The curve of Spee and its levelling in different vertical skeletal patterns: A 3D set-up study.

Objectives: Primary: to compare intra-arch measurements [Curve Of Spee (COS) depth, space required to level the COS (δ), uprighting angles of 1st and 2nd lower molars (αM1 & αM2), respectively] between different vertical skeletal patterns. Secondary: to explore the relationship between those measurements in each vertical skeletal pattern.

Methods: 90 Plaster models were scanned and transformed into digital models using 3Shape Ortho System™ intra oral scanner. COS depth, (δ), αM1 & αM2 were measured on digital models using 3Shape Ortho Analyser™ software. The latter was used to achieve the virtual set-up. The sample was dividied into 3 groups according to the vertical skeletal pattern which was identified by measuring the FMA˚ angle (Frankfort mandibular plane) on a lateral cephalogram using Dolphin imaging software.

Results: The deepest COS was present in the hypodivergent group (P-value=0.001). No difference for the additional arch space required to level the COS (δ) between different vertical skeletal patterns (P-value=0.063). αM2 is the same in all 3 groups, whereas αM1 is greater in normodivergents compared to hyperdivergents (P-value=0.012). A positive correlation exists between the deepest point of the COS and the uprighting angles, αM1 and αM2. No correlation between (δ) and the different parameters evaluated (deepest point of the COS, αM1 and αM2).

Conclusions: Additional arch space required to level the COS presented no correlation with the vertical skeletal pattern. Levelling the lower arch was carried out by extrusion of the premolars and uprighting of the posterior teeth; confirming that well-planned orthodontic mechanics can minimize side effects (flaring of the lower incisors) encountered during treatment.