Introduction
Dens invaginatus, formerly known as “dens in dente“ (tooth in tooth) is a rare developmental abnormality, when enamel and dentin are infolded inside the tooth. According to F. A. Oehlers there are three types: type I- invagination is limited to coronal part of the tooth, but does not cross the cemento-enamel junction (CEJ). Type II crosses the CEJ, but ends up blindly. It may or may not communicate with pulp chamber. Due to bacterial invasion, there is a loss of periodontal tissues and bone resorption present along the invagination. Type III- invagination is present throughout the tooth root and creates a separate periodontal opening, located close to apex or elsewhere on the wall of the tooth root. Through the invagination “canal“ the bacteria invade apical part of periodontal space leading to periapical inflammation. The pulp itself can remain vital or due to infection pulpitis or impairment of peridontium can occur.4

In humans, the prevalence is around 2%. Mostly affected are second incisors, followed by premolars of both jaws. Etiology is unknown. From pathophysiologic point of view it is a “hamartoma“, where ectoderm of the crown infolds deeply into the mesoderm of the dental pulp. Clinically the crown shape can be distorted, but often the diagnosis can be based only on radiologic examination. It is necessary to seal possible communication with composite restoration as soon as the tooth erupts. Affected teeth are predisposed to infection, caries and related complications. Eventual endodontic therapy is complicated, and if not successfull, extraction might be necessary.4

Thanks to similar radiologic appearance and clinical consequences of infected endodontium with periapical pathology the anomaly of mandibular carnassial teeth in dogs was considered a “dens in dente“ for a long time. A case of diagnosed dens invaginatus was described in a chimpanzee.1 Dens invaginatus was diagnosed bilaterally on CBCT examination in first molar of a chimpanzee skull as well as multiple calcifications in incisor teeth in this study. In each of the mandibular incisors a solitary flame-shaped or conical structure was detected, comprising more than 80% of the pulp chamber.

A recent study3 aimed to study mandibular first molar with an anomaly formerly called dens invaginatus or “enamel pearls” clinically, radiologically and histologically as well as with micro CT examination. Histologic examination was completed with immunohistochemistry using antibodies against amelogenin. Six dogs with eleven affected teeth were included in this study. Five dogs were affected bilaterally, one only unilaterally. Average body weight of affected teeth was 4 kg. Typical radiologic feature of affected teeth was often divergence of roots or parallel roots, present in six of eleven teeth. Dilacerated roots were present in three out of eleven teeth. 

Eight out of eleven affected teeth were extracted due to infection and periapical pathology or periodontal disease. Micro CT revealed heterogenic tissue of transition between dentin and enamel opacity. Histologically it was not dentin neither enamel. Many tubules of 20-40 μm were present, communicated with enamel fissures in two of eight teeth and with pulp in four out of eight teeth. Amelogenin was not detected with immunohistochemistry in any of the samples. Authors of this publication revealed different characteristics of malformation formerly formerly called “dens in dente” of first mandibular molar in dogs. The developmental origin as well as any factors linked to it´s development were not revealed. It was concluded this abnormality resembles malformation of molars and incisors in humans.2

Case No. 1
The patient was russion bolonka, 2,5 years old, female, weighing 5,1 kg. Originally the dog was presented for right mandibular canine tooth being not fully erupted. This tooth due to labioversion combined with lingvoversion caused traumatic malocclusion (type I). Dental examination revealed furcation defect of grade two in both first mandibular molars. Radiologicaly there was evident abnormal structure of dentin radiopacity within pulp chambers in furcation area. There was evident beginning periapical lucency at both roots. The roots of affected teeth were convergent and dilacerated. It was a bilateral finding. (Picture 1,2) The owner did not agree with recommended extraction of the affected teeth and another radiologic recheck was planned in six months with possible subsequent therapeutic measures. Radiologic recheck in six months showed mild to significant enlargement of periapical lucencies (Picture 3,4), confirming ongoing endodontal infection of both teeth. After obtaining owner´s agreement, the teeth were surgically extracted. The clinical appearance of the tooth prior to extraction was not remarkable apart from mild gingival recession at the furcation area (Picture No. 5), but after flap elevation abnormal formation of furcation region of the first molar was evident. (Picture No.6)

Case No. 2
Seven months old intact male dachshund weighing 7 kg was presented for radiographic examination and in both mandibular first molars radiodense small structures were detected. (Picture No. 7) Apexogenesis due to young age was not completed, there were no signs of endodontal infection evident. Radiographic monitoring in six-months interval was suggested to the owner.

Case No.3
Nine year old female patient of japanese chin, weighing 9,2 kg was presented for advanced periodontal disease. Marked furcation defect (grade 3) was noted in both first mandibular molars on dental exam. Radiologicaly this finding was supported by loss of alveolar bone in furcation area, both roots of those teeth had periapical lucencies evident. This finding was more profound in the right mandibular first molar, where in the distal root at the apex external inflammatory root resorption was evident due to longstanding inflammation. In the area of furcation, in the crowns of both teeth there was evident abnormal structure of dentin radiodensity. None of the teeth had convergent roots. (Picture 8,9)

Case No 4.
5 months old male sheltie puppy, weight 5,2 kg was presented for lingvoversion of deciduous mandibular canines. On dental examination no tooth revealed any abnormal shape. Radiologically, as an accidental finding, there were evident extensive dentin-opacity structures in crowns of both mandibular first molars. In this dog similar structures were found also in first mandibular and maxillary premolars, maxillary fourth premolars and first molars. (Picture 10,11,12) The pathology was bilateral in all teeth. This patient was suggested to be monitored radiologically for possible endodontal infection development.

Discussion
Malformation of mandibular carnassial teeth in newly named pathology, which in many patients leads to endodontal infection development with consequent periapical abscedation. Above described case reports illustrate and emphasize the importance of full mouth radiologic examination in every patient as a basic part of dental examination. Animals with accidental finding should be monitored radiographically and if signs of infection appear, appropriate treatment should follow. No other therapy than extraction is recommended, considering histologic presence of tubules/canals in pathologic tissue possibly leading infection into endodontium. It can not be assumend endodontic therapy could be successful as it can be in dens invaginatus cases in people. For succesfull endodontic therapy an excellent sealing of the tooth is necessary, in standard situation by composite restoration. In case the communication of endodontal system is present (not sealed) after the therapy, bacterial invasion into the pulp canals continues and there is no chance of succesfull root canal treatment. This disease is often manifested as furcation periodontal involvement as illustrated in above described case reports. Due to the possibility of communication of endodontal system with oral cavity, it could be considered a specific endo-perio lesions, meaning infection of peridoncium could be caused primary by spread of infection from infected endodotium. Other additional etiologic factor could be increased plaque retention in malformed area just above furcation, leading to periodontal disease progress in that specific area.

In conclusion it will be very interesting to follow scientific progress in this topic, as similar structures to those documented in mandibular first molar can be present in other teeth as well as documented in case No.4. It will be necessary to clear out etiopathogenesis of this malformation, for instance there is an interesting question why in some individuals it leads to endodontal infection and in some not. It is definitelly adviceable to recommend full mouth radiologic examination and regular radiologic monitoring in patients with radiologically accidentally diagnosed malformation of mandibular first molar.

References
1. AL-AMERY. Unusual Dental Morphology in a Chimpanzee: A Case Report Utilizing Cone-Beam Computed Tomography. Journal of Veterinary Dentistry 2018
2. LEE, H.-S. A new type of dental anomaly: molar-incisor malformation (MIM). Oral Surg Oral Med Oral Pathol Oral Radio l2014;(6)
3. NG, K. Mandibular Carnassial Tooth Malformations in 6 Dogs-Micro-Computed Tomography and Histology Findings. Frontiers in Veterinary Science2019
4. ŠEDÝ J. Kompedium stomatologie II. Praha: Triton 2016

Author´s address
MVDr. Kateřina Slabá, Veterinární stomatologie, Písek, www.veterinarni-stomatologie.cz