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The dental pulp is an important soft connective tissue which is able to produce dentin over time as a reaction on external stimuli. It also maintains the biological and physiological vitality of the dentin. Due to this the pulp is essential for teeth homeostasis. However, dental caries is still one of the most prevalent health problems in dentistry and therefore, one major cause for early loss of the dental pulp vitality and subsequent tooth extractions. Meanwhile the potential for successful pulp regeneration therapy is increasing due to advances in the field of regenerative endodontics. Thus, adequate experimental animal models are required for testing and validating these new regenerative therapies. Rodents and rats in particular, are relevant models for experimental periodontal research. The breeding and housing costs of rats are relatively low facilitating studies with sufficient numbers for statistical analysis in comparison to bigger sized mammals like beagle dogs, miniature pigs or monkeys. Additionally, rat molar teeth and pulps are characterized by similar anatomical, histological, biological and physiological features to human teeth. Essential biological reactions of the pulp tissue and the interaction during the different stages of wound healing of rat molar teeth are comparable to that of other mammals. However, despite of the multiple research activities in the field of regenerative endodontics and the above mentioned advantages of the rat model only rare in vivo studies are published. Therefore, the presented study aimed to introduce the rat molar teeth as a valid model for studying dental pulp stem cell based endodontic tissue regeneration. Human dental pulp stem cells were implanted into the pulp of immunodeficient rats (RNU rats). Cell growth was supported by a collagenous membrane, which was applied on top of the cells after implantation. After closing the pulpal cavity with a light-polymerisable resin human dental pulp stem cells were able to maintain cell viability in the rat molar pulp niche for at least three weeks. This demonstrated the suitability of immunodeficient RNU rats for non-autologous dental stem cell based endodontic tissue engineering approaches.

Objectives. The majority of human maxillary first molars is usually described as having three roots, but different morphologies were documented in several studies and case reports. One very rare and less investigated anatomical anomaly is the occurrence of four radicular structures in the upper first molars. This communication aimed to define the prevalence of four-rooted maxillary first molars on a meta-analytical basis. The external and internal morphology of these teeth was described by the collection of published case reports. Materials and Methods. Six electronic databases were accessed to collect case reports dealing with four-rooted maxillary first molars, as well as population-based cone-beam computed tomography (CBCT) studies. Afterward, the publications were selected according to predefined inclusion/exclusion criteria and evaluated using the Joanna Briggs Institute Critical Appraisal tool. The teeth of the chosen case reports were then independently analyzed by two dental professionals according to different dental classifications. Furthermore, the population studies were meta-analyzed to calculate the global and regional prevalence of four-rooted maxillary molars. Results. Included were forty-nine population-based CBCT studies containing 26663 maxillary first molars. Upon these data, the global incidence of four-rooted maxillary molars was meta-analytically determined as 0.047% (95%-CI:0.011–0.103%). In combination with the case reports, it was pointed out that this anomaly is distributed worldwide. Furthermore, forty-eight case reports were included containing fifty-three maxillary molars with four roots. The analyzed teeth exhibited Versiani´s pulpal chamber floor Types A and B. The majority of four-rooted maxillary first molars were classified as Type I regarding Christie's configuration. But, also 7.54% of the altered teeth could not be described by this classification. 62.34% exhibited four root canals, but also variations with five, six, or seven canals were identified. Furthermore, a significant difference was found in the occurrence rate between male and female patients. Conclusion. Due to the worldwide occurrence, dental professionals should be aware of this rare anomaly to avoid treatment errors, especially during endodontic or surgical therapies.

The present communication describes the endodontic retreatment of a rare four-rooted maxillary second molar. A 25-year-old patient was referred to our dental practice requesting an apicoectomy because of continuous and permanent pain reaction six months after the first endodontic treatment. The sent radiograph demonstrated three filled root canals (one mesial, two distal) and four radiographically superimposing roots (two mesial, two distal). Due to the diagnosed chronic apical periodontitis and based on the visible untreated root canal, we decided to endodontically re-treat the quadrangular tooth against the referred apicoectomy. In the first session, only the previously untreated mesiopalatal root canal was mechanically prepared and filled with a corticosteroid- and tetracycline-containing paste. After two symptom-free weeks, the gutta-percha was removed from the other canals and calcium hydroxide was applied. Another two weeks later, the four root canals, whose orifices were “irregular quadrilateral” shaped on the pulp chamber floor representing Versiani Type A, were obturated. After the verification of this rare anatomy by cone beam computed tomography, the tooth was classified as Christie’s radicular type II. One characteristic of this type is parallel running buccal and palatal roots, which caused a radiographic superimposition and probably led to the endodontic mistreatment in the first case.