# The Pathology of the First Permanent Molar during the Mixed Dentition Stage--Review
## 1 Introduction  
Mixed dentition, characterized by the coexistence of deciduous and permanent teeth in the oral cavity, is one of the most active periods in the development and growth of the stomatognathic system [1]. This stage extends between the ages of 6 to 13 years, and it is further divided into early mixed and late mixed dentition [2]. The dynamic biological and functional events that mark the mixed dentition phase include not only the teeth but also the alveolar process, which remodels to accommodate the newly erupted permanent teeth, and the relationships that are established between the maxillary and mandibular teeth (the dental occlusion). The onset of mixed dentition is marked at around the age of six by the eruption of the first permanent molar (FPM), called accessional tooth because it emerges posterior to the primary teeth, different from the so-called successional teeth, which replace the previous deciduous ones. The transition from primary to permanent dentition includes three stages [3]:  
* The first stage corresponds to the first transition in which the FPMs erupt, the primary incisors exfoliate, and the permanent incisors erupt. When the permanent incisors have reached the plane of occlusion, the first transition ends, and we move to the second stage, called the intertransitional stage, in which intense remodeling of the alveolar process occurs simultaneously with the continuation of the development of the permanent teeth and with the resorption of the roots of the deciduous canines and molars. During the intertransitional period, the permanent incisors, primary canines, primary molars, and the FPMs cohabit in the oral cavity. * The third stage corresponds to the second transition in which the deciduous molars and canines exfoliate, and their successors erupt (the permanent premolars and canines), whereupon the permanent second molars erupt. The development of the FPM begins in intrauterine life [4]. As radiographic investigations have shown, the crypts of the FPMs can be seen at birth in the territory of the second primary molars, in addition to the germs of the deciduous teeth [5]. At first, the crypts of the maxillary FPMs are oriented high in the maxilla, above and behind the germ of the second deciduous molar, and the crypts of the mandibular ones are found below the anterior border of the ramus of the mandible [5]. Osseous growth occurs through the apposition of bone to the maxillary tuberosity and resorption of the anterior border of the mandibular ramus, providing space for the FPMs to erupt [5]. Consequently, the FPMs perform intramuscular and intramandibular movements, traverse the bone and end up taking the form of two plateaus covered by the gingiva, which can be observed behind the deciduous dental arches at around three to five years of age [5]. Once the occlusal surface is free of gingiva, the path of eruption is guided by the terminal plane (the relationships of the distal surfaces of the second primary molars, when viewed from the side). The FPMs erupt almost perpendicular to the occlusal plane [3]. The FPM plays a dominant role in the development of dental arches and occlusion during the mixed dentition period:  
* The eruption of the FPM is a unique morphological and functional event because it simultaneously establishes the distal limit of the canine and premolar segment and the mesial limit of the molar teeth [6]. It not only achieves the only stable posterior occlusal stops, but it contributes to accomplishing the consistency between the anterior determinant (the teeth) and the posterior determinant (the temporomandibular joint) of occlusion [7]. * Being the first permanent tooth that erupts, the FPM pushes the jaws apart, increasing the height of the lower third of the face [8]. * It changes its vertical position to compensate for the mandibular rotation pattern, thus establishing the vertical dimension of occlusion [9; 10].