Preliminary comparison of three-dimensional reconstructed palatal morphology in subjects with different sagittal and vertical patterns

Seventy-six skeletal Class II subjects with retrusive mandible (38 females, 38 males; mean age 25.63 ± 4.76 years) and 85 skeletal Class I subjects (45 females, 40 males; mean age 23.95 ± 4.45 years) were collected retrospectively from Department of Oral Maxillofacial Surgery and Department of Orthodontics, Peking University School and Hospital of Stomatology. Biomedical Ethics Committee of Peking University School and Hospital of Stomatology has approved this study (Number: PKUSSIRB-201946086). Written informed consent was obtained from all participants included in the study.

The inclusion criteria for skeletal Class II subjects with retrusive mandible were the following: (1) Mongolian, (2) aged 18–35 years, (3) 78.8° < SNA < 86.8°, SNB < 76.2°, ANB > 4.7°, NP-FH < 81.7° (according to Chinese cephalometric norms), (4) no previous orthodontic or orthognathic treatment. Exclusion criteria for skeletal Class II with retrusive mandible included: (1) missing permanent teeth, (2) retained deciduous teeth, (3) impacted teeth, (4) severe periodontitis, (5) history of palatal surgery, (6) cleft lip and/or palate, (7) craniofacial syndromes. According to Chinese cephalometric norms, skeletal Class I subjects with 78.8° < SNA < 86.8°, 76.2° < SNB < 84°, 0.7° < ANB < 4.7°, and 81.7° < NP-FH < 89.1° were enrolled in this study. The other inclusion criteria and exclusion criteria for skeletal Class I subjects were the same as those for skeletal Class II subjects with retrusive mandible.

According to the values of SN-MP, FH-MP and S-Go/N-Me, skeletal Class II subjects with retrusive mandible and skeletal Class I subjects were divided into the following sub-groups:

Hyperdivergent: SN-MP > 37.7°, FH-MP > 32°, S-Go/N-Me< 62%;

Normodivergent: 27.3° < SN-MP < 37.7°, 22° < FH-MP < 32°, 62% < S-Go/N-Me< 68%;

Hypodivergent: SN-MP < 27.3°, FH-MP < 22°, S-Go/N-Me> 68%.

Descriptions of six groups (Class II-hype, Class II-norm, Class II-hypo, Class I-hype, Class I-norm, and Class I-hypo) were shown in Table 1.

NewTom Scanner (NewTom AG, Marburg, Germany) was used to take CBCT images for the subjects. All images were taken with 0.3-mm axial slice thickness, 15 × 15-cm field of view, 3.6-s scan time, 110-kV tube voltage, and 2.81-mA tube current. CBCT were exported in digital imaging and communications in medicine (DICOM) format [14]. Lateral cephalograms were generated from CBCT images by Dolphin 3D Imaging software (version 11.8, Dolphin Imaging and Management Solutions, Chatsworth, Calif).

To set identical 3D reference planes in different groups, CBCT images were reoriented and exported by Dolphin software to obtain palatal morphology. Horizontal plane was the plane tangent to the most inferior slice of maxillary alveolar bone. Sagittal plane was the plane that passed through ANS-PNS line and was perpendicular to horizontal plane. Coronal plane was the plane perpendicular to the above two planes [8, 15] (Fig. 1).

Reoriented CBCT images were imported into ProPlan CMF 1.4 software (Materialise, Leuven, Belgium) for 3D reconstruction [16]. The landmarks are defined in Table 2 and Fig. 2a. Firstly, 3D skull was obtained by threshold segmentation. To separate maxilla from 3D skull, the boundary of maxilla was defined by the following specific planes: the lowermost horizontal plane was through U1’, U6L’ and U6R’; the foremost coronal plane was through U1’, the backmost coronal plane was through U7L’or U7R’; and the uppermost horizontal plane was through ANS (Fig. 2b). Then 3D shape of maxilla was segmented and exported as standard tessellation language (STL) format document (Fig. 2c). To transfer reconstructed 3D object of maxilla into 3D palatal morphology, STL format documents were imported into Geomagic Studio 11.0 software (Raindrop Geomagic, Inc., NC, USA). Through the method of plane cutting, the lowermost horizontal plane and backmost coronal plane of maxilla were used to separate palate from maxilla, and then the palate was selected to create a bounded component, and the residual sections were removed. Finally, 3D closed figure of palate was obtained by filling the boundary hole (Fig. 3).

In order to compare palatal morphology between different groups, it is necessary to obtain the average morphology of each group and put them into the same 3D coordinate system. U1’ was set as the origin; horizontal plane passing through U1’ was set as plane XY; sagittal plane passing through U1’ was set as plane YZ; and the backmost coronal plane perpendicular to the above two planes was set as plane XZ (Fig. 4a). All 3D palatal models were put into this coordinate system and then an average 3D palatal morphology for each group were established with the method of average calculation (Fig. 4b).

Palatal volume (PV) was calculated as the volume of 3D closed figure of palate, and palatal area (PA) was calculated as the surface area of 3D closed figure of palate using Geomagic studio (Fig. 5a). Besides, palatal width (PW) was measured as the width of the bounding box, palatal height (PH) was measured as the height of the bounding box, and palatal length (PL) was measured as the length of the bounding box using Geomagic studio (Fig. 5b, c, d) [4].

In Class II-hype group, the posterior part of palate was flatter than that in Class I-hype group for both gender, with the difference of 0.63–2.13 mm in males and 0.63–1.75 mm in females (Fig. 9a, g). Furthermore, PH of males in Class II-hype group showed significantly smaller than that in Class I-hype group (p < 0.05, Table 5).

In Class II-norm group, the posterior part of palate was also flatter than that in Class I-norm group, and the difference was 0.63–2.50 mm (Fig. 9c, i). PH in Class II-norm group showed significantly smaller than that in Class I-norm group (male: p < 0.05; female: p = 0.05, Table 5).

In Class II-hypo group, the posterior part of palate was flatter than that in Class I-hypo group, with the difference of more than 2.50 mm in males and 0.63–2.50 mm in females (Fig. 9e, k).

As for the width, the posterior part of palate in Class II groups was narrower than that in Class I groups with the similar vertical pattern for males and females (Fig. 9b, d, f, h, j, l). However, comparison of PV between skeletal Class II with retrusive mandible and skeletal Class I subjects showed no significant difference (p > 0.05, Table 5).

In subjects with skeletal Class II and retrusive mandible, males with hyperdivergent and normodivergent showed narrower in the posterior part of palate. These results were in agreement with Parcha’s study, in which to explore the correlation between palatal morphology and skeletal pattern using GMM based on cephalograms and maxillary dental casts, and they reported skeletal Class II subjects with hyperdivergent pattern tended to have higher and narrower palate and emphasized that the most significant principal component related to palatal shape was vertical dimension [5].

According to the previous studies, it was speculated that one of the reasons to affect palatal morphology in different vertical patterns might be the function of masticatory muscles [20‐22]. Lione et al. used ultrasound method to compare masticatory muscles in growing subjects among different vertical patterns and concluded that masticatory muscles of hyperdivergent subjects were relatively weak [21]. Besides, Kiliaridis et al. demonstrated that the weaker the function of masticatory muscles, the narrower maxillary dental arch might be [22]. Animal studies on growing rats also elucidated that decreased bone apposition on midpalatal suture and deficiency in transverse growth of maxilla was caused by decreased function of masticatory muscles [23]. All these results suggested that hyperdivergent subjects would be expected to have weak muscle function and narrow and high palatal shape [24]. Therefore, the clinician should pay attention to the exercise of muscles activity in hyperdivergent growing subjects during orthopedic maxillary expansion.

In our study, the posterior part of palate in subjects with skeletal Class II and retrusive mandible was narrower than that of skeletal Class I subjects. Accumulated literatures reported the consistent results that palate of Class II subjects was narrower compared with Class I subjects [1, 25, 26]. Alarashi et al. analyzed maxillary base width with thin-plate spline analysis based on the cephalograms and revealed that constricted maxilla was associated with Class II subjects [1]. As for palatal height, our study observed the posterior part of palate in skeletal Class II subjects with retrusive mandible was flatter than that in skeletal Class I subjects of similar vertical pattern. However, Alarashi et al. reported the opposite result that Class II subjects had an increased palatal height when compared with Class I subjects [1]. The reasons leading to the different results might be contributed to the fact that vertical pattern was ignored in Alarashi’s study when palatal shape was compared among different sagittal patterns. According to Paoloni’s research, the most significant factor to affect palatal morphology was vertical pattern, and the higher the mandibular plane angle in skeletal Class II subject, the narrower and higher the palate was [4]. Furthermore, previous studies found that the proportion of hyperdivergent subjects in skeletal Class II with retrusive mandible was higher than normodivergent and hypodivergent subjects [27, 28]. Therefore, ignoring vertical factors might lead to inaccurate interpretation of the results. When discussing the differences of palatal morphology in our study between skeletal Class II subjects with retrusive mandible and skeletal Class I subjects, our conclusion was based on the similar vertical pattern, which might be more convincing, but still need further study.