A 6 years old horse sustained a traumatic mandibular fracture and damage to 3 incisors surgical repositioning resulted in successful heeling. The horse was followed over a 9 year period with radiographs taken every 2nd year.

Resorption of the apices of two of the affected teeth (401 and 402) occurred but was complete 1 year after the trauma. A retained loose root-fragment (from 403) was totally resorped after 7 years. After 2 years and until last examination, the dentin was blood-stained at the occlusal surface of 401 and 402. There was no dental treatment of the incisors during the 9 years of observation.

The horse was used for dressage, jumping and riding without any complaints during the entire study.

Key words: Equine incisor fracture, Equine retained root, replacement resorption in Equine teeth. Replantation Equine dentistry

The general goal for treatment of retained tooth root fragments is to remove them. However, sometimes the position of the fractured root is such that removal is technically difficult and the chances of damage to adjacent tissues can outweigh the benefit of removing the fragmented root [1].

In humans, the treatment of horizontal positioned, impacted third mandibular molars has been managed by coronectomy and the rest of the root remained untreated. Only in very few cases of reaction in the soft tissue e.g. inflammation or if the socket failed to heal, will the patient be treated by complete root removal [2, 3]. Even in horses partial crown removal has an excellent prognosis [4].

Among humans a study [5] showed that out of 2000 patients referred for removal of a retained root fragment only 16.2 % were associated with symptoms or pathology. Treatment was reserved for patients with clinical symptoms related to their retained root fragment.

Eruption of the permanent canine teeth in dogs and cats does not replace the primary teeth, but the permanent canine teeth will erupt lingual to the primary mandibular canine teeth. The primary teeth will undergo resorption starting on the lingual part of the teeth in the sub gingival area, eventually fracture transversally. The retained part of the root will normally undergo resorption very rapidly, so that the permanent teeth can move into its normal position. This resorption and fracture process very seldom results in any clinical problem. The dead root fragment in normal developmental evolution can easily be resorped normally without problem.

A 6 year old horse, gelding, was brought in to the Teaching Hospital for Large Animal at the University of Copenhagen with a history of fractured mandible.

Examination of the horse at presentation in the Clinic the horse showed that the 3 right mandibular incisors (401, 402 and 403) were ventrally displaced. On radiographs mandibular fracture was displaced by approximately 1 cm, and there was a fracture of 403 about 1 cm from the apex (see fig 1 and 2). All radiographic pictures are taken according to “open mouth technique” [6].

The horse was anaesthetized and the mandibular fracture was reduced and stabilized with steel wire around the incisors fixed to the right mandibular premolar teeth (fig 3) using an established technique [7]. The horse was discharged from the Hospital after 3 days, and the owners were instructed to allow liquid food only for the following week. The wound healed uneventfully and the horse returned to the clinic for a routine checks after 1 month (fig 3), 3 months post operatively (fig 4). At the 3 month check, the mandibular fracture had healed and the steel-wires were removed.

Recheck was performed at the owner’s premises every 2nd year with the final check 9 years after the acute fracture (figs 5, 6, 7, 8, 9). It was observed, that the free fracture-element of apex of 403 persisted for 6 years whereas the resorption of the nonfractured apex of 401 and 402 was completed after13 months. After 2 years black colored dentin area was observed in the occlusal surface of 401 and 402. This persisted during the rest of the follow up period (7 years) (fig 10, 11 and 12). In the latest final follow up visits, photographs of the occlusal surface of the mandible were taken (Fig 10, 11, 12). These showed normal alignment of the mandibular incisors arcade.

The length of the mandibular incisors at the fracture site (401 to 403) was gradually reduced, but the pulp diameters seem to be of the same size as the noninvolved incisors. It is probable that vascularisation to the roots was restored. The alveolar bone adjacent to the affected incisors was replaced with normal trabucular bone tissue. (fig 7 and 8 compared with fig 3) after the resorption of the apical end of the teeth.

Studies of human injuries, avulsion and severely intruded single root incisors show that only between 36 and 54 % of the periodontal ligament remained on the damaged teeth [8]. The importance of intact periodontal ligament for survival of reimplanted teeth has been shown [9].

The reason for the rapid resorption of the apical part of 401 and 402 is the lack of nutrition through the periodontal ligament (PDL) which is seen on fig 3, 4 and 5. The fractured apex of 403 stayed in place and therefore the PDL was not destroyed and so this “intact” part of the root remained vital for several years. It was resorped totally after 7 years (fig 9) without any infection or discomfort for the horse. In man this resorption is followed by replantation in 42.3 % of the teeth within the first year after replantation [9]. Another study [10] showed resorption of replanted roots occurred in 51 % of teeth and inflammatory root resorption 23 %, even if the replanted teeth were treated with disinfection material (tetracycline) before the replantation.

The intact PDL is very important for survival of the root [12] which also is observed in this case. The fractured apical fragment of 403 persisted for 7 years and very slowly resorped during the last 4 years. At the beginning the fragment was filled with dentin in the original pulp and surrounded by cement as may be seen in similar human cases [13]. Therefore the fragment was observed to be more and more opaque on the radiographs and eventually showed irregular borders. In the following years total resorption occurred (fig 7 and 8) with the roots replaced with normal structured trabecular bone that merges with the surroundings.

Among humans [1, 5] retained roots after tooth extraction is a very small problem and when they do occur they are often considered to be incidental findings. Therefore it is recommended to carefully consider the extraction of minor toot root fragments [1]. The present study supports this idea of allowing retained tooth root fragments to be resorped naturally in horses with acute injuries to healthy incisor teeth. In cases with diseased teeth the problems are not the same. In cases with pathology related to fractured teeth the remnants should be removed [14, 15].

In the horse described in this report, the pulp was ruptured and blood was partly resorped during the revascularization which appeared in connection with the fracture heeling. However, some black miscoloring in the new dentin formed gradually and became visible when the occlusion surface weared down. The black color could be observed after 2 years (fig 10) and persisted during the rest of the observation period (7 years) (fig 9 and 10).

Some studies [11] showed that some teeth have continued root formation in the periapical tissue, which was explained by the mesenchymal stem cells in the apical papilla of permanent immature teeth. These cells are the source of odontoblasts and can develop root dentin. The conclusion was therefore, if infection can be prevented tooth root healing can occur - even grove in some instances. It is still not yet clear if the odontclasts responsible for the physiological resorption of the roots are the same as those responsible for bone resorption [15].

The horse in this report was used for dressage and jumping riding without any complaints during the entire period of study (9 years). There had been no teeth anomaly after the fracture incidence, and after 2 years the alignment and the occusal surface were normal. There were no clinical findings related to the fracture except for the black colored dentin in 401 and 402.

1. Nayyar J, Clarke M, O'Sullivan M, Stassen LF. Fractured root tips during dental extractions and retained root fragments. A clinical dilemma? Br Dent J. 2015; 13: 218(5):285-90. Doi: 10.1038/sj.bdj.2015.147
2. Cilasun U, Guzeldemir E, Pektas ZO. Coronectomy in patients with high risk of inferior alveolar nerve injury diagnosed by computed tomography. J Oral Maxillofac Surg. 2011;69(6):1557-61. Doi: 10.1016/j.joms.2010.10.026
3. Patel V, Sproat C, Kwok J, Beneng K, Thavaraj S, McGurk M. Histological evaluation of mandibular third molar roots retrieved after coronectomy. Br J Oral Maxillofac Surg. 2014; 52:415-9. Doi: 10.1016/j.bjoms.2014.02.016
4. Rice MK, Henry TJ. Standing intraoral extractions of cheek teeth aided by partial crown-removal in 165 horses (2010-2016) equine Vet J. 2018: 50(1); 48-53. Doi: 10.1111/evj.12727
5. Helsham RW. Some observations on the subject of root of teeth retained in the jaws as a result of incomplete exodontias. Aust Dent J. 1960; 5: 70-77.
6. Arnbjerg J, Radiographing the Incisor and Canine Teeth of Horses. J Vet Dent 1996;13:74-75
7. Knox PM, Crabill MR, Honnas CM. Chapter 20 in Equine Dentistry, 2nd ed. Ed. Baker GJ, Easley J. Elsevier/Saunders, 2005, Edinburgh 316-320.
8. Haas M, Kenny JD, Casas MJ, Barret EJ. Characterization of root surface periodontal ligament following avulsion, severe intrusion or extraction: preliminary observations. Dent Traum 2008; 24(4):404-9. Doi: 10.1111/j.1600-9657.2008.00626.x
9. Zhou Y, Li Y, Mao L, Peng H, Periodontal healing by periodontal ligament cell sheets in a teeth replantation model. Arch Oral Biol 2012: 57(2); 169-76. Doi: 10.1016/j.archoralbio.2011.08.008
10. Chappis V, von Arx T. Replantation of 45 avulsed permanent teeth: a 1 year follow-up study. Dent Traumatol. 2005; 21(5):289-96. Doi: 10.1111/j.1600-9657.2005.00330.x
11. Souza BDM, Dutra KL, Kuntze MM, Bortoluzzi EA, Flores-Mir C, Reyes-Carmona J, Felippe WT, Porporatti AL, De Luca Canto G. Incidence of Root Resorption after the Replantation of Avulsed Teeth: A Meta-analysis. J Endodont. 2018; 44(8):1216–1227. Doi: 10.1016/j.joen.2018.03.002
12. Pohl Y, Filippi A, Kirschner H. Results after replantation of avulsed permanent teeth. II. Periodontal healing and the role of physiologic storage and antiresorptive-regenerative therapy. Dent Traumatol. 2005; 21(2):93-101. Doi: 10.1111/j.1600-9657.2004.00298.x
13. Staszy C, Bienert A, Kreutzer R, Wohlsein P, Simhofer H. Equine odontoclastic tooth resorption and hypercementosis. Vet J. 2008; 178(3): 372-379. Doi: 10.1016/j.tvjl.2008.09.017
14. Arnbjerg J. Dental resorption in an old pony: case report. Dent Oral Craniofac Rec 2016; 2: 276-279. Doi: 10.15761/DOCR.1000161
15. Harokopakis-Hajishengallis E. Physiologic root resorption in primary teeth: molecular and histological events E J Oral Sci. 2007; 49(1): 1-12.