Introduction

Angle Class II, division 1 with mandibular retrognathia is the most frequent malocclusion in central Europe [8]. Orthodontic treatment usually includes removable or fixed functional appliances [7, 30]. The functional appliance treatment period is approximately 6–9 months [50]. When the functional appliance (FA) is inserted, the condyles are moved to an anterior position on the articular tubercle, which is capable of adaptation [19]. Thus, it was hypothesized that morphological changes may occur [20].

In their systematic review, Ivorra-Carbonell et al. [19] evaluated the main effects of different functional appliances on temporomandibular joints (TMJ). A total of 401 articles were identified. However, only 21 papers (1 review and 20 clinical studies) were finally included: 10 of which reported on fixed functional appliances (FFAs; [1, 17, 23, 24, 27, 28, 30, 34, 35, 50]), 10 on removable functional appliances (RFAs; [3, 4, 10, 11, 16, 20, 48, 53,54,55]) and 1 study compared the treatment effects of FFAs and RFAs [7]. Ivorra-Carbonell et al. [19] concluded from their review that treatment with FAs leads to a more advanced position the condyle, with remodelling of the condyle and adaptation of the morphology of the mandibular fossa. Still, the articles included in the review showed a lack of methodological homogeneity.

The TMJ is the body’s most complex joint [2]. Different two-dimensional (2D) and three-dimensional (3D) imaging modalities for TMJ diagnosis have been advocated, including plain films, cephalograms [33, 35, 41], cone beam computed tomography (CBCT), magnetic resonance imaging (MRI) and tomography [5, 9].

The most frequently investigated fixed functional appliance (FFA) treatment effects are those after treatment with a Herbst appliance [36] or Functional Mandibular Advancer (FMA; [26]). Numerous studies of FFA patients investigated therapy-induced effects upon the TMJ, particularly focusing on condylar position, morphology and disco-condylar relationship [27, 28, 40, 42,43,44,45,46,47]. Although some studies investigated morphological changes of the condyles using tomograms [37], little attention has been paid towards potential changes of the mandibular fossa position and shape following FFA therapy.

Ruf and Pancherz [43, 45] first suspected possible mandibular fossa remodelling in their investigation of Herbst appliance treatment effects. Only a few studies attempted to assess these morphologic changes of the condyles and fossae not only visually but also metrically. Kinzinger et al. [25] recently investigated possible changes of the mandibular fossae using MRI datasets. The anatomical structures were assessed visually and metrically in the sagittal plane.

Tomograms are suitable for certain aspects of TMJ diagnosis [12]. Compared to MRI scans, tomograms have the advantage that additional osseous structures anterior and posterior to the fossa mandibularis and the articular tubercle, e.g. porion or pterygopalatine fossa, can be used as reference points for different linear measurements [22]. Considering these structures as stable during the short period of functional treatment during growth, particularly the evaluation of positional changes of the mandibular fossa, the so-called fossa shift, seems feasible.

This study used tomograms to investigate the following:

  • if treatment with the Functional Mandibular Advancer (FMA) led to changes in (a) shape and (b) width and depth of the mandibular fossae in the sagittal plane,

  • if treatment with the FMA led to a fossa shift, i.e. changes of the topographic relation between porion, mandibular fossa, articular tubercle and pterygopalatine fossa, and

  • if including porion and pterygopalatine fossa on the tomogram measurements may usefully complement MRI measurements [25].

The investigation also tested the hypothesis that visual classification of mandibular fossa shape and different linear measurements led to results comparable to those of a previously published study using MRI for the same purpose.

Materials and methods

Patients

The study included 25 patients with a skeletal Class II malocclusion (14 females, 11 males). They were the same used for a previous study (Kinzinger et al. [25]). Mean age was 16 years (range 12.0–27.6 years) at the beginning of treatment. Average functional treatment time with the Functional Mandibular Advancer (Forestadent, Pforzheim, Germany) was 7.3 months (range 6–9.5 months). All patients were treated by one experienced orthodontist and received a single-step advancement (SSA) protocol to protrude the mandible to an edge to edge position. After treatment, all patients showed a bilateral Class I molar relationship. Tomograms were available prior to (T1) and after (T2) FMA treatment.

Further inclusion criteria were the following: complete permanent dentition without third molars, no tooth loss during treatment, no history of previous orthodontic treatment, molar relationship of at least ½ cusp width distal, and pretreatment ANB angle ≥4°. Exclusion criteria were craniofacial anomalies, congenital agenesis or permanent tooth loss, or planned extraction protocol.

A total of 50 mandibular fossae measurements on parasagittal slices of MRIs of a study by Kinzinger et al. [25] served as controls for comparison.

Visual classification of mandibular fossae

The mandibular fossa shape was always assessed ipsi- and contralaterally on tomograms at T1 and T2. According to Katsavrias [21], mandibular fossa shape was recorded as either round (A), oval (B), trapezoidal (C) or triangular (D; Fig. 1).

Fig. 1 Abb. 1
figure 1

Classification of mandibular fossa shape according to Katsavrias: round (A), oval (B), trapezoidal (C) or triangular (D; [21])

Einteilung der Formen der Fossa mandibularis nach Katsavaris: rund (A), oval (B), trapezförmig (C) bzw. dreieckig (D; [21])

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Radiographic material and metric analysis

Always two digital tomograms (DTs) taken with the same device (Orthophos®, Sirona, Bensheim, Germany) were available for each patient. Positioning of the patients within the x‑ray unit was performed according to the manufacturer’s instructions using a standard bite block [14]. The first DT (T1) was taken as part of the pretreatment diagnostics. The second DT (T2) was recorded at the end of FMA treatment.

A total of 50 DTs were evaluated, for a total of 100 TMJs at T1 and T2. Metric analysis was performed by one single blinded investigator using a dedicated software (fr-win®, version 7.0; Computer Konkret, Falkenstein, Germany). This dedicated tracing software is capable of measuring to two decimal places.

The metric analysis of the mandibular fossae was performed according to Katsavrias and Voudouris [22]. Seven reference points were traced on the DTs per TMJ (Fig. 2):

  • Porion, Po (uppermost point of auditory meatus);

  • Fossa posterior, F post (the top of postmandibular process);

  • Roof of the mandibular fossa, F roof (the highest point of the mandibular fossa);

  • Articular tubercle midpoint, AE mp (the middle point between roof of the mandibular fossa and the height of the articular tubercle);

  • Height of the articular tubercle, AE top;

  • Pt (top of the pterygopalatine fissure) and

  • Ptm (bottom point of the pterygopalatine fissure).

A total of 20 distances were measured using these reference points.

Fig. 2 Abb. 2
figure 2

Seven reference points in the sagittal plane referring to porion, mandibular fossa, articular tubercle (AE articular eminence) and pterygoid fossa. See text for description of points/abbreviations

Sieben Referenzpunkte in der Sagittalen zur Erfassung von Porion, Fossa mandibularis, Tuberculum articulare („articular eminence”, AE) und Fossa pterygopalatina. Messpunkte und Abkürzungen s. Text

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These 20 measurements were

  1. 1.

    F post-AE top (fossa width),

  2. 2.

    F roof on F post-AE top (fossa depth; [38]; Fig. 3a).

Fig. 3 Abb. 3
figure 3

a F post-AE top (mandibular fossa width, 1), F roof on F post-AE top (mandibular fossa depth, 2). b Measurements of the mandibular fossa and the articular tubercle (AE): F post-F roof (3), F post-AE mp (4), F roof-AE top (5), AE mp–AE top. See text for description of points/abbreviations

a „F post-AE top” (Breite der Fossa mandibularis, 1), F roof on F post-AE top (Tiefe der Fossa mandibularis, 2). b Messungen an der Fossa mandibularis und am Tuberculum articulare (AE): F post-F roof (3), F post-AE mp (4), F roof-AE top (5), AE mp–AE top (6). Messpunkte und Abkürzungen s. Text

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Additionally, the ratio between mandibular fossa depth and width was calculated for each TMJ.

Measurements of the mandibular fossa and the articular tubercle (eminence) were

  1. 3.

    F post-F roof,

  2. 4.

    F post-AE mp,

  3. 5.

    F roof-AE top,

  4. 6.

    AE mp–AE top (Fig. 3b),

  5. 7.

    Po-Pt between porion and pterygopalatine fossa,

  6. 8.

    Po-Ptm between porion and pterygopalatine fossa (Fig. 4a).

Fig. 4 Abb. 4
figure 4

a Measurements Po-Pt (7) between porion and pterygopalatine fossa, and Po-Ptm (8) between porion and pterygopalatine fossa. b Measurements between porion to mandibular fossa and to articular tubercle (AE): Po-F roof (9), Po-AE mp (10), Po-AE top (11), Po-F post (12). c Measurements between the pterygopalatine fossa (point Pt) to reference points of the mandibular fossa and to articular tubercle (AE): F roof-Pt (13), F post-Pt (14), AE mp-Pt (15), AE top-Pt (16). d Measurements between the pterygopalatine fossa (point Ptm) to reference points of the mandibular fossa and to articular tubercle (AE): AE top-Ptm (17), AE mp-Ptm (18), F roof-Ptm (19), F post-Ptm (20). See text for description of points/abbreviations

a Messungen von Po-Pt (7) zwischen Porion und Fossa pterygopalatina sowie Po-Ptm (8) zwischen Porion und Fossa pterygopalatina. b Messungen zwischen Porion, Fossa mandibularis und Tuberculum articulare (AE): Po-F roof (9), Po-AE mp (10), Po-AE top (11), Po-F post (12). c Messungen zwischen Fossa pterygopalatina (Punkt Pt) und Referenzpunkten an Fossa mandibularis und Tuberculum articulare (AE): F roof-Pt (13), F post-Pt (14), AE mp-Pt (15), AE top-Pt (16). d Messungen zwischen Fossa pterygopalatina (Punkt Ptm) und Referenzpunkten der Fossa mandibularis und dem Tuberculum articulare (AE): AE top-Ptm (17), AE mp-Ptm (18), F roof-Ptm (19), F post-Ptm (20). Messpunkte und Abkürzungen s. Text

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From porion to mandibular fossa and to articular tubercle included the distances

  1. 9.

    Po-F roof,

  2. 10.

    Po-AE mp,

  3. 11.

    Po-AE top,

  4. 12.

    Po-F post (Fig. 4b).

From the pterygopalatine fossa (point Pt) to reference points of the mandibular fossa and to articular tubercle included the distances

  1. 13.

    F roof-Pt,

  2. 14.

    F post-Pt,

  3. 15.

    AE mp-Pt,

  4. 16.

    AE top-Pt (Fig. 4c).

From the pterygopalatine fossa (point Ptm) to reference points of the mandibular fossa and to articular tubercle included the distances

  1. 17.

    AE top-Ptm,

  2. 18.

    AE mp-Ptm,

  3. 19.

    F roof-Ptm,

  4. 20.

    F post-Ptm (Fig. 4d).

All visual morphology classifications and measurements on the DTs were performed under constant environmental conditions, including an officially certified image viewing system for radiographic diagnostics.

Statistical analysis

The same blinded examiner remeasured the DTs and repeated the visual classifications of mandibular fossae and condyles after an interval of 3 months. The method error (ME) was then calculated using the Dahlberg formula (ME = √(∑d2/2n)) [13] where d is the difference of the repeated measurement pairs and n the number of measurements. The ME was <1 for all measurements. Intrarater reliability (IRR) for the visual classification was assessed using Cohen’s kappa. For both mandibular fossae and condyles kappa was = 1. Data was recorded using a spreadsheet software (Excel®, Microsoft Corp., Redmond, WA, USA). The Kolmogorov–Smirnov test confirmed normal distribution of the data. Homogeneity of variance was tested using Levene’s method. Treatment related changes were analysed with paired Student’s t‑tests. Descriptive statistics mean (M) and standard deviation (SD) were recorded for each variable. All statistical analyses were performed using SPSS® version 22.0 (IBM Corp., Armonk, NY, USA) for Windows® (Microsoft Corp.). Statistical significance was set at p < 0.05.

Results

Visual classification of fossa shape

Visual classification of the sagittal DT slices showed no differences in mandibular fossa shape between T1 and T2 in all 50 mandibular fossae. In 24 patients, fossa shape of the right and left fossa was identical. Only one patient showed side differences of mandibular fossa morphology (patient no. 25; Table 1). In total, 26 of the mandibular fossae were round, 11 ovoid, 5 trapezoid and 8 triangular.

Table 1 Tab. 1 Visual classification of mandibular fossa shapeVisuelle Klassifikation der Kontur der Fossa mandibularis
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Visual classification of mandibular fossa morphology on tomograms and MRI

Similar visual classification of 50 mandibular fossae were also performed on parasagittal slices of MRI (figure 3 in Kinzinger et al. [25]). No morphological changes were observed. Slightly different to our present investigation, fossa morphology of the right and left TMJ was always the same in each of the 25 patients. The MRI diagnosis also revealed primarily round and ovoid fossa shapes: 34 out of 50 compared to 37 out of 50, respectively, in the present study.

Metrical analysis

Tables 2 and 3 show the results of all measurements. Only few differences between T1 and T2 (∆T2–T1) were statistically significant (p < 0.05). Small treatment related changes ranging from −0.89 mm (decrease) to 1.25 mm (increase) occurred. None of the comparisons between the right and left TMJ were significant (Table 4).

Table 2 Tab. 2 Measurements within the mandibular fossa and the articular eminence (according to Fig. 3)Messungen innerhalb der Fossa mandibularis und des Tuberculum articulare (analog Abb. 3)
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Table 3 Tab. 3 Measurements of the anatomical structures porion and mandibular fossa (according to Fig. 4)Messungen an den anatomischen Strukturen Porion und Fossa mandibularis (analog Abb. 4)
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Table 4 Tab. 4 P-value comparison between right and left sideVergleich zwischen rechter und linker Seite, p-Werte
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Comparison of linear measurements on tomograms and MRI

Ten of the linear measurements were taken for the same patients on DTs (Figs. 3 and 4b of the present study) and on MRIs (figures 4 and 5 in Kinzinger et al. [25]). DTs and MRIs were taken at comparable points in time, i.e. before and after FMA treatment. The results of our present study (Tables 2 and 3) and the MRI study (tables 3–6 in Kinzinger et al. [25]) were almost identical for 8 of the 10 linear measurements. Only 2 out of 10 linear measurements exhibited minor insignificant differences.

Discussion

We have examined DTs to reveal changes of mandibular fossae after FMA treatment. The patients investigated in our present study were equal to those evaluated in a previous MRI investigation [25]. Hence, direct comparison between results obtained from DTs and MRI scans were possible. This has not been previously published. Therefore, our study adds new data to the literature.

Like other investigators [18, 22], we used DTs to quantify possible transformations of the mandibular fossa after FFA treatment. The patients were positioned within the x‑ray unit according to the manufacturer’s instructions, using a standard bite block [14]. In agreement with similar studies [37], upper and lower incisors were moved to an edge-to-edge position. The subsequent anterior displacement of the mandible caused anterior and downward movement of the condyles, which ensured complete visual assessment of the mandibular fossae without superimposition of anatomical structures, e.g. the zygomatic arc.

We used DTs which were part of diagnostic procedures during fixed orthodontic appliance treatment. This complied with the ALARA (as low as reasonably achievable) principle [15] because no additional radiographic examinations of the patients were conducted. Still, DTs are associated with radiation burden for the patient. On the other hand, MRI is available as a radiation-free alternative. However, recording an MRI is time consuming, costly and not available in the orthodontist’s office. Moreover, the comparatively small field of view in an MRI shows only a limited area around the TMJ. Compared to MRI scans, tomograms have the advantage that additional osseous structures anterior and posterior to the mandibular fossa and the articular eminence can be used as reference points for different linear measurements [22]. In our study, we used the porion and the pterygopalatine fossa for this purpose. The distance between these two structures remained stable during the observed treatment period, allowing these to be used as stable anatomical landmarks. Given that apposition and resorption in the fossae occur in a structured manner, the depth and width of the fossae would reveal no differences in sagittal measurement comparison. This alone does not allow the conclusion that no changes occurred. However, adding measurements of porion and pterygopalatine fossa allows a comparison with stable anatomical landmarks. If the relative position between mandibular fossa and these structures change, a fossa shift could be proven. However, this was not the case in the present tomogram investigation.

Translation of the mandibular fossa has been identified as contributor to mandibular positional changes after FFA treatment in animal studies [31, 32, 39, 51, 52]. Signs of possible fossa remodelling after functional treatment were found in isolated incidents [49]. Other studies described remodelling in the posterior–superior portion of the condyle and/or the fossa on the PT in selected Herbst appliance patients [6, 37].

Bone remodelling is not visible in conventional radiography (PT) until mineralization has occurred. The opposite is true in MRI [43, 45]: cartilage that is formed during treatment is visible. The results of a recent MRI study in the same population [25] did not show any alterations of fossa width or depth. If any remodelling had occurred, it must have happened symmetrically. However, in both the tomogram and the MRI study, distance measurements relating to the porion revealed no changes.

Results of both the tomogram and the MRI investigation do not allow the conclusion that FFA treatment does not alter the mandibular fossa and adjacent regions but indicate that changes are so minuscule that they cannot be recorded with the methods described, or that they occur within methological error.

Ruf and Pancherz [43, 45] have already described that the amount of fossa remodelling accounts for less effect than condyle remodelling. Using dedicated 3D rendering software [29], possible remodelling/alterations of the mandibular fossae might be revealed [25]. To reveal possible age-related influences, further research is needed.

Conclusions

A total of 25 patients with skeletal class II malocclusion were successfully treated with a fixed functional appliance (FMA). The mandibular fossae received pre- and posttreatment metric analysis and visual classification of tomograms, and allow the following conclusions:

  • No visible changes in fossa morphology were found in the sagittal plane.

  • The majority (24 out of 25) showed bilateral similarity of TMJ structures.

  • No metric changes could be recorded for width, depth and ratio thereof, neither between T1 and T2 nor between the different sides.

  • No metric changes could be found for another 18 linear parameters, neither between T1 and T2 nor between the different sides.

  • There is no indication of a fossa shift.

  • Absolute values are identical for tomogram and MRI measurements within the same patient group.

The standard tomographic radiograph appears to be a valuable research tool for the sagittal analysis of mandibular fossa changes during treatment with a fixed functional appliance. Provided that the tomogram is recorded with an x‑ray unit adjusted according to the manufacturer’s instructions, an additional MRI is not necessary to evaluate sagittal fossa changes.