development in all groups (4.6 versus 4.4 mm, respectively 3.2 mm). Conclusion In the long term, Herbst treatment resulted in a significant post-treatment increase of PA width, possibly due to an increased lower posterior facial height development compared to untreated individuals. Introduction During the past years, growing interest has evolved regarding the influence of conservative and surgical mandibular advancement procedures on pharyngeal airway (PA) dimensions for addressing sleep disordered breathing (SDB) (1, 2). SDB is a collective term referring to several symptoms associated with constricted upper airway dimensions, ranging from snoring to obstructive sleep apnea (OSA) (3–5). SDB can be categorized into pediatric or adult variants (6). While the first are often associated with large tonsils or adenoids but also craniofacial syndromes (7), a great risk factor for the latter is constriction of upper airways due to obesity (6, 8). In general, there are reported correlations between OSA and increased morbidity, fatigue, disturbed growth in children, deviant behavior, hypertension, bruxism, and headache (9, 10). SDB and OSA are associated with larger tongues as well as both reduced oropharyngeal areas and anteroposterior dimensions (11, 12). Removable functional appliances such as Fränkel-1, Activator or Bionator have been shown to increase oropharyngeal dimensions during the treatment period by advancing the mandible and thus moving the hyoid bone, tongue, and soft palate forward (13, 14). The positive effect on PA dimensions using headgear activator can be maintained at least until age 22 (15). For fixed functional appliances, there has been found a short-term increase in PA width when a Herbst appliance and rapid maxillary expansion were combined (16). A 3D-cone-beam computed tomography analysis demonstrated that during the treatment period the increase in PA volume was significantly higher in Class II patients treated with a Herbst appliance than in Class I patients that received edgewise treatment only (17). Similar results were reported for treatments with a mandibular anterior repositioning appliance (MARA) (18). However, the present data do not provide any information about possible long-term changes in the PA achieved by fixed functional appliance treatment for Class II correction. Therefore, the aim of this study was to evaluate the changes in PA width from the beginning of treatment until the end of growth in Class II patients treated with a Herbst appliance in comparison to two historic untreated age- and gender-matched control groups with Classes I or II malocclusion. The null hypothesis was that treatment with a Herbst appliance would not lead to different changes in upper airway dimensions compared to untreated controls. Materials and methods Ethical approval for this study was granted by the Ethic committee of the Medical faculty of the University of Giessen (No. 126/15). Subjects The subjects of the treatment group (n = 13) were selected from the archives at the Department of Orthodontics at the University of Malmö, Sweden. The sample size of this explorative study was predefined by the availability of long-term lateral cephalograms, therefore no sample size calculation was performed. The group consisted of male Class II patients that had been treated with the Herbst appliance between 1977 and 1982. The intervals T1–T2 included a phase of active treatment with the Herbst appliance (0.5 years on average) as well as a subsequent passive phase of occlusal settling without any appliance (1.3 years on average) except for two persons in whom four premolars were extracted and a fixed appliance was used to close the extraction spaces. During active treatment, the Herbst appliance was initially activated to an edge to edge incisor relationship. After active treatment, retention appliances were used in nine patients: three patients were provided with a passive activator for 2 years, while six got a combination of cuspid retainers and removable plates. Lateral cephalograms of these patients were present from before Herbst treatment (T1, mean age: 12.4 years), on average 1.3 years after Herbst treatment (T2, mean age: 14.2 years) and from the end of the long-term follow up with the patient being at least 18 years of age (T3, mean age: 20.2 years). For the historic control groups, the lateral cephalograms of 26 white Northern American males were derived with permission from the ‘Oregon Growth Study’ and the ‘Denver Growth Study’ through the AAOF Legacy Collection (www.aaoflegacycollection.org). All lateral cephalograms had been taken in an upright standing position in habitual occlusion. The state of the respiratory circle during radiography, however, could not be found out retrospectively. All lateral cephalograms of the Herbst group were taken between 1977 and 1988, those of the Class I control group were taken between 1935 and 1977 and those of the Class II control group were taken between 1935 and 1972 (Supplementary Table). Inclusion criteria for both control groups were no history of past orthodontic treatment and the presence of three lateral cephalograms of good quality at similar age to the patients of the treatment group for T1, T2, and T3, respectively. Additionally, the Class I subjects (n = 13) had to present a Class I molar relationship (according to the data given in the AAOF Legacy collection) plus an ANB angle between 0° and 3° at T1. The Class II controls (n = 13) had to have a Class II molar relationship plus an ANB angle > 4. Unfortunately, the AAOF Legacy collection does not provide information about the severity of the Class II molar relationship. Method The radiographs of the treatment group were scanned at 300 dpi (Dual Lens System V750 Pro, Epson, Nagano, Japan). The cephalograms of the control groups were delivered from the AAOF Legacy Collection in digital form at 300 dpi. The resulting 117 digitized lateral cephalograms were traced and measured using the software Ivoris for Windows, version 8.1.22 (Computer Konkret AG, Falkenstein, Germany). To enable the comparability among the groups, a correction for linear radiographic magnification was performed for all images, according to the settings of the respective X-Ray unit, respectively the instructions provided by the AAOF Legacy Collection. The measured angles and distances are shown in Figure 1. Figure 1. View largeDownload slide Cephalometric reference points, lines, and measurements used for the assessment of (a) pharyngeal airway: p (narrowest distance between the soft palate and the posterior pharyngeal wall), t (narrowest distance between the base of the tongue and the posterior pharyngeal wall) (b) sagittal jaw base relationship: SNA, SNB, ANB, Wits, overjet (OJ), and (c) vertical jaw base relationship: ML/NSL, NL/NSL, ML/NL, Ar-Go, N-Me, NL-Me, overbite (OB). Figure 1. View largeDownload slide Cephalometric reference points, lines, and measurements used for the assessment of (a) pharyngeal airway: p (narrowest distance between the soft palate and the posterior pharyngeal wall), t (narrowest distance between the base of the tongue and the posterior pharyngeal wall) (b) sagittal jaw base relationship: SNA, SNB, ANB, Wits, overjet (OJ), and (c) vertical jaw base relationship: ML/NSL, NL/NSL, ML/NL, Ar-Go, N-Me, NL-Me, overbite (OB). Statistical method All statistical analyses were performed with MS Excel 2007 (Microsoft, Redmond, Washington, USA) and SPSS for Windows, version 20 (IBM SPSS, Armonk, New York, USA). Means and standard deviations were calculated for descriptive analyses. The Kolmogorov–Smirnoff test was performed to determine the presence of a normal distribution. A one-way analysis of variance (ANOVA) was used to investigate differences between the three groups. Post hoc Tukey test was applied to identify the groups that were the cause for the significant differences. In case of variables without normal distribution, the non-parametric Kruskal–Wallis H-test was used instead of the ANOVA. A two-sample t-test was applied in order to demonstrate longitudinal changes within each of the groups. In case of variables without normal distribution, the non-parametric Wilcoxon test for paired samples was used instead. Results with P-values ≤ 0.05 were considered statistically significant. Method error All measurements of the first cephalogram of each of the 39 patients were repeated 12 weeks after the first tracing by the same author (CD). To assess the reproducibility of the measurements, Cronbach’s reliability test was applied. Cronbach’s alpha coefficients for intra-class correlation were found to be within a range of 0.938–0.995 with the minimum value of 0.938 for overbite. Values for airway parameters were 0.976 for p and 0.991 for t, indicating a very good repeatability of the measurements. Results The cephalometric variables of the Herbst- and control groups from the three time points are given in Table 1. At T1, the values for the PA width for the given airway parameters were similar in all three groups. The dento-skeletal characteristics of the three groups differed in accordance with the selection criteria. The Wits appraisal was significantly greater in the Herbst group than in both control groups (4.5 versus 2.7 versus 2.7 mm, P = 0.05). The SNB angle was significantly smaller in the Herbst group than in the Class I controls at T1 (75.7 versus 78.7°, P = 0.05) but not different to the Class II controls. Correspondingly, the ANB angle was largest in the Herbst group, followed by the Class II controls and smallest in the Class I controls (∆ANB: 3.6° and 4.5°, P = 0.000). Both the overjet (P = 0.000) and the overbite (P = 0.001) were largest in the Herbst and smallest in the Class I controls. The vertical jaw base relationship did not differ between the groups. Table 1. Means, standard deviations, and P-values (one-way ANOVA) comparing the Herbst group and the controls at the three time points. n  T1  T2  T3  Herbst  Class I  Class II    Herbst  Class I  Class II    Herbst  Class I  Class II    13  13  13  P  13  13  13  P  13  13  13  P  Age (years)  12.4 ± 0.9  12.3 ± 0.6  12.1 ± 0.5  0.711  14.2 ± 1.2  14.3 ± 0.7  14.2 ± 0.6  0.865  20.2 ± 1.0  19.8 ± 2.0  19.8 ± 2.3  0.827  p (mm)  7.5 ± 1.8  8.0 ± 1.7  7.7 ± 1.9  0.818  8.1 ± 1.2  8.5 ± 2.3  8.5 ± 2.4  0.836  10.3 ± 2.9  9.0 ± 2.6  9.1 ± 2.1  0.335  t (mm)  9.4 ± 2.1  9.6 ± 2.4  8.7 ± 3.6  0.698  9.5 ± 2.9  10.9 ± 4.4  9.0 ± 3.7  0.405  12.9 ± 2.9  11.4 ± 5.0  10.6 ± 3.8  0.224  SNA (°)  82.1 ± 3.6  80.5 ± 3.1  81.9 ± 3.6  0.447  81.0 ± 3.7  81.5 ± 3.1  82.1 ± 4.0  0.729  82.3 ± 4.0  81.6 ± 3.5  82.6 ± 3.9  0.804  SNB (°)  75.7 ± 3.3  78.7 ± 3.0  76.3 ± 3.0  0.052*  76.0 ± 3.8  79.8 ± 2.9  77.1 ± 3.3  0.020*  77.6 ± 3.8  80.7 ± 2.7  78.4 ± 3.7  0.069  ANB (°)  6.4 ± 1.8  1.9 ± 1.1  5.5 ± 1.4  0.000*  5.0 ± 1.4  1.7 ± 1.4  5.1 ± 1.5  0.000*  4.7 ± 1.5  1.0 ± 1.8  4.2 ± 1.7  0.000*  Wits(mm)  4.5 ± 2.5  2.7 ± 1.9  2.7 ± 1.7  0.051*  1.8 ± 1.1  2.8 ± 1.8  3.3 ± 1.9  0.090  2.9 ± 1.7  3.1 ± 2.0  3.4 ± 1.9  0.746  OJ (mm)  7.1 ± 1.6  3.3 ± 1.2  4.7 ± 1.1  0.000*  3.8 ± 1.1  3.2 ± 1.0  4.4 ± 0.8  0.018*  4.1 ± 1.0  3.0 ± 1.5  4.0 ± 1.0  0.225  ML/NSL (°)  31.1 ± 5.8  32.4 ± 5.4  32.1 ± 3.8  0.780  31.9 ± 6.7  31.2 ± 5.3  31.4 ± 4.4  0.940  27.4 ± 6.8  29.0 ± 5.2  28.4 ± 5.5  0.770  ML/NL (°)  23.6 ± 4.8  25.6 ± 5.6  24.7 ± 4.2  0.567  23.1 ± 5.8  24.5 ± 5.2  23.9 ± 5.2  0.819  19.5 ± 5.5  22.3 ± 5.2  21.6 ± 6.1  0.428  NL/NSL (°)  7.6 ± 2.6  6.8 ± 2.0  7.1 ± 3.5  0.777  8.8 ± 2.8  6.7 ± 2.8  7.5 ± 4.0  0.276  7.9 ± 3.4  6.4 ± 1.8  6.9 ± 3.7  0.461  OB (mm)  4.5 ± 1.5  2.3 ± 2.1  4.7 ± 1.1  0.001*  2.8 ± 1.2  2.8 ± 1.7  4.0 ± 2.3  0.160  3.4 ± 0.9  2.3 ± 2.3  4.2 ± 1.9  0.042*  N-Me (mm)  106.6 ± 4.5  104.5 ± 4.8  104.4 ± 4.1  0.356  113.5 ± 7.6  110.0 ± 5.9  110.6 ± 5.5  0.356  119.3 ± 6.2  115.8 ± 5.9  115.7 ± 6.0  0.225  NL-Me (mm)  56.8 ± 2.6  57.1 ± 3.9  56.1 ± 3.3  0.717  61.3 ± 4.7  60.4 ± 4.9  60.0 ± 4.4  0.759  65.9 ± 4.0  64.8 ± 4.6  63.1 ± 4.8  0.299  Ar-Go (mm)  41.3 ± 3.3  41.1 ± 4.0  40.0 ± 2.3  0.523  44.8 ± 5.0  44.7 ± 4.0  43.9 ± 3.1  0.826  53.0 ± 5.0  50.5 ± 3.6  49.3 ± 2.4  0.054*  n  T1  T2  T3  Herbst  Class I  Class II    Herbst  Class I  Class II    Herbst  Class I  Class II    13  13  13  P  13  13  13  P  13  13  13  P  Age (years)  12.4 ± 0.9  12.3 ± 0.6  12.1 ± 0.5  0.711  14.2 ± 1.2  14.3 ± 0.7  14.2 ± 0.6  0.865  20.2 ± 1.0  19.8 ± 2.0  19.8 ± 2.3  0.827  p (mm)  7.5 ± 1.8  8.0 ± 1.7  7.7 ± 1.9  0.818  8.1 ± 1.2  8.5 ± 2.3  8.5 ± 2.4  0.836  10.3 ± 2.9  9.0 ± 2.6  9.1 ± 2.1  0.335  t (mm)  9.4 ± 2.1  9.6 ± 2.4  8.7 ± 3.6  0.698  9.5 ± 2.9  10.9 ± 4.4  9.0 ± 3.7  0.405  12.9 ± 2.9  11.4 ± 5.0  10.6 ± 3.8  0.224  SNA (°)  82.1 ± 3.6  80.5 ± 3.1  81.9 ± 3.6  0.447  81.0 ± 3.7  81.5 ± 3.1  82.1 ± 4.0  0.729  82.3 ± 4.0  81.6 ± 3.5  82.6 ± 3.9  0.804  SNB (°)  75.7 ± 3.3  78.7 ± 3.0  76.3 ± 3.0  0.052*  76.0 ± 3.8  79.8 ± 2.9  77.1 ± 3.3  0.020*  77.6 ± 3.8  80.7 ± 2.7  78.4 ± 3.7  0.069  ANB (°)  6.4 ± 1.8  1.9 ± 1.1  5.5 ± 1.4  0.000*  5.0 ± 1.4  1.7 ± 1.4  5.1 ± 1.5  0.000*  4.7 ± 1.5  1.0 ± 1.8  4.2 ± 1.7  0.000*  Wits(mm)  4.5 ± 2.5  2.7 ± 1.9  2.7 ± 1.7  0.051*  1.8 ± 1.1  2.8 ± 1.8  3.3 ± 1.9  0.090  2.9 ± 1.7  3.1 ± 2.0  3.4 ± 1.9  0.746  OJ (mm)  7.1 ± 1.6  3.3 ± 1.2  4.7 ± 1.1  0.000*  3.8 ± 1.1  3.2 ± 1.0  4.4 ± 0.8  0.018*  4.1 ± 1.0  3.0 ± 1.5  4.0 ± 1.0  0.225  ML/NSL (°)  31.1 ± 5.8  32.4 ± 5.4  32.1 ± 3.8  0.780  31.9 ± 6.7  31.2 ± 5.3  31.4 ± 4.4  0.940  27.4 ± 6.8  29.0 ± 5.2  28.4 ± 5.5  0.770  ML/NL (°)  23.6 ± 4.8  25.6 ± 5.6  24.7 ± 4.2  0.567  23.1 ± 5.8  24.5 ± 5.2  23.9 ± 5.2  0.819  19.5 ± 5.5  22.3 ± 5.2  21.6 ± 6.1  0.428  NL/NSL (°)  7.6 ± 2.6  6.8 ± 2.0  7.1 ± 3.5  0.777  8.8 ± 2.8  6.7 ± 2.8  7.5 ± 4.0  0.276  7.9 ± 3.4  6.4 ± 1.8  6.9 ± 3.7  0.461  OB (mm)  4.5 ± 1.5  2.3 ± 2.1  4.7 ± 1.1  0.001*  2.8 ± 1.2  2.8 ± 1.7  4.0 ± 2.3  0.160  3.4 ± 0.9  2.3 ± 2.3  4.2 ± 1.9  0.042*  N-Me (mm)  106.6 ± 4.5  104.5 ± 4.8  104.4 ± 4.1  0.356  113.5 ± 7.6  110.0 ± 5.9  110.6 ± 5.5  0.356  119.3 ± 6.2  115.8 ± 5.9  115.7 ± 6.0  0.225  NL-Me (mm)  56.8 ± 2.6  57.1 ± 3.9  56.1 ± 3.3  0.717  61.3 ± 4.7  60.4 ± 4.9  60.0 ± 4.4  0.759  65.9 ± 4.0  64.8 ± 4.6  63.1 ± 4.8  0.299  Ar-Go (mm)  41.3 ± 3.3  41.1 ± 4.0  40.0 ± 2.3  0.523  44.8 ± 5.0  44.7 ± 4.0  43.9 ± 3.1  0.826  53.0 ± 5.0  50.5 ± 3.6  49.3 ± 2.4  0.054*  *P ≤ 0.05. View Large The longitudinal changes of the pharyngeal dimension are shown in Table 2 and Figures 2 and 3. Table 2. Means, standard deviations, and P-values (one-way ANOVA) for longitudinal changes in the Herbst- and both control groups. n  T1–T2 (treatment)  T2–T3 (post-treatment)  T1–T3 (total)  Herbst  Class I  Class II    Herbst  Class I  Class II    Herbst  Class I  Class II    13  13  13  P  13  13  13  P  13  13  13  P  Age (years)  1.8 ± 0.6  2.1 ± 0.2  2.0 ± 0.2    6.0 ± 0.8  5.5 ± 2.1  5.7 ± 2.2    7.8 ± 0.5  7.5 ± 2.1  7.7 ± 2.1    ∆p (mm)  0.6 ± 2.0  0.5 ± 1.5  0.8 ± 2.2  0.837  2.3 ± 2.8**  0.5 ± 2.3  0.7 ± 1.8  0.112  2.9 ± 2.8**  1.0 ± 2.8  1.5 ± 2.0  0.172  ∆t (mm)  0.2 ± 3.2  1.3 ± 3.5  0.2 ± 3.7  0.639  3.3 ± 3.9**  0.5 ± 5.9  1.6 ± 3.9  0.309  3.5 ± 3.7**  1.8 ± 4.9  1.8 ± 4.5  0.536  ∆ SNA (°)  –1.1 ± 1.1**  0.9 ± 1.2**  0.3 ± 0.9  0.000*  1.2 ± 1.7**  0.2 ± 1.7  0.5 ± 1.6  0.250  0.1 ± 1.7  1.1 ± 1.5**  0.7 ± 1.7  0.297  ∆ SNB (°)  0.2 ± 1.1  1.1 ± 1.0**  0.7 ± 0.9**  0.115  1.5 ± 1.5**  0.9 ± 1.0**  1.3 ± 1.6**  0.514  1.7 ± 1.7**  2.0 ± 1.0**  2.0 ± 1.9**  0.878  ∆ ANB (°)  –1.4 ± 0.7**  –0.2 ± 1.1  –0.4 ± 0.8  0.004*  –0.3 ± 1.1  –0.7 ± 1.4  –0.9 ± 0.8  0.443  –1.7 ± 1.2**  –0.9 ± 1.6  –1.3 ± 1.2**  0.358  ∆Wits(mm)  –2.6 ± 1.7**  0.2 ± 1.5  0.6 ± 1.0  0.000*  1.0 ± 1.4**  0.3 ± 1.6  0.2 ± 1.2  0.665  –1.6 ± 2.5**  0.5 ± 1.1  0.8 ± 1.9  0.006*  ∆ OJ (mm)  –3.4 ± 2.2**  –0.1 ± 0.8  –0.3 ± 0.9  0.000*  0.3 ± 1.1  –0.3 ± 0.9  –0.4 ± 0.7  0.160  –3.1 ± 1.6**  –0.3 ± 1.1  –0.7 ± 1.0**  0.000*  ∆ML/NSL (°)  0.8 ± 2.2  –1.3 ± 1.5**  –0.8 ± 1.7  0.019*  –4.5 ± 3.0**  –2.1 ± 1.7**  –2.9 ± 2.0**  0.033*  –3.7 ± 3.1**  –3.8 ± 2.1**  –3.7 ± 2.6**  0.998  ∆ML/NL (°)  –0.4 ± 2.3  –1.2 ± 1.7**  –0.8 ± 2.0  0.657  –3.7 ± 2.8**  –2.2 ± 2.1**  –2.3 ± 1.7**  0.207  –4.1 ± 2.8**  –3.4 ± 2.3**  –3.1 ± 2.8**  0.625  ∆NL/NSL (°)  1.2 ± 0.8**  –0.1 ± 1.2  0.4 ± 1.1  0.012*  –0.9 ± 1.7  –0.3 ± 2.0  –0.6 ± 1.5  0.798  0.3 ± 1.6  –0.4 ± 1.3  –0.2 ± 1.6  0.798  ∆OB (mm)  –1.7 ± 1.8**  0.5 ± 1.9  –0.7 ± 2.1  0.020*  0.6 ± 1.2  –0.5 ± 1.0  0.2 ± 1.6  0.124  –1.1 ± 1.1**  0.0 ± 2.3  –0.5 ± 1.9  0.304  ∆N-Me (mm)  6.8 ± 3.9**  5.5 ± 2.0**  6.3 ± 2.1**  0.611  5.9 ± 4.0**  6.4 ± 2.4**  5.0 ± 2.8**  0.532  12.7 ± 3.6**  11.9 ± 2.4**  11.3 ± 2.7**  0.486  ∆NL-Me (mm)  4.5 ± 2.6**  3.4 ± 1.8**  3.9 ± 1.7**  0.552  4.6 ± 3.1**  4.4 ± 1.9**  3.2 ± 1.9**  0.264  9.1 ± 2.8**  7.7 ± 2.0**  7.1 ± 1.7**  0.076  ∆Ar-Go (mm)  3.5 ± 3.0**  3.6 ± 1.5**  4.0 ± 2.6**  0.882  4.6 ± 3.1**  5.8 ± 2.4**  5.4 ± 2.2**  0.015*  11.7 ± 3.0**  9.4 ± 2.1**  9.4 ± 2.1**  0.029*  n  T1–T2 (treatment)  T2–T3 (post-treatment)  T1–T3 (total)  Herbst  Class I  Class II    Herbst  Class I  Class II    Herbst  Class I  Class II    13  13  13  P  13  13  13  P  13  13  13  P  Age (years)  1.8 ± 0.6  2.1 ± 0.2  2.0 ± 0.2    6.0 ± 0.8  5.5 ± 2.1  5.7 ± 2.2    7.8 ± 0.5  7.5 ± 2.1  7.7 ± 2.1    ∆p (mm)  0.6 ± 2.0  0.5 ± 1.5  0.8 ± 2.2  0.837  2.3 ± 2.8**  0.5 ± 2.3  0.7 ± 1.8  0.112  2.9 ± 2.8**  1.0 ± 2.8  1.5 ± 2.0  0.172  ∆t (mm)  0.2 ± 3.2  1.3 ± 3.5  0.2 ± 3.7  0.639  3.3 ± 3.9**  0.5 ± 5.9  1.6 ± 3.9  0.309  3.5 ± 3.7**  1.8 ± 4.9  1.8 ± 4.5  0.536  ∆ SNA (°)  –1.1 ± 1.1**  0.9 ± 1.2**  0.3 ± 0.9  0.000*  1.2 ± 1.7**  0.2 ± 1.7  0.5 ± 1.6  0.250  0.1 ± 1.7  1.1 ± 1.5**  0.7 ± 1.7  0.297  ∆ SNB (°)  0.2 ± 1.1  1.1 ± 1.0**  0.7 ± 0.9**  0.115  1.5 ± 1.5**  0.9 ± 1.0**  1.3 ± 1.6**  0.514  1.7 ± 1.7**  2.0 ± 1.0**  2.0 ± 1.9**  0.878  ∆ ANB (°)  –1.4 ± 0.7**  –0.2 ± 1.1  –0.4 ± 0.8  0.004*  –0.3 ± 1.1  –0.7 ± 1.4  –0.9 ± 0.8  0.443  –1.7 ± 1.2**  –0.9 ± 1.6  –1.3 ± 1.2**  0.358  ∆Wits(mm)  –2.6 ± 1.7**  0.2 ± 1.5  0.6 ± 1.0  0.000*  1.0 ± 1.4**  0.3 ± 1.6  0.2 ± 1.2  0.665  –1.6 ± 2.5**  0.5 ± 1.1  0.8 ± 1.9  0.006*  ∆ OJ (mm)  –3.4 ± 2.2**  –0.1 ± 0.8  –0.3 ± 0.9  0.000*  0.3 ± 1.1  –0.3 ± 0.9  –0.4 ± 0.7  0.160  –3.1 ± 1.6**  –0.3 ± 1.1  –0.7 ± 1.0**  0.000*  ∆ML/NSL (°)  0.8 ± 2.2  –1.3 ± 1.5**  –0.8 ± 1.7  0.019*  –4.5 ± 3.0**  –2.1 ± 1.7**  –2.9 ± 2.0**  0.033*  –3.7 ± 3.1**  –3.8 ± 2.1**  –3.7 ± 2.6**  0.998  ∆ML/NL (°)  –0.4 ± 2.3  –1.2 ± 1.7**  –0.8 ± 2.0  0.657  –3.7 ± 2.8**  –2.2 ± 2.1**  –2.3 ± 1.7**  0.207  –4.1 ± 2.8**  –3.4 ± 2.3**  –3.1 ± 2.8**  0.625  ∆NL/NSL (°)  1.2 ± 0.8**  –0.1 ± 1.2  0.4 ± 1.1  0.012*  –0.9 ± 1.7  –0.3 ± 2.0  –0.6 ± 1.5  0.798  0.3 ± 1.6  –0.4 ± 1.3  –0.2 ± 1.6  0.798  ∆OB (mm)  –1.7 ± 1.8**  0.5 ± 1.9  –0.7 ± 2.1  0.020*  0.6 ± 1.2  –0.5 ± 1.0  0.2 ± 1.6  0.124  –1.1 ± 1.1**  0.0 ± 2.3  –0.5 ± 1.9  0.304  ∆N-Me (mm)  6.8 ± 3.9**  5.5 ± 2.0**  6.3 ± 2.1**  0.611  5.9 ± 4.0**  6.4 ± 2.4**  5.0 ± 2.8**  0.532  12.7 ± 3.6**  11.9 ± 2.4**  11.3 ± 2.7**  0.486  ∆NL-Me (mm)  4.5 ± 2.6**  3.4 ± 1.8**  3.9 ± 1.7**  0.552  4.6 ± 3.1**  4.4 ± 1.9**  3.2 ± 1.9**  0.264  9.1 ± 2.8**  7.7 ± 2.0**  7.1 ± 1.7**  0.076  ∆Ar-Go (mm)  3.5 ± 3.0**  3.6 ± 1.5**  4.0 ± 2.6**  0.882  4.6 ± 3.1**  5.8 ± 2.4**  5.4 ± 2.2**  0.015*  11.7 ± 3.0**  9.4 ± 2.1**  9.4 ± 2.1**  0.029*  One-way ANOVA was used for intergroup comparison concerning the extent of longitudinal changes between the groups: *P ≤ 0.05 (significant differences in the extent of longitudinal changes between groups). Paired t-test was used for identification of significant longitudinal changes within a group: **P ≤ 0.05 (significant longitudinal changes within group). View Large Figure 2. View largeDownload slide Longitudinal mean changes in distance ‘p’ in mm. *Significant change within group (P ≤ 0.05). Figure 2. View largeDownload slide Longitudinal mean changes in distance ‘p’ in mm. *Significant change within group (P ≤ 0.05). Figure 3. View largeDownload slide Longitudinal mean changes in distance ‘t’ in mm. *Significant change within group (P ≤ 0.05). Figure 3. View largeDownload slide Longitudinal mean changes in distance ‘t’ in mm. *Significant change within group (P ≤ 0.05). Despite some absolute differences for the values of the distances p and t between the three groups, especially at T3, the large inter-individual variations resulted in no statistically significant differences at any observation point. Nevertheless, during the post-treatment interval (T2–T3) a significant increase in the distances p and t was exclusively seen in the Herbst group (∆p: + 2.3 mm, P = 0.012, ∆t: + 3.3 mm, P = 0.010). Also the overall changes (T1–T3) were significant in the Herbst group exclusively (∆p: + 2.9 mm, P = 0.003, ∆t: + 3.5 mm, P = 0.005) (Table 1). The longitudinal dento-skeletal changes are presented in Tables 1 and 2. The initial difference in SNB angle between the Herbst group and the Class I controls did not remain significant at T3 (P = 0.069). Also the initially significant difference in the Wits appraisal between the Herbst group and the two control groups at T1 did not remain at T3, where all three groups showed similar values (2.9 versus 3.1 versus 3.4 mm, P = 0.746). This was due to a significant decrease of the Wits-appraisal that occurred exclusively in the Herbst group during T1–T2 (–2.6 mm, P = 0.000), and remained significant considering the whole period T1–T3 (–1.6 mm, P = 0.039). During the treatment period (T1–T2), the ANB angle was significantly reduced in the Herbst group only (–1.4°, P = 0.001). Although this decrease could also be noted in the Herbst group during the entire observation period (–1.7°, P = 0.002), a similar ANB decrease (–1.3°, P = 0.002) was also seen in the untreated Class II controls. The absolute value of the ANB angle at T3 still differed significantly between the Class I control group and both the Herbst group and the Class II controls (∆: 3.7°, 3.2°, P = 0.000). The Herbst group showed a significant reduction in overjet during the interval T1–T2 (–3.4 mm, P = 0.001) which remained over the entire observation period T1–T3 (–3.1 mm, P = 0.001). At T2, the only difference in overjet was between both control groups (∆: 1.2 mm, P = 0.018) whereas at T3 all three groups showed similar values for overjet (4.1 versus 3.0 versus 4.0 mm, P = 0.225). A significant difference in overbite at T3 existed only between both control groups (∆: 1.9 mm, P = 0.04). Absolute values for all vertical skeletal parameters were similar in the three groups over the entire period of observation, except for the lower posterior face height (Ar-Go) which developed a significant difference between the Herbst group and the Class II controls at T3 (∆: 3.7 mm, P = 0.05, Table 1; Figure 4). This was due to a significantly larger increase of this parameter in the Herbst group in comparison to the two control groups during T2–T3 (8.2 versus 5.8 mm, respectively, 5.4 mm, P = 0.015, Figure 4). Over the same period, the mandibular plane angle (ML/NSL) showed a significant decrease in all groups, with the extent of the decrease being significantly greater in the Herbst group compared to the controls (–4.5°versus –2.1° versus –2.9°, P = 0.033). During the treatment interval, changes in ML/NSL and NL/NSL angles differed significantly between the Herbst group and both control groups (∆ML/NSL: 0.8° versus –1.3°, respectively, –0.8°, P = 0.019, ∆NL/NSL: 1.2° versus –0.1°, respectively, 0.4°). Regarding the overall (N-Me) and lower anterior face heights (NL-Me), all groups showed similar developments with no significant differences at any of the time points (Figure 5). Figure 4. View largeDownload slide Longitudinal mean changes in lower posterior face height (Ar-Go) in mm. *Significantly higher increase than other groups (P ≤ 0.05). **Significant absolute difference between marked groups at time point (P ≤ 0.05). Figure 4. View largeDownload slide Longitudinal mean changes in lower posterior face height (Ar-Go) in mm. *Significantly higher increase than other groups (P ≤ 0.05). **Significant absolute difference between marked groups at time point (P ≤ 0.05). Figure 5. View largeDownload slide Longitudinal mean changes in lower anterior face height (NL-Me) in mm. *Significant change within group (P ≤ 0.05). Figure 5. View largeDownload slide Longitudinal mean changes in lower anterior face height (NL-Me) in mm. *Significant change within group (P ≤ 0.05). Discussion In this study, a significant increase of the distances p and t occurred only during the post-treatment interval and exclusively in the Herbst group, while remaining similar in both untreated groups. During the same period, posterior face height showed a significantly larger increase in the Herbst group than in both control groups, whereas anterior face height showed a similar development in all groups. The treatment of Class II malocclusions with functional appliances has been reported to have a positive effect on PA dimensions (13, 14, 19). Similar short-term results were shown for fixed functional appliances like Herbst (16, 17) or MARA appliances (18). However, to date information is lacking about the long-term stability of such changes. The long-term design of the present study required the use of lateral cephalograms, limiting the evaluation of the PA to two-dimensional measurements, because for ethical reasons 3D long-term data can neither be obtained for treated controls, nor for untreated individuals. While the superiority of analyzing airway width in a three-dimensional way using CT-imaging seems obvious, previous studies showed significant correlations between cephalometric measurements of the airway to those that were achieved by three-dimensional CT imaging (20, 21). An investigation of the reproducibility of measuring airway dimensions on lateral cephalograms found this method to be highly accurate (22). A comparison of the 3D cone-beam computed tomography measurements of OSA- to those of non-OSA patients found the smallest cross-section area to be the only significant difference between the groups, demonstrating the clinical relevance of this two-dimensional parameter (23). In a systematic review, the capability of lateral cephalograms in diagnosing posterior nasopharyngeal airways was evaluated. From a variety of landmarks and lines examined, the McNamara’s line (distance ‘p’ in the present study) was found to be consistently reliable (24). Although distance ‘t’, representing the oropharyngeal area behind the base of the tongue, was not among the examined parameters in this review, it seems safe to say that conventional lateral cephalograms are a legitimate tool for the assessment of upper airway dimensions as is further confirmed by actual current (25, 26). In the literature, airway dimension has been found to be an average of 10–12 mm for distance ‘t’ and 9–10 mm for distance ‘p’ from age 15 onwards (21, 27, 28). These values are consistent with the measurements in the present study in particular at T3. One study attempted to create reference values for the distances ‘p’ and ‘t’ at different age groups based on a large sample of lateral cephalograms of untreated individuals. The authors mention a high inter-individual variability, however, their average values were similar to the corresponding age groups of the present study (29). Comparing the longitudinal changes of ‘p’ and ‘t’ during the treatment interval (T1–T2), our results showed no significant changes in any of the groups. At first sight, this finding seems contradictory for the Herbst group, because most comparable studies indicate a significant short-term improvement in PA width after functional Class II treatment (13, 14, 16–18). First of all, some of the studies showing dimensional increases were using different two-dimensional parameters for PA dimensions (13, 16). From a wide range of possible two-dimensional parameters for PA dimensions, only the distances ‘p’ and ‘t’ were chosen for our study because of the ease of their identification as well as their central role in airflow dynamics, being the most constricted sites in the critical area. Furthermore, it must be considered that in the present study the data for T2 were taken about 1.3 years after removal of the Herbst appliance including a phase of occlusal settling, while one of the mentioned studies used data from immediately after functional treatment and thus examined a shorter treatment interval (14). Nevertheless, three-dimensional studies found significant enlargements of PA dimension after similar treatment periods to the present investigation (17, 18). During the post-treatment period, reaching from average ages 14.2 to 20.2 years, both parameters representing the PA showed a significant increase in the Herbst group, whereas they remained similar in both control groups. During the same period, the only skeletal measures that showed significant differences between the Herbst group and the controls were the mandibular plane angle (ML/NSL) and the lower posterior face height (Ar-Go). While the first showed a significant decrease in all groups, the extent of the decrease was significantly higher in the Herbst group compared to the controls. On the contrary, the lower posterior face height increased to a significantly greater extent in the Herbst group in comparison to the controls. This finding might suggest a long-term modification of the post-treatment growth pattern towards a more pronounced counter-clockwise rotation of the mandible, induced by treatment with the Herbst appliance. A maxillo-mandibular advancement (MMA) has been reported to be the most effective surgical treatment for OSA (30–32). In bimaxillary orthognatic surgery, the traditional and predominant operational sequence starts with the anterior replacement of the maxilla followed by adjusting the position of the mandible whereas the alternative approach, the ‘Mandible-First’ sequence is applied less frequently (33). However, newer efforts are reported, modifying the technique in order to minimize the risk of bimaxillary prognathism that occurs after a classical MMA in OSA patients whose jaws are orthognathic or even prognathic before treatment (34). These modifications set a focus on a counter-clockwise rotation of the maxillo-mandibular complex instead of a sheer sagittal advancement (35, 36). The counter-clockwise rotation of the mandible leads to a larger advancement of the chin in comparison with the teeth. Thereby, the genial tubercles, e.g. the points of origin of geniohyoid-, suprahyoid-, and genioglossal muscles, are pulled further forward than would occur with traditional sagittal advancement of the jaws, thus maximizing the forward movement of the hyoid bone, the base of the tongue and the associated soft tissues, which in turn leads to an increase of the PA space (35). The post-treatment changes of ML/NSL and Ar-Go in the present Herbst patients might suggest a growth modification towards a comparable pattern. Another possible cause for the long-term widening of the PA could be Herbst-induced morphologic changes of the mandible. A tendency towards an increased posterior mandibular height in Herbst patients compared to untreated controls has been found in a long-term study (37). This was due to increased bone apposition at the lower border of the mandible in the gonial area that did not occur in the control group. The authors explained this reaction of the mandible with an increased activity of the M. masseter, as had been described in earlier studies (38–40). As a result of such a morphologic change, the point of insertion of the medial pterygoid muscle (i.e. the medial surface of the mandibular margin in the area of the gonial angle) is also moved slightly caudally. Originating at the lateral pterygoid plate, this muscle spans through the oral cavity, thus deliminating the space for the tongue and oral soft tissues. A caudal movement of its insertion point could lead to a decrease of this space, forcing the tongue into a slightly upward and forward position and thus widening the airway space at the level of the distance ‘t’. One could argue that the significant group differences during the post-treatment phase from 14 to 20 years of age are likely to be caused by the general difference in body height and weight between the Herbst group and the controls, because Swedish men are on average taller than American men. This possible bias could be minimized, because from the wide range of different races and ethnicities within the USA, the control groups in our study consisted only of Caucasian American individuals that are ethnically comparable to Swedish Individuals. Nevertheless, differences due to secular height trends cannot completely be ruled out. Considering the impact of body weight (BMI) on airway size, snoring and sleep apnea, it could be argued that the current prevalence of obesity is lower in Sweden than in the USA (22.5% of males in Sweden, 32.6% in the USA) (41) and therefore this could have led to a bias in airway measurements between the Herbst group and the control groups. This can be ruled out considering the prevalence of obesity in the USA between 1960 and 1962 of only 13.4% (approximately the time corresponding to the middle of the T3 period from our American control groups) (42) which is not likely to be greater than that of the Swedish Herbst group. For ethical reasons, a matched, untreated Swedish group with long-term cephalograms is and will not be available as far as to be forseen. Limitations of the study are the relatively low sample size as well as the great inter-individual variance in airway measurements (29). Also the fact that different states of the respiratory circle during radiography have an effect on airway measurements and the actual status during which the cephalograms were taken is unknown is a limiting factor. Furthermore, information on possible adenoidectomy is lacking for all groups. Further studies will be needed to confirm the long-time influence of Herbst treatment on the vertical skeletal parameters and its possible effects on PA width. Conclusion In the long term, Herbst treatment resulted in a significant post-treatment increase of PA width, possibly due to a more pronounced lower posterior facial height development compared to untreated individuals. Supplementary material Supplementary material is available at European Journal of Orthodontics online. Conflict of Interest statement None to declare. Acknowledgements We thank the American Association of Orthodontists Foundation (AAOF) for providing the highly valuable Legacy Collection of historic cephalograms. 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Published by Oxford University Press on behalf of the European Orthodontic Society. All rights reserved. For permissions, please email: journals.permissions@oup.com http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png The European Journal of Orthodontics Oxford University Press

Long-term effects of Class II Herbst treatment on the pharyngeal airway width

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© The Author(s) 2017. Published by Oxford University Press on behalf of the European Orthodontic Society. All rights reserved. For permissions, please email: journals.permissions@oup.com
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10.1093/ejo/cjx032
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Abstract

Summary Objective The aim was to assess the long-term effects of Class II malocclusion treatment with the Herbst appliance on the pharyngeal airway (PA) width in comparison to untreated individuals with Classes I and II malocclusion. Methods Lateral cephalometric radiographs of 13 male Class II patients from before (T1) and after (T2) treatment with the Herbst appliance as well as after the end of growth (T3) were retrospectively analyzed and compared to two untreated age- and gender-matched samples with Class I (n = 13) or Class II (n = 13) malocclusion. The PA dimensions were measured using the parameters p (narrowest distance between the soft palate and the posterior pharyngeal wall) and t (narrowest distance between the base of the tongue and the posterior pharyngeal wall). In addition, standard cephalometric measurements were performed. Results Relevant changes in PA dimensions were only seen for the post-treatment period, during which the distances p and t showed a significant increase in the Herbst group only (∆p: 2.3 mm, ∆t: 3.3 mm) while remaining similar in both untreated groups (∆p: 0.5 mm, ∆t: 0.5 mm, respectively, ∆p: 0.7 mm, ∆t: 1.6 mm). During the same period, posterior face height showed a significantly larger increase in the Herbst group than in both control groups (8.2 versus 5.8 mm, respectively, 5.4 mm), whereas anterior face height (NL-Me) showed a similar development in all groups (4.6 versus 4.4 mm, respectively 3.2 mm). Conclusion In the long term, Herbst treatment resulted in a significant post-treatment increase of PA width, possibly due to an increased lower posterior facial height development compared to untreated individuals. Introduction During the past years, growing interest has evolved regarding the influence of conservative and surgical mandibular advancement procedures on pharyngeal airway (PA) dimensions for addressing sleep disordered breathing (SDB) (1, 2). SDB is a collective term referring to several symptoms associated with constricted upper airway dimensions, ranging from snoring to obstructive sleep apnea (OSA) (3–5). SDB can be categorized into pediatric or adult variants (6). While the first are often associated with large tonsils or adenoids but also craniofacial syndromes (7), a great risk factor for the latter is constriction of upper airways due to obesity (6, 8). In general, there are reported correlations between OSA and increased morbidity, fatigue, disturbed growth in children, deviant behavior, hypertension, bruxism, and headache (9, 10). SDB and OSA are associated with larger tongues as well as both reduced oropharyngeal areas and anteroposterior dimensions (11, 12). Removable functional appliances such as Fränkel-1, Activator or Bionator have been shown to increase oropharyngeal dimensions during the treatment period by advancing the mandible and thus moving the hyoid bone, tongue, and soft palate forward (13, 14). The positive effect on PA dimensions using headgear activator can be maintained at least until age 22 (15). For fixed functional appliances, there has been found a short-term increase in PA width when a Herbst appliance and rapid maxillary expansion were combined (16). A 3D-cone-beam computed tomography analysis demonstrated that during the treatment period the increase in PA volume was significantly higher in Class II patients treated with a Herbst appliance than in Class I patients that received edgewise treatment only (17). Similar results were reported for treatments with a mandibular anterior repositioning appliance (MARA) (18). However, the present data do not provide any information about possible long-term changes in the PA achieved by fixed functional appliance treatment for Class II correction. Therefore, the aim of this study was to evaluate the changes in PA width from the beginning of treatment until the end of growth in Class II patients treated with a Herbst appliance in comparison to two historic untreated age- and gender-matched control groups with Classes I or II malocclusion. The null hypothesis was that treatment with a Herbst appliance would not lead to different changes in upper airway dimensions compared to untreated controls. Materials and methods Ethical approval for this study was granted by the Ethic committee of the Medical faculty of the University of Giessen (No. 126/15). Subjects The subjects of the treatment group (n = 13) were selected from the archives at the Department of Orthodontics at the University of Malmö, Sweden. The sample size of this explorative study was predefined by the availability of long-term lateral cephalograms, therefore no sample size calculation was performed. The group consisted of male Class II patients that had been treated with the Herbst appliance between 1977 and 1982. The intervals T1–T2 included a phase of active treatment with the Herbst appliance (0.5 years on average) as well as a subsequent passive phase of occlusal settling without any appliance (1.3 years on average) except for two persons in whom four premolars were extracted and a fixed appliance was used to close the extraction spaces. During active treatment, the Herbst appliance was initially activated to an edge to edge incisor relationship. After active treatment, retention appliances were used in nine patients: three patients were provided with a passive activator for 2 years, while six got a combination of cuspid retainers and removable plates. Lateral cephalograms of these patients were present from before Herbst treatment (T1, mean age: 12.4 years), on average 1.3 years after Herbst treatment (T2, mean age: 14.2 years) and from the end of the long-term follow up with the patient being at least 18 years of age (T3, mean age: 20.2 years). For the historic control groups, the lateral cephalograms of 26 white Northern American males were derived with permission from the ‘Oregon Growth Study’ and the ‘Denver Growth Study’ through the AAOF Legacy Collection (www.aaoflegacycollection.org). All lateral cephalograms had been taken in an upright standing position in habitual occlusion. The state of the respiratory circle during radiography, however, could not be found out retrospectively. All lateral cephalograms of the Herbst group were taken between 1977 and 1988, those of the Class I control group were taken between 1935 and 1977 and those of the Class II control group were taken between 1935 and 1972 (Supplementary Table). Inclusion criteria for both control groups were no history of past orthodontic treatment and the presence of three lateral cephalograms of good quality at similar age to the patients of the treatment group for T1, T2, and T3, respectively. Additionally, the Class I subjects (n = 13) had to present a Class I molar relationship (according to the data given in the AAOF Legacy collection) plus an ANB angle between 0° and 3° at T1. The Class II controls (n = 13) had to have a Class II molar relationship plus an ANB angle > 4. Unfortunately, the AAOF Legacy collection does not provide information about the severity of the Class II molar relationship. Method The radiographs of the treatment group were scanned at 300 dpi (Dual Lens System V750 Pro, Epson, Nagano, Japan). The cephalograms of the control groups were delivered from the AAOF Legacy Collection in digital form at 300 dpi. The resulting 117 digitized lateral cephalograms were traced and measured using the software Ivoris for Windows, version 8.1.22 (Computer Konkret AG, Falkenstein, Germany). To enable the comparability among the groups, a correction for linear radiographic magnification was performed for all images, according to the settings of the respective X-Ray unit, respectively the instructions provided by the AAOF Legacy Collection. The measured angles and distances are shown in Figure 1. Figure 1. View largeDownload slide Cephalometric reference points, lines, and measurements used for the assessment of (a) pharyngeal airway: p (narrowest distance between the soft palate and the posterior pharyngeal wall), t (narrowest distance between the base of the tongue and the posterior pharyngeal wall) (b) sagittal jaw base relationship: SNA, SNB, ANB, Wits, overjet (OJ), and (c) vertical jaw base relationship: ML/NSL, NL/NSL, ML/NL, Ar-Go, N-Me, NL-Me, overbite (OB). Figure 1. View largeDownload slide Cephalometric reference points, lines, and measurements used for the assessment of (a) pharyngeal airway: p (narrowest distance between the soft palate and the posterior pharyngeal wall), t (narrowest distance between the base of the tongue and the posterior pharyngeal wall) (b) sagittal jaw base relationship: SNA, SNB, ANB, Wits, overjet (OJ), and (c) vertical jaw base relationship: ML/NSL, NL/NSL, ML/NL, Ar-Go, N-Me, NL-Me, overbite (OB). Statistical method All statistical analyses were performed with MS Excel 2007 (Microsoft, Redmond, Washington, USA) and SPSS for Windows, version 20 (IBM SPSS, Armonk, New York, USA). Means and standard deviations were calculated for descriptive analyses. The Kolmogorov–Smirnoff test was performed to determine the presence of a normal distribution. A one-way analysis of variance (ANOVA) was used to investigate differences between the three groups. Post hoc Tukey test was applied to identify the groups that were the cause for the significant differences. In case of variables without normal distribution, the non-parametric Kruskal–Wallis H-test was used instead of the ANOVA. A two-sample t-test was applied in order to demonstrate longitudinal changes within each of the groups. In case of variables without normal distribution, the non-parametric Wilcoxon test for paired samples was used instead. Results with P-values ≤ 0.05 were considered statistically significant. Method error All measurements of the first cephalogram of each of the 39 patients were repeated 12 weeks after the first tracing by the same author (CD). To assess the reproducibility of the measurements, Cronbach’s reliability test was applied. Cronbach’s alpha coefficients for intra-class correlation were found to be within a range of 0.938–0.995 with the minimum value of 0.938 for overbite. Values for airway parameters were 0.976 for p and 0.991 for t, indicating a very good repeatability of the measurements. Results The cephalometric variables of the Herbst- and control groups from the three time points are given in Table 1. At T1, the values for the PA width for the given airway parameters were similar in all three groups. The dento-skeletal characteristics of the three groups differed in accordance with the selection criteria. The Wits appraisal was significantly greater in the Herbst group than in both control groups (4.5 versus 2.7 versus 2.7 mm, P = 0.05). The SNB angle was significantly smaller in the Herbst group than in the Class I controls at T1 (75.7 versus 78.7°, P = 0.05) but not different to the Class II controls. Correspondingly, the ANB angle was largest in the Herbst group, followed by the Class II controls and smallest in the Class I controls (∆ANB: 3.6° and 4.5°, P = 0.000). Both the overjet (P = 0.000) and the overbite (P = 0.001) were largest in the Herbst and smallest in the Class I controls. The vertical jaw base relationship did not differ between the groups. Table 1. Means, standard deviations, and P-values (one-way ANOVA) comparing the Herbst group and the controls at the three time points. n  T1  T2  T3  Herbst  Class I  Class II    Herbst  Class I  Class II    Herbst  Class I  Class II    13  13  13  P  13  13  13  P  13  13  13  P  Age (years)  12.4 ± 0.9  12.3 ± 0.6  12.1 ± 0.5  0.711  14.2 ± 1.2  14.3 ± 0.7  14.2 ± 0.6  0.865  20.2 ± 1.0  19.8 ± 2.0  19.8 ± 2.3  0.827  p (mm)  7.5 ± 1.8  8.0 ± 1.7  7.7 ± 1.9  0.818  8.1 ± 1.2  8.5 ± 2.3  8.5 ± 2.4  0.836  10.3 ± 2.9  9.0 ± 2.6  9.1 ± 2.1  0.335  t (mm)  9.4 ± 2.1  9.6 ± 2.4  8.7 ± 3.6  0.698  9.5 ± 2.9  10.9 ± 4.4  9.0 ± 3.7  0.405  12.9 ± 2.9  11.4 ± 5.0  10.6 ± 3.8  0.224  SNA (°)  82.1 ± 3.6  80.5 ± 3.1  81.9 ± 3.6  0.447  81.0 ± 3.7  81.5 ± 3.1  82.1 ± 4.0  0.729  82.3 ± 4.0  81.6 ± 3.5  82.6 ± 3.9  0.804  SNB (°)  75.7 ± 3.3  78.7 ± 3.0  76.3 ± 3.0  0.052*  76.0 ± 3.8  79.8 ± 2.9  77.1 ± 3.3  0.020*  77.6 ± 3.8  80.7 ± 2.7  78.4 ± 3.7  0.069  ANB (°)  6.4 ± 1.8  1.9 ± 1.1  5.5 ± 1.4  0.000*  5.0 ± 1.4  1.7 ± 1.4  5.1 ± 1.5  0.000*  4.7 ± 1.5  1.0 ± 1.8  4.2 ± 1.7  0.000*  Wits(mm)  4.5 ± 2.5  2.7 ± 1.9  2.7 ± 1.7  0.051*  1.8 ± 1.1  2.8 ± 1.8  3.3 ± 1.9  0.090  2.9 ± 1.7  3.1 ± 2.0  3.4 ± 1.9  0.746  OJ (mm)  7.1 ± 1.6  3.3 ± 1.2  4.7 ± 1.1  0.000*  3.8 ± 1.1  3.2 ± 1.0  4.4 ± 0.8  0.018*  4.1 ± 1.0  3.0 ± 1.5  4.0 ± 1.0  0.225  ML/NSL (°)  31.1 ± 5.8  32.4 ± 5.4  32.1 ± 3.8  0.780  31.9 ± 6.7  31.2 ± 5.3  31.4 ± 4.4  0.940  27.4 ± 6.8  29.0 ± 5.2  28.4 ± 5.5  0.770  ML/NL (°)  23.6 ± 4.8  25.6 ± 5.6  24.7 ± 4.2  0.567  23.1 ± 5.8  24.5 ± 5.2  23.9 ± 5.2  0.819  19.5 ± 5.5  22.3 ± 5.2  21.6 ± 6.1  0.428  NL/NSL (°)  7.6 ± 2.6  6.8 ± 2.0  7.1 ± 3.5  0.777  8.8 ± 2.8  6.7 ± 2.8  7.5 ± 4.0  0.276  7.9 ± 3.4  6.4 ± 1.8  6.9 ± 3.7  0.461  OB (mm)  4.5 ± 1.5  2.3 ± 2.1  4.7 ± 1.1  0.001*  2.8 ± 1.2  2.8 ± 1.7  4.0 ± 2.3  0.160  3.4 ± 0.9  2.3 ± 2.3  4.2 ± 1.9  0.042*  N-Me (mm)  106.6 ± 4.5  104.5 ± 4.8  104.4 ± 4.1  0.356  113.5 ± 7.6  110.0 ± 5.9  110.6 ± 5.5  0.356  119.3 ± 6.2  115.8 ± 5.9  115.7 ± 6.0  0.225  NL-Me (mm)  56.8 ± 2.6  57.1 ± 3.9  56.1 ± 3.3  0.717  61.3 ± 4.7  60.4 ± 4.9  60.0 ± 4.4  0.759  65.9 ± 4.0  64.8 ± 4.6  63.1 ± 4.8  0.299  Ar-Go (mm)  41.3 ± 3.3  41.1 ± 4.0  40.0 ± 2.3  0.523  44.8 ± 5.0  44.7 ± 4.0  43.9 ± 3.1  0.826  53.0 ± 5.0  50.5 ± 3.6  49.3 ± 2.4  0.054*  n  T1  T2  T3  Herbst  Class I  Class II    Herbst  Class I  Class II    Herbst  Class I  Class II    13  13  13  P  13  13  13  P  13  13  13  P  Age (years)  12.4 ± 0.9  12.3 ± 0.6  12.1 ± 0.5  0.711  14.2 ± 1.2  14.3 ± 0.7  14.2 ± 0.6  0.865  20.2 ± 1.0  19.8 ± 2.0  19.8 ± 2.3  0.827  p (mm)  7.5 ± 1.8  8.0 ± 1.7  7.7 ± 1.9  0.818  8.1 ± 1.2  8.5 ± 2.3  8.5 ± 2.4  0.836  10.3 ± 2.9  9.0 ± 2.6  9.1 ± 2.1  0.335  t (mm)  9.4 ± 2.1  9.6 ± 2.4  8.7 ± 3.6  0.698  9.5 ± 2.9  10.9 ± 4.4  9.0 ± 3.7  0.405  12.9 ± 2.9  11.4 ± 5.0  10.6 ± 3.8  0.224  SNA (°)  82.1 ± 3.6  80.5 ± 3.1  81.9 ± 3.6  0.447  81.0 ± 3.7  81.5 ± 3.1  82.1 ± 4.0  0.729  82.3 ± 4.0  81.6 ± 3.5  82.6 ± 3.9  0.804  SNB (°)  75.7 ± 3.3  78.7 ± 3.0  76.3 ± 3.0  0.052*  76.0 ± 3.8  79.8 ± 2.9  77.1 ± 3.3  0.020*  77.6 ± 3.8  80.7 ± 2.7  78.4 ± 3.7  0.069  ANB (°)  6.4 ± 1.8  1.9 ± 1.1  5.5 ± 1.4  0.000*  5.0 ± 1.4  1.7 ± 1.4  5.1 ± 1.5  0.000*  4.7 ± 1.5  1.0 ± 1.8  4.2 ± 1.7  0.000*  Wits(mm)  4.5 ± 2.5  2.7 ± 1.9  2.7 ± 1.7  0.051*  1.8 ± 1.1  2.8 ± 1.8  3.3 ± 1.9  0.090  2.9 ± 1.7  3.1 ± 2.0  3.4 ± 1.9  0.746  OJ (mm)  7.1 ± 1.6  3.3 ± 1.2  4.7 ± 1.1  0.000*  3.8 ± 1.1  3.2 ± 1.0  4.4 ± 0.8  0.018*  4.1 ± 1.0  3.0 ± 1.5  4.0 ± 1.0  0.225  ML/NSL (°)  31.1 ± 5.8  32.4 ± 5.4  32.1 ± 3.8  0.780  31.9 ± 6.7  31.2 ± 5.3  31.4 ± 4.4  0.940  27.4 ± 6.8  29.0 ± 5.2  28.4 ± 5.5  0.770  ML/NL (°)  23.6 ± 4.8  25.6 ± 5.6  24.7 ± 4.2  0.567  23.1 ± 5.8  24.5 ± 5.2  23.9 ± 5.2  0.819  19.5 ± 5.5  22.3 ± 5.2  21.6 ± 6.1  0.428  NL/NSL (°)  7.6 ± 2.6  6.8 ± 2.0  7.1 ± 3.5  0.777  8.8 ± 2.8  6.7 ± 2.8  7.5 ± 4.0  0.276  7.9 ± 3.4  6.4 ± 1.8  6.9 ± 3.7  0.461  OB (mm)  4.5 ± 1.5  2.3 ± 2.1  4.7 ± 1.1  0.001*  2.8 ± 1.2  2.8 ± 1.7  4.0 ± 2.3  0.160  3.4 ± 0.9  2.3 ± 2.3  4.2 ± 1.9  0.042*  N-Me (mm)  106.6 ± 4.5  104.5 ± 4.8  104.4 ± 4.1  0.356  113.5 ± 7.6  110.0 ± 5.9  110.6 ± 5.5  0.356  119.3 ± 6.2  115.8 ± 5.9  115.7 ± 6.0  0.225  NL-Me (mm)  56.8 ± 2.6  57.1 ± 3.9  56.1 ± 3.3  0.717  61.3 ± 4.7  60.4 ± 4.9  60.0 ± 4.4  0.759  65.9 ± 4.0  64.8 ± 4.6  63.1 ± 4.8  0.299  Ar-Go (mm)  41.3 ± 3.3  41.1 ± 4.0  40.0 ± 2.3  0.523  44.8 ± 5.0  44.7 ± 4.0  43.9 ± 3.1  0.826  53.0 ± 5.0  50.5 ± 3.6  49.3 ± 2.4  0.054*  *P ≤ 0.05. View Large The longitudinal changes of the pharyngeal dimension are shown in Table 2 and Figures 2 and 3. Table 2. Means, standard deviations, and P-values (one-way ANOVA) for longitudinal changes in the Herbst- and both control groups. n  T1–T2 (treatment)  T2–T3 (post-treatment)  T1–T3 (total)  Herbst  Class I  Class II    Herbst  Class I  Class II    Herbst  Class I  Class II    13  13  13  P  13  13  13  P  13  13  13  P  Age (years)  1.8 ± 0.6  2.1 ± 0.2  2.0 ± 0.2    6.0 ± 0.8  5.5 ± 2.1  5.7 ± 2.2    7.8 ± 0.5  7.5 ± 2.1  7.7 ± 2.1    ∆p (mm)  0.6 ± 2.0  0.5 ± 1.5  0.8 ± 2.2  0.837  2.3 ± 2.8**  0.5 ± 2.3  0.7 ± 1.8  0.112  2.9 ± 2.8**  1.0 ± 2.8  1.5 ± 2.0  0.172  ∆t (mm)  0.2 ± 3.2  1.3 ± 3.5  0.2 ± 3.7  0.639  3.3 ± 3.9**  0.5 ± 5.9  1.6 ± 3.9  0.309  3.5 ± 3.7**  1.8 ± 4.9  1.8 ± 4.5  0.536  ∆ SNA (°)  –1.1 ± 1.1**  0.9 ± 1.2**  0.3 ± 0.9  0.000*  1.2 ± 1.7**  0.2 ± 1.7  0.5 ± 1.6  0.250  0.1 ± 1.7  1.1 ± 1.5**  0.7 ± 1.7  0.297  ∆ SNB (°)  0.2 ± 1.1  1.1 ± 1.0**  0.7 ± 0.9**  0.115  1.5 ± 1.5**  0.9 ± 1.0**  1.3 ± 1.6**  0.514  1.7 ± 1.7**  2.0 ± 1.0**  2.0 ± 1.9**  0.878  ∆ ANB (°)  –1.4 ± 0.7**  –0.2 ± 1.1  –0.4 ± 0.8  0.004*  –0.3 ± 1.1  –0.7 ± 1.4  –0.9 ± 0.8  0.443  –1.7 ± 1.2**  –0.9 ± 1.6  –1.3 ± 1.2**  0.358  ∆Wits(mm)  –2.6 ± 1.7**  0.2 ± 1.5  0.6 ± 1.0  0.000*  1.0 ± 1.4**  0.3 ± 1.6  0.2 ± 1.2  0.665  –1.6 ± 2.5**  0.5 ± 1.1  0.8 ± 1.9  0.006*  ∆ OJ (mm)  –3.4 ± 2.2**  –0.1 ± 0.8  –0.3 ± 0.9  0.000*  0.3 ± 1.1  –0.3 ± 0.9  –0.4 ± 0.7  0.160  –3.1 ± 1.6**  –0.3 ± 1.1  –0.7 ± 1.0**  0.000*  ∆ML/NSL (°)  0.8 ± 2.2  –1.3 ± 1.5**  –0.8 ± 1.7  0.019*  –4.5 ± 3.0**  –2.1 ± 1.7**  –2.9 ± 2.0**  0.033*  –3.7 ± 3.1**  –3.8 ± 2.1**  –3.7 ± 2.6**  0.998  ∆ML/NL (°)  –0.4 ± 2.3  –1.2 ± 1.7**  –0.8 ± 2.0  0.657  –3.7 ± 2.8**  –2.2 ± 2.1**  –2.3 ± 1.7**  0.207  –4.1 ± 2.8**  –3.4 ± 2.3**  –3.1 ± 2.8**  0.625  ∆NL/NSL (°)  1.2 ± 0.8**  –0.1 ± 1.2  0.4 ± 1.1  0.012*  –0.9 ± 1.7  –0.3 ± 2.0  –0.6 ± 1.5  0.798  0.3 ± 1.6  –0.4 ± 1.3  –0.2 ± 1.6  0.798  ∆OB (mm)  –1.7 ± 1.8**  0.5 ± 1.9  –0.7 ± 2.1  0.020*  0.6 ± 1.2  –0.5 ± 1.0  0.2 ± 1.6  0.124  –1.1 ± 1.1**  0.0 ± 2.3  –0.5 ± 1.9  0.304  ∆N-Me (mm)  6.8 ± 3.9**  5.5 ± 2.0**  6.3 ± 2.1**  0.611  5.9 ± 4.0**  6.4 ± 2.4**  5.0 ± 2.8**  0.532  12.7 ± 3.6**  11.9 ± 2.4**  11.3 ± 2.7**  0.486  ∆NL-Me (mm)  4.5 ± 2.6**  3.4 ± 1.8**  3.9 ± 1.7**  0.552  4.6 ± 3.1**  4.4 ± 1.9**  3.2 ± 1.9**  0.264  9.1 ± 2.8**  7.7 ± 2.0**  7.1 ± 1.7**  0.076  ∆Ar-Go (mm)  3.5 ± 3.0**  3.6 ± 1.5**  4.0 ± 2.6**  0.882  4.6 ± 3.1**  5.8 ± 2.4**  5.4 ± 2.2**  0.015*  11.7 ± 3.0**  9.4 ± 2.1**  9.4 ± 2.1**  0.029*  n  T1–T2 (treatment)  T2–T3 (post-treatment)  T1–T3 (total)  Herbst  Class I  Class II    Herbst  Class I  Class II    Herbst  Class I  Class II    13  13  13  P  13  13  13  P  13  13  13  P  Age (years)  1.8 ± 0.6  2.1 ± 0.2  2.0 ± 0.2    6.0 ± 0.8  5.5 ± 2.1  5.7 ± 2.2    7.8 ± 0.5  7.5 ± 2.1  7.7 ± 2.1    ∆p (mm)  0.6 ± 2.0  0.5 ± 1.5  0.8 ± 2.2  0.837  2.3 ± 2.8**  0.5 ± 2.3  0.7 ± 1.8  0.112  2.9 ± 2.8**  1.0 ± 2.8  1.5 ± 2.0  0.172  ∆t (mm)  0.2 ± 3.2  1.3 ± 3.5  0.2 ± 3.7  0.639  3.3 ± 3.9**  0.5 ± 5.9  1.6 ± 3.9  0.309  3.5 ± 3.7**  1.8 ± 4.9  1.8 ± 4.5  0.536  ∆ SNA (°)  –1.1 ± 1.1**  0.9 ± 1.2**  0.3 ± 0.9  0.000*  1.2 ± 1.7**  0.2 ± 1.7  0.5 ± 1.6  0.250  0.1 ± 1.7  1.1 ± 1.5**  0.7 ± 1.7  0.297  ∆ SNB (°)  0.2 ± 1.1  1.1 ± 1.0**  0.7 ± 0.9**  0.115  1.5 ± 1.5**  0.9 ± 1.0**  1.3 ± 1.6**  0.514  1.7 ± 1.7**  2.0 ± 1.0**  2.0 ± 1.9**  0.878  ∆ ANB (°)  –1.4 ± 0.7**  –0.2 ± 1.1  –0.4 ± 0.8  0.004*  –0.3 ± 1.1  –0.7 ± 1.4  –0.9 ± 0.8  0.443  –1.7 ± 1.2**  –0.9 ± 1.6  –1.3 ± 1.2**  0.358  ∆Wits(mm)  –2.6 ± 1.7**  0.2 ± 1.5  0.6 ± 1.0  0.000*  1.0 ± 1.4**  0.3 ± 1.6  0.2 ± 1.2  0.665  –1.6 ± 2.5**  0.5 ± 1.1  0.8 ± 1.9  0.006*  ∆ OJ (mm)  –3.4 ± 2.2**  –0.1 ± 0.8  –0.3 ± 0.9  0.000*  0.3 ± 1.1  –0.3 ± 0.9  –0.4 ± 0.7  0.160  –3.1 ± 1.6**  –0.3 ± 1.1  –0.7 ± 1.0**  0.000*  ∆ML/NSL (°)  0.8 ± 2.2  –1.3 ± 1.5**  –0.8 ± 1.7  0.019*  –4.5 ± 3.0**  –2.1 ± 1.7**  –2.9 ± 2.0**  0.033*  –3.7 ± 3.1**  –3.8 ± 2.1**  –3.7 ± 2.6**  0.998  ∆ML/NL (°)  –0.4 ± 2.3  –1.2 ± 1.7**  –0.8 ± 2.0  0.657  –3.7 ± 2.8**  –2.2 ± 2.1**  –2.3 ± 1.7**  0.207  –4.1 ± 2.8**  –3.4 ± 2.3**  –3.1 ± 2.8**  0.625  ∆NL/NSL (°)  1.2 ± 0.8**  –0.1 ± 1.2  0.4 ± 1.1  0.012*  –0.9 ± 1.7  –0.3 ± 2.0  –0.6 ± 1.5  0.798  0.3 ± 1.6  –0.4 ± 1.3  –0.2 ± 1.6  0.798  ∆OB (mm)  –1.7 ± 1.8**  0.5 ± 1.9  –0.7 ± 2.1  0.020*  0.6 ± 1.2  –0.5 ± 1.0  0.2 ± 1.6  0.124  –1.1 ± 1.1**  0.0 ± 2.3  –0.5 ± 1.9  0.304  ∆N-Me (mm)  6.8 ± 3.9**  5.5 ± 2.0**  6.3 ± 2.1**  0.611  5.9 ± 4.0**  6.4 ± 2.4**  5.0 ± 2.8**  0.532  12.7 ± 3.6**  11.9 ± 2.4**  11.3 ± 2.7**  0.486  ∆NL-Me (mm)  4.5 ± 2.6**  3.4 ± 1.8**  3.9 ± 1.7**  0.552  4.6 ± 3.1**  4.4 ± 1.9**  3.2 ± 1.9**  0.264  9.1 ± 2.8**  7.7 ± 2.0**  7.1 ± 1.7**  0.076  ∆Ar-Go (mm)  3.5 ± 3.0**  3.6 ± 1.5**  4.0 ± 2.6**  0.882  4.6 ± 3.1**  5.8 ± 2.4**  5.4 ± 2.2**  0.015*  11.7 ± 3.0**  9.4 ± 2.1**  9.4 ± 2.1**  0.029*  One-way ANOVA was used for intergroup comparison concerning the extent of longitudinal changes between the groups: *P ≤ 0.05 (significant differences in the extent of longitudinal changes between groups). Paired t-test was used for identification of significant longitudinal changes within a group: **P ≤ 0.05 (significant longitudinal changes within group). View Large Figure 2. View largeDownload slide Longitudinal mean changes in distance ‘p’ in mm. *Significant change within group (P ≤ 0.05). Figure 2. View largeDownload slide Longitudinal mean changes in distance ‘p’ in mm. *Significant change within group (P ≤ 0.05). Figure 3. View largeDownload slide Longitudinal mean changes in distance ‘t’ in mm. *Significant change within group (P ≤ 0.05). Figure 3. View largeDownload slide Longitudinal mean changes in distance ‘t’ in mm. *Significant change within group (P ≤ 0.05). Despite some absolute differences for the values of the distances p and t between the three groups, especially at T3, the large inter-individual variations resulted in no statistically significant differences at any observation point. Nevertheless, during the post-treatment interval (T2–T3) a significant increase in the distances p and t was exclusively seen in the Herbst group (∆p: + 2.3 mm, P = 0.012, ∆t: + 3.3 mm, P = 0.010). Also the overall changes (T1–T3) were significant in the Herbst group exclusively (∆p: + 2.9 mm, P = 0.003, ∆t: + 3.5 mm, P = 0.005) (Table 1). The longitudinal dento-skeletal changes are presented in Tables 1 and 2. The initial difference in SNB angle between the Herbst group and the Class I controls did not remain significant at T3 (P = 0.069). Also the initially significant difference in the Wits appraisal between the Herbst group and the two control groups at T1 did not remain at T3, where all three groups showed similar values (2.9 versus 3.1 versus 3.4 mm, P = 0.746). This was due to a significant decrease of the Wits-appraisal that occurred exclusively in the Herbst group during T1–T2 (–2.6 mm, P = 0.000), and remained significant considering the whole period T1–T3 (–1.6 mm, P = 0.039). During the treatment period (T1–T2), the ANB angle was significantly reduced in the Herbst group only (–1.4°, P = 0.001). Although this decrease could also be noted in the Herbst group during the entire observation period (–1.7°, P = 0.002), a similar ANB decrease (–1.3°, P = 0.002) was also seen in the untreated Class II controls. The absolute value of the ANB angle at T3 still differed significantly between the Class I control group and both the Herbst group and the Class II controls (∆: 3.7°, 3.2°, P = 0.000). The Herbst group showed a significant reduction in overjet during the interval T1–T2 (–3.4 mm, P = 0.001) which remained over the entire observation period T1–T3 (–3.1 mm, P = 0.001). At T2, the only difference in overjet was between both control groups (∆: 1.2 mm, P = 0.018) whereas at T3 all three groups showed similar values for overjet (4.1 versus 3.0 versus 4.0 mm, P = 0.225). A significant difference in overbite at T3 existed only between both control groups (∆: 1.9 mm, P = 0.04). Absolute values for all vertical skeletal parameters were similar in the three groups over the entire period of observation, except for the lower posterior face height (Ar-Go) which developed a significant difference between the Herbst group and the Class II controls at T3 (∆: 3.7 mm, P = 0.05, Table 1; Figure 4). This was due to a significantly larger increase of this parameter in the Herbst group in comparison to the two control groups during T2–T3 (8.2 versus 5.8 mm, respectively, 5.4 mm, P = 0.015, Figure 4). Over the same period, the mandibular plane angle (ML/NSL) showed a significant decrease in all groups, with the extent of the decrease being significantly greater in the Herbst group compared to the controls (–4.5°versus –2.1° versus –2.9°, P = 0.033). During the treatment interval, changes in ML/NSL and NL/NSL angles differed significantly between the Herbst group and both control groups (∆ML/NSL: 0.8° versus –1.3°, respectively, –0.8°, P = 0.019, ∆NL/NSL: 1.2° versus –0.1°, respectively, 0.4°). Regarding the overall (N-Me) and lower anterior face heights (NL-Me), all groups showed similar developments with no significant differences at any of the time points (Figure 5). Figure 4. View largeDownload slide Longitudinal mean changes in lower posterior face height (Ar-Go) in mm. *Significantly higher increase than other groups (P ≤ 0.05). **Significant absolute difference between marked groups at time point (P ≤ 0.05). Figure 4. View largeDownload slide Longitudinal mean changes in lower posterior face height (Ar-Go) in mm. *Significantly higher increase than other groups (P ≤ 0.05). **Significant absolute difference between marked groups at time point (P ≤ 0.05). Figure 5. View largeDownload slide Longitudinal mean changes in lower anterior face height (NL-Me) in mm. *Significant change within group (P ≤ 0.05). Figure 5. View largeDownload slide Longitudinal mean changes in lower anterior face height (NL-Me) in mm. *Significant change within group (P ≤ 0.05). Discussion In this study, a significant increase of the distances p and t occurred only during the post-treatment interval and exclusively in the Herbst group, while remaining similar in both untreated groups. During the same period, posterior face height showed a significantly larger increase in the Herbst group than in both control groups, whereas anterior face height showed a similar development in all groups. The treatment of Class II malocclusions with functional appliances has been reported to have a positive effect on PA dimensions (13, 14, 19). Similar short-term results were shown for fixed functional appliances like Herbst (16, 17) or MARA appliances (18). However, to date information is lacking about the long-term stability of such changes. The long-term design of the present study required the use of lateral cephalograms, limiting the evaluation of the PA to two-dimensional measurements, because for ethical reasons 3D long-term data can neither be obtained for treated controls, nor for untreated individuals. While the superiority of analyzing airway width in a three-dimensional way using CT-imaging seems obvious, previous studies showed significant correlations between cephalometric measurements of the airway to those that were achieved by three-dimensional CT imaging (20, 21). An investigation of the reproducibility of measuring airway dimensions on lateral cephalograms found this method to be highly accurate (22). A comparison of the 3D cone-beam computed tomography measurements of OSA- to those of non-OSA patients found the smallest cross-section area to be the only significant difference between the groups, demonstrating the clinical relevance of this two-dimensional parameter (23). In a systematic review, the capability of lateral cephalograms in diagnosing posterior nasopharyngeal airways was evaluated. From a variety of landmarks and lines examined, the McNamara’s line (distance ‘p’ in the present study) was found to be consistently reliable (24). Although distance ‘t’, representing the oropharyngeal area behind the base of the tongue, was not among the examined parameters in this review, it seems safe to say that conventional lateral cephalograms are a legitimate tool for the assessment of upper airway dimensions as is further confirmed by actual current (25, 26). In the literature, airway dimension has been found to be an average of 10–12 mm for distance ‘t’ and 9–10 mm for distance ‘p’ from age 15 onwards (21, 27, 28). These values are consistent with the measurements in the present study in particular at T3. One study attempted to create reference values for the distances ‘p’ and ‘t’ at different age groups based on a large sample of lateral cephalograms of untreated individuals. The authors mention a high inter-individual variability, however, their average values were similar to the corresponding age groups of the present study (29). Comparing the longitudinal changes of ‘p’ and ‘t’ during the treatment interval (T1–T2), our results showed no significant changes in any of the groups. At first sight, this finding seems contradictory for the Herbst group, because most comparable studies indicate a significant short-term improvement in PA width after functional Class II treatment (13, 14, 16–18). First of all, some of the studies showing dimensional increases were using different two-dimensional parameters for PA dimensions (13, 16). From a wide range of possible two-dimensional parameters for PA dimensions, only the distances ‘p’ and ‘t’ were chosen for our study because of the ease of their identification as well as their central role in airflow dynamics, being the most constricted sites in the critical area. Furthermore, it must be considered that in the present study the data for T2 were taken about 1.3 years after removal of the Herbst appliance including a phase of occlusal settling, while one of the mentioned studies used data from immediately after functional treatment and thus examined a shorter treatment interval (14). Nevertheless, three-dimensional studies found significant enlargements of PA dimension after similar treatment periods to the present investigation (17, 18). During the post-treatment period, reaching from average ages 14.2 to 20.2 years, both parameters representing the PA showed a significant increase in the Herbst group, whereas they remained similar in both control groups. During the same period, the only skeletal measures that showed significant differences between the Herbst group and the controls were the mandibular plane angle (ML/NSL) and the lower posterior face height (Ar-Go). While the first showed a significant decrease in all groups, the extent of the decrease was significantly higher in the Herbst group compared to the controls. On the contrary, the lower posterior face height increased to a significantly greater extent in the Herbst group in comparison to the controls. This finding might suggest a long-term modification of the post-treatment growth pattern towards a more pronounced counter-clockwise rotation of the mandible, induced by treatment with the Herbst appliance. A maxillo-mandibular advancement (MMA) has been reported to be the most effective surgical treatment for OSA (30–32). In bimaxillary orthognatic surgery, the traditional and predominant operational sequence starts with the anterior replacement of the maxilla followed by adjusting the position of the mandible whereas the alternative approach, the ‘Mandible-First’ sequence is applied less frequently (33). However, newer efforts are reported, modifying the technique in order to minimize the risk of bimaxillary prognathism that occurs after a classical MMA in OSA patients whose jaws are orthognathic or even prognathic before treatment (34). These modifications set a focus on a counter-clockwise rotation of the maxillo-mandibular complex instead of a sheer sagittal advancement (35, 36). The counter-clockwise rotation of the mandible leads to a larger advancement of the chin in comparison with the teeth. Thereby, the genial tubercles, e.g. the points of origin of geniohyoid-, suprahyoid-, and genioglossal muscles, are pulled further forward than would occur with traditional sagittal advancement of the jaws, thus maximizing the forward movement of the hyoid bone, the base of the tongue and the associated soft tissues, which in turn leads to an increase of the PA space (35). The post-treatment changes of ML/NSL and Ar-Go in the present Herbst patients might suggest a growth modification towards a comparable pattern. Another possible cause for the long-term widening of the PA could be Herbst-induced morphologic changes of the mandible. A tendency towards an increased posterior mandibular height in Herbst patients compared to untreated controls has been found in a long-term study (37). This was due to increased bone apposition at the lower border of the mandible in the gonial area that did not occur in the control group. The authors explained this reaction of the mandible with an increased activity of the M. masseter, as had been described in earlier studies (38–40). As a result of such a morphologic change, the point of insertion of the medial pterygoid muscle (i.e. the medial surface of the mandibular margin in the area of the gonial angle) is also moved slightly caudally. Originating at the lateral pterygoid plate, this muscle spans through the oral cavity, thus deliminating the space for the tongue and oral soft tissues. A caudal movement of its insertion point could lead to a decrease of this space, forcing the tongue into a slightly upward and forward position and thus widening the airway space at the level of the distance ‘t’. One could argue that the significant group differences during the post-treatment phase from 14 to 20 years of age are likely to be caused by the general difference in body height and weight between the Herbst group and the controls, because Swedish men are on average taller than American men. This possible bias could be minimized, because from the wide range of different races and ethnicities within the USA, the control groups in our study consisted only of Caucasian American individuals that are ethnically comparable to Swedish Individuals. Nevertheless, differences due to secular height trends cannot completely be ruled out. Considering the impact of body weight (BMI) on airway size, snoring and sleep apnea, it could be argued that the current prevalence of obesity is lower in Sweden than in the USA (22.5% of males in Sweden, 32.6% in the USA) (41) and therefore this could have led to a bias in airway measurements between the Herbst group and the control groups. This can be ruled out considering the prevalence of obesity in the USA between 1960 and 1962 of only 13.4% (approximately the time corresponding to the middle of the T3 period from our American control groups) (42) which is not likely to be greater than that of the Swedish Herbst group. For ethical reasons, a matched, untreated Swedish group with long-term cephalograms is and will not be available as far as to be forseen. Limitations of the study are the relatively low sample size as well as the great inter-individual variance in airway measurements (29). Also the fact that different states of the respiratory circle during radiography have an effect on airway measurements and the actual status during which the cephalograms were taken is unknown is a limiting factor. Furthermore, information on possible adenoidectomy is lacking for all groups. Further studies will be needed to confirm the long-time influence of Herbst treatment on the vertical skeletal parameters and its possible effects on PA width. Conclusion In the long term, Herbst treatment resulted in a significant post-treatment increase of PA width, possibly due to a more pronounced lower posterior facial height development compared to untreated individuals. Supplementary material Supplementary material is available at European Journal of Orthodontics online. Conflict of Interest statement None to declare. Acknowledgements We thank the American Association of Orthodontists Foundation (AAOF) for providing the highly valuable Legacy Collection of historic cephalograms. 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Published by Oxford University Press on behalf of the European Orthodontic Society. All rights reserved. For permissions, please email: journals.permissions@oup.com

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The European Journal of OrthodonticsOxford University Press

Published: Feb 1, 2018

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