Network analysis of three-dimensional hard-soft tissue relationships in the lower 1/3 of the face: Skeletal Class I-normodivergent malocclusion versus Class II-hyperdivergent malocclusion

Abstract Background The determining effect of facial hard tissues on soft tissue morphology in orthodontic patients has yet to be explained. The aim of this study was to clarify the hard-soft tissue relationships of the lower 1/3 of the face in skeletal Class II-hyperdivergent patients compared with those in Class I-normodivergent patients using network analysis. Methods Fifty-two adult patients (42 females, 10 males; age, 26.58 ± 5.80 years) were divided into two groups: Group 1 (G1), 25 subjects, skeletal Class I normodivergent pattern with straight profile (Class I-norm-straight); Group 2 (G2), 27 subjects, skeletal Class II hyperdivergent pattern with convex profile (Class II-hype-convex). Pretreatment cone-beam computed tomography (CBCT) and three-dimensional (3D) facial scans were taken and superimposed, on which 3D landmarks were identified manually, and their coordinate values were used for network analysis. Results (1) In sagittal direction, G2 correlations were generally weaker than G1. In both the vertical and sagittal directions of G1, the most influential hard tissue landmarks to soft tissues were located between the level of cemento-enamel junction (CEJ) of upper teeth and root apex of lower teeth. In G2, the hard tissue landmarks with the greatest influence in vertical direction were distributed more forward and downward than in G1. (2) In G1, all the coefficients in the correlation matrix of sagittal and vertical positions of hard tissue landmarks were positive. In G2, the values of those coefficients decreased. In G1, all the correlations for vertical-hard tissue to sagittal-soft tissue position and sagittal-hard tissue to vertical-soft tissue position were positive. However, G2 correlations between vertical-hard tissue and sagittal-soft tissue positions were mostly negative. Between sagittal-hard tissue and vertical-soft tissue positions, G2 correlations were negative for mandible, and were positive for maxilla and teeth. Conclusion Compared with Class I-norm-straight patients, Class II-hype-convex patients had more variations in soft tissue morphology in sagittal direction. In vertical direction, the most relevant hard tissue landmarks on which soft tissue predictions should be based were distributed more forward and downward in Class II-hype-convex patients. Class II-hype-convex pattern was an imbalanced phenotype concerning sagittal and vertical positions of maxillofacial hard and soft tissues.