Evaluation depth of the curve of Spee in class I, class II, and class III malocclusion: A cross sectional study.

Occlusal plane is an essential consideration when multiple long-span posterior restorations are designed. When restorations are added to an existing tooth arrangement characterized by rotated, tipped, or extruded teeth, excursive interferences may be incorporated, resulting in detrimental squeal. The curve of Spee, which exists in the ideal natural dentition, allows harmony to exist between the anterior tooth and condylar guidance. This curve exists in the sagittal plane and is the best viewed from a lateral aspect. It permits total posterior disclusion on mandibular protrusion, given proper anterior tooth guidance. It is unclear that whether the curve of Spee is a description of the occlusal surface of each arch separately or in maximal intercuspation. The purpose of this study was to examine the differences in the depth of curve of Spee between the class I, class II, class III and to investigate the relationship of depth of curve of Spee with over jet, over-bite.

The curve of Spee was first described by F Graf Von Spee in 1890, who used skulls with abraded teeth to define a line of occlusion. This line lies on a cylinder that is tangent to the anterior border of the condyle, the occlusal surface of the second molar, and the incisal edges of the mandibular incisors. Spee located the center of this cylinder in the midorbital plane and hence that it had a radius of 6.5–7.0 cm. The functional significance of the curvature has not been completely understood. However, it has been suggested that the curve of Spee has a biomechanical function during food processing by increasing the crush-shear ratio between the posterior teeth and the efficiency of occlusal forces during mastication.[1]

Leveling of the curve of Spee represents a routine procedure in orthodontic practice. The deviation of the occlusal plane from a flat plane has practical consequences when considering the arch circumference requirements necessary to flatten the curve. In disputably, a curved arch has a greater circumference than a flat arch. However, the amount of additional arch circumference required to flatten the curve is not as apparent. A popular theory is that 1 mm of arch circumference is needed to level each millimeter of the curve of Spee. Yet, Germane et al., found that less 1 mm of arch circumference is required to level each millimeter of the curve of Spee. According to Woods, the amount needed is variable depending on the type of mechanics used.

Later on, the curve of Spee and/or leveling of this has related to incisor over-bite and lower arch circumference. A deep curve of Spee is usually associated with an increased over-bite. Orthodontic correction of the over-bite often involves leveling the curve of Spee by anterior intrusion, posterior extrusion or a combination of these actions. The purpose of this study was to determine the depth of the curve of Spee in class I, class II, and class III occlusions and to investigate the relationship of depth of curve of Spee with over jet, over-bite.[23]

Materials and Methods

This study was performed using the dental casts of 30 subjects. The samples are selected, are the students of Sree Balaji Dental College and hospital Chennai, Tamil Nadu with the following criteria, the subjects are selected with age group between 18 and 20 years with complete permanent dentition, including the second molars (at least 28 teeth) all teeth present except third molar, no history of trauma and no previous orthodontic treatment.

Alignate impression was taken and cast is prepared then Standardized digital images of the maxillary and mandibular dental casts will be made with a digital camera fixed on a tripod. The camera-to-tooth distance was 40 cm for all pictures to eliminate image distortion. Each digital image of the dental casts is transferred to a personal computer for analysis. And the Images were analyzed using Auto-Cad software (Auto-cad 2011, version 18.1, Autodesk inc., California, United states).

The depth of curve of Spee was measured as the perpendicular distance between the deepest cusp tip and a flat plane that was laid on the top of the mandibular dental cast, touching the incisal edges of the central incisors and the distal cusp tips of the most posterior teeth in the lower arch [Figure 1]. The measurement was made on the right and left side of the dental arch and the mean value of these two measurements was used as the depth of the curve of Spee.

Figure 1

Showing the depth of curve of Spee

The overjet was measured as the distance (in millimeters) along a horizontal plane between the incisal edge of the labial surface of the mandibular central incisor and incisal edge of the labial surface of the most labially positioned maxillary central incisor.

The over-bite was measured as the vertical distance (in millimeters) between the incisal edge of the maxillary central incisor and the incisal edge of the mandibular central incisor.

Results

The mean, standard deviation values, of all variables, overjet and over-bite used in this study of three groups are presented in Table 1 and Figure 2.

Table 1

Showing the mean values of class I, class II, and class III

Figure 2

Bar diagram showing mean values

Discussion

The morphological arrangement of the teeth in the sagittal plain has been related to the slope of the articular eminence, craniofacial morphology, lower incisor proclination, the incisor over-bite, the molar cusp height and lower arch circumference. However, it is suggested that the curve of Spee has a biomechanical function during food processing by increasing the crush-shear ratio between the posterior teeth and the efficiency of occlusal forces during mastication.

The curve of Spee is an important guideline in prosthetic management of a patient as it determines the position of the tooth in the sagittal plane. From the result of this study, it is clear that the curve is deeper in class II cases while it is flatter in class III cases. This study is in correlation with Orthlieb's finding that the radius of the curve of Spee is less in class II cases while it is more in class III occlusion.

Thus, it has a direct relation with the antero-posterior position of maxilla and mandible. Furthermore, curve of Spee is directly proportional to the overjet and over-bite of an individual. In prosthodontic management of full mouth rehabilitation cases, it is necessary to maintain the curve of Spee at an ideal depth which will determine the incisal guidance. A deep curve will require a steep incisal guidance to provide disocclusion of the posterior tooth during protrusive movement. Correlation is appeared between curve of Spee and over-bite indicated that increased in over-bite is coinciding with increasing in the depth of the curve of Spee.

Conclusion

Correlation coefficient of the variables shows that there is a significant correlation between curve of Spee, over-bite and overjet. The curve of Spee for class II subjects shows a significant difference than that of class I and III subjects. Curve of Spee is the deepest in class II subjects and flatter in class III subject.