The importance of upper first permanent molars position for the orthognatic occlusion

Abstract

Development of the dental arches and occlusion in permanent dentition can be divided into several stages and has to be observed regularly. The first permanent molar eruption is related to the onset of significant changes in the developing occlusion. Although this tooth is seen as the `key to occlusion` its value as an anchorage is debatable. The aim of the article is to study the correct position of the upper first molars in the two planes of space - the sagittal and transverse planes. In this article the position of the first upper molar is examined with the aid of diagnostic records, such as study cast, orthopantomogram (OPG), and lateral cephalometrics. A literature review includes Bulgarian and foreigner authors. Angle, who in 1899 referred to the maxillary first permanent molars as the `key to occlusion`, was the first to mention their importance within the dentition. According to Angle, the line passing through the middle of the mesiobuccal cusp of the upper first molar coincides with the line passing through the buccal groove of the lower first molar. After Angle, other authors have discussed the position of upper molars from different point of view, such as their relation or position in the maxilla, anteroposterior axial inclination and rotation. As indicated by Lamons and Holmes molar rotations commonly exist in Class II malocclusions. The molars are usually rotated around an axis lingual to their central fossae. In an ideal occlusion the buccal surfaces of the upper first molars are usually parallel to each other. On the OPG Kurol and Bjerklin measured the axial mesial inclination of upper first molar. The tipping of the molars is measured by the angle formed between the tangent line to the mesial surfaces of the root and crown and the line through the lower margins of the left and right orbits. According to Sassouni, the mesial contour of upper first molar should to lie on the 4th arc - the temporal arc. If the molar is anterior to this arc, a treatment with distalization could be initiated. The temporal arcnasion distance measured on the radius is equal to the distance from point ANS to the upper first molar. The position of the upper first molar varies with the position of the upper central incisors. The basic hypothesis is that if the upper first molar has a fixed position in the face, any increase in the total upper dental arch length will be transferred to the incisor area. Any change in the anteroposterior position of the upper first molar could influence the position of the mandibular- leading to Class II malocclusion. Ricketts pointed out that the average distance from the pterygoid vertical (PTV) to the distal surface of upper first molar is the sum of the age of the patient + 3mm, in a growing patient. This diagnostic method can help the orthodontist to decide whether to extract teeth or to distalize the molars. Any loss of space in the arch is a justification for early orthodontic treatment. Mediopalatal rotation of the upper molar is an additional problem in the final phase as well. The rotation of upper first molars is measured by the angles formed by the intersection of lines going over the tips of the mesiopalatal and distobuccal cusps of each molar (Ricketts line) with a straight line marked over the palatine raphe. The problem of reduced arch length has an impact on the final treatment stage when the major orthodontic goal is establishing a tight teeth intercuspation. The molars influence the transfer of occlusal forces to the facial skeleton. The upper first molar tolerates more changes in the position than the lower one. The correct position of the upper molar ensures a stable occlusion with significantly low grade of relapse.