Introduction
Molar root-incisor malformation (MRIM) is a dental anomaly mostly affecting permanent first molars and is characterized by a short, thin, and tapered shape of the root and narrowed pulp chamber with cervical constriction [1]. Similar features have been observed in primary second molars, and reports on permanent central incisors mentioned a thin root and a wedge-shaped cervical enamel notch [2]. Histologically, an atypical mineralized plate called cervical mineralized diaphragm (CMD) is found at the level of the cemento-enamel junction of the tooth, which may contribute to root malformation [1].
Differential diagnosis of MRIM from other dentin anomalies is necessary because of several shared radiographic and clinical characteristics. Dentin dysplasia (DD) type I is a well-known dentin anomaly with similar characteristics to MRIM, including a short root and narrowed pulp chamber with a normal crown appearance. DD type I is hereditarily expressed in the whole dentition, whereas MRIM is non-hereditarily expressed in certain teeth. Regional odontodysplasia is a nonhereditary disorder exhibiting root anomalies; however, it is expressed in all or some quadrants of dentition [3].
Although the etiology of MRIM is not yet clearly defined, it is thought to be associated with environmental factors rather than genetic factors [4]. The majority of reported cases of MRIM are related to medical histories in the first year of life, such as neurological conditions, prematurity or low birthweight, medications, and surgeries [5]. However, the occurrence of these medical histories in the first year of life that does not match the timing of root formation, which begins approximately at the age of 3 years or later, is still in question[6].
Complications of MRIM include spontaneous pain, periapical abscess, and tooth mobility, which can cause early loss of permanent teeth [7]. Therefore, proper management of MRIM is important to minimize these complications. There has been growing interest in MRIM, and studies have been made to demonstrate the etiology and clinical features of MRIM. However, to the best of our knowledge, limited cases have been reported that focus on disease patterns and treatment guidelines for MRIM. Therefore, this study aimed to investigate the clinical features of MRIM and contribute to the limited number of cases providing clinical considerations about its treatments.
Materials and Methods
This retrospective study was approved by the Institutional Review Board (IRB No.S-D20240013) of School of Dentistry, Seoul National University, Korea.
1. Study subjects
Patients with MRIM who visited Seoul National University Dental Hospital (SNUDH) and had panoramic radiographs taken between January 2010 and February 2024 were included in this study. Patients exhibiting generalized or segmental tooth malformation, including dentin dysplasia, dentinogenesis imperfecta, amelogenesis imperfecta, and regional odontodysplasia, and patients with a history of radiation or chemotherapy were excluded. A total of 64 patients with MRIM were included.
2. Methods
The criteria for radiographic diagnosis of MRIM were the presence of more than one molar tooth with a normal crown appearance but with underdeveloped roots and a constricted pulp chamber, or the presence of a permanent incisor tooth with enamel defects on the cervical area. Clinical data, including age, gender, follow-up period, medical history, distribution of the affected teeth, complications, treatment methods, and prognosis, were obtained from patients’ electronic medical records and panoramic radiographs. Radiographic diagnosis of MRIM-whether the tooth is affected or not-from 64 patients’ panoramic radiographs was done by two educated dentists, and interobserver reliability was assessed. Intraclass correlation coefficients were used to test the reliability of the distribution of MRIM teeth. The interobserver reliability was high with a value of 1.000 (p < 0.0001).
Results
1. Clinical features of patients with MRIM
A total of 64 patients with MRIM (25 males and 39 females) were analyzed. Females were affected more than males at a 1.56:1 ratio. The age of the patients at their first visit ranged from 2.2 to 15 (average, 8.2) years. The follow-up period ranged from 0 to 14.7 (average, 5) years. Among 64 patients, 49 patients (76.6%) were checked for MRIM more than 1 year, and 11 patients (17.2%) were lost to follow-up after the first visit.
Clinical features of MRIM, including short and slender roots of affected permanent and primary molars with constricted cervical areas, were noticed in panoramic radiographs. The malformation of the crown or dental caries and enamel hypomineralization were not observed. Affected incisors showed an enamel notch on the cervical area, and a malformed root was observed. Several complications related to MRIM were found, including periapical lesions, periapical abscess, root resorption, and alveolar bone loss. The patient’s clinical symptoms, such as pain, discomfort, and mobility, were also noticed. However, the severity of these complications varied between patients, from absence of complications to severe complications. There were asymptomatic patients without any signs of radiographic lesions, but the majority of patients were symptomatic with radiographic lesions. Of the 64 patients, 46 (71.9%) exhibited dental complications related to MRIM teeth.
Eruption disturbance was also observed in patients with MRIM. Eruption disturbance of adjacent teeth was noted in 24 patients (37.5%). Mesial angulation of the eruption path of the permanent first and second molars was observed when the primary second and permanent first molars had MRIM, respectively. The crown of the erupting tooth was locked at the cemento-enamel junction of the MRIM tooth, and resorption of the MRIM tooth root was observed (Fig. 1).
The medical histories were reported in 53 patients (82.8%). The most prevalent medical history was premature or low birthweight in 22 patients (34.4%), followed by neurological conditions including meningitis, myelomeningocele, hydrocephalus, cerebral hemorrhage, cerebral infarction, and cephalohematoma with 17 (26.6%). Among these neurological conditions, meningitis and myelomeningocele were the most frequent with 9 patients, followed by hydrocephalus, cerebral infarction, cerebral hemorrhage, and cephalohematoma. Surgeries in the first year of life were reported in 7 patients (10.9%), and receiving medications in the first year of life were in 6 patients (9.4%). Only 1 patient (1.6%) had a history of urinary tract infection. No medical history was reported in 11 patients (17.2%, Table 1).
2. Distribution of teeth with MRIM
Mandibular permanent first molars were affected in all 64 patients (100%), of whom 56 patients (87.5%) had also affected maxillary permanent first molars. Thus, all 4 permanent first molars were affected in 55 patients (85.9%). Maxillary permanent central incisors were affected in 24 patients (37.5%), and maxillary permanent lateral incisors were affected in only 1 patient (1.6%). No mandibular permanent central and lateral incisors were affected. Maxillary permanent canines were affected in 6 patients (9.4%), of whom 2 patients (3.1%) had also affected mandibular permanent canines. Moreover, 40 and 39 patients had discernible primary second molars and primary first molars, respectively. Maxillary primary second molars were affected in 36 patients (90%), and mandibular primary second molars were affected in 35 patients (87.5%) out of 40 patients. Primary first molars were affected in 5 patients (12.8%) out of 39 patients (Table 2, Fig. 2).
From the 64 patients included in this study, 466 MRIM teeth were found, including 239 permanent first molars, 48 permanent central incisors, 2 permanent lateral incisors, 15 permanent canines, 20 primary first molars, and 142 primary second molars.
3. Treatment methods and prognosis according to the treatment method
Treatment methods of MRIM included endodontic therapy, extraction, and periodic recall checks. Except for 11 patients who were lost to follow-up after the first visit, 6 patients (11.3%) received endodontic treatment, 32 patients (60.4%) had extraction, and 15 patients (28.3%) were doing periodic recall checks (Fig. 3). Endodontic treatment and extraction were done when there were periapical lesions or abscesses and clinical pain and discomforts. Extraction was also done when ectopic eruption of adjacent teeth was observed to allow the eruption of adjacent teeth. In the absence of any dental complications and symptoms, a periodic recall check was done.
In 6 patients who received endodontic treatment, 11 MRIM teeth were treated, including 2 permanent central incisors and 9 permanent first molars. Except for 1 patient, 5 patients had taken cone-beam computed tomography (CBCT) before the initiation of endodontic treatment. Of the 11 endodontically treated MRIM teeth, only 2 permanent first molars (18.2%) were healed without periapical lesions after treatment. Permanent central incisors were fractured at the cervical notch area after treatment, and other permanent first molars were hopeless with persistent periapical lesions and pain. The 2 teeth healed after endodontic treatment were both mandibular permanent first molars with 2 roots. The roots were thin and tapered but had a straight shape without curvature. From CBCT images, 3 main canals with 1 accessory canal and 2 main canals with 2 accessory canals were observed, respectively. Patency was not achieved in all accessory canals because these canals were not connected to the pulp chamber or partially obliterated inside the root. Although canal negotiation of all canals was not achieved, periapical lesions were healed after the treatment.
Moreover, 32 patients had more than one permanent tooth extracted, totaling 88 teeth, consisting of 85 permanent first molars (96.6%) and 3 permanent central incisors (3.4%). The average age at extraction was 10.9 years. To be more specific, the average age at extraction was 11.2 years, ranging from 7.8 to 16.6 in maxillary permanent first molars and 10.6 years, ranging from 8 to 16.5 in mandibular permanent first molars. After the extraction of permanent central incisors, a fixed or removable space maintainer with pontics was delivered. The management of posterior extraction sites included spontaneous space closure by the eruption of permanent second molars, space closure by orthodontic treatment, and space maintenance. Of the 27 patients, except for 5 patients who were lost to follow-up after extraction, spontaneous space closure was achieved in 23 patients, orthodontic treatment was given on 5 patients, and space maintenance was done on 3 patients, allowing duplicates (Fig. 4). When the prognosis of each extracted permanent first molar is measured, spontaneous space closure was achieved in 55 teeth, orthodontic treatment was done on 20 teeth, and space maintenance was done on 4 teeth. Space maintenance was done when there is no permanent third molar in the same quadrant.
Periodic recall checks were done on 15 patients. Among 15 patients who did not receive any treatment on MRIM teeth, 7 (46.7%) were asymptomatic with absence of radiographic lesions. Clinical characteristics of MRIM, such as thin and short roots with cervical constriction and root resorption of affected teeth, were noticed, but no signs of endodontic and periodontal lesions were found. Panoramic radiographs revealed periapical lesions or root resorption in the other 8 patients, yet they received no treatment. The reason for delaying the treatment included the absence of clinical symptoms or discomforts such as pain and mobility and the absence of a permanent third molar.
Discussion
MRIM is a dental anomaly affecting certain tooth types, including the permanent first molars, permanent incisors, and primary molars. In this study, females were affected 1.56 times more than males. However, in the study by Vargo et al. [8], males (56.3%) were affected more than females (43.7%), and in the study by Jensen et al. [9], males were affected 1.16 times more than females. Although the gender ratio in this study was different from previous studies, the gender difference in the prevalence of MRIM is challenging to define because of the lack of cases.
In accordance with previous studies, short and underdeveloped roots of molars with normal crown appearance and wedge-shaped enamel defects of incisors were noticed in this study [1]. MRIM teeth are related to various dental complications such as pain, mobility, periapical lesions or abscesses, root resorption, and alveolar bone loss [5]. In this study, the incidence rate of complications reached 71.9%, and the degree of complications was different between patients, affecting the treatment method and prognosis. Interestingly, complications related to MRIM teeth occur independently of dental caries. Lee et al. [7] investigated the pathogenesis of MRIM complications and reported that periodontitis in MRIM appeared to be localized tooth-related periodontitis caused by the anatomical structure of MRIM teeth. A concave cervical area could make plaque accumulation easier and difficult to keep clean.
Eruption disturbance of the adjacent teeth was another clinical finding. Ectopic eruption of the permanent first molar caused by the affected primary second molar was reported [10]. Moreover, Kim et al. [11] reported mesial locking of the permanent second molar and resorption of the distal root of the permanent first molar. Distal eruption of a second premolar with the unresorbed mesial root of the primary second molar was also reported. A similar phenomenon was observed in the present study, and the incidence rate was 37.5%. The etiology of ectopic eruption in MRIM patients is not completely understood. Generally, the etiology of the ectopic eruption of the permanent first molar is considered multifactorial, including family history, large tooth size, mesial angulation of the eruption path, undesirable shape of the primary second molar, eruption space deficiency, and insufficient marginal adaptation of the stainless-steel crown restored on primary second molars [12,13]. Ectopic eruption is also reported to be related to systemic and genetic diseases [14]. For instance, ectopic eruption or impaction of permanent molars were observed in patients with osteogenesis imperfecta. In this case, the posterior position of the maxilla or bulbous shape of crowns affected by dentinogenesis imperfecta was supposed to be the cause of the ectopic eruption [15,16]. Also, the prevalence of ectopic eruption of permanent first molar was higher in patients with the history of anticancer therapy, especially in patients who received anticancer therapy before 3 years old [17]. The anticancer therapy given before 3 years old may have disturbed the growth of teeth and eruption pattern of the permanent first molar. Likewise, the ectopic eruption in MRIM patients may be attributed to the bulbous shape of the crown due to cervical constriction and thin roots, and medical conditions in the first year of life may have disrupted the normal eruption of the permanent first molar.
The etiology of MRIM is uncertain; however, environmental stressors related to the patient’s medical history are suggested rather than genetic factors [4,18]. Furthermore, epigenetic factors were suggested as probable causes of MRIM because, in a previous study, only one of the monozygotic twins had MRIM [19]. According to Jensen et al. [9], a medical history was reported in 95.3% of 130 cases, and neurological conditions were the most prevalent (48.5%), followed by premature birth, surgery in the first year of life, and medications prescribed in the first year of life. In another study, 75% of the patients had medical conditions, and the most common was premature birth, followed by meningitis [8]. Other neurological conditions, including stroke, hydrocephalus, cranial injury, brain abscess, and cerebral palsy, were reported. These align with the findings of the present study that premature birth was the most prevalent medical history followed by neurological conditions, and meningitis was the most common neurological condition. From these results, premature birth and neurological conditions appear to be the most common medical conditions related to MRIM. However, finding the direct relationship between medical conditions and MRIM is still difficult because the timing of root development does not match the onset of medical conditions that mostly occur in the first year of life [6]. Root development of the permanent first molar, which is the most frequently affected tooth, begins approximately at the age of 3 years. Epigenetic consideration is suggested; however, further studies are needed to determine the exact etiology of MRIM.
Permanent first molars are the most commonly affected, followed by primary second molars and permanent central incisors [20]. In previous studies, permanent first molars were always involved, except for one case, and mandibular permanent first molars were all affected. Primary second molars and permanent central incisors were affected in approximately 35 - 40% of cases. Very few cases involving the permanent lateral incisors, canines, first premolars, second molars, and primary first molars were reported [8,9]. In accordance with previous studies, all mandibular permanent first molars were affected in the present study with a very high prevalence in maxillary permanent first molars. A similar distribution was observed in permanent incisors and canines; however, in this study, mandibular permanent incisors, premolars, and permanent second molars were not affected. Another difference was the high prevalence in primary molars, particularly in primary second molars. The relatively low prevalence in previous studies may be attributed to the difficulty in determining root deformity and cervical constriction because of physiologic root resorption by the eruption of the permanent successor. In addition, primary molars with MRIM may be lost earlier because of the short and thin root and ectopic eruption of the adjacent permanent first molars.
MRIM can cause various clinical problems that can lead to the early loss of permanent teeth; thus, proper MRIM management is important. The complex anatomical structure of root canals makes conventional endodontic treatment of MRIM teeth challenging [4]. The presence of CMD and accessory canals hampers the identification of all canal orifices, and some root canals are not connected to the pulp chamber. Thus, the possibility of perforation is also high. The CBCT and microscopy are recommended for successful endodontic treatment of MRIM [21,22]. However, reinfections can occur because of the complex structures, and long-term survival is not guaranteed [23]. In the present study, only a few teeth were endodontically treated with the aid of CBCT, and the success rate was 18.2%, supporting the reported difficulty in achieving successful endodontic management of MRIM teeth.
In severe MRIM, many of the affected teeth were eventually extracted; thus, extraction at proper timing was suggested as a treatment method [4,24]. In this study, more than half of the patients had their permanent teeth extracted. Because permanent first molars are critical for occlusion and function, careful diagnosis before extraction is important. When the development of a permanent third molar is observed, the extraction of the affected permanent first molar is favorable because the permanent second molar can replace the permanent first molar [24,25]. For bodily mesial movement of the permanent second molar, the extraction of the permanent first molar before the eruption of the permanent second molar is recommended [26]. The bodily mesial shift of the permanent second molar occurs better in the maxilla; however, in the mandible, the mesial inclination of the permanent second molar may occur because of high bone density. The recommended age at extraction is 8 - 10.5 years in the maxilla and 8 - 11.5 years in the mandible, and in common, the time when the permanent second molar is still within the bone would be beneficial [27]. The permanent third molar is first observed in panoramic radiographs at the age of 9.34 ± 1.35 years which is similar to the recommended age of permanent first molar extraction [28]. However, it can vary from 6 to 12 years among individuals, so periodic panoramic radiography is recommended.
In the absence of permanent third molars, it is valuable to maintain the affected tooth and delay the extraction; however, periodic recall checks should also be done because the MRIM teeth can cause various clinical problems such as pain, mobility, and endodontic or periodontal lesion [24]. In this case, a bilateral fixed space maintainer such as a mandibular lingual arch could be considered to stabilize the teeth with discomfort or mobility [29]. When extraction is planned, the extracted space should be preserved for future prosthetic treatment. Space maintenance can be achieved by using a removable or fixed-type appliance, and prevention of the extrusion of the opposing tooth should be considered in appliance design. In the case of dental crowding, the extraction space can be used for orthodontic treatment; thus, orthodontic analysis could be considered before extraction [24].
Overall, patient-specific treatment planning for MRIM is needed to avoid unnecessary discomforts. Based on the findings in this study, we suggest a recommended treatment guideline for MRIM (Fig. 5). MRIM can cause various clinical problems, including endodontic and periodontal lesions, but it also can be asymptomatic. The severity of symptoms is not equal in all patients. Therefore, evaluating the severity of symptoms would be the first step in choosing a treatment method. In patients without pain, periapical lesions, or abscesses, it is recommended to periodically re-evaluate the symptoms and follow-up. In symptomatic patients, conservative endodontic treatment and extraction could be considered. However, conventional endodontic treatment of MRIM teeth is challenging due to the complex root structure, and the risk of failure is high, so assessing the viability of canal negotiation and the severity of the lesion and alveolar bone loss with CBCT is necessary [21-23]. When extraction is planned, evaluating the development of permanent third molars is recommended. If the permanent third molar develops, extraction at the age of 8 - 11 years is favorable for mesial shift of the permanent second molar [27]. Otherwise, orthodontic treatment for space closure or space maintenance for future prosthetics could be considered.
Due to the rareness of the disease, limited cases of MRIM are available, and therefore the treatment strategy for MRIM is not completely established. The findings of the current study add to the limited number of MRIM cases and provide evidence for establishing treatment guidelines for MRIM. The etiology is not completely revealed, but reported medical histories in this study support the connection between the MRIM and medical conditions that occurred in the first year of life. Furthermore, the distribution of MRIM teeth in permanent and primary dentitions, especially the high prevalence in primary molars, was an important finding. However, as a retrospective study, this study had several limitations. First, bias may occur because the study participants were patients attending a university hospital. Furthermore, panoramic radiographs have limitations in providing clinical symptoms of MRIM such as pain and mobility. These clinical complications were collected from electronic medical records, so underestimation was possible, and symptom severity was not considered. Medical history was also based on electronic medical records obtained from parents, indicating the possibility of omission because parents may have forgotten events in the first year of life. Moreover, treatments were provided by different clinicians; thus, the criteria for deciding the treatment method could vary between clinicians.
Conclusion
MRIM exhibits diverse clinical symptoms and can lead to early loss of permanent teeth, which is critical for function, occlusion, and esthetic. Early diagnosis and proper treatment are needed to minimize MRIM complications. Interest in MRIM has increased since the first discovery; however, clinical information and appropriate treatment methods for MRIM are insufficient. This study contributes to the limited number of cases in the literature regarding clinical features of MRIM and proposes possible treatment options. Pediatric dentists should be aware of the clinical and radiographic characteristics of MRIM for early detection and provide timely treatment to prevent clinical problems. Further research is warranted to determine the exact etiology of MRIM and establish treatment standards.
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