Endodontic Treatment of Incisors and Canines

Endodontic treatment is the treatment of the dentino-pulp complex. This should be distinguished from restorative treatment of caries, which in horses is largely confined to treatment of infundibular cemental caries, a disease which is often symptomatic and this treatment aims to arrest progression of caries [1]. The indications for endodontic treatment are pulpitis of one or more pulp horns, with the aim of salvaging the tooth in order to reduce morbidity associated with exodontia, including operative complications, occlusal consequences and cost.

Incisors and canine teeth in horses and donkeys have different structures and functions compared with the cheek teeth, an observation that was appreciated by early veterinary authors who referred to them as ‘nippers, tushes and grinders’ respectively.

Common pathologies of dental tissues including caries, periodontitis and pulpitis can be identified involving both the incisors and canines, and they are more vulnerable to traumatic fracture. Wear disorders of the incisors can occur in horses grazing low grass on rocky soil, or associated with crib-biting, that leads to accelerated occlusal attrition and asymmetry of the incisors.

Incisors particularly, and to a lesser extent canines, are sensitive and consequently apparently minor lesions of these teeth can cause pain. This can manifest in a range of signs which often go overlooked, that can include slower grazing, sensitivity to being bitted, asymmetric food prehension, equitation problems, headshyness, weight loss, halitosis and hypersalivation.

Examination of the incisors and canines is straightforward but often overlooked in the zeal of the recently qualified veterinarian or technician excited to place the speculum, or in the old complacent equine vet, who may overlook their sensitivity. Visual inspection commences by parting the horse’s lips to reveal the labial surfaces and gingiva, and digital pressure on the hard palate in the interdental space encourages mouth opening to enable inspection of the occlusal surfaces. Palpation of the bars of the mouth for sensitive or any unerupted canines completes the initial appraisal. Incisor specula made from malleable materials are available but are often poorly tolerated by unsedated horses.

In humans, a study of 1,312 patients treated endodontically with a mean follow-up of 3.6 years reported that 3.3% of the index teeth were extracted, 2.2% underwent retreatment, 3.6% had pain on percussion and 10.6% had periapical radiolucencies, for a combined failure rate of 19.1% [2]. In another study, endodontic treatment was considered successful with normal periapical structures in 64% of the cases. However, only 41% of the root fillings were of optimal quality [3].

In dogs 69% of roots treated were considered to have been successful based on pre and post-endodontic treatment radiography, with 39% showing no evidence of failure and 6% failures [4].

Equine hypsodont teeth are very different in anatomy from brachydont teeth [5,6] and this complexity presents particular challenges for any attempted endodontic treatment. Clinical signs of apical pulpitis include periapical coronitis, remodelling of periapical bone, gingival, transcutaneous or intrasinus suppuration, and the presence of fissures in subocclusal secondary dentine. Periapical changes are detected on radiographs and, with the assistance of computed tomography (CT), endodontic changes can be detected. CT has been shown to be considerably more sensitive than radiography for maxillary cheek tooth pulpitis, but good quality digital radiographs are sensitive for detecting signs of pulpitis in canines and incisors. Changes include incisive or rostral mandibular bone lysis, intrapulpal gas attenuation, endodontic dentinal loss of attenuation, periapical granuloma and identification of apical endodontic tracts [7-9]. Traditionally teeth affected by chronic pulpitis have been extracted but awareness of the morbidity and complications associated with this has ignited interest in endodontic therapy.

Endodontic therapy has been performed in incisors for fractured teeth and those with pulpitis, canines and premolars and molars. In these teeth that are accessible with relatively uncomplicated dentino-pulp systems, outcome seems anecdotally to be superior to endodontics in cheek teeth, although data on outcomes are sparse. Complex fractures involving the pulp of incisors should be treated as early as practically possible. Pulp capping using calcium hydroxide powder or paste may help to protect vital exposed pulp and induce dentogenesis. Exact time expectancy for salvage of exposed or contaminated pulp is currently lacking, although anecdotal case reports suggest possible viability for at least 48 hours. Exposed pulp should be debrided (pulpotomy) using a burr and if the pulp is inflamed (as indicated by ongoing capillary haemorrhage after debridement) then total pulpectomy using files should be considered. In teeth suspected to be afflicted by pulpitis without occlusal pulp exposure, exposure of the diseased pulp is achieved using a burr from the occlusal surface. Secondary dentine is removed to widen the cavity until the pulp canal is exposed. Thereafter, widening of the approach to enable instrumentation with H and K files is done. Files are used manually and with rotary instruments to remove and debride all necrotic material from pulp canals. Once this has been achieved as thoroughly as possible, and confirmed radiographically, chemical disinfection using Dakin’s solution or a similar product is performed, followed by extensive irrigation with isotonic saline until the pulp cavity is deemed to be sufficiently clean. Vital pulp can be partially resected and, if not inflamed, treated using a pulp sealer to disinfect and induce odontogenesis. Where vital pulp remains, partial pulpectomy is followed by mechanical and chemical debridement of the affected pulp canal, followed by root sealing and canal obturation. Root sealers commonly used are calcium hydroxide-containing products, although MTA is also suitable. Canal obturation is performed and temporary luting cement appears to be suitable as etch bonding is not necessary. This is compacted to maintain good contact with endodontic dentine. Thereafter, occlusal restoration is performed when an orthograde approach is used, using etch bonding techniques in multiple layers with flowable or light cured dental composites in 2 mm layers.

There are notable challenges for equine endodontic therapy. Detection of pulpitis at an early stage is elusive. Signs including pain and sensitivity in humans are poorly recognised in horses and subtle periapical radiolucency on radiographs is not always apparent. The very complex anatomy and narrow pulp horns do not lend themselves to orthograde pulp canal debridement, and the access to the occlusal surface is very limited in equine cheek teeth. Instrumentation for pulp canal debridement is made for humans (although recently equine files have become available) but complete mechanical debridement of diseased pulp is challenging. Deposition of root canal sealers such as calcium hydroxide paste apically in equine pulp horns is difficult, with apices being up to 9 cm from the occlusal surface. Materials used for pulp canal obturation include temporary luting cements and resin cements and, although clinical evidence is encouraging, no testing of these materials in equine dentition has been conducted. Unlike brachydonts where restorations can be semipermanent, the occlusal attrition intrinsic to hypsosont mastication means that any restorations will undergo attrition, risking breaking of occlusal seals and will require replacement ultimately. In vivo, apicectomy is reported in only a few cases with isolation, haemostasis and drying of the treated apex being very challenging. In addition, the apex of caudal maxillary and mandibular teeth is very inaccessible, and impossible except in anaesthetised horses. Extraction, apicectomy, endodontic treatment and re-implantation of the restored tooth has been successfully performed [10] and might offer a suitable alternative to orthograde approaches in young horses where there is relatively little dentine relative to pulp volume in the dental structure. To date, success rates reported are encouraging although these have yet to be widely replicated [11,12]. Further developments in diagnostic techniques to identify cases earlier, research on performance of materials in equine teeth, validation of canal debridement efficacy and outcome data are needed before these techniques can become widespread among suitably-trained experts.

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