May 29, 2022
The positions of the human teeth, such that they lie in perfect intercuspation with each other are governed by multiple factors. Each of these factors influence the position of teeth in the sagittal, vertical and transverse plane. The sagittal plane is a reference for growth of the face in front to back direction. Similarly, the vertical plane is a reference for height and the transverse plane is a reference for the breadth.
In 1890, Ferdinand Graf von Spee described the relation of the teeth to the growth of the face in the sagittal plane. He described a natural curve formed by the lower teeth that ensures a normal bite and masticatory system. He termed this the Curve of Spee. If we look at the side profile of the skull, and draw a line connecting the tip of the canine, the cusps of the premolars and molars till a point on the front border of the mandible.
The Curve of Spee changes in depth as the dentition develops from deciduous to permanent. The way the Curve of Spee develops depends a lot on the direction in which the teeth erupt, the sequence in which the teeth erupt and the way come in contact with the teeth of the opposing arch. In 1972, in one of the most important literary paper of Dental Sciences, it was stated that a flat or mild Curve of Spee is one of the seven main factors affecting the normal occlusion.
The significance of Curve of Spee is of importance in planning treatment of teeth that involves aligning the teeth and while giving restorations (fillings) in areas that bears the maximum chewing force. The strongest muscle for mastication is the masseter. It is present perpendicular to the Curve of Spee. Hence, a flat or a mild Curve of Spee ensures that no undue forces are exerted on the lower teeth if the Curve of Spee is maintained.
The distance between the upper and lower front teeth horizontally, is called the overjet. The amount of upper teeth that covers the lower when viewed from the front is called overbite. The Curve of Spee also acts as a determinant of overjet and overbite. Deeper the Curve of Spee, more will be the overjet and overbite.
Apart from overjet and overbite, an altered Curve of Spee might also lead to improper bite where canines might hinder it mouth closure.
In cases where the Curve of Spee is severe, it needs to be flattened. Usually, a severe Curve of Spee is observed when:
• the molars are erupted lower than their normal position
• when the incisors are erupted above their normal position
• a combination of both
• when the lower front teeth are placed backward.
Levelling the Curve of Spee
The levelling of Curve of Spee can be done by countering each of the reasons mentioned above.
Extrusion of Molars
Patients with extrusion of molars often have a lower facial height. Treatment in these patients must be done when their growth and development has not ceased yet. Placement of archwires or acrylic bite planes in the anterior region can help in flattening the Curve of Spee.
Intrusion of Incisors
Incisors are intruded at a force of 12-15 g with the help of arch wires. It is usually done in patients with a longer facial height.
Proclination of incisors
Incisors that are tipped towards the tongue are pushed forwards with the help of acrylic bite planes. 1 mm of forward movement of incisors causes a 4° increase in Curve of Spee.
Post-treatment retention
The treatment to flatten the Curve of Spee might relapse. In order to avoid this, patients are asked to wear a retainer plate continuously for 4 to 6 months. This plate does not interfere with normal speech or mastication and also promotes regular oral hygiene maintenance. The main function of the retainer is to maintain the corrected Curve of Spee.
The Curve of Spee is an important component of a normal bite. Orthodontists study the Curve of Spee an individual by taking radiographs (lateral cephalogram) and by studying replicas (models). In growing patients, the Curve of Spee should thus be considered during treatment involving alignment of teeth with relation to the growth of face.