Evidence-Based Equine Dentistry: 5 Years of Peer-Reviewed Literature (2008 - 2013)

Take Home Message—It is of paramount importance that the equine dental profession approach dental problems in the horse with an open mind. The field of equine dentistry is beginning to benefit from multifaceted research. In recent years, basic biomechanical research has been undertaken with respect to dentition and the temporomandibular joint. Large retrospective studies evaluating diastema treatment, exodontia and sinus disease have been reported, however, the “gold-standard” – the randomized, controlled, blinded clinical trial is lacking in most facets of this highly important (yet often underrated) area of equine health care.

I. Introduction

Equine dentistry is one of the most common tasks performed by large animal practitioners.¹ Dental publications exist from circa 600 BC. Despite this incredible history, equine dentistry is only just becoming more science than art. Evidence-based approaches to clinical equine dentistry are rare for a number of reasons. For the majority of clinical situations there is no evidence upon which to base a clinical decision. It is even difficult to prospectively research this evidence satisfactorily because of the perceived importance of the intervention by veterinarians, equine dental technicians and owners. An example of this is odontoplasty (also known as dental “rasping” or floating), the most common dental procedure performed in the horse.² Amazingly, the importance of this common procedure has yet to be fully determined.

Galloway and Easley (2008) ³ wrote that “.... science progresses in a continuum which can be divided into four historical ‘Phases of Knowledge’ (Bader and Shugars, 2006).⁴ Through Phase 1, the ‘Age of Experts’ knowledge is accumulated through experience and uncontrolled observation, and is shared informally from person to person through apprenticeships. During Phase 2, the ‘Age of Professionalism,’ the opinions of the experts is disseminated through textbooks.

In Phase 3, the ‘Age of Science,’ knowledge is acquired through clinical trials and literature reviews. Scientific study allows an investigator to question the opinions of experts, and literature review exposes scientific investigation to peer scrutiny. This process is essential to define ‘factual’ information. Once a body of factual information is established, practitioners can move to Phase 4, the ‘Age of Evidence,’ where individual patient care decisions can be made based on the current best scientific evidence available.”

There has been a veritable explosion of interest in the field of equine dentistry in the last 15 to 20 years and the amount of new information being published is incredible. In 2008 Galloway and Easley suggested that equine dentistry was in the early stages of Phase 3 and that most of the knowledge being taught was “the unproven opinion of ‘experts.’” As a profession, we are moving deeper into Phase 3, however, studies on the necessity for and the benefits of commonly advocated dental procedures in horses remain lacking. Additionally, longitudinal studies following the majority of our “interventions” are completely lacking (except for two single reports following horses after sinus surgery and exodontia) and in these areas we fall back to late Phase 1 or early Phase 2.

The presentation will focus on peer-reviewed data from the last 5 years (in as many languages as can be accessed using Pubmed and CABI abstract sources) and be presented in discipline-basedsections.

II. General oral and dental anatomy

There have been several publications focused on dental anatomy, in particular endodontic anatomy. Windley et al. (2009)^5,6 examined 2 and 3D CT reconstructions of 126 equine cheek teeth showing that the number of interpulpar communications and pulpar volume decreased with tooth age. The internal anatomy of the maxillary teeth was significantly more complicated than the mandibular teeth. However, both subgroups showed consistent patterns in their pulpar and enamel morphology. This data appears at odds with that subsequently published by Kopke et al. (2012)⁷ using high resolution micro-computed tomography to examine the dental cavities of cheek teeth. These authors found a common pulp chamber in all teeth less than 5 years of age, but there was also one found in a tooth of 9 years. A wide variety of pulp configurations was found, typically however mesial and distal pulp compartments were seen. Maxillary teeth had up to 4 compartments but the number was not related to age. A segmented pulp was seen in 72% of mandibular teeth. The latter authors suggest that the improved resolution of the μCT above that of the clinical CT enabled them to visualize smaller inter-pulpar communications than those seen by Windley et al. (2009).⁵

Dacre et al. (2008a,b,c,d,e)^8-12 published a series of five seminal papers examining apical infections of cheek teeth, reporting on the normal endodontic anatomy, thickness of dentine in normal and infected teeth and etiopathological features of apical infection in both mandibular and maxillary teeth. This research group subsequently went on to publish on the thickness of sub-occlusal dentine (SOD) in horses of different ages (White and Dixon 2010) stating that there was a wide variation in the depth of this tissue above pulp horns, even within the same cheek tooth.¹³ Mandibular cheek teeth had more SOD than maxillary teeth, but no other differences were found. Further to this, Marshall et al. (2012) published on the amount of sub-occlusal dentine present in overgrowth cheek teeth, reporting overall that the mean thickness was 12.14mm (with a range from 1.87-36mm) in overgrowth teeth and 10.25 (range 2.64 – 17.2mm) in normal teeth.¹⁴ There was no significant difference between normal and overgrown mandibular cheek teeth; however in the maxillary teeth overgrown teeth had more sub-occlusal dentine. The most important point of this study was to state that in 49% of overgrown teeth, there was less subocclusal dentine, with the result that if these teeth were to be reduced to the level of their adjacent teeth, pulp exposure would have occurred in 58% of cases.

Casey and Tremaine (2010) examined the prevalence of secondary dentinal lesions (occlusal defects noted on oral examination with a mirror or endoscope) in cheek teeth from horses with clinical signs of pulpitis (as defined by the presence of bony swellings, external or oral discharging tracts or unilateral nasal discharge) and compared these to control teeth.¹⁵ They found that secondary dentinal lesions were significantly overrepresented in teeth that had pulpitis 56.5% of mandibular diseased teeth versus none of the controls and 57% of maxillary teeth compared to 1.6% of controls.

Fitzgibbon et al. (2010) examined the infundibulae of normal horses showing that variations in cementum was common and that in some cases localized cemental hypoplasia may have subsequently led to the development of infundibular caries.¹⁶ Severe infundibular caries were present in 8% of clinically normal horses. The authors found convincing evidence of an apical blood supply to the infundibulum in young horses. The high percentage of infundibular caries in molar teeth suggesting that the hypothesis of premature disruption of the deciduous caps leading to this condition is unlikely. Additionally, the highly irregular shape of the infundibula, incompletely filled with cementum, calls into question the practice of filling these with restorative materials.

III. General physiology and biomechanics

Huthmann et al. (2008a, 2009a, 2009b) published on position and curvature of equine cheek teeth and the effect of age, as well as on the Curve of Spee.^17,18 This then led to the biomechanical calculation of masticatory forces on the equine cheek teeth during mastication using a computer based model.¹⁹ This research group then published several finite-element analysis papers showing that due to age related changes in periodontal ligament elastic properties, the stress level on the ligament and surrounding bone increases with age and the level of intrusion (movement in a vertical direction within the alveolus) increases (Cordes et al. 2012a).²⁰ The second paper showed uniform distribution of stresses and strains during the closing stroke of the masticatory cycle, however, during the power stroke stresses and strains concentrated at the alveolar crest and periapical regions (Cordes et al. 2012b).²¹ Suggestions were that these force concentrations may set up conditions of local inflammation and necrosis suitable for microorganism growth.

IV. Ageing

Two papers were found on the subject of aging horses by their teeth. The first by Gaspardy et al. (2009) characterized age dependent alterations in the infundibular cup of incisor teeth of Hungarian and German horses.²² They measured the depth of the cup and the rate of yearly attrition and based on these results suggest altering the age of cup disappearance from 6-7-8 to 5-7-9 years. In the second paper, Lszcynski et al. (2011) reported on using the incisors for ageing Hucul horses and found, unsurprisingly that their use (when compared to actual breeding documentation) were not accurate enough alone. The accuracy fell substantially between ages 6 and 11.²³

In regard to cheek teeth, Ramzan et al. (2009) reported the chronology and sequence of emergence of permanent premolar teeth in the horse. A total of 508 premolar ‘caps’ were removed from 207 race horses in England. Age at removal was 35.1, 37.7 and 45.1months for PM2 (Triadan 06), PM3(07) and PM4(08) respectively. Later “cap” removal was significantly associated with caudal teeth, the upper jaw and the female gender.²⁴

V. Imaging

Equine dental imaging studies featured heavily in the last 5 years and spanned a number of different modalities.  [...]