This investigation evaluated the palatal and craniofacial morphological covariation in Class III growing patients by means of GMM.
In literature, Parcha et al.14 studied this kind of shape covariation in a general orthodontic population, while Paoloni et al.15 used the same methodology for a Class II malocclusion mixed dentition orthodontic population. To our knowledge, this is the first attempt to analyse the covariation between palatal and craniofacial shape in a group of growing subjects with Class III malocclusion.
Treatment outcomes for subjects with Class III malocclusion are dependent on multiple factors including growth characteristics, facial morphology, environmental factors, direction and magnitude of corrective forces, treatment timing and duration, and patient’s compliance4 − 6. While several studies7 − 10 have been useful to understand Class III growth patterns, Class III craniofacial skeletal pattern needed more research, as it is a complex biological nonlinear system in which one component’s action changes the context for other components25.
In fact, the variations in transverse, vertical, anteroposterior skeletal factors, and palatal morphology are complex and are related in various directions16.
The first attempt of three-dimensional evaluation of mandibular changes in Class III malocclusion subjects was made by Singh et al.26 on lateral cephalograms. The aim of their study was to apply the finite-element morphometry (FEM) to human mandibular configurations and determine local size- and shape-change differences in subjects with normal and Class III malocclusion between 5 to 11 years. FEM analysis revealed that the combination of a longer mandibular corpus and shorter ramus, associated with acute mandibular and symphyseal angles, distinguished a Class III mandible from a normal one.
Then, the thin-plate spline analysis (TPS) was used to evaluate the mandibular deformations in Class III subjects (adults and children) when compared to normal occlusion subjects11. The study showed in the Class III group a longer mandibular body and a narrower ramus, allied with a larger mandibular angle, combined to a longer mandibular total length with upward and forward extension of the ascending ramus and forward and downward extension of the mandibular symphysis11.
Bui et al.27 used a cluster analysis and a principal component analysis based on cephalometric variables to evaluate the most significant changes of the craniofacial complex in Class III malocclusion adults (mean age 19.10 years). 3 PCs were selected: the first principal component consisted of sagittal parameters; the second principal component was significant for vertical measurements and for lower incisor position; the third principal component consists of variables related in both anteroposterior and vertical dimensions.
The studies described above, however, assessed only the craniofacial deformations, in particular the mandibular ones, without correlation to the palatal changes and they collected samples of Class III malocclusion adult patients.
Recently, Ahn et al.16 used the SEM analysis, applied to CBCT and maxillary study models, to study the relationship between the morphology of the palate and the facial skeletal patterns in Class III malocclusion adult patients (mean age 22.12 years). The Authors observed that the palatal shape was narrow, deep, and long, or was wide, shallow, and short, depending on the transverse facial skeletal pattern. In contrast, the anteroposterior latent variable had a low influence on the principal component, in that the variation of the palatal morphology: even if the posterior facial height is long, its influence on palatal shape variation would not be significant.
On the contrary, our study focused on growing subjects with Class III malocclusion and used the two-block PLS method to evaluate the covariance between palatal and craniofacial components.
In according to Ahn et al.16, in our study palatal morphological changes occurred in all the three space dimensions (Fig. 3): a wide palate was related to a shallow palatal shape, while a narrow palate was associated with a high palatal vault. As for the morphology of the craniofacial complex (Fig. 5), the most significant morphological variability referred to the vertical and not to the sagittal plane. The analysis of the pattern of covariation demonstrated a statistically significant relation between the divergence of the craniofacial complex and the shape of the palate.
To our knowledge, in literature only two studies evaluated the covariation between the morphology of the palate and the facial skeletal patterns using different sample of malocclusion. Parcha et al.14, analysing the palatal morphology and its relationship to skeletal pattern in a general orthodontic population, underlined that high and narrow palatal vaults were principally associated to a hyperdivergent skeletal pattern while shallow and wide palates to a hypodivergent one. Paoloni et al.15 evaluated a group of Class II malocclusion growing patients, showing that the tendency to develop a transverse deficit of the maxilla was more easily recognizable in Class II subjects with high angle mandibular pattern. Despite the different analysed subjects, both the two authors found covariation results that were similar to the ones found in our study. We confirmed that beyond the kind of sagittal malocclusion there is a strong correlation between the maxillary morphology and the vertical facial skeletal pattern.
Limitations
Since the records were obtained from Class III individuals who chose to seek treatment and subjects who present for correction of their malocclusion, they may represent a more severe phenotype than occurs within the normal population. Moreover the study included only Caucasians so these results will not apply to other ethnic groups As recommended by Parcha et al.14, the palatal vault was assessed up to the gingival margin as described by previous studies in order to eliminate the influence of dental inclination and position in the alveolar bone.